ArticlePDF AvailableLiterature Review

Abstract and Figures

Background: Sports nutrition is a constantly evolving field with hundreds of research papers published annually. In the year 2017 alone, 2082 articles were published under the key words 'sport nutrition'. Consequently, staying current with the relevant literature is often difficult. Methods: This paper is an ongoing update of the sports nutrition review article originally published as the lead paper to launch the Journal of the International Society of Sports Nutrition in 2004 and updated in 2010. It presents a well-referenced overview of the current state of the science related to optimization of training and performance enhancement through exercise training and nutrition. Notably, due to the accelerated pace and size at which the literature base in this research area grows, the topics discussed will focus on muscle hypertrophy and performance enhancement. As such, this paper provides an overview of: 1.) How ergogenic aids and dietary supplements are defined in terms of governmental regulation and oversight; 2.) How dietary supplements are legally regulated in the United States; 3.) How to evaluate the scientific merit of nutritional supplements; 4.) General nutritional strategies to optimize performance and enhance recovery; and, 5.) An overview of our current understanding of nutritional approaches to augment skeletal muscle hypertrophy and the potential ergogenic value of various dietary and supplemental approaches. Conclusions: This updated review is to provide ISSN members and individuals interested in sports nutrition with information that can be implemented in educational, research or practical settings and serve as a foundational basis for determining the efficacy and safety of many common sport nutrition products and their ingredients.
Content may be subject to copyright.
R E V I E W Open Access
ISSN exercise & sports nutrition review
update: research & recommendations
Chad M. Kerksick
1*
, Colin D. Wilborn
2
, Michael D. Roberts
3
, Abbie Smith-Ryan
4
, Susan M. Kleiner
5
, Ralf Jäger
6
,
Rick Collins
7
, Mathew Cooke
8
, Jaci N. Davis
2
, Elfego Galvan
9
, Mike Greenwood
10
, Lonnie M. Lowery
11
,
Robert Wildman
12
, Jose Antonio
13
and Richard B. Kreider
10*
Abstract
Background: Sports nutrition is a constantly evolving field with hundreds of research papers published annually. In
the year 2017 alone, 2082 articles were published under the key words sport nutrition. Consequently, staying current
with the relevant literature is often difficult.
Methods: This paper is an ongoing update of the sports nutrition review article originally published as the lead paper
to launch the Journal of the International Society of Sports Nutrition in 2004 and updated in 2010. It presents a well-
referenced overview of the current state of the science related to optimization of training and performance
enhancement through exercise training and nutrition. Notably, due to the accelerated pace and size at which
the literature base in this research area grows, the topics discussed will focus on muscle hypertrophy and
performance enhancement. As such, this paper provides an overview of: 1.) How ergogenic aids and dietary
supplements are defined in terms of governmental regulation and oversight; 2.) How dietary supplements are
legally regulated in the United States; 3.) How to evaluate the scientific merit of nutritional supplements; 4.)
General nutritional strategies to optimize performance and enhance recovery; and, 5.) An overview of our current
understanding of nutritional approaches to augment skeletal muscle hypertrophy and the potential ergogenic value of
various dietary and supplemental approaches.
Conclusions: This updated review is to provide ISSN members and individuals interested in sports nutrition
with information that can be implemented in educational, research or practical settings and serve as a foundational
basis for determining the efficacy and safety of many common sport nutrition products and their ingredients.
Keywords: Sports nutrition, Performance nutrition, Position stand, Review, Recommendations, Efficacy, Double-blind,
Randomized, Placebo-controlled, Dietary supplements, Ergogenic aids, Weight gain, Hypertrophy, Strength, Capacity,
Power
Background
Evaluating the scientific merit of articles and advertise-
ments about exercise and nutrition products is a key
skill that all sports nutrition professionals must possess.
To assist members and other advocates of the
International Society of Sports Nutrition (ISSN) in
keeping up to date about the latest findings in sports
nutrition, the ISSN Exercise & Sports Nutrition
Review: Research & Recommendations has been up-
dated. The initial version of this paper was the first
publication used to help launch the Journal of the
International Society of Sports Nutrition (JISSN, ori-
ginally called the Sports Nutrition Review Journal).
This paper provides a definition of ergogenic aids
and dietary supplements and discusses how dietary
supplements are legally regulated. Other sections
highlight how to evaluate the scientific merit of nu-
tritional supplements and provide general nutritional
strategies to optimize performance and enhance re-
covery. Finally, a brief overview of the efficacy sur-
rounding many supplements commonly touted to
* Correspondence: ckerksick@lindenwood.edu;rbkreider@tamu.edu
1
Exercise and Performance Nutrition Laboratory, School of Health Sciences,
Lindenwood University, St. Charles, MO, USA
10
Exercise & Sports Nutrition Lab, Human Clinical Research Facility, Texas
A&M University, College Station, TX, USA
Full list of author information is available at the end of the article
© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38
https://doi.org/10.1186/s12970-018-0242-y
promote skeletal muscle hypertrophy and improve phys-
ical performance is provided. Based upon the available sci-
entific literature testing the efficacy and safety of the
nutritional supplements discussed herein, all nutritional
supplements discussed in this paper have been placed into
three categories based upon the quality and quantity of
scientific support available:
A) Strong Evidence to Support Efficacy and Apparently
Safe
B) Limited or Mixed Evidence to Support Efficacy
C) Little to No Evidence to Support Efficacy and/or
Safety
Since the last published version of this document in
2010 [1], the general approach to categorization has not
changed, but several new supplements have been intro-
duced to the market and are subsequently reviewed in
this article. In this respect, many supplements have had
additional studies published that has led to some supple-
ments being placed into a different category or removed
from the review altogether. We understand and expect
that some individuals may not agree with our interpreta-
tions of the literature or what category we have assigned
a particular supplement, but it is important to appreciate
that some classifications may change over time as more
research becomes available.
Definition of an ergogenic aid
An ergogenic aid is any training technique, mechanical
device, nutritional ingredient or practice, pharmacological
method, or psychological technique that can improve
exercise performance capacity or enhance training adapta-
tions [24]. Ergogenic aids may help prepare an individual
to exercise, improve exercise efficiency, enhance recovery
from exercise, or assist in injury prevention during intense
training. Although this definition seems rather straightfor-
ward, there is considerable debate regarding the ergogenic
value of various nutritional supplements. A consensus
exists to suggest that a nutritional supplement is ergogenic
if peer-reviewed studies demonstrate the supplement
significantly enhances exercise performance following
weeks to months of ingestion (e.g., promotes increases in
maximal strength, running speed, and/or work during a
given exercise task). On the other hand, a supplement
may also have ergogenic value if it acutely enhances the
ability of an athlete to perform an exercise task or
enhances recovery from a single exercise bout. The ISSN
has adopted a broader view regarding the ergogenic value
of supplements. While the muscle building and perform-
ance enhancing effects of a supplement on a single bout
of exercise may lead to eventual ergogenic effects or opti-
mized training adaptations, our view is that such evidence
does not warrant Excellent Evidence to Support Efficacy
if there is a lack of long-term efficacy data. Herein, we have
adopted the view that a supplement is clearly ergogenic if
most of human studies support the ingredient as being
effective in promoting further increases in muscle hyper-
trophy or performance with exercise training. Conversely,
supplements that fall short of this standard and are only
supported by preclinical data (e.g., cell culture or rodent
studies) are grouped into other categories.
Definition and regulation of dietary supplements
The Dietary Supplement Health and Education Act (DSHEA)
and the safety of dietary supplements
Congress passed the Dietary Supplement Health and Edu-
cation Act of 1994 (DSHEA), placing dietary supplements
in a special category of foods. In October 1994, President
Clinton signed DSHEA into law. This statute was enacted
amid claims that the Food and Drug Administration (FDA)
was distorting the then-existing provisions of the Food,
Drug, and Cosmetic Act (FDCA) to improperly deprive the
public of safe and popular dietary supplement products.
The law defines a dietary supplementas a product that
is intended to supplement the diet and contains a dietary
ingredient. By definition, dietary ingredientsin these
products may include vitamins, minerals, herbs or other
botanicals, amino acids, and substances such as enzymes,
organ tissues, and glandular extracts. Further, dietary ingre-
dients may also include extracts, metabolites, or concen-
trates of those substances. Dietary supplements may be
found in many forms such as tablets, capsules, softgels, gel-
caps, liquids, or powders, but may only be intended for oral
ingestion. Dietary supplements cannot be marketed or pro-
moted for sublingual, intranasal, transdermal, injected, or
in any other route of administration except oral ingestion.
A supplement can be in other forms, such as a bar, as long
as the information on its label does not represent the prod-
uct as a conventional food or a sole item of a meal or diet.
Indeed, DSHEA clearly defines dietary supplements
and dietary ingredients,it sets certain criteria for new
dietary ingredients,and the law prevents the FDA from
overreaching. Additionally, and contrary to widespread
opinion, DSHEA did not leave the industry unregulated.
