Pros and cons of CLA consumption: an insight
from clinical evidences
Benjamin et al.
Benjamin et al. Nutrition & Metabolism 2015, 12:4
R E V I E W Open Access
Pros and cons of CLA consumption: an insight
from clinical evidences
, Priji Prakasan
, Sajith Sreedharan
, Andre-Denis G Wright
and Friedrich Spener
This comprehensive review critically evaluates whether supposed health benefits propounded upon human
consumption of conjugated linoleic acids (CLAs) are clinically proven or not. With a general introduction on the
chemistry of CLA, major clinical evidences pertaining to intervention strategies, body composition, cardio-vascular
health, immunity, asthma, cancer and diabetes are evaluated. Supposed adverse effects such as oxidative stress, insulin
resistance, irritation of intestinal tract and milk fat depression are also examined. It seems that no consistent result
was observed even in similar studies conducted at different laboratories, this may be due to variations in age,
gender, racial and geographical disparities, coupled with type and dose of CLA supplemented. Thus, supposed
promising results reported in mechanistic and pre-clinical studies cannot be extrapolated with humans, mainly
due to the lack of inconsistency in analyses, prolonged intervention studies, follow-up studies and international
co-ordination of concerted studies. Briefly, clinical evidences accumulated thus far show that CLA is not eliciting
significantly promising and consistent health effects so as to uphold it as neither a functional nor a medical food.
Keywords: Conjugated linoleic acids, CLA, Review, Clinical evidences
Conjugated linoleic acids (CLAs) encompass a group of
positional and geometric isomers of octadecadienoic
acids (18:2) –naturally occurring polyunsaturated fatty
acids or PUFA- synthesized in the rumen of cattle, deer,
sheep and goat by microbial biotransformation of
forage-derived fatty acids (FAs) such as oleic acid (OA),
linoleic acid (LA) and α-linolenic acid (ALA) ultimately
into saturated stearic acid (SA) [1,2]. Although CLA is
formed as an intermediate during ruminal biohydro-
genation of OA, LA and ALA, its primary source in vivo
is endogenous (de novo) synthesis by the activity of Δ
desaturase from the monounsaturated FA (MUFA), the
vaccenic acid (trans-11,18:1; VA), another intermediate
in ruminal biohydrogenation . It is also synthesized
endogenously in humans from dietary VA by the activity
-desaturase [4,5] (Figure 1). The Δ
referred to as stearoyl-CoA desaturase; EC 126.96.36.199)
catalyzes the addition of a cis-9 double bond on the VA,
and was shown to be present in several tissues, including
the mammary gland, adipose, liver, and intestine; during the
process, cis-9, trans-11-CLA (designated as 9-CLA, the
rumenic acid) is formed from VA . Thus, VA is
the pivotal precursor of 9-CLA in ruminants (probably
in mammals too); therefore, anessentialFAinhumans.
In fact, most of the commercially available CLAs are
produced by the alkaline isomerization of LA-rich oils,
such as sunflower oil, and tend to contain an equimolar
mixture of 9- and 10-CLAs, together with a mixture of
variable quantities (up to 30%) of other geometrical and
positional isomers of CLA, and that 100% pure CLA
isomer is not available on the market . Therefore, the
primary focus of all mechanistic, preclinical and clinical
studies pertaining to CLA were on 9- and 10-CLAs –
especially a mixture of both or rarely one of these two
isomers (with others isomers as impurities) [8,9]. During
the past couple of decades, hundreds of reports - princi-
pally based on in vitro, microbial, animal, and of late
clinical studies on humans - have been accumulating
with the highlights of contrasting biological activities of
CLA isomers, especially of 9- and 10-CLAs [9,10].
There has been an overwhelming consumer interest
toward the health improving role of specific foods or
food components, the so-called ‘functional foods’.
* Correspondence: email@example.com
Biotechnology Division, Department of Botany, Enzyme Technology
Laboratory, University of Calicut, Kerala 673 635, India
Full list of author information is available at the end of the article
© 2015 Benjamin et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly credited. 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.
Benjamin et al. Nutrition & Metabolism 2015, 12:4
The term ‘functional food’is often used as a generic
description for the beneficial health effects of ingested
foods that go beyond their traditional nutritive values.
The supposed health benefit of CLA was discovered
nearly three decades ago, i.e., Ha and his co-workers
found that ground beef contained an anti-carcinogenic
factor that consisted of a series of conjugated dienoic iso-
mers of LA . As the biomedical studies with CLA ex-
panded, it became apparent that CLA showed a range of
positive health effects in experimental animal models.
Such supposed beneficial health effects were attributed to
suppressing cancer, reducing body fat accretion, delaying
the onset of type II diabetes, retarding the development of
atherosclerosis, improving the mineralization of bone and
modulating the immune system [12-14]. Therefore, CLA-
rich foods may be considered as functional foods (a food
offers an additional function in the form of health-
promotion or disease prevention in combination with
some supporting ingredients); and that CLAs per se are
neither a food nor a functional food but a FA class with
some bioactive properties.
In the light of the aforesaid background, this review
critically examines whether the health benefits attributed
to CLA on humans are clinically proven or not. Keeping
this in mind, this review is categorized into different sec-
tions with appropriate illustrations wherever necessary.
The thrust areas addressed include: structure of CLA,
intervention strategies, physiological effects of CLA
consumption associated with diseases such as obesity,
cardiovascular disorders, diabetes, cancer, immune disor-
ders; certain adverse effects of CLA consumption such
as oxidative stress, abdominal irritations, milk fat
depression, insulin resistance, coupled with possible
drawbacks such as ignoring the ingredients in placebo;
differences in the duration of study, dosage of CLA,
food/life styles, and selection of subjects.
Structure of CLA
Very often, the CLAs are erroneously classified as
omega-6 (abbreviated as ω-6 or n-6) FAs. In fact, CLA is
a class of FAs comprising as many as 56 isomers with
conjugated (juxtaposed or neighboring) double bond
pairs (i.e., at positions 6,8-; 7,9-; 8,10-; 9,11-; 10,12-;
11,13-; 12,14-; and 13,15- with cis-cis, cis-trans, trans-cis
or trans-trans geometric configurations) varying along
octadecadienoic acid (18:2)  (Figure 1). Regarding
geometric isomers, the cis and trans configurations un-
equivocally indicate the steric relations around a (given)
double bond; however, instead of cis and trans, the sym-
bols Z(from German zusammen, means together) and E
(from German entgegen, means opposite), respectively
are used in some classifications. It should not be
confused with the counting of carbon position from ω
)or–COOH terminus along the acyl chain; the
former is used in ωclassification, while the latter in
systemic nomenclature. From this, it is evident that only
Figure 1 Major ωfatty acids with their common names, structures and systemic names.
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 2 of 20
the isomers with the conjugated double bonds on carbon
positions starting at 10 (from –COOH terminus) have
indeed their first C-C double bond at the ω-6 position,
while counting from the methyl (−CH
) terminus (i.e.
from the last or omega-C in the acyl chain) (Figure 1).
Thus, trans-10, cis-12-CLA (designated as 10-CLA) is a
typical ω-6 CLA; while the biologically active and pre-
dominantly (natural) occurring 9-CLA is a typical ω-7
FA . Though, VA (the principal precursor of 9-CLA)
is devoid of conjugated double bonds; it is also an ω-6
FA, while counting from the methyl terminus on the
acyl chain (Figure 1).