The dietary supplement industry is in fact regulated by
the FDA as a result of DSHEA. The law ensures the au-
thority of the FDA to provide legitimate protections for
the public health. The Federal Trade Commission (FTC)
also continues to have jurisdiction over the marketing
claims that dietary supplement manufacturers or compan-
ies make about their products. The FDA and FTC operate
in a cooperative fashion to regulate the dietary supple-
ment industry. In this respect, the extent to which infor-
mation is shared and jurisdiction between these two
entities overlaps with regard to marketing and advertising
dietary supplements continues to increase.
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 2 of 57
In the United States, dietary supplements are classified
as food products, not drugs, and there is generally no
mandate to register products with the FDA or obtain FDA
approval before producing or selling supplements to con-
sumers. However, if a dietary supplement manufacturer is
making a claim about their product, the company must
submit the claims to FDA within 30 days of marketing the
product. Compare this, for example, with Canada where
under the Natural Health Product (NHP) Regulations
enacted in 2004 supplements must be reviewed, approved,
and registered with Health Canada. The rationale for the
U.S. model is based on a presumed long history of safe use;
hence there is no need to require additional safety data.
DSHEA also requires supplement marketers to include
on any label displaying structure/function claims (i.e.,
claims that the product affects the structure or function of
the body) the mandatory FDA disclaimer This statement
has not been evaluated by the Food and Drug Administra-
tion. This product is not intended to diagnose, treat, cure,
or prevent any disease.Opponents of dietary supplements
often cite this statement as evidence that the FDA does
not review or approve dietary supplements. However, most
dietary ingredients have been grandfathered inas
DSHEA-compliant ingredients due to a long history of safe
use, and those products containing new ingredients must
be submitted by a notification to the FDA for a safety re-
view prior to being brought to market. Although many
dietary ingredients have been introduced into dietary sup-
plements since October 1994 and have not been submitted
to the FDA for a safety review, nutritional supplementation
writ large is generally safe. In this regard, while there are
over 50,000 dietary supplements registered with the Office
of Dietary SupplementsDietary Supplement Label Data-
base, a 2013 Annual Report (released in 2015) of the
American Association of Poison Control Centers revealed
zero fatalities occurred due to dietary supplements
compared to 1692 deaths due to drugs [5]. Perhaps more
alarming is a 2015 report by the Centers for Disease
Control suggesting 2,287,273 emergency room visits were
due to prescription drug-related events which dwarfs the
3266 emergency room visits due to dietary supplements
(adjusted from 23,000 visits after excluding cases of older
adults choking on pills, allergic reactions, unsupervised
children consuming too many vitamins, and persons con-
suming ingredients not defined by DSHEA as a dietary
supplement) [5]. Furthermore, a recent Healthcare Cost
and Utilization Project Statistical Brief by Lucado et al. [6]
reported approximately one in six Americans suffered
from food borne illnesses in 2010, and food borne illnesses
were associated with over 3.7 million treat-and-release
emergency department visits, 1.3 million inpatient hospital
stays, and 3000 deaths. Notwithstanding, there have been
case reports of liver and kidney toxicity potentially caused
by supplements containing herbal extracts [7]aswellas
overdoses associated with pure caffeine anhydrous inges-
tion [8]. Collectively, the aforementioned statistics and case
reports demonstrate that while generally safe, as with food
or prescription drug consumption, dietary supplement
consumption can lead to adverse events in spite of DSHEA
and current FDA regulations described below.
New dietary ingredients
Recognizing that new and untested dietary supplement
products may pose unknown health issues, DSHEA distin-
guishes between products containing dietary ingredients
that were already on the market and products containing
new dietary ingredients that were not marketed prior to
the enactment of the law. A new dietary ingredient
(NDI) is defined as a dietary ingredient that was not mar-
keted in the United States before October 15, 1994.
DSHEA grants the FDA greater control over supplements
containing NDIs. A product containing an NDI is deemed
adulterated and subject to FDA enforcement sanctions un-
less it meets one of two exemption criteria: either (1) the
supplement in question contains only dietary ingredients
which have been present in the food supply as an article
used for food in a form in which the food has not been
chemically altered;or(2)thereisahistory of use or other
evidence of safetyprovided by the manufacturer or dis-
tributor to the FDA at least 75 days before introducing the
product into interstate commerce. The first criterion is si-
lent as to how and by whom presence in the food supply
as food articles without chemical alteration is to be estab-
lished. The second criterionapplicable only to new diet-
ary ingredients that have not been present in the food
supplyrequires manufacturers and distributors of the
product to take certain actions. Those actions include sub-
mitting, at least 75 days before the product is introduced
into interstate commerce, information that is the basis on
which a product containing the new dietary ingredient is
reasonably expected to be safe.That information would
include: (1) the name of the new dietary ingredient and, if
it is an herb or botanical, the Latin binomial name; (2) a
description of the dietary supplement that contains the
new dietary ingredient, including (a) the level of the new
dietary ingredient in the product, (b) conditions of use of
the product stated in the labeling, or if no conditions of
use are stated, the ordinary conditions of use, and (c) a
history of use or other evidence of safety establishing that
the dietary ingredient, when used under the conditions
recommended or suggested in the labeling of the dietary
supplement, is reasonably expected to be safe.
In July 2011, the FDA released a Draft Guidance for
Industry, entitled Dietary Supplements: New Dietary
Ingredient Notifications and Related Issues.While a
guidance does not carry the authority or the enforceabil-
ity of a law or regulation, the FDAs NDI draft guidance
represented the agencys current thinking on the topic.
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 3 of 57
The guidance prompted great controversy, and FDA
agreed to issue a revised draft guidance to address some
of the issues raised by industry. In August 2016, FDA re-
leased a revised Draft Guidance that replaced the 2011
Draft Guidance. The purpose of the 2016 Draft Guid-
ance was to help manufacturers and distributors decide
whether to submit a premarket safety notification to
FDA, help prepare NDI notifications in a manner that
allows FDA to review and respond more efficiently and
quickly, and to improve the quality of NDI notifications.
The 2016 Draft Guidance has been criticized by industry
and trade associations for its lack of clarity and other
problems. Some of these issues include the lack of clarity
regarding Pre-DSHEA, (Grandfathered), ingredients and
FDA requiring an NDI notification even if another
manufacturer has submitted a notification.
The lack of clarity surrounding the newDraft Guid-
ance has led to many NDI notifications being rejected by
FDA for lack of safety data and other issues. Other com-
panies have opted to utilize the Self-Affirmed Generally
Recognized as Safe (GRAS)route in order to bypassthe
NDI notification process. Self-Affirmed GRAS is when a
company has a team of scientific experts evaluate the safety
of their ingredient. There is no requirement that the safety
dossier be submitted to FDA but is used by the company
as an internal document that may be relied upon if the
ingredient is challenged by the FDA. FDA has expressed
its concern with this practice and does not encourage diet-
ary supplement manufacturers to use Self-Affirmed GRAS
to avoid submitting NDI notifications. In any event, the
likelihood of another revised Draft Guidance from FDA
becoming available in the future is high, and possibly more
enforcement actions taken against companies that market
an NDI without submitting a notification.
Adverse event reporting
In response to growing criticism of the dietary supplement
industry, the 109th Congress passed the first mandatory
Adverse Event Reporting (AER) legislation for the dietary
supplement industry. In December 2006, President Bush
signed into law the Dietary Supplement and Nonprescrip-
tion Drug Consumer Protection Act, which took effect on
December 22, 2007. After much debate in Congress and
input from the FDA, the American Medical Association
(AMA), many of the major supplement trade associations,
and a host of others all agreed that the legislation was ne-
cessary and the final version was approved by all. In short,
the Act requires that all serious adverse eventsregarding
dietary supplements be reported to the Secretary of Health
and Human Services. The law strengthens the regulatory
structure for dietary supplements and builds greater con-
sumer confidence, as consumers have a right to expect
that if they report a serious adverse event to a dietary
supplement marketer the FDA will be advised about it.
An adverse event is any health-related event associated
with the use of a dietary supplement that is adverse. A
serious adverse event is an adverse event that (A) results in
(i) death, (ii) a life-threatening experience, (iii) inpatient
hospitalization, (iv) a persistent or significant disability or
incapacity, or (v) a congenital anomaly or birth defect; or
(B) requires, based on reasonable medical judgment, a
medical or surgical intervention to prevent an outcome
described under subparagraph (A). Once it is determined
that a serious adverse event has occurred, the manufac-
turer, packer, or distributor (responsible person) of a dietary
supplement whose name appears on the label of the
supplement shall submit to the Secretary of Health and
Human Services any report received of the serious adverse
event accompanied by a copy of the label on or within the
retail packaging of the dietary supplement. The responsible
person has 15 business days to submit the report to FDA
after being notified of the serious adverse event. Following
the initial report, the responsible person must submit
follow-up reports of new medical information that they
receive for one-year.
Adulterated supplements
The FDA has various options to protect consumers from
unsafe supplements. The Secretary of the Department of
Health and Human Services (which falls under FDA over-
sight) has the power to declare a dangerous supplement to
be an imminent hazardto public health or safety and
immediately suspend sales of the product. The FDA also
has the authority to protect consumers from dietary
supplementsthatdonotpresentanimminenthazardto
the public but do present certain risks of illness or injury
to consumers. The law prohibits introducing adulterated
products into interstate commerce. A supplement shall be
deemed adulterated if it presents a significant or unrea-
sonable risk of illness or injury. The standard does not
require proof that consumers have actually been harmed
or even that a product will harm anyone. It was under this
provision that the FDA concluded that dietary supplements
containing ephedra, androstenedione, and DMAA pre-
sented an unreasonable risk. Most recently, FDA imposed
an importation ban on the botanical Mitragyna speciose,
better known as Kratom. In 2016, FDA issued Import Alert
#5415, which allows for detention without physical exam-
ination of dietary supplements and bulk dietary ingredients
that are, or contain, Kratom. Criminal penalties are present
for a conviction of introducing adulterated supplement
products into interstate commerce. While the harms asso-
ciated with dietary supplements may pale in comparison to
those linked to prescription drugs, recent pronouncements
from the U.S. Department of Justice confirm that the
supplement industry is being watched vigilantly to protect
the health and safety of the American public.
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 4 of 57
Good manufacturing practices
When DSHEA was passed in 1994, it contained a
provision requiring that the FDA establish and enforce
current Good Manufacturing Practices (cGMPs) for diet-
ary supplements. However, it was not until 2007 that the
cGMPs were finally approved, and not until 2010 that the
cGMPs applied across the industry, to large and small
companies alike. The adherence to cGMPs has helped
protect against contamination issues and should serve to
improve consumer confidence in dietary supplements.
The market improved as companies became compliant
with cGMPs, as these regulations imposed more stringent
requirements such as Vendor Certification, Document
Control Procedures, and Identity Testing. These compli-
ance criteria addressed the problems that had damaged
the reputation of the industry with a focus on quality con-
trol, record keeping, and documentation.
However, it does appear that some within the industry
continue to struggle with compliance. In Fiscal Year 2017,
it was reported that approximately 23.48% of the FDAs
656 total cGMP inspections resulted in citations for failing
to establish specifications for the identity, purity, strength,
and composition of dietary supplements. Further, 18.47%
of those inspected were cited for failing to establish and/
or follow written procedures for quality control opera-
tions. Undoubtedly, relying on certificates of analysis from
the raw materials supplier without further testing, or fail-
ing to conduct identity testing of a finished product, can
result in the creation of a product that contains something
it should not contain such as synthetic chemicals or even
pharmaceutical drugs. All members of the industry need
to ensure compliance with cGMPs.
Marketing claims
According to the 1990 Nutrition Labeling and Education
Act (NLEA), the FDA can review and approve health
claims (claims describing the relationship between a food
substance and a reduced risk of a disease or health-related
condition) for dietary ingredients and foods. However,
since the law was passed it has only approved a few claims.
The delay in reviewing health claims of dietary supplement
ingredients resulted in a lawsuit, Pearson v. Shalala,filed
in 1995. After years of litigation, in 1999 the U.S. Court of
Appeals for the District of Columbia Circuit ruled that
qualified health claims may be made about dietary supple-
ments with approval by FDA, as long as the statements are
truthful and based on adequate science. Supplement or
food companies wishing to make health claims or qualified
health claims about supplements can submit research
evidence to the FDA for review.
The FTC also regulates the supplement industry. Un-
substantiated claims invite enforcement by the FTC (along
with the FDA, state district attorney offices, groups like
the Better Business Bureau, and plaintiff s lawyers who file
class action lawsuits). The FTC has typically applied a sub-
stantiation standard of competent and reliable scientific
evidenceto claims about the benefits and safety of dietary
supplements. FTC case law defines competent and reliable
scientific evidenceas tests, analyses, research, studies, or
other evidence based on the expertise of professionals in
the relevant area, that has been conducted and evaluated
in an objective manner by persons qualified to do so, using
procedures generally accepted in the profession to yield
accurate and reliable results.The FTC has claimed that
this involves providing at least two clinical trials showing
efficacy of the actual product, within a population of
subjects relevant to the target market, supporting the
structure/function claims that are made. While the exact
requirements are still evolving, the FTC has acted against
several supplement companies for misleading advertise-
ments and/or structure/function claims.
A safer industry ahead
As demonstrated, while some argue that the dietary
supplement industry is unregulatedand/or may have
suggestions for additional regulation, manufacturers and
distributors of dietary supplements must adhere to several
federal regulations before a product can go to market. Fur-
ther, before marketing products, they must have evidence
that their supplements are generally safe to meet all the
requirements of DSHEA and FDA regulations. For this
reason, over the last 20 years, many established supple-
ment companies have employed research and develop-
ment directors who help educate the public about
nutrition and exercise, provide input on product develop-
ment, conduct preliminary research on products, and/or
assist in coordinating research trials conducted by inde-
pendent research teams (e.g., university-based researchers
or clinical research sites). These companies also consult
with marketing and legal teams with the responsibility to
ensure structure/function claims do not misrepresent re-
sults of research findings. This has increased job oppor-
tunities for sports nutrition specialists as well as enhanced
external funding opportunities for research groups inter-
ested in exercise and nutrition research.
While some companies have falsely attributed research
on different dietary ingredients or dietary supplements to
their own products, suppressed negative research findings,
and/or exaggerated results from research studies, the
trend in the sports nutrition industry has been to develop
scientifically sound supplements. This trend toward
greater research support is the result of: (1) attempts to
honestly and accurately inform the public about results;
(2) efforts to obtain data to support safety and efficacy on
products for the FDA and the FTC; and/or, (3) endeavors
to provide scientific evidence to support advertising claims
and increase sales. While the push for more research is
due in part to greater scrutiny from the FDA and FTC, it
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 5 of 57
is also in response to an increasingly competitive market-
place where established safety and efficacy attracts more
consumer loyalty and helps ensure a longer lifespan for
the product in commerce. Companies that adhere to these
ethical standards tend to prosper while those that do not
will typically struggle to comply with FDA and FTC guide-
lines resulting in a loss of consumer confidence and an
early demise for the product.
Product development and quality assurance
A common question posed by athletes, parents, and profes-
sionals surrounding dietary supplements relates to how
they are manufactured and perceived supplement quality.
In several cases, established companies who develop dietary
supplements have research teams who scour the medical
and scientific literature looking for potentially effective
nutrients. These research teams often attend scientific
meetings and review the latest patents, research abstracts
presented at scientific meetings, and research publications.
Leading companies invest in basic research on nutrients
before developing their supplement formulations and often
consult with leading researchers to discuss ideas about diet-
ary supplements and their potential for commercialization.
Other companies wait until research has been presented in
patents, research abstracts, or publications before develop-
ing nutritional formulations featuring the nutrient. Upon
identification of new nutrients or potential formulations,
the next step is to contact raw ingredient suppliers to see if
the nutrient is available, if it is affordable, how much of it
can be sourced and what is the available purity. Sometimes,
companies develop and pursue patents involving new pro-
cessing and purification processes because the nutrient has
not yet been extracted in a pure form or is not available in
large quantities. Reputable raw material manufacturers
conduct extensive tests to examine purity of their raw in-
gredients. When working on a new ingredient, companies
often conduct series of toxicity studies on the new nutrient
once a purified source has been identified. The company
would then compile a safety dossier and communicate it to
the FDA as a New Dietary Ingredient submission, with the
hopes of it being allowed for lawful sale.
When a powdered formulation is designed, the list of in-
gredients and raw materials are typically sent to a flavoring
house and packaging company to identify the best way to
flavor and package the supplement. In the nutrition indus-
try, several main flavoring houses and packaging companies
exist who make many dietary supplements for supplement
companies. Most reputable dietary supplement manufac-
turers submit their production facilities to inspection from
the FDA and adhere to GMP, which represent industry
standards for good manufacturing of dietary supplements.
Some companies also submit their products for independ-
ent testing by third-party companies to certify that their
products meet label claims and that the product is free of
various banned ingredients. For example, the certification
service offered by NSF International includes product test-
ing, GMP inspections, ongoing monitoring and use of the
NSF Mark indicating products comply with inspection
standards, and screening for contaminants. More recently,
companies have subjected their products for testing by
third party companies to inspect for banned or unwanted
substances. These types of tests help ensure that the dietary
supplement made available to athletes do not contained
substances banned by the International Olympic Commit-
tee or other athletic governing bodies (e.g., NFL, NCAA,
MLB, NHL, etc.). While third-party testing does not guar-
antee that a supplement is void of banned substances, the
likelihood is reduced (e.g., Banned Substances Control
Group, Informed Choice, NSF, etc.). Moreover, consumers
can request copies of results of these tests and each prod-
uct that has gone through testing and earned certification
can be researched online to help athletes, coaches and sup-
port staff understand which products should be consid-
ered. In many situations, companies who are not willing to
provide copies of test results or certificates of analysis
should be viewed with caution, particularly for individuals
whose eligibility to participate might be compromised if a
tainted product is consumed.