Clinical studies are generally of two categories: the
cross-over and non cross-over (parallel or between pa-
tients) designs; in the former, the subjects are random-
ized into two groups of which the first receive X (e.g., a
drug) followed by Y (e.g., a placebo), and the second
group receive Y first, followed by X; while in the non
cross-over category also two groups will be there –one
of the groups (the test group) will invariably receive X
and the other group (the control group) receive Y, then
the results are compared between the test and control
groups (i.e., parallel). In cross-over study, the influence
of covariates is reduced as each cross-over patient serves
as her or his own control; while in non cross-over study,
the treatment groups may be unbalanced on certain
covariates . Unlike non cross-over study, cross-over
studies are statistically efficient, and hence fewer sub-
jects are required for the study. In fact, in non cross-
over design, the placebo effect will not be mixed up with
the effect of the test material. Thus, both methods have
some advantages and disadvantages in different situa-
tions, and hence, selection of study design depends on
the situation of the study.
Generally, the clinical studies on the efficacy of CLA
are randomized, double-blind and placebo-controlled
designs comprising two groups, i.e., one group of sub-
jects (the experimental group) receive the active CLA
isomers (i.e., 9- or 10-CLA) or its mixture; and the other
half (control group) receive a placebo, designed without
CLA or its isomeric mixture. In such experiments, nei-
ther the researcher nor the subjects know whether they
received CLA or the placebo designed for the purpose
(i.e., they are “blind”) till all the data are recorded. Such
types of clinical studies ensure that the personal expec-
tations of neither the researcher nor the subjects influ-
ence the results, making it more dependable, and thus
eliminate possible treatment bias. Nevertheless, in case
of some life-threatening diseases like cancer, analyses of
the implications on the direct supplementation of CLA
seem to be difficult. In such cases, epidemiological
studies were performed in which intake data derived
from a validated food-frequency questionnaire are
linked to an existing or freshly established nutrient
databases containing analytical data of specific FA
[16-18]. These studies may be limited by the variability
of CLA in the food supply as well as the difficulty of
assessing the intake of these minor dietary FAs, the
CLA. Some of the commercially available CLA prepara-
tions, commonly used for clinical studies are detailed
in Table 1.
Most of the clinical studies monitored the effect of
commercially available CLA supplements, which usually
contain a mixture of 9- and 10-CLAs at approximately
50:50 ratio, whereas some other researchers used natur-
ally CLA-enriched dairy products for evaluating the
biological activities. Typical level of CLA in cow’s milk
fat is about 0.5% [26,27]; which would vary considerably
depending on the composition of diet [e.g., green forage,
organic forage, nature and age of forage (young leaves of
grasses better), altitude of grazing, silage and concen-
trates supplemented to the feed for fortification],
coupled with stage of lactation, lactation number,
breed, parity, animal’s health, climate, ruminal mico-
biota (especially bacteria and protozoa), etc. [26-30].
Feeding animals with plant oils rich in LA or ALA
(such as sunflower, soybean or linseed oil) is shown to
enhance the 9-CLA content in the milk fat, which can
subsequently be used to make CLA-enriched dairy
products , i.e., an in situ approach . For in-
stance, the CLA content could be enhanced up to
2.08% of total milk FAs, if 4% (in terms of the dry
weight of feed) soybean oil (is supplemented as concen-
trate in the feed) ; as reported in the case, wherein
flaxseed or fish oil supplemented to the diet .
Naturally CLA-enriched (in situ) dairy products such
as butter, cheese, etc. were incorporated in a variety of
recipes such as those used for making muffins, cakes,
sauces, and very often used as spreads.
CLA and body composition
Overweight or obesity - one of the typical syndromes of
lifestyle diseases - is referred to as the excessive fat
accumulation that impacts health. The occurrence of
overweight and obesity has been augmented as the most
common health issue of modern food style. Obesity is
considered as a cause for many health problems such as
heart diseases, infertility and insulin resistance .
With a view to lose weight and improve the body com-
position (i.e., increase in fat-free mass and decrease in
body fat, many diet supplements, health preparations/
formulations or weight-loss drugs were made available
on the market by various companies. Among them,
CLAs draw more attention, since many pre-clinical stud-
ies in animal models proved its inverse relationship with
obesity. The significant clinical studies investigating the
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 3 of 20
effect of CLA on body composition and their interven-
tion strategies are shown in Table 2.
In clinical studies, several methods or techniques were
applied to measure the body composition. The simplest
method is that, to measure the thickness of subcutane-
ous fat in multiple places on the body such as abdominal
area, arms, sub-scapular region (large triangular muscle
near shoulder bone), buttocks and thighs [25,51,52].
Other commonly used measures are bioelectrical imped-
ance analysis , hydrodensitometry and dual-energy
X-ray absorptiometry (DEXA) [53-55]. Among them,
DEXA is the most widely used method in clinical studies
to assess body composition due to CLA consumption. In
fact, DEXA measures total body composition and fat
content with a high degree of accuracy and is considered
as the gold standard for measuring the body compos-
ition, since one can get an image of the entire body .
Another commonly used method is the measurement of
body mass index, i.e., a measure of body weight to
height index of a person which is calculated as weight in
kilograms divided by the square of height in meters
). According to World Health Organization ,
a BMI greater than or equal to 25 is overweight, and a
BMI greater than or equal to 30 is considered as obes-
ity. In fact, BMI is not necessarily a good measure,
especially in terms of body composition; for instance,
individuals like athletes with strong bone and greater
muscle (lean) mass have a higher BMI than non-
athletes, and hence different BMI classifications of
overweight is warranted while assessing obesity in
terms of body girth .
Some of the clinical studies suggested a positive asso-
ciation of the intake of 3.4 to 6.8 g/d isomeric mixture
of CLA (mainly 9- and 10-CLA) supplementation for 12
wk for overweight and obese volunteers (BMI, 25 to 35
) of either sex in reducing the body fat mass
(BFM) significantly . In another study, supplementa-
tion of 4.2 g/d isomeric mixture of 9- and 10-CLAs for 4
wk decreased the sagittal abdominal diameter in obese
individuals; but, body weight and BMI remained un-
affected [59,60]. In a different study comprising 60 over-
weight or obese volunteers including men and women
who received 3.4 g/d CLA for 12 wk showed reduction
in the mean weight and mean BMI, and these results
Table 1 Commercially available common CLA mixtures with their FA composition, trade name and manufacturer
Trade name CLA content (%) Total
Clarinol™9-CLA (37.3); 79.4 20.6 capsule Loders Croklaan, 
10-CLA (37.6); The Netherlands
other CLAs (4.5)
CLA-80 9-CLA (41.6); 82 18 capsule Cognis Corporation, 
10-CLA (40.4) The USA
Tonalin-TG 9-CLA (37.49); 80.8 19.2 capsule Natural lipids, 
10-CLA (38.02); Norway
other CLAs (5.26)
Tonalin™9-CLA (39.2); 79.6 20.4 capsule Cognis 
10-CLA (38.5); Corporation, The USA
other CLAs (1.9)
Tonalin® 9-CLA (11.4); 65 35 capsule Pharmanutrients, Inc. 
10-CLA (14.7); Lake Bluff, IL
other CLAs (38.9)
CLA-Capsules 9-CLA (21.7) ; 56.6 43.4 capsule Fitness Pharma, Norway 
other CLAs (15.8)
9-CLA (14.6); 19.3 drink NIZO Food Research, 
10-CLA (3.3) The Netherlands
other CLAs (1.4)
CLA-70 9-CLA (33.8); 69 31 capsule TrofoCell, Hamburg, 
10-CLA (35.2) Germany
*includes FAs such as OA, LA, ALA, SA, palmitic acid, etc.in varying concentrations.