Evaluation of nutrition ergogenic aids
The ISSN recommends that potential consumers under-
take a systematic process of evaluating the validity and sci-
entific merit of claims made when assessing the ergogenic
value of a dietary supplement [1,4]. This can be accom-
plished by examining the theoretical rationale behind the
supplement and determining whether there is any well-
controlled data showing the supplement is effective. Sup-
plements based on sound scientific rationale with direct,
supportive research showing effectiveness may be worth
trying or recommending. However, those based on un-
sound scientific results or offer little to no data supporting
the ergogenic value of the actual supplement/technique
may not be worthwhile. Sports nutrition specialists should
be a resource to help their clients interpret the scientific
and medical research that may impact their welfare and
help them train more effectively. The following are recom-
mended questions to ask when evaluating the potential
ergogenic value of a supplement.
Does the theory make sense?
Most supplements that have been marketed to improve
health or exercise performance are based on theoretical
applications derived from basic science or clinical research
studies. Based on these preliminary studies, a dietary
approach or supplement is often marketed to people
proclaiming the benefits observed in these basic research
studies. Although the theory may appear relevant, critical
analysis of this process often reveals flaws in the scientific
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 6 of 57
logic or that the claims made do not quite match up with
the cited literature. By evaluating the literature one can
discern whether or not a dietary approach or supplement
has been based on sound scientific evidence. To do so, one
is recommended to first read reviews about the training
method, nutrient, or supplement from researchers who
have been intimately involved in the available research and
consult reliable references about nutritional and herbal
supplements [1,9]. To aid in this endeavour, the ISSN has
published position statement on topics related to creatine
[10], protein [11], beta-alanine [12], nutrient timing [13],
caffeine [14], HMB [15], meal frequency [16], energy drinks
[17], and diets and body composition [18]. Each of these
documents would be excellent resources for any of these
topics. In addition, other review articles and consensus
statements have been published by other researchers and
research groups that evaluate dietary supplements, offer
recommendations on interpreting the literature, and
discuss the available findings for several ingredients that
are discussed in this document [1921]. We also advise
consumers to conduct a search on the nutrient, key ingre-
dients or the supplement itself on the National Library of
Medicines Pub Med Online (https://www.ncbi.nlm.nih.-
gov/pubmed/). A quick look at these references will often
help determine if the theoretical impetus for supplement-
ing with an ingredient is plausible or not. Proponents of
ergogenic aids often overstate claims made about training
devices and dietary supplements while opponents of ergo-
genic aids and dietary supplements are often either un-
aware or are ignorant of research supporting their use.
Sports nutrition specialists have the responsibility to know
the literature and search available databases to evaluate
the level of merit surrounding a proposed ergogenic aid.
Is the supplement legal and safe?
An initial question that should be asked is whether the
supplement is legal and/or safe. Some athletic associations
have banned the use of various nutritional supplements
(e.g., prohormones, ephedra that contains ephedrine,
muscle buildingsupplements, etc.) and many professional
sports organization have now written language into their
collective bargaining agreements that products made avail-
able by the team must be NSF certified as safe for sport.
Obviously, if the supplement is banned, the sports nutrition
specialist should discourage its use. In addition, many sup-
plements lack appropriate long-term safety data. People
who consider taking nutritional supplements should be
well aware of the potential side effects so they can make an
informed decision whether to use a supplement. Addition-
ally, they should consult with a knowledgeable physician to
see if any underlying medical problems exist that may
contraindicate its use. When evaluating the safety of a sup-
plement, it is suggested to determine if any side effects
have been reported in the scientific or medical literature. In
particular, we suggest determining how long a particular
supplement has been studied, the dosages evaluated, and
whether any side effects were observed. We also recom-
mend consulting the Physicians Desk Reference (PDR) for
nutritional supplements and herbal supplements to see if
anysideeffectshavebeenreportedand/orifthereareany
known drug interactions. If no side effects have been re-
ported in the scientific/medical literature, we generally will
view the supplement as safe for the length of time and dos-
ages evaluated. Unfortunately, many available supplements
have not had basic safety studies completed that replicate
thelengthoftimeanddosagesbeingused.
Is there any scientific evidence supporting the ergogenic value?
The next question to ask is whether any well-controlled
data are available showing effectiveness of the proposed
ergogenic aid in athletic populations or people regularly
involved in exercise training. The first place to look is the
list of references cited in marketing material supporting
their claims. Are the abstracts or articles cited just general
references or specific studies that have evaluated the
efficacy of the nutrients included in the formulation or of
the actual supplement? From there, one can critically
evaluate the cited abstracts and articles by asking a series
of questions:
Are the studies basic research done in animals/clinical
populations or have the studies been conducted on
athletes/trained subjects? For perspective, studies
reporting improved performance in rats or an
individual diagnosed with type 2 diabetes may be
insightful, but research conducted on non-diabetic
athletes is much more practical and relevant.
Were the studies well controlled? For ergogenic aid
research, the gold standard study design is a
randomized, double-blind, placebo controlled
clinical trial. This means that neither the researcher
nor the subject is aware which group received the
supplement or the placebo during the study and that
the subjects were randomly assigned into the
placebo or supplement group. An additional element
of rigor is called a cross-over design, where each
subject, at different times (separated by an interval
known as a washout period), is exposed to each of
the treatments. While utilization of a cross-over
design is not always feasible, it reduces the element
of variability within a participant and subsequently,
increases the strength of studys findings. At times,
supplement claims have been based on poorly
designed studies (i.e., small groups of subjects, no
control group, use of unreliable tests, etc.) or
testimonials which make interpretation more
difficult. Well-controlled clinical trials provide
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 7 of 57
stronger evidence as to the potential ergogenic value
and importantly how the findings can best be used.
Do the studies report statistically significant results
or are claims being made on non-significant means
or trends? Appropriate statistical analysis of research
results allows for an unbiased interpretation of data.
Although studies reporting statistical trends may be
of interest and lead researchers to conduct additional
research, studies reporting statistically significant
results are obviously more convincing. With this said,
it is important for people to understand that
oftentimes the potential effect a dietary supplement or
diet regimen may have above and beyond the effect
seen from the exercise bout or an accepted dietary
approach is quite small. In addition, many studies
examining a biochemical or molecular biology
mechanism can require invasive sampling techniques
or the study population being recruited is unique
(very highly trained) resulting in a small number of
study participants. When viewed together, the
combination of these two considerations can result in
statistical outcomes that do not reach statistical
significance even though large mean changes were
observed. In these situations, the reporting of
confidence intervals on the mean change, individual
responses from all participants to the investigated
treatment and/or effect sizes are additional pieces of
information that can allow for a more accurate
interpretation. In all such cases, additional research is
warranted to further examine the potential ergogenic
aid before conclusions can be made.
Do the results of the cited studies match the claims
made about the supplement or do they accurately
portray the response of the supplement against an
appropriate placebo or control group? It is not
unusual for marketing claims to greatly exaggerate
the results found in the actual studies and do so by
focusing upon just the outcomes within the
supplement (treatment) group as opposed to how
the supplement group changed in comparison to
how a placebo group changed. Similarly, it is not
uncommon for ostensibly compelling results, that
may indeed be statistically significant, to be
amplified while other relevant findings of significant
consumer interest are obscured or omitted (e.g. a
dietary supplement showing statistically significant
increases in circulating testosterone yet changes in
body composition or muscular performance were
not superior to a placebo). The only way to
determine this is to read the entire article versus
focusing an entire studys interpretation on the
provided abstract or even the article citation, and
compare results observed in the studies to the
available marketing claims. Reputable companies
accurately and completely report results of studies
so that consumers can make informed decisions
about using a product.
Were results of the study presented at a reputable
scientific meeting and/or published in a peer-reviewed
scientific journal? At times, claims are based on
research that has either never been published or only
published in an obscure journal. The best research is
typically presented at respected scientific meetings
and/or published in reputable peer-reviewed journals.
Three ways to determine a journals reputation is
either: 1) identify the publisher, 2) the impact factor
of the journal or 3) whether or not the journal is
indexed and subsequently available for review on Pub
Med (https://www.ncbi.nlm.nih.gov/pubmed/). Many
peer-reviewedjournals are published by companies
with ties to, or are actually owned by, companies that
do business with various nutritional products (even
though they may be available on PubMed). Therefore,
we recommend looking up the publisherswebsite
and see how many other journals they publish. If you
see only a few other journals this is a suggestion that
the journal is not a reputable journal. Additionally,
one can also look up how many articles have been
published by the journal in the last 612 months and
how many of these articles are well-conducted studies.