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 4 of 20
Table 2 List of clinical trials investigating the effect of CLA consumption on body composition; ↑- increased; ↓- decreased; ↔no change in
Subjects Body type Age BMI
Composition Placebo Duration Measurement Effect of CLA Country/
1.7, 3.4, 5.1 or
9 and 10-CLA
Olive oil 12 wk Dual-energy X-ray
↓bodyfat mass Norway 
54 Overweight 20-50 27.81 ± 1.5 1.8 or 3.6 Tonalin™OA 13 wk Hydrodensitometry/
17 Normal 20-41 - 3 CLA
Sunflower oil 64 d Dual x-ray
↔body weight The USA 
>18 27.5-39.0 3.4 Tonalin™Olive oil 12 wk bioelectrical impedance,
Dual x-ray absorptiometry
weight & BMI
18-50 30-35 3.2 or 6.4 9 & 10-CLA (50:50) Safflower oil 12 wk Dual-energy X-ray
↑LBM The USA 
24 Normal 19–
>30 0.7-1.4 9 & 10-CLA (50:50) Soybean oil 8 wk Skinfold thickness ↓bodyfat mass - 
20 Normal 18-30 >25 1.8 Tonalin™Hydrogel 12 wk Near infrared light utilizing
a Futrex 5000 A instrument
23 Normal 3 Tonalin™Olive oil 28 d Dual-energy X-ray
↔fat mass The USA 
35-55 >25 3.2 Tonalin™Safflower oil 12 wk Computed tomography ↔visceral adipose
The USA 
18-65 28–32 3.4 Clarinol™Olive oil 6 mnt Waist–hip ratio ↓bodyfat mass Norway 
45-68 25–35 4.5 Tonalin TG 80 Safflower/
4 wk Waist/hip ratio ↓body weight Germany 
32 Normal 1.3 naturally or
with 9 & 10-CLA
8 wk Magnetic resonance
↔fat mass Canada 
81 overweight 35-65 25-30 1.5/3 Dairy drink with
9 & 10-CLA
18 wk Dual-energy X-ray
60 overweight 35-65 25–35 3 Milk with 9
Skimmed milk 12 wk Dual-energy X-ray
↓body fat mass Spain 
55 Obese ≤70 >30 8 CLA mixture Safflower oil 36 wk Anthropometry ↔BMI; ↑lean mass The USA 
76 Normal 18 -
- 5 Tonalin Sunflower oil 14 wk Air displacement
↑lean tissue mas;
↓body fat mass
>35 4.7 9 & 10-CLA (50:50) Olive oil 8 wk Dual energy X-ray
↓total fat mass and
lower body fat mass
33 Type-2 diabetes
35-50 25-30 8 9 & 10-CLA (50:50) Soybean oil 8 wk Bioelectrical impedance,
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 5 of 20
indicated that CLA in the given dose was safe in healthy
populations with regard to the safety parameters investi-
gated . Steck et al.  examined the effect of 2
doses of CLA (3.2 g/d or 6.4 g/d) for 12 wk (mixture of
9- and 10 CLA in 50:50 ratio) on body composition in
obese individuals, who were free of chronic diseases.
They concluded that lean body mass (LBM) increased by
the higher dose after 12 wk of intervention. Supple-
mentation of 9- and 10-CLAs at a dosage of 1.7 g/d for
12 wk in overweight and class I (low risk) obese
subjects (i.e., BMI = 30.0 –34.9) of Chinese population
showed lower obesity indices with no obvious adverse
effects . Interestingly, CLA was found to be effect-
ive in reducing the weight gain associated with psychi-
atric medications - one of the major side effects in
psychological treatments. Consumption of CLA at a
dosage of 3.4 g/d along with green tea extract signifi-
cantly reduced total body fat percentage in psychiatric
patients by 5.1 to 8.1% and increased LBM by 4.4 to
11% . A possible explanation for this effect is that
green tea extract high in pigallocatechin-gallate can
directly inhibit gastric and pancreatic lipases, thereby
increasing the thermogenesis and possibly prevent the
enzymatic degradation of catechol O-methyltransferase,
an enzyme which plays a role in the respiration rate of
brown adipose tissue .
Due to overeating and sedentary life, the incidence of
weight gain during holiday season (i.e., obesity and over-
weight) has increased considerably during the past two
decades and currently affects majority of the adult popu-
lation. For instance, supplementation of CLA (3.2 g/d
for 6 month) for 40 healthy overweight adults (18–44 yr;
BMI: 25–30 kg/m
), significantly reduced body fat (by
1.0-2.2 kg) and prevented weight gain during their holi-
day season . Thus, all these studies suggest some po-
tential beneficial effects upon consumption of isomeric
CLA mixture (2 to 6 g/d) in body composition of obese
or overweight individuals.
On the contrary, an inverse relationship between CLA
and body composition has been demonstrated. One of
the first studies demonstrating negative effects of CLA
was performed with 71 subjects including obese men
and women of 20 to 50 yr of age. The subjects were
instructed to take 90% pure CLA (2.7 g/d active 9- and
10-CLA isomers in equal ratio) daily for 26 wk, and
compared the effects to 3 g/d safflower oil as placebo.
Body was measured by hydrodensitometry, but the re-
sults did not show any effect on body composition .
In sedentary young women, intake of 2.1 g CLA/d for 45
d did not induce any changes in body composition .
Likewise, consumption of 4.5 g/d CLA isomeric mixture
showed no decrease in body weight, in comparison to
the consumption of safflower oil as placebo in 85 over-
weight and obese male subjects .
Some of the studies observed gender specific effects of
CLA intake. Riserus et al.  showed that CLA (4.2 g/d)
supplementation for 4 wk in 14 obese men (BMI: 32 ± 2.7
;39–64 yr old) with the metabolic syndrome may
decrease abdominal fat, without concomitant effects on
overall obesity or other cardiovascular risk factors. The
small sample size and short duration were the major limi-
tations of this study; thus, the effects of CLA in abdominal
obesity need to be investigated further in larger studies
with longer duration. Long-term (1 yr) supplementation
(daily dose of CLA was 3.6 g; the mixture contained 39%
9-CLA, 41% 10-CLA) with CLA in free FA or triacylglyc-
erol (TAG) did not show significant improvement on the
BFM in healthy overweight adults (higher standard
deviation found in the reported value makes the report
neutral) . In this double-blind placebo-controlled
study, 180 (female 149 and male 31) volunteers with
BMI of 25–30 were included. In another study, healthy
adult women were examined for the effects of an intake
of 3 g/d CLA for 64 d on body composition, but no
differences were found in the measured parameters like
fat-free mass, BFM and percentage of BFM, body compos-
ition, energy expenditure, fat oxidation and respiratory
exchange ratio against sunflower oil as placebo . In a
bicentric study (conducted simultaneously at Clermont-
Ferrand, France and Maastricht, The Netherlands), eighty-
one middle-aged, overweight, healthy men and women
were enrolled, and all subjects consumed a drinkable dairy
product containing 3 g of high OA sunflower oil daily
(for 6 wk, the run-in period) .Volunteers were then
randomized over five groups receiving daily either 3 g
of high OA sunflower oil, 1.5 or 3 g each of 9- or 10-
CLA administrated as TAG in a drinkable dairy prod-
uct for 18 wk. Percentage BFM, fat and LBM were
assessed at the end of the run-in and experimental
periods by DEXA. Dietary intake was also recorded. It
was concluded that, a daily consumption of a drinkable
dairy product containing up to 3 g of CLA isomers for
18 wk had no significant effect on body composition in
overweight, middle-aged men and women .
A study from Greece reported that CLA administered
first at 0.7 g/d for 4 wk and thereafter at 1.4 g/d for the
next 4 wk, decreased BFM in healthy volunteers .
Raff et al.  compared the effects of the supplementa-
tion of 5.5 g/d CLA mixture (50:50 mixture of 9- and
10-CLA) or only 9-CLA for 16 wk and assessed the
change in total and regional fat mass in healthy post-
menopausal women and concluded that the consump-
tion of 9- and 10-CLA mixture resulted in the reduction
of total and lower BFMs. Likewise, supplementation of
3 g/d of 80% CLA (50:50 ratio of 9- and 10-CLA) for 7
month attenuated the increase in LBM by 0.5 ± 0.8
(SD is on higher side). This study gained more import-
ance, as it was reported in children aged between 6
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 6 of 20
and 10 yr, who were overweight or obese, but otherwise
healthy . Interestingly, in a study to examine improve-
ment on the reduction of BFM, CLA (500 mg/d) was sup-
plemented in conjunction with 50 mg γ-oryzanol, which
effectively reduced BFM by 1.14 kg (against 0.36 kg reduc-
tion in CLA group) in healthy overweight Korean women
(n = 51, BMI > 23) . It is known that the γ-oryzanol is
a phytochemical having several biological activities like
anti-oxidant activity, anti-atherogenic effect, lowering
triglycerides and improves LBM . This report also
indicates that CLA per se was less efficient to improve
BFM; for instance, a recent non cross-over clinical
study conducted on 66 non-trained healthy male stu-
dents for 2 month showed that CLA supplementation
had no effect on LBM, BFM, trunk and visceral fats,
and waist circumference .
From the aforesaid reports, it seems that a minimum
daily dose (about 3 g/human) is required to induce
reduction in fat. Some clinical studies suggested that ad-
ministration of CLA might be the most effective strategy
in controlling regionalized reduction of fat mass rather
than its constitutional reduction, i.e., uniformly in the
whole body. For instance, administration of 3.4 g/d CLA
for 6 month reduced fat mass significantly in legs .
Waist-to-hip ratio also decreased significantly in healthy,
overweight and obese men, compared with placebo
group. Interestingly, these effects were produced inde-
pendent of diet and specific lifestyle.
Effect of CLA on exercise
Exercising individuals often add nutritional supplements
to their diet to accelerate the increase in muscle mass
and strength from heavy resistance-exercise training.
Some short- and long-term studies employing high doses
of CLA in healthy and obese, sedentary and exercised
adults have shown beneficial effects of CLA in reducing
fat mass and increasing LBM. A daily supplementation
of 1.8 mg CLA for 12 wk reduced body fat (measured
using near infrared lights); but not body weight in
healthy exercising humans of normal body weight, com-
pared to the placebo group who received hydrogel .
In this study, physical exercise was standardized as 90
min in gym, three times a wk; and concluded that CLA
reduces the deposition of fat. These results seem to be
encouraging, because much lower dose of CLA pro-
duced the expected results - when compared to other
studies - wherein comparatively 2 to 4 folds higher con-
centrations of CLA were used. Effect of CLA (Clarinol
A-80) supplementation in conjunction with 6 wk of
aerobic exercise training on 33 untrained to moderately
trained men (average age 21.6 ± 2.8 yr) was investigated
. CLA showed no ergogenic benefits on neuromus-
cular fatigue, and field tests of muscular endurance and
CLA also gained attention among resistance-trained
athletes as agents for reducing catabolism, body fat and
improving muscle mass during training, but supplemen-
tation of 6 g/d of CLA coupled with 3 g/d of FAs in the
formulation (Tonalin®) against 9 g/d olive oil placebo for
28 d showed no significant ergogenic value, as it did not
significantly affect changes in total body mass, fat-free
mass, fat mass, percent body fat, bone mass, strength,
serum substrates, or general markers of catabolism dur-
ing training . But in contrast, administration of 6 g/d
CLA (in combination with 5 g/d creatine monohydrate)
followed by resistance exercise training in older adults
(above 65 yr, comprising 19 men and 20 women) for 6
month enhanced strength gains and improved body
composition . This combined strategy showed that
supervised resistance exercise training is safe and effect-
ive for increasing strength in older adults, because aging
is associated with lower muscle mass and an increase in
body fat. In another study, CLA (6 g/d) supplementation
along with creatine (9 g/d) and whey protein (36 g/d)
also found beneficial for improving body strength and
LBM during heavy resistance training in well-trained
young adults (both men and women; aged 22.5 ± 2.5 yr)
with no changes in oxidative stress and kidney function
. Similarly, in another double blind and placebo con-
trolled resistance-training study, no decrease in visceral
adipose tissue was observed; however a significant
reduction in the cross-sectional area of visceral adipose
tissue was noticed in the placebo group . In this
study, 30 overweight and moderately obese, but other-
wise healthy middle-aged (35 to 55 yr) male subjects
received 3.2 g/d CLA for 4 wk . Pinkoski et al. 
showed that supplementation of CLA (5 g/d) or placebo
for 7 wk while resistance training (3 d/wk), which re-
sulted in relatively lesser changes in body composition,
accompanied by a lessening of the catabolic effect of
training on muscle protein. Thus, some of the studies
showed the effectiveness of CLA in fat mass reduction
in subjects during resistance-training program. Contrary
to this, no CLA-specific effects were observed on body
composition, energy expenditure or appetite in non-
obese, regularly exercising individuals (comprising 25
men and 27 women), who received either 3.9 g/d CLA
or 3.9 g/d oleic acid rich sunflower oil (placebo) for 12
wk . More recently, it was observed that CLA sup-
plementation at a dosage of 6 g/d increased the level of
total testosterone in the blood of human males, but no
significant change was observed before or after each re-
sistance exercise bout . It suggests that CLA supple-
mentation may promote testosterone synthesis through
a molecular pathway that should be investigated in
detail. Furthermore, this study becomes much relevant,
since the correlation between the production of testos-
terone and body building still remains a controversy.
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 7 of 20
CLA on fat-mass regain
Some pre-clinical studies showed that 10-CLA reduces
fat uptake into adipocytes by lowering the activities of
lipoprotein lipase and Δ
-desaturase, instead of enhan-
cing lipolysis [14,74,75]. Based upon this background, a
few clinical investigations were made on the effect of
CLA on fat-mass regain after weight loss - with an as-
sumption that CLA could block body fat gain. To check
this, overweight adults were administered a very low-
calorie diet for 3 wk, followed by CLA supplementation
at a dosage of either 1.8 or 3.6 g/d for an intervention
period of 13 wk . Subjects took CLA in either dose
showed increased regain of fat-free mass and resting
metabolic rate, thereby lowering the regain of body fat
relative to the control subjects. Interestingly, they con-
cluded in later findings that the measures of appetite
(hunger, satiety and fullness) favorably and dose-
independently affected by the same dose of CLA but had
no effect on energy intake at breakfast or improved
body-weight maintenance after weight loss .