Alternatively, one can also inquire about the impact
factor, a qualitative ranking determined by the
number of times a journals articles are cited. Impact
factors are determined and published by Thomson
Reuters under Journal Citation Reports® (a
subscription service available at most university
libraries). Most journals list their impact factor on the
journal home page. Historically, those articles that are
read and cited the most are the most impactful
scientifically.
Have the research findings been replicated? If so,
have the results only been replicated at the same
laboratory? The best way to know an ergogenic aid
works is to see that results have been replicated in
several studies preferably by several separate, distinct
research groups. The most reliable ergogenic aids
are those in which multiple studies, conducted at
different labs, have reported similar results of safety
and efficacy. Additionally, replication of results by
different, unaffiliated labs with completely different
authors also removes or reduces the potentially
confounding element of publication bias (publication
of studies showing only positive results) and
conflicts of interest. A notable number of studies on
ergogenic aids are conducted in collaboration with
one or more research scientists or co-authors that
have a real or perceived economic interest in the
outcome of the study. This could range from being a
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 8 of 57
co-inventor on a patent application that is the
subject of the ergogenic aid, being paid or receiving
royalties from the creation of a dietary supplement
formulation, providing consulting services for the
company or having stock options or shares in a
company that owns or markets the ergogenic aid
described in the study. An increasing number of
journals require disclosures by all authors of
scientific articles, and including such disclosures
in published articles. This is driven by the aim of
providing greater transparency and research
integrity. It is important to emphasize that disclosure
of a conflict of interest does not alone discredit or
dilute the merits of a research study. The primary
thrust behind public disclosures of potential conflicts
of interest is first and foremost transparency to the
reader and second to prevent a later revelation of
some form of confounding interest that has the
potential of discrediting the study in question, the
findings of the study, the authors, and even the
research center or institution where the study was
conducted.
Classifying and categorizing supplements
Dietary supplements may contain carbohydrate, protein,
fat, minerals, vitamins, herbs, enzymes, metabolic interme-
diates (i.e., select amino acids), or various plant/food
extracts. Supplements can generally be classified as
convenience supplements (e.g., energy bars, gels, blocks,
meal replacement powders, or ready to drink supplements)
designed to provide a convenient means of meeting neces-
sary energy or macronutrientneedswhilealsoproviding
support towards attempts at managing caloric intake,
weight gain, weight loss, and/or performance enhancement.
As discussed previously, evaluating the available scientific
literature is an important step in determining the efficacy of
any diet, diet program or dietary supplement. In consider-
ing this, nutritional supplements can be categorized in the
following manner:
I. Strong Evidence to Support Efficacy and
Apparently Safe: Supplements that have a sound
theoretical rationale with the majority of available
research in relevant populations using appropriate
dosing regimens demonstrating both its efficacy and
safety.
II. Limited or Mixed Evidence to Support Efficacy:
Supplements within this category are characterized
as having a sound scientific rationale for its use, but
the available research has failed to produce consistent
outcomes supporting its efficacy. Routinely, these
supplements require more research to be completed
before researchers can begin to understand their
impact. Importantly, these supplements have no
available evidence to suggest they lack safety or
should be viewed as harmful.
III. Little to No Evidence to Support Efficacy and/or
Safety: Supplements within this category generally
lack a sound scientific rationale and the available
research consistently shows it to lack efficacy.
Alternatively, supplements that may be harmful to
ones health or to lack safety are also placed in this
category.
Several factors are evaluated when beginning to counsel
individuals who regularly complete exercise training. First,
a clear understanding of the athletes goals and the time
with which they have to meet those goals is important. In
addition to monitoring load and recovery, an evaluation of
the individuals diet and training program should also be
completed. To accomplish this, one should make sure the
athlete is eating an energy balanced, nutrient dense diet
that meets their estimated daily energy needs and that
they are training intelligently. Far too many athletes or
coaches focus too heavily upon supplementation or appli-
cations of supplementation and neglect these key funda-
mental aspects. Following this, we suggest that they
generally only recommend supplements in category I (i.e.,
Strong Evidence to Support Efficacy and Apparently
Safe). If an athlete is interested in trying supplements in
category II (i.e., Limited or Mixed Evidence to Support
Efficacy), the athlete should make sure they understand
these supplements are more experimental and they may
or may not see the type of results claimed. Obviously, the
ISSN does not support athletes taking supplements in
category III (i.e., Little to No Evidence to Support Efficacy
and/or Safety). We believe this approach is scientifically
substantiated and offers a balanced view as opposed to
simply dismissing the use of all dietary supplements.
General dietary guidelines for active individuals
A well-designed diet that meets energy intake needs and
incorporates proper timing of nutrients is the foundation
upon which a good training program can be developed
[22,23]. Research has clearly shown that lacking suffi-
cient calories and/or enough of the right type of macro-
nutrients may impede an athletes training adaptations,
while athletes who consume a balanced diet that meets
energy needs can augment physiological training adapta-
tions. Moreover, maintaining an energy deficient diet
during training may lead to loss of muscle mass,
strength, and bone mineral density in addition to an in-
creased susceptibility to illness and injuries, disturbances
in immune, endocrine and reproductive function, and an
increased prevalence of overreaching and/or overtrain-
ing. Incorporating good dietary practices as part of a
training program is one way to help optimize training
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 9 of 57
adaptations and prevent overtraining. The following is
an overview of energy intake recommendations and
major nutrient needs for active individuals.
Energy needs
The primary component to optimize training and perform-
ance through nutrition is to ensure the athlete is consum-
ing enough calories to offset energy expenditure [2226].
People who participate in a general fitness program (e.g.,
exercising 3040 min per day, 3 times per week) can typic-
ally meet nutritional needs following a normal diet (e.g.,
18002400 kcals/day or about 2535 kcals/kg/day for a
5080 kg individual) because their caloric demands from
exercise are not too great (e.g., 200400 kcals/session).
However, athletes involved in moderate levels of intense
training (e.g., 23 h per day of intense exercise performed
56timesperweek)orhighvolumeintensetraining(e.g.,
36 h per day of intense training in 12workoutsfor5
6 days per week) may expend 6001200 kcals or more per
hour during exercise [24]. For this reason, their caloric
needs may approach 4070 kcals/kg/day (20007000
kcals/day for a 50100 kg athlete). For elite athletes, energy
expenditure during heavy training or competition will
further exceed these levels [27,28]. For example, energy
expenditure for cyclists to compete in the Tour de France
has been estimated as high as 12,000 kcals/day (150200
kcals/kg/day for a 6080 kg athlete) [29,30]. Additionally,
caloric needs for large athletes (i.e., 100150 kg) may range
between 6000 and 12,000 kcals/day depending on the
volume and intensity of different training phases [31].
Although some argue that athletes can meet caloric
needs simply by consuming a well-balanced diet, it is often
very difficult for larger athletes and athletes engaged in
high volume/intense training to be able to eat enough food,
on a daily basis, to meet caloric needs [2,29,30,3234].
This point was clearly highlighted in a review by Burke
who demonstrated that carbohydrate needs are largely un-
met by high-level athletes [22]. Additionally it is difficult to
consume enough food and maintain gastrointestinal com-
fort to train or race at peak levels [35]. Maintaining an
energy deficient diet during training often leads to a num-
ber of physical (i.e., loss of fat-free mass, illness, reduced
sleep quality, incomplete recovery, hormonal fluctuations,
increased resting heart rate, etc.) and psychological (i.e.,
apathy towards training, heightened stress) adverse out-
comes [23,27]. Nutritional analyses of athletesdiets have
revealed that many are susceptible to maintaining negative
energy intakes during training. It is still a question whether
there may be specific individualized occasions when nega-
tive energy balance may enhance performance in the days
prior to running performance [36]. Populations susceptible
to negative energy balance include runners, cyclists, swim-
mers, triathletes, gymnasts, skaters, dancers, wrestlers,
boxers, and athletes attempting to lose weight too quickly
[37]. Additionally, female athletes are at particular risk of
under fueling due to both competitive and aesthetic
demands of their sport and their surrounding culture. Fe-
male athletes have been reported to have a high incidence
of eating disorders [38]. Low or reduced energy availability
(LEA) is linked to functional hypothalamic oligomenor-
rhea/amenorrhea (FHA), which is frequently reported in
weightsensitivesports.ThismakesLEAamajornutri-
tional concern for female athletes [39]. Consequently, it is
important for the sports nutrition specialist working with
athletes to assess athletes individually to ensure that ath-
letes are well fed according to the goals of their sport and
their health, and consume enough calories to offset the in-
creased energy demands of training, and maintain body
weight. Although this sounds relatively simple, intense
training often suppresses appetite and/or alters hunger
patterns so that many athletes do not feel like eating
[37,38]. Some athletes prefer not to exercise within sev-
eral hours after eating because of sensations of fullness
and/or a predisposition to cause gastrointestinal distress.
Further, travel and training schedules may limit food avail-
ability or the types of food athletes are accustomed to eat-
ing. This means that care should be taken to plan meal
times in concert with training, as well as to make sure ath-
letes have sufficient availability of nutrient dense foods
throughout the day for snacking between meals (e.g.,
fluids, carbohydrate/protein-rich foods and supplemental
bars, etc.) [2,33,40]. For this reason, sports nutritionists
often recommend that athletes consume four to six meals
per day and snacks in between meals to meet energy
needs. Due to these practical concerns, the use of nutrient
dense energy foods, energy bars and high calorie carbohy-
drate/protein supplements provides a convenient way for
athletes to supplement their diet in order to maintain en-
ergy intake during training.
Carbohydrate
Beyond optimal energy intake, consuming adequate
amounts of carbohydrate, protein, and fat is important for
athletes to optimize their training and performance. In par-
ticular and as it relates to exercise performance, the need
for optimal carbohydrates before, during and after intense
and high-volume bouts of training and competition is evi-
dent [41]. Excellent reviews [42,43] and original investiga-
tions [4449] continue to highlight the known dependence
on carbohydrates that exists for athletes competing to win
various endurance and team sport activities. A complete
discussion of the needs of carbohydrates and strategies to
deliver optimal carbohydrate and replenish lost muscle and
liver glycogen extend beyond the scope of this paper, but
the reader is referred to several informative reviews on the
topic [23,41,5053].
As such, individuals engaged in a general fitness program
and are not necessarily training to meet any type of
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 10 of 57
performance goal can typically meet daily carbohydrate
needs by consuming a normal diet (i.e., 4555% CHO [3
5 g/kg/day], 1520% PRO [0.81.2 g/kg/day], and 2535%
fat [0.51.5 g/kg/day]). However, athletes involved in mod-
erate and high-volume training need greater amounts of
carbohydrate and protein (discussed later) in their diet to
meet macronutrient needs [50]. In terms of carbohydrate
needs, athletes involved in moderate amounts of intense
training (e.g., 23 h per day of intense exercise performed
56 times per week) typically need to consume a diet con-
sisting of 58 g/kg/day or 2501200 g/day for 50150 kg
athletes of carbohydrate to maintain liver and muscle
glycogen stores [23,24,50]. Research has also shown that
athletes involved in high volume intense training (e.g.,
36 h per day of intense training in 12 daily workouts for
56 days per week) may need to consume 810 g/day of
carbohydrate (i.e., 4001500 g/day for 50150 kg athletes)
in order to maintain muscle glycogen levels [50]. Prefera-
bly, the majority of dietary carbohydrate should come from
whole grains, vegetables, fruits, etc. while foods that empty
quickly from the stomach such as refined sugars, starches
and engineered sports nutrition products should be
reserved for situations in which glycogen resynthesis needs
to occur at accelerated rates [53]. In these situations, the
absolute delivery of carbohydrate (> 8 g of carbohydrate/
kg/day or at least 1.2 g of carbohydrate/kg/hour for the
first four hours into recovery) takes precedence over other
strategies such as those that may relate to timing or con-
comitant ingestion of other macronutrients (e.g., protein)
or non-nutrients (e.g., caffeine) or carbohydrate type (i.e.,
glycemic index) [50].
When considering the carbohydrate needs throughout
an exercise session, several key factors should be consid-
ered. Previous research has indicated athletes undergoing
prolonged bouts (23 h) of exercise training can oxidize
carbohydrates at a rate of 11.1gperminuteorabout60g
perhour[41]. Several reviews advocate the ingestion of
0.7 g of carbohydrate/kg/hr. during exercise in a 68% so-
lution (i.e., 68 g per 100 ml of fluid) [41,42,50,54]. It is
now well established that different types of carbohydrates
can be oxidized at different rates in skeletal muscle due to
the involvement of different transporter proteins that result
in carbohydrate uptake [5559]. Interestingly, combina-
tions of glucose and sucrose or maltodextrin and fructose
have been reported to promote greater exogenous rates of
carbohydrate oxidation when compared to situations when
single sources of carbohydrate are ingested [5563]. These
studies generally indicate a ratio of 11.2 for maltodextrin
to 0.81.0 fructose seems to support the greatest rates of
carbohydrate oxidation during exercise. Additional re-
search on high molecular weight amylopectin indicates that
there may be a benefit to the lower osmolality of the starch,
allowing for greater consumption (100 g/hour) and pos-
sibly greater oxidation rates and performance improvement
[6467]. In addition to oxidation rates and carbohydrate
types, the fasting status and duration of the exercise bout
also function as key variables for athletes and coaches to
consider. When considering duration, associated reviews
have documented that bouts of moderate to intense exer-
cise need to reach exercise durations that extend well into
90th minute of exercise before carbohydrate is shown to
consistently yield an ergogenic outcome [41,68,69]. Of
interest, however, not all studies indicate that shorter (60
75 min) bouts of higher intensity work may benefit from
carbohydrate delivery. Currently the mechanisms sur-
rounding these findings are, respectively, thought to be re-
placement of depleted carbohydrate stores during longer
duration of moderate intensity while benefits seen during
shorter, more intense exercise bouts are thought to operate
in a central fashion. Moreover, these reviews have also
pointed to the impact of fasting status on documentation
of ergogenic outcomes [41,68,69]. In this respect, when
studies require study participants to commence exercise in
a fasted state, ergogenic outcomes are more consistently
reported, yet other authors have questioned the ecological
validity of this approach for competing athletes [43].
As it stands, the need for optimal carbohydrates in the
diet for those athletes seeking maximal physical perform-
ance is unquestioned. Daily consumption of appropriate
amounts of carbohydrate is the first and most important
step for any competing athlete. As durations extend into
2 h, the need to deliver carbohydrate goes up, particularly
when commencing exercise in a state of fasting or incom-
plete recovery. Once exercise ceases, several dietary strat-
egies can be considered to maximally replace lost muscle
and liver glycogen, particularly if a limited window of re-
covery exists. In these situations, the first priority should
lie with achieving aggressive intakes of carbohydrate while
strategies such as ingesting protein with lower carbohy-
drate amounts, carbohydrate and caffeine co-ingestion or
certain forms of carbohydrate may also help to facilitate
rapid assimilation of lost glycogen.
Protein
Considerable debate exists surrounding the amount of
protein needed in an athletesdiet[7074]. Initially, it was
recommended that athletes do not need to ingest more
than the RDA for protein (i.e., 0.8 to 1.0 g/kg/d for
children, adolescents and adults). However, research
spanning the past 30 years has indicated that athletes
engaged in intense training may benefit from ingesting
about two times the RDA of protein in their diet (1.4
1.8 g/kg/d) to maintain protein balance [11,70,71,73,
7580]. If an insufficient amount of protein is consumed,
an athlete will develop and maintain a negative nitrogen
balance, indicating protein catabolism and slow recovery.
Over time, this may lead to muscle wasting, injuries,
illness, and training intolerance [76,77,81].
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 11 of 57
For people involved in a general fitness program or sim-
ply interested in optimizing their health, recent research
suggests protein needs may also be above the RDA. Phillips
and colleagues [76], Witard et al. [82], Jager et al. [11]and
Tipton et al. [79] report that current evidence indicates op-
timal protein intakes in the range of 1.22.0 g/kg/day
should be considered. In this respect, Morton and investi-
gators [83] performed a meta-review and meta-regression
involving 49 studies and 1863 participants and concluded
that a daily protein intake of 1.62 g/kg/day may be an ideal
place to start, with intakes beyond that providing no further
contribution to increases in fat-free mass. In addition and
in comparison to the RDA, non-exercising, older individ-
uals (5371 years) may also benefit from a higher daily pro-
tein intake (e.g., 1.01.2 g/kg/day of protein). Recent
reports suggest that older muscle may be slower to respond
and less sensitive to protein ingestion, typically requiring
40 g doses to robustly stimulatemuscleproteinsynthesis
[8486]. Studies in younger individuals, however, have indi-
cated that in the absence of exercise, a 20 g dose can
maximize muscle protein synthesis [87,88]andifcon-
sumed after a multiple set workout consisting of several ex-
ercises that target large muscle groups a 40 g dose might be
needed [89]. Consequently, it is recommended that athletes
involved in moderate amounts of intense training consume
1.22.0 g/kg/day of protein (60300g/dayfora50150 kg
athlete) while athletes involved in high volume, intense
training consume 1.72.2 g/kg/day of protein (85330 g/
dayfora50150 kg athlete) [78,90]. This protein need
would be equivalent to ingesting 315 three-ounce servings
of chicken or fish per day for a 50150 kg athlete [78]. Al-
though smaller athletes typically can ingest this amount of
protein, on a daily basis, in their normal diet, larger athletes
oftenhavedifficultyconsuming this much dietary protein.
Additionally, a number of athletic populations are
known to be susceptible to protein malnutrition (e.g.,
runners, cyclists, swimmers, triathletes, gymnasts,
dancers, skaters, wrestlers, boxers, etc.) and conse-
quently, additional counseling and education may be
needed to help these athletes meet their daily protein
needs. To this point, the periods of energy restriction
to meet weight or aesthetic demands of their sports
that are seemingly a part of the sports fabric creates an
arguably greater need to understand that protein intake,
quality and timing as well as combination with carbo-
hydrate is particularly important to maintain lean body
mass, training effects, and performance [25]. Overall, it
goes without saying that care should be taken to ensure
that athletes consume a sufficient amount of quality
protein in their diet to maintain nitrogen balance.