CLA as dairy products
Apart from a few studies that investigated the effects
of CLA supplementation in humans, there were some
experiments designed to supplement CLA-enriched
dairy products. Consumption of dairy products such as
ultra-heat treated milk, butter, and cheese enriched
with 1.42 g of 9-CLA did not significantly affect BFM
and bodyweight in healthy, middle-aged men . An-
other experiment compared the effects of the con-
sumption of a modified butter, naturally enriched with
CLA (4.22 g /100 g butter fat) on body composition in
overweight and obese men, wherein abdominal adipose
tissue area was measured by computed tomography
, which found no differences in the accumulation
of abdominal or subcutaneous adipose tissue, com-
pared to the control group who consumed butter fat
with low CLA (0.38 g /100 g butter fat) content.
Consumption of a drinkable dairy product containing
up to 3 g of 10-CLA isomer for 18 wk did not result in
any significant effect on body composition in over-
weight, middle-aged men and women .
Venkatramanan et al.  examined the role of CLA
enriched (so as to get 1.3 g/d) milk (i.e., naturally
enriched with only 9-CLA or synthetically with a mix-
ture of 9- and 10-CLA isomers) in modulating body
composition of moderately overweight, borderline hyper-
lipidemic individuals. More precisely, consumption of
CLA-enriched milks (in either form) failed to alter TAG
concentrations in the blood; body weight or fat compos-
ition . Supplementation (14 wk) of 9- and 10-CLA
isomers (in equal proportion, 70% purity) as TAG form
blended in flavored yoghurt-like products was also found
unable to alter body composition, although a significant
increase in the resting metabolic rate was induced .
In contrast, a study conducted in Spain involving 60
healthy men and women (aged 35–65 yr) with signs of
metabolic syndrome (BMI, 25–35 kg/m
) showed signifi-
cant reduction (2-3%) of fat mass in overweight, but not
in obese subjects; upon daily intake of 500 ml milk sup-
plemented with 9- and 10-CLA mixture (3 g/d) for 12
wk . Thus, in general, dairy products enriched with
either of 9- or 10-CLA isomers or its mixture failed to
establish a consistent effect on body composition.
Long-term consumption of CLA
The question of inconclusive results on efficacy and
effectiveness of CLA on body composition and obesity
may answer from long-term intervention studies. Effects
of any dietary supplement or food ingredient on body
composition should be assessed over an extended period
of time to conclude the results, because crash diet pro-
cedures seem inappropriate. In most of the studies, the
intervention period lasted only for a few wk, and long-
term studies were very few. In a study, 134 subjects in-
cluding men and women were supplemented with 3.4 g
CLA/d in the TAG or free FA form for 12 month, and
extension study was also conducted in the same subjects
for next 12 month . During the first 12 month, sig-
nificant reduction in BFM and leptin levels was reported.
These changes in body composition were not related to
diet and exercise. Most of the effects on BFM were ob-
served during the first 6 month of CLA supplementation
and the extension study concluded that CLA may be
beneficial in preventing weight regain and long-term
maintenance of BFM and LBM. These studies seem to
be important as most of weight loss studies in over-
weight and obese subjects have demonstrated that
most subjects will regain the lost weight within the
next 1 to 2 yr [43,80]. Gaullier et al.  also reported
similar effects in another study with CLA supplemen-
tation for 1 yr in healthy overweight adults. Energy
expenditure, substrate utilization and dietary fat oxidation
were measured before and after 6 month of CLA supple-
mentation, which showed that fat oxidation and energy
expenditure increased during sleep in subjects received
CLA, in comparison to placebo . Supplementation of
CLA (6.4 g/d) for 36 wk) reduced BMI and total adipose
fat mass without altering LBM in obese postmenopausal
women with type 2 diabetes, who were not also on a
weight-loss diet or exercise plan . Such long-term
studies have to be conducted in a cross-over design (by
including men and women of different age groups) to
generalize the beneficial effects of CLA.
From these evidences, a few clinical studies pertaining
to the long/short term effects of CLA in obese,
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 8 of 20
sedentary, healthy or exercising humans have shown
some beneficial effects of CLA in reducing body fat and
improving body composition. However, all of them failed
to reproduce the dramatic results reported in animal
and in vitro models, especially mice. The extensive con-
troversies in clinical studies limit from proposing a def-
inite statement regarding the beneficial effects of CLA
on body composition, so as to address the increasing
concerns of health professionals, body builders and
CLA and cardiovascular health
Hyper-triacylglycerolemia and elevated plasma choles-
terol are suggested as the major risk factors for athero-
sclerosis and cardio-vascular diseases (CVD), and that
blood lipid profile, blood pressure, BMI and blood sugar
are generally considered as the indicators of heart health.
The lipid profile is a panel of blood tests performed on
the patient to determine the risk of CVD. These tests
are good indicators of whether someone is likely to have
a heart attack or stroke caused by blockage of blood
vessels or hardening of the arteries (atherosclerosis). The
lipid profile typically includes the baseline measurements
of total cholesterol; high density lipoprotein cholesterol
(HDL-C), often called good cholesterol; low density lipo-
protein cholesterol (LDL-C), called bad cholesterol; and
TAG in plasma . Normal cholesterol levels vary by
age and sex. LDL-C is the major cholesterol carrier in
the blood, and if too much it is in circulation, it can
slowly build up in the walls of the arteries of heart and
brain leading to arteriosclerotic vascular diseases. Ac-
cording to American Heart Association, a high TAG
level combined with low HDL-C or high LDL-C in-
creases the risk of CVD . The major circulatory
markers associated with heart health are C-reactive
protein (CRP), tumor necrosis factor-α(TNF-α), 15-
keto-dihydro prostaglandinF2 (PGF2), 8-iso-prostaglan-
dinF2-α(PGF2α), leptin, interleukin (IL)-6, plasma alanine
transaminase, and total bilirubin . Variations in
the concentration of these markers in blood plasma
(from the normal level) indicate dysfunctions of human
system. The major circulatory markers associated with
heart health and their normal levels in blood are listed
in Table 3.
Some animal studies suggest the health benefits (anti-
CVD effects) of CLA such as anti-sclerotic and im-
provements in blood lipid profile, hypolipidaemic and
anti-oxidative effects [84-86]. Two different isomers of
CLA (i.e., 9- and 10-CLA) have different or opposing
effects on atherosclerosis . The 10-CLA is pro-
atherogenic and induces pathways involved in the devel-
opment of insulin resistance, whereas 9-CLA is associated
with reduced risk of CVD [87-89].
Effect of CLA on blood lipid profile
Epidemiological studies showed that plasma HDL-C
concentrations have an inverse relationship with the risk
of CVD, and it is anticipated that raising plasma HDL-C
levels might protect against atherosclerosis . Sup-
posed effects of CLA supplementation on blood lipid
profile also remains inconclusive. Supplementation of an
isomeric blend of CLA (9- and 10-CLAs in 50:50 ratio)
for 12 wk (1.7 to 6.8 g/d) decreased total cholesterol,
HDL-C and LDL-C . Similarly, a dose of 0.7 or 1.4
g/d CLA decreased serum HDL-C and TAG significantly
and increased the CLA content of serum lipids . In
this study, 22 volunteers were enrolled and they were
divided into study and control groups in a doubly blind
design; the study group received 0.7 g of CLA for four
wk and 1.4 g of CLA for the next four wk, while the
control group received similar dose placebo throughout.
Diet was controlled, and no significant differences in
energy or macronutrient intake were found between the
two groups. A significant reduction of HDL-C was ob-
served when 6.4 g/d CLA was consumed, whereas no
change was observed in total cholesterol or LDL-C. But
2.1 g/d of CLA (9- and 10 CLAs in equal proportion)
for 45 d showed no significant difference in serum TAG,
total cholesterol, HDL-C in healthy non-obese sedentary
women . In contrast, CLA (9- and 10-CLA in 50:50
ratio) in a dose of 3.0 g/d for 8 wk increased total HDL-
C concentrations by 8%. And the ratio of LDL-C to
HDL-C was significantly reduced in subjects with stable,
diet-controlled type 2 diabetes .
Supplementation (3 g/d) of a 50:50 isomeric blend of
9- and 10–CLAs or 80:20 blend of 9- and 10-CLAs for 8
Table 3 Major parameters of blood profile analysis, their
normal level and variations in blood in relation to heart
health ↑- increased; ↓- decreased
Type Normal Indication
Total cholesterol Below 200 mg/dL ↑risk of heart disease
LDL-C 100-129 mg/dL ↓risk of heart disease
HDL-C 40-50 mg/dL (men) ↑protective against
50-60 mg/dL (women)
Triglyceride 100-150 mg/dL ↑risk of heart disease
VLDL-C 2-30 mg/dL ↑risk of coronary
C-reactive protein 0-10 mg/L ↑Inflammation/heart
3-12 ng/L ↑inflammation
8-iso-PGF2-α150 ng/L ↑oxidative stress
Leptin 1-5 ng/dL (men) ↑inflammation
7-13 ng/dL (women)
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 9 of 20
wk showed that the former combination significantly
reduced plasma TAG concentrations in synergy, whereas
the latter blend significantly reduced the VLDL-C .
In this non cross-over double-blind, placebo-controlled
and randomized study, 51 normolipidaemic subjects
were enrolled. These results further suggested that
CLA supplementation significantly improved the lipid
profile in human subjects without any adverse effects
on body weight, plasma glucose and insulin concentra-
tions; and thus indicates the supposed cardio-protective
effects of CLA. Contrary to this, the opposing effects 9-
and 10-CLAs were observed by Tricon et al. . In
this cross-over study, 49 healthy men (20–47 yr; BMI 18–
) were enrolled, and supplemented with either
79.3% (2.38 g/d of 9-CLA) 9-CLA or 84.1% (2.52 g/d of
10-CLA) 10-CLA for 8 wk consecutively. It showed that
10-CLAincreased the ratios of LDL-C to HDL-C and total
to HDL-C, whereas 9-CLA decreased them, suggesting
the beneficial effects of CLA on blood lipid profile .
But, later, the same group showed that dairy products
enriched with 9-CLA (1.42 g/d) had no significant effect
on blood lipid profile .
Some studies observed neither a beneficial nor an
adverse effect of an isomeric blend of 9- and 10-CLA in
a ratio other than 50:50. A 93 d long study in 17 healthy
female volunteers to observe the effect of dietary CLA
(on blood lipids, lipoproteins, and tissue FA compos-
ition) showed that daily supplementation of 3.9 g/d CLA
(the mixture contained 11.4% 9-CLA, 14.7% 10-CLA
and 38.9% other CLAs) did not alter the blood choles-
terol or lipoprotein levels of healthy, normo-lipidemic
subjects . Furthermore, no adverse effect of CLA
supplementation was reported in this study, though
plasma concentration of CLA was increased during
the intervention period, i.e., 95.77% of the total CLA
consumed was metabolized in the body . In a
cross-over study (for 6 month, n = 401, aged 40–70 yr
and with a BMI > or = 25), consumption of 4 g CLA/d
(4:1 ratio of 9- and 10-CLAs) did not influence aortic
pulse wave velocity (marker of atherosclerosis), blood
pressure, anthropometric characteristics, and concentra-
tions of fasting lipid, glucose, insulin, and C-reactive protein
(CRP), briefly it neither supported an anti-atherosclerotic
effect nor an effect on CVD risk factors of 9- CLA .
Some studies investigated the effect of dairy products
on lipid profile. However, human studies with the sup-
plementation of CLA-enriched dairy products (in situ
enrichment) produced contradictory results. Intake of
1.3 g/d of CLA in the form of naturally enriched milk
(containing only 9-CLA) or milk enriched with a syn-
thetic mixture of 9- and 10-CLAs for 8 wk did not alter
the levels of cholesterol, LDL-C, HDL-C or TAG con-
centrations in blood samples of moderately overweight,
borderline hyperlipidemic individuals ; likewise, in
another cross-over study (contained healthy middle-aged
men, n= 32 for 6 wk), supplementation of dairy prod-
ucts (heat-treated milk, butter or cheese) enriched with
9-CLA and VA (1.421 g/d) appeared to have no effect on
the blood lipid profile of men . However, levels of
CLA and VA in human milk can be modulated if breast-
feeding mothers replace conventional dairy and meat
products with organic dairy products (enriched by nat-
ural feeding) . Recently, Penedo et al.  showed
that intake of butter, naturally enriched with 9-CLA
(1.02 ± 0.167 g/d) for 8 wk induced beneficial changes
in immune modulators associated with sub-clinical
inflammation in overweight individuals. Furthermore,
sheep cheese naturally enriched in VA, CLA and ALA
improved the lipid profile and reduced anandamide
(an endogenous cannabinoid neurotransmitter and
obesity marker) in adults with diagnosed mildly hyper-
CLA on circulatory markers
CRP is synthesized by liver in response to inflammation.
Inflammations may be due to a variety of reasons such
as cancer, diabetes, cardiovascular diseases, etc. [98,99].
Normal concentration of CRP in healthy human serum
is usually lower than 10 mg/l, but slightly increasing
with aging. Increased plasma concentration of CRP, a
circulatory inflammation marker helps in predicting
CVD . Supplementation with10-CLA for 12 wk
markedly increased the levels of PGF
α(578%) and CRP
(110%), compared with placebo in 60 men with meta-
bolic syndrome . A dose of 3 g 9-CLA/d significantly
elevated the levels of urinary PGF2 and PGF
markers of in vivo inflammation and oxidative stress, re-
spectively; after the supplementation for 3 month with
25 abdominally obese men against olive oil as placebo
. A mixture of 9- and 10-CLA isomers with equal
proportions also reported an increased CRP, but not of
the other inflammatory markers, i.e., TNF-α, TNF-αre-
ceptors 1 and 2, and vascular cell adhesion molecule
(VCAM)-1 . Another study concluded that a mix-
ture of 9- and 10-CLAs had more adverse effects on
CVD markers, while 9-CLA isomer appeared to be more
neutral in healthy postmenopausal women. Daily supple-
mentation of 5.5 g of CLA mixture significantly elevated
the level of CRP, fibrinogen, and plasminogen activator
inhibitor-1 in plasma . The CLA mixture at a dose
of 3.0 g/d reduced fibrinogen concentrations but had no
effect on other inflammatory markers of CVD like CRP
and interleukins (IL) in subjects with type 2 diabetes
. High dose of CLA consumption (6.4 g/d of 9- and
10-CLA in 50:50 ratio) for 12 wk markedly increased the
levels of CRP and IL-2, suggesting an increase in inflam-
mation during short-term supplementation. In contrast,
CLA in the same composition (50:50 ratio), but in lower
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 10 of 20
dose (i.e., 3.0 g/d showed) no effect on the inflammatory
markers of CVD (CRP and IL-6) .
Outcome from some recent studies suggested that
CLA did not increase the risk of CVD. Pfeuffer et al.
 assessed the effect of CLA against safflower oil on
endothelial function and markers of CV risk in over-
weight and obese men, i.e., by the consumption of 4.5
g/d of the CLA isomeric mixture for 4 wk. It was ob-
served that CLA did not impair endothelial function.
Other parameters associated with metabolic syndrome
and oxidative stress were not changed or slightly im-
proved. Interestingly, it was observed that oral supple-
mentation of CLA along with calcium reduced the
incidence of pregnancy-induced hypertension without
changing the plasma levels of other circulatory markers
such as PGF2α, CRP and IL-6 . Forty eight healthy
primigravidas with a family history of preeclampsia and
with diastolic notch were included in this double-blind
and placebo-controlled non cross-over study. Partici-
pants were randomized to daily oral doses of elemental
calcium (0.6 g/d) with CLA (0.45 g/d) or lactose-starch
placebo from wk 18 to wk 22 of gestation until
The controversial beneficial and detrimental effects of
CLA on heart health observed during clinical studied
are summarized in Figure 2. All these studies were too
randomized in dosage, composition and duration, which
make difficult to conclude the positive effects of CLA on
heart health. Moreover, there is a complete lack of uni-
formity in assessing the effects CLA on heart health, i.e.,
some studies were focused on lipid profiles, while others
on circulatory markers; but none of them was reported
to have a consistent effect. Even though the isomeric
mixture of 9- and 10 CLA (1:1) was found to exert some
positive effects, it is necessary to elucidate the mechan-
ism of action to ascertain which of these isomers elicited
CLA and immune function
Different studies show that the effects of dietary CLA on
immune functions in animal as well as human models
are highly variable and inconsistent (Table 4). For in-
stance, a 93 d long study in 17 young women upon
feeding with 3.9 g/d CLA isomeric blend showed no
alteration in any of the indices of immune status such as
circulating white blood cells, granulocytes, monocytes
and lymphocytes . Even after immunization with
influenza vaccine, the delayed type hypersensitivity
response and serum antibody titers were not altered
during the intervention period. These data suggest that
short-term CLA supplementation in healthy volunteers
was safe, but it showed no added benefit to their im-
mune status . Moreover, short-term consumption
of CLA produced no observable physiological change in
blood coagulation and platelet function in healthy adult
females . CLA supplementation (3.9 g/d of a mixture
of CLA isomers: 17.6% 9-CLA, 10-CLA, 23.6% cis-11,
trans-13 CLA, 16.6% trans-8,cis-10 CLA and other iso-
mers 19.6%) resulted in an eight folds increase (0.12 to
0.97, wt %) in the concentration of CLA in the lipid frac-
tion of peripheral blood mononuclear cells (PBMC)
without changing the concentration of other FAs; but,
increased concentration of CLA did not alter the func-
tions of PBMCs, i.e., secretion of PGE2, leukotriene B4,
IL-1βor TNFα. Supplementation with 9- to10
CLA in the ratio 50:50 or 80:20, respectively resulted in
35% increased CLA levels in PBMC . Interestingly,
in this non cross-over study, 62% of the subjects, who
consumed 9- and 10-CLA mixture in the ratio 50:50
showed increased titers of protective antibody levels
after hepatitis B vaccination. Although the overall effect
was not significant, the results at least suggested that
50:50 CLA might have a biologically relevant enhancing
effect on the response to hepatitis B vaccination, which
warrants further study . Contrary to this, supple-
mentation with the 9- and 10-CLA isomers (80:20 blend,
respectively) significantly enhanced phyto-hemagglutinin
(PHA) content, a T-cell mitogen-induced lymphocyte
proliferator. CLA decreased basal IL-2 secretion, but in-
creased PHA-induced IL-2 and TNF-αproduction - when
55 healthy volunteers received 3 g/d of 9- and 10-CLAs
blend in ratios 50:50 and 80:20, respectively . Plasma
IgA and IgM levels were found increased upon supple-
mentation with 9- and 10-CLAs (50:50), but decreased the
levels of IgE, TNF-αand IL-1β. In addition to these effects,
Figure 2 Proposed effects of CLA consumption on heart health.
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 11 of 20
delayed hypersensitivity response was decreased during
CLA supplementation .
CLA vs. asthma
CLA is reported to modify the inflammatory responses
associated with allergic airway disease, primarily in
animal models. A prominent study in this regard came
from the group of MacRedmond et al. , which
demonstrated that supplementation of 4.5 g/d CLA as
an adjunct to usual care in overweight mild asthmatics
(28 subjects; aged 19–40 yr involved) for 12 wk was
well tolerated, which was associated with improve-
ments in airway hyper-responsiveness . However,
daily supplementation of 4.8 g CLA for 8 wk did not
attenuate airway inflammation or hyperpnea-induced
broncho constriction in asthmatic individuals .
One of the early studies in this direction measured the
mean serum phospholipid esterified 9-CLA concentra-
tion in peripheral blood; observed it as significantly
higher in 98 patients with chronic stable asthma, and
25 patients with acute severe asthma. Thus the sup-
posedroleofoxygen-derivedfree-radical activity in in-
flamed lung tissue was envisaged . It shows that,
some attempts were made to estimate the effect of
CLA on immunity with reference to asthma, but none
of them succeeded in reproducing the positive effects
such as enhancement of immune function, down regu-
lation of autoimmunity and increased proliferation of
lymphocytes,consistently in clinical studies [112-114].
Furthermore, activation of peroxisome proliferator-
activated receptors (PPARs, a group of nuclear recep-
tors), especially PPAR-γin the human airway smooth
muscle would be a possible strategy to treat airway dis-
eases ; therefore, targeting PPAR-γ,9-CLAmight
show therapeutic value in alleviating airway disease by
affecting epithelial and eosinophil functions .
CLA and cancer
The interest on CLA mainly sprouted from the discovery
of the anticancer property of CLAs . Nevertheless,
only a few studies have examined the isomers-specific
effects of CLA in humans. In fact, no clinical studies
have been conducted to relate CLA consumption with
the incidence of cancer, but the data available in this
regard are only from epidemiological studies. Such data
can be viewed as a collection of statistical tools used to
elucidate the associations of CLA exposures to health
outcomes. Regarding clinical studies on cancer, many re-
searchers focused on human breast cancer; for instance,
in an elaborate follow-up study using Cox proportional
hazards models; Larsson et al.  showed that the
dietary intake of CLA had no evidence for a protective
role against breast cancer development in women.
Chajes et al.  conducted a case–control study
among 297 women treated for breast cancer or benign
breast disease at the University Hospital of Tours,
France, to evaluate the hypothesis that CLA protects
against breast cancer, and they could not show a link for
the negative association between adipose tissue CLA
(predominantly 9-CLA) and the risk of breast cancer.