Proteins differ based on their source, amino acid pro-
file, and the methods of processing or isolating the pro-
tein undergoes [11]. These differences influence the
availability of amino acids and peptides, which may
possess biological activity (e.g., α-lactalbumin, ß-lacto-
globulin, glycomacropeptides, immunoglobulins, lacto-
peroxidases, lactoferrin, etc.). Additionally, the rate of
digestion and/or absorption and metabolic activity of the
protein also are important considerations [91]. For
example, different types of proteins (e.g., casein, whey,
and soy) are digested at different rates, which may affect
whole body catabolism and anabolism and acute stimu-
lation of muscle protein synthesis (MPS) [9196]. There-
fore, care should be taken not only to make sure the
athlete consumes enough protein in their diet but also
that the protein is high quality. The best dietary sources
of low fat, high quality protein are light skinless chicken,
fish, egg whites, very lean cuts of beef and skim milk
(casein and whey) while protein supplements routinely
contain whey, casein, milk and egg protein. In what is
still an emerging area of research, various plant sources
of protein have been examined for their ability to stimu-
late increases in muscle protein synthesis [77,97] and
promote exercise training adaptations [98]. While amino
acid absorption from plant proteins is generally slower,
leucine from rice protein has been found to be absorbed
even faster than from whey [99], while digestive enzymes
[100], probiotics [101] and HMB [102] can be used to
overcome differences in protein quality. Preliminary
findings suggest that rice [98] and pea protein [103] may
be able to stimulate similar changes in fat-free mass and
strength as whey protein, although the reader should
understand that many other factors (dose provided,
training status of participants, duration of training and
supplementation, etc.) will ultimately impact these
outcomes and consequently more research is needed.
While many reasons and scenarios exist for why an ath-
lete may choose to supplement their diet with protein pow-
ders or other forms of protein supplements, this practice is
not considered to be an absolute requirement for increased
performance and adaptations. Due to nutritional, societal,
emotional and psychological reasons, it is preferable for
the majority of daily protein consumed by athletes to occur
as part of a food or meal. However, we recognize and
embrace the reality that situations commonly arise where
efficiently delivering a high-quality source of protein takes
precedence. Jager and colleagues [11] published an updated
position statement of the International Society of Sports
Nutrition that is summarized by the following points:
1) An acute exercise stimulus, particularly resistance
exercise and protein ingestion both stimulate muscle
protein synthesis (MPS) and are synergistic when
protein consumption occurs before or after resistance
exercise
2) For building and maintaining muscle mass, an overall
daily protein intake of 1.42.0 g/kg/d is sufficient for
most exercising individuals
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 12 of 57
3) Higher protein intakes (2.33.1 g/kg fat-free mass/
d) may be needed to maximize the retention of lean
body weight in resistance trained subjects during
hypocaloric periods
4) Higher protein intakes (> 3.0 g protein/kg body
weight/day) when combined with resistance exercise
may have positive effects on body composition in
resistance trained individuals (i.e., promote loss of fat
mass)
5) Optimal doses for athletes to maximize MPS are
mixed and are dependent upon age and recent
resistance exercise stimuli. General recommendations
are 0.250.55 g of a high-quality protein per kg of
body weight, or an absolute dose of 2040 g.
6) Acute protein doses should contain 7003000 mg
of leucine and/or a higher relative leucine content,
in addition to a balanced array of the essential
amino acids (EAAs)
7) Protein doses should ideally be evenly distributed,
every 34 h, across the day
8) The optimal time period during which to ingest
protein is likely a matter of individual tolerance;
however, the anabolic effect of exercise is
long-lasting (at least 24 h), but likely diminishes
with increasing time post-exercise
9) Rapidly digested proteins that contain high
proportions of EAAs and adequate leucine, are
most effective in stimulating MPS
10) Different types and quality of protein can affect
amino acid bioavailability following protein
supplementation; complete protein sources deliver all
required EAAs
Fat
The dietary recommendations of fat intake for athletes are
similar to or slightly greater than dietary recommendations
made to non-athletes to promote health. Maintenance of
energy balance, replenishment of intramuscular triacylglyc-
erol stores and adequate consumption of essential fatty
acids are important for athletes, and all serve as reasons
for an increased intake of dietary fat [104]. Depending
upon the athletes training status or goals, the amount of
dietary fat recommended for daily intake can change. For
example, higher-fat diets appear to maintain circulating
testosterone concentrations better than low-fat diets
[105107]. Additionally, higher fat intakes may provide
valuable translational evidence to the documented testos-
terone suppression which can occur during volume-type
overtraining [108]. Generally, it is recommended that ath-
letes consume a moderate amount of fat (approximately
30% of their daily caloric intake), while proportions up to
50% of daily calories can be safely ingested by athletes dur-
ing regular high-volume training [104]. In situations where
an athlete may be interested in reducing their body fat,
dietary fat intakes ranging from 0.5 to 1 g/kg/day have
been recommended results in situations where daily fat in-
take might comprise as little as 20% of total calories in the
diet [2]. This recommendation stems largely from avail-
able evidence in weight loss studies involving non-athletic
individuals that people who are most successful in losing
weight and maintaining the weight loss are those who in-
gest reduced amounts of fat in their diet [109,110]al-
though this is not always the case [111]. Strategies to help
athletes manage dietary fat intake include teaching
them which foods contain various types of fat so that
they can make better food choices and how to count
fat grams [2,33].
For years, high-fat diets have been used by athletes with
the majority of evidence showing no ergogenic benefit and
consistent gastrointestinal challenges [112]. In recent
years, significant debate has swirled regarding the impact
of increasing dietary fat. One strategy, train low, compete
high, refers to an acute pattern of dietary periodization
whereby an athlete first follows a high-fat, low carbohy-
drate diet for one to 3 weeks while training before
reintroducing carbohydrates back into the diet. While
intramuscular adaptations result that may theoretically
impact performance [113,114], no consistent, favorable
impact on performance has been documented [112,115].
A variant of high-fat diets, ketogenic diets, have increased
in popularity. While no exact prescription exists, nearly all
ketogenic diet prescriptions derive at least 7080% of their
daily calories from dietary fat, prescribe a moderate
amount of protein (2025% total calories or 2.02.5 g/kg/
day) and are largely devoid of carbohydrate (1040 g per
day). This diet prescription leads to a greater reliance on
ketones as a fuel source. Currently, limited and mixed evi-
dence remains regarding the overall efficacy of a ketogenic
diet for athletes. In favor, Cox et al. [116] demonstrated
that ketogenic dieting can improve exercise endurance by
shifting fuel oxidation while Burke and colleagues [115]
failed to show an increase in performance in a cohort of
Olympic-caliber race walkers. Additionally, Jabekk and col-
leagues [117] reported decreases in body fat with no
change in lean mass in overweight women who resistance
trained for 10 weeks and followed a ketogenic diet. In light
of the available evidence being limited and mixed, more
human research needs to be completed before appropriate
recommendations can be made towards the use of high fat
diets for athletic performance.
Strategic eating and refueling
In addition to the general nutritional guidelines described
above, research has also demonstrated that timing and
composition of meals consumed may play a role in opti-
mizing performance, training adaptations, and preventing
overtraining [2,25,40]. In this regard, it takes about 4 h
for carbohydrate to be digested and assimilated into
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38 Page 13 of 57
muscle and liver tissues as glycogen. Consequently,
pre-exercise meals should be consumed about four to 6 h
before exercise [40]. This means that if an athlete trains in
the afternoon, breakfast can be viewed to have great
importance to top off muscle and liver glycogen levels. Re-
search has also indicated that ingesting a light carbohy-
drate and protein snack 30 to 60 min prior to exercise (e.g.,
50 g of carbohydrate and 5 to 10 g of protein) serves to in-
crease carbohydrate availability toward the end of an in-
tense exercise bout [118,119]. This also serves to increase
availability of amino acids, decrease exercise-induced
catabolism of protein, and minimize muscle damage
[120122]. Additionally, athletes who are going through
periods of energy restriction to meet weight or aesthetic
demands of sports should understand that protein intake,
quality and timing as well as combination with carbo-
hydrate is particularly important to maintain lean
body mass, training effects, and performance [25].
When exercise lasts more than 1 h and especially as
duration extends beyond 90 min, athletes should in-
gest glucose/electrolyte solutions (GES) to maintain
blood glucose levels, prevent dehydration, and reduce
the immunosuppressive effects of intense exercise [40,
123128]. Notably, this strategy becomes even more
important if the athlete is under-fueled prior to the
exercise task or is fasted vs. unfasted at the start of
exercise [68,69,129]. Following intense exercise, ath-
letes should consume carbohydrate and protein (e.g.,
1 g/kg of carbohydrate and 0.5 g/kg of protein)
within 30 min after exercise and consume a high
carbohydrate meal within 2 h following exercise [2,74].