High-fat dairy food and CLA intake were examined in
60,708 women of age 40 to 76 (Swedish mammography
cohort study) with 14.8 yr follow-up. It was found that
women who consumed four or more servings of high-fat
dairy foods per day (including whole milk, full-fat
cultured milk, cheese, cream, sour cream and butter)
showed half the risk of developing colorectal cancer,
compared to women who consumed less than one serv-
ing per day . Concerning CLA intake, they found it
was associated with an almost 30 percent reduction in
the risk of colorectal cancer . Similarly, the possible
role of CLA in preventing testicular cancer was depicted
by the decreased CLA content in mitochondrial fractions
of testicular cancer as against the normal testicular cells;
Table 4 Major clinical trials investigating the effect of CLA consumption on immune status; ↑- increased; ↓- decreased;
↔no change in
Subjects Dose Duration Observation Reference
17 women 3.9 g CLA (Tonalin)/d 93 d ↔immune status 
17 women 3.9 g CLA (Tonalin)/d 93 d ↑PBMC 
71 males 1.7 g 9 & 10-CLA (50:50), Clarinol™/d 12 wk ↑protective antibodies upon vaccination for hepatitis B 
49 healthy men 2.38 g/d 9-CLA or 8 wk ↓mitogen-induced T-lymphocyte activation 
2.52 g/d 10-CLA ↔circulatory cytokines
55 healthy volunteers 2 g 9 & 10-CLA (50:50)/d or 8 wk ↔markers of human immune function 
1.76 g 9 & 10-CLA (80:20)/d
28 men and women 3 g 9 & 10-CLA (50:50)/d 12 wk ↑levels of IgA, IgM and IL-10 
↓TNF-α, IL-1βand delayed type hypersensitivity response
28 mild asthmatic adults 4.5 g CLA/d 12 wk ↑airway hyperresponsiveness 
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 12 of 20
and that CLA incorporation into nuclei and cytosol was
significantly higher than its incorporation into plasma
membranes and mitochondria . Tumors in estrogen
receptor (ER)-negative epithelial cells in the breast are
common among premenopausal women . McCann
et al.  demonstrated that the protective effect of
9-CLA in women with its higher intake, i.e., the num-
ber of ER-negative cells to ER-positive was found de-
creased in such women. Another epidemiological study
(the Netherlands cohort) with 6.4 yr of follow-up eval-
uated the relation between intakes of CLA and other
FAs failed to confirm the anti-carcinogenic property of
CLA in humans with breast cancer incidence .
A few studies examined the relationship between
dietary or serum CLA in women and the risk of breast
cancer. Such studies found an inverse association be-
tween dietary and serum CLA and risk of breast cancer
in postmenopausal women . But in contrast, the
adipose tissue extracts from a population of French
patients with invasive breast carcinoma failed to reveal
any positive correlation between adipose tissue CLA
and the incidence of breast cancer . Since CLA
accumulates in body fat stores, the adipose tissue of
breast cancer obtained at the time of surgery could be
used as a qualitative biomarker for CLA intake.
Thus, the available human clinical studies could not
ascertain the anti-cancer property of CLA. A major
limitation in the epidemiological studies is the diffi-
culty in obtaining accurate estimates of dietary CLA
intake. Most of the studies were carried out in small
populations, where the diversity in food habits was
less. Moreover, no clinical studies evaluated the effects
of pure CLA preparations or individual isomers on the
incidences of cancer. It focuses that well-defined and
controlled studies are required to fully understand the
effects of CLA intake on the incidence of human
CLA and diabetes
The life style epidemics, diabetes and obesity are consid-
ered as the major causes of morbidity and mortality all
over the world; and that obesity and weight gain are as-
sociated with an increased risk of diabetes . The
hormone, insulin is responsible for regulating glucose
concentration in blood. Insulin resistance is a state in
which cells do not respond properly to insulin (even if it
is available in the blood), which leads to hyperinsuline-
mia (high blood insulin). Some animal studies demon-
strated that CLA supplementation enhances insulin
sensitivity; however, the mechanism underlying this ef-
fect is unclear [126,127].
Relatively few studies have examined the anti-diabetic
properties of CLA in humans. Supplementation of 3.0 g/d
of CLA (in 24 women) for 64 d showed no significant
changes in the levels of circulatory glucose or insulin
. CLA isomeric blends at the same dose also
showed no significant effects on plasma glucose or insu-
lin levels in healthy human subjects ; in such studies,
fasting blood glucose and/or insulin generally showed
little demonstrable effect. The gold-standard for quanti-
fying blood glucose is the ‘hyperinsulinemic-euglycemic
clamp test’, which measures the amount of glucose ne-
cessary to compensate for an increased insulin level
without causing hypoglycemia . However, another
study utilizing a euglycemic/hyperinsulinemic clamp in
abdominally obese male subjects indicated a decline in
insulin sensitivity after supplementation with both mixed
and purified 9- and 10-CLA isomers at a dose of 3.4 g/d
for 12 wk . The 10-CLA supplementation increases
oxidative stress and inflammatory biomarkers in obese
men . Oxidative stress seems closely related to
induced insulin resistance, which suggests a link be-
tween the FA-induced lipid peroxidation; these unfavor-
able effects of 10-CLA might be of clinical relevance
with regard to CVD . Recently, Shadman et al. 
showed that supplementation of CLA (50:50 isomer
blend of 9- and 10-CLA) with or without vitamin E for
8 wk showed a trend to increase in malondialdehyde
(a marker of lipid peroxidation) in overweight type 2
In non-diabetic abdominally obese men, 3.4 g/d 10-CLA
supplementation for 12 wk induced hyperproinsulinaemia
(plasma proinsulin, insulin, C-peptide and adiponectin
concentrations, including their associations with change in
insulin sensitivity assessed), which was related to impaired
insulin sensitivity, independently of changes in insulin
concentrations . These results are of clinical interest,
as hyperproinsulinaemia predicts diabetes and cardiovas-
cular diseases. The same investigators also showed that
the other active isomer of CLA (9-CLA) also increased in-
sulin resistance in abdominally obese individuals upon its
supplementation at a dose of 3 g/d for 3 month. .
But, the isomeric mixture of 9- and 10-CLA (3.4 g/d
for 6 month) showed no significant effect on glucose
metabolism or insulin levels in overweight or obese
individuals . All these studies failed to support the
anti-diabetic property of CLA in humans; however admin-
istration of 4 g/d of mixed CLA isomers improved insulin
sensitivity in young sedentary humans . Sixteen indi-
viduals (age, 21.5 ± 0.4 yr; body mass, 77.6 ± 3.4 kg) were
involved in this study. Ten subjects received 4 g/d of mixed
CLA isomers (35.5% 9-CLA; 36.8%10-CLA) for 8 wk,
whereas six subjects received placebo (safflower oil); but
small to offer a generalized effect .
Clinical studies regarding the anti-diabetic effects of
CLA are inconclusive. Rather, some of them speculated
the reduction in insulin sensitivity; which attract
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 13 of 20
immediate attention of the medical practitioners, because
the increased consumption of CLA through dietary
supplements might be ill-advised.
Adverse effects of CLA consumption
It seems that the use of weight-loss supplements con-
taining 9-CLA, 10-CLA or both as mixture is worrying,
because most of the clinical studies presented in the
previous sections provide mostly neutral or inconclusive
results with very few favorable impacts (Table 5). In
association with this, a few studies reported some
adverse effects such as oxidative stress, insulin resist-
ance, gastrointestinal irritation, etc., but no serious ad-
verse effects were reported at the time of intervention
except the relapse of asthma on consumption of 3.4 g/d
of CLA . Therefore, most of these side effects could
be categorized as ‘mild to moderate’.
Many studies showed increase in the plasma concentra-
tion of CLA, which was directly proportional to the
quantity of CLA consumed [23,133]. Therefore, the
immediate expected biological effect is oxidative stress.
Oxidative stress is the reflection of an imbalance
between the systemic manifestation of reactive oxygen
species and the ability of the body system to readily de-
toxify them or to repair the resulting damage imparted
to cell components like proteins, lipids and nucleic acids.
It is known that the free radicals such as reactive oxygen
formed by lipid peroxidation would ‘steal