This nutritional strategy has been found to accelerate
glycogen resynthesis as well as promote a more ana-
bolic hormonal profile that may hasten recovery
[120,130,131], but as mentioned above only when
rapid glycogen restoration is needed or if the carbo-
hydrate intake in the diet is adequate (< 6 g/kg/day)
[53,132]. In other words, the total carbohydrate
consumption and timing of carbohydrate consump-
tion should be individualized to each athletes needs
according to the goals of the training cycle and bout
[112]. Finally, for two to 3 days prior to competition, ath-
letes should taper training by 30 to 50% and consume an
additional 200 to 300 g of carbohydrate each day in their
diet. This eating strategy has been shown to supersaturate
carbohydrate stores prior to competition and improve en-
durance exercise capacity [2,40]. Thus, the type of meal,
amount of carbohydrate consumed, and timing of eating
are important factors to maximize glycogen storage and in
maintaining carbohydrate availability during training while
also potentially decreasing the incidence of overtraining.
The ISSN has adopted a position stand on nutrient timing
in 2008 [133] that has been subsequently revised [13]and
can be summarized with the following points:
1. Intramuscular and hepatic glycogen stores are best
maximized by consumption of a high-carbohydrate
diet (812 g/kg/day). Strategies such as aggressive
carbohydrate feedings (~ 1.2 g/kg/hour) that favor
high-glycemic (> 70) carbohydrates, addition of
caffeine (38mg/kg) and combining a moderate
carbohydrate dose (0.8 g/kg/h) with protein (0.2
0.4 g/kg/h) have been shown to promote rapid
restoration of glycogen stores.
2. High intensity (> 70% VO
2
Max) exercise bouts that
extend beyond 90 min challenge fuel supply and
fluid regulation. In these situations, it is advisable to
consume carbohydrate at a rate of 3060 g of
carbohydrate/hour in a 68% carbohydrate-electrolyte
solution (612 fluid ounces) every 1015 min
throughouttheentireexercisebout.Theimportance
of this strategy is increased when poor feeding or
recovery strategies were employed prior to exercise
commencement. Consequently, when carbohydrate
delivery is inadequate, adding protein may help
increase performance, mitigate muscle damage,
promote euglycemia, and facilitate glycogen
re-synthesis.
3. Consuming a diet that delivers adequate energy
(minimum of 2730 kcal/kg) and protein (1.6
1.8 g/kg/day), preferably with evenly spaced (every
34 h) protein feedings (0.250.40 g/kg/dose)
during the day, should be considered for all exercising
individuals.
4. Ingesting efficacious doses (1012 g) of essential
amino acids (EAAs) either in free form or as a
protein bolus in 2040 g doses (0.250.40 g/kg/
dose) will maximally stimulate muscle protein
synthesis (MPS).
5. Pre- and/or post-exercise nutritional interventions
(carbohydrate + protein or protein alone) can be an
effective strategy to support improvements in strength
and body composition. However, the size (0.25
0.40 g/kg/dose) and timing (04 h) of a pre-exercise
meal may impact the benefit derived from the
post-exercise protein feeding.
6. Post-exercise ingestion (immediately-post to 2 h
post) of high-quality protein sources stimulates
robust increases in MPS. Similar increases in MPS
have been found when high-quality proteins are
ingested immediately before exercise.
Vitamins
Vitamins are essential organic compounds that serve to
regulate metabolic and neurological processes, energy
synthesis, and prevent destruction of cells. Fat-soluble
vitamins include vitamins A, D, E, & K and the body stores
fat-soluble vitamins in various tissues, which can result in
toxicity if consumed in excessive amounts. Water-soluble
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vitamins consist of the entire complex of B-vitamins and
vitamin C. Since these vitamins are water-soluble, excessive
intake of these vitamins are eliminated in urine, with few
exceptions (e.g. vitamin B6, which can cause peripheral
nerve damage when consumed in excessive amounts).
Table 1describes the RDA, proposed ergogenic benefit,
and summary of research findings for fat and water-soluble
vitamins. Research has demonstrated that specific vitamins
possess various health benefits (e.g., Vitamin E, niacin, folic
acid, vitamin C, etc.), while few published studies have re-
ported to find an ergogenic value of vitamins for athletes
[134138]. Alternatively, if an athlete is deficient in a vita-
min, supplementation or diet modifications to improve
vitamin status can consistently improve health and per-
formance [139]. For example, Paschalis and colleagues
[140] supplemented individuals who were low in vitamin C
for 30 days and reported these individuals had significantly
lower VO
2
Max levels than a group of males who were high
in vitamin C. Further, after 30 days of supplementation,
VO
2
Max significantly improved in the low vitamin C co-
hort as did baseline levels of oxidative stress of oxidative
stress. Importantly, one must consider that some vitamins
may help athletes tolerate training to a greater degree by
reducing oxidative damage (Vitamin E, C) and/or help to
maintain a healthy immune system during heavy training
(Vitamin C). Alternatively, conflicting evidence has accu-
mulated that ingesting high doses of Vitamins C and E
may negatively impact intracellular adaptations seen in re-
sponse to exercise training [141144], which may conse-
quently negatively impact an athletes performance.
Furthermore, while optimal levels of vitamin D have been
linked to improved muscle health [145] and strength [146]
in general populations, research studies conducted in ath-
letes generally fail to report on the ergogenic impact of
vitamin D in athletes [147,148]. However, equivocal evi-
dence from Wyon et al. [149] suggests vitamin D supple-
mentation in elite ballet dancers improved strength and
reduced risk for injuries. The remaining vitamins reviewed
appear to have little ergogenic value for athletes who con-
sume a normal, nutrient dense diet. Since dietary analyses
of athletes commonly indicate that athletes fail to consume
enough calories and subsequently may not be consuming
adequate amounts of each vitamin, many sport dietitians
and nutritionists recommend that athletes consume a
low-dose daily multivitamin and/or a vitamin enriched
post-workout carbohydrate/protein supplement during pe-
riods of heavy training [150]. Finally, athletes may desire to
consume a vitamin or mineral for various health (non-per-
formance) related reasons including niacin to elevate high
density lipoprotein (HDL) cholesterol levels and decrease
risk of heart disease (niacin), vitamin E for its antioxidant
potential, vitamin D for its ability to preserve musculoskel-
etal function, or vitamin C to promote and maintain a
healthy immune system.
Minerals
Minerals are essential inorganic elements necessary for
a host of metabolic processes. Minerals serve as struc-
ture for tissue, important components of enzymes and
hormones, and regulators of metabolic and neural con-
trol. In athletic populations, some minerals have been
found to be deficient while other minerals are reduced
secondary to training and/or prolonged exercise. Not-
ably, acute changes in sodium, potassium and magne-
sium throughout a continued bout of moderate to high
intensity exercise are considerable. In these situations,
athletes must work to ingest foods and fluids to replace
these losses, while physiological adaptations to sweat
composition and fluid retention will also occur to pro-
mote a necessary balance. Like vitamins, when mineral
status is inadequate, exercise capacity may be reduced
and when minerals are supplemented in deficient ath-
letes, exercise capacity has been shown to improve
[151]. However, scientific reports consistently fail to
document a performance improvement due to mineral
supplementation when vitamin and mineral status is
adequate [134,152,153]. Table 2describes minerals
that have been purported to affect exercise capacity in
athletes. Of the minerals reviewed, several appear to
possess health and/or ergogenic value for athletes
under certain conditions. For example, calcium supple-
mentation in athletes susceptible to premature osteo-
porosis may help maintain bone mass [151]. For years,
the importance of iron status in female athletes has
been discussed [154] and more recent efforts have
highlighted that iron supplementation in athletes prone
to iron deficiencies and/or anaemia can improve exer-
cise capacity [155,156]. Sodium phosphate loading can
increase maximal oxygen uptake, anaerobic threshold,
and improve endurance exercise capacity by 8 to 10%
[157]. Increasing dietary availability of salt (sodium
chloride) during the initial days of exercise training in
the heat helps to maintain fluid balance and prevent
dehydration. The American College of Sports Medicine
(ACSM) recommendations for sodium levels (340 mg)
represent the amount of sodium in less than 1/8 tea-
spoon of salt and recommended guidelines for sodium
ingestion during exercise (300600mgperhouror
1.72.9 g of salt during a prolonged exercise bout)
[158161]. Finally, zinc supplementation during train-
ing can support changes in immune status in response
to exercise training. Consequently, several minerals
may enhance exercise capacity and/or training adapta-
tions for athletes under certain conditions. However,
there is little evidence that boron, chromium, magne-
sium, or vanadium affect exercise capacity or training
adaptations in healthy individuals eating a normal diet.
Sport nutritionists and dietitians should be aware of
the specialized situations in which different types of
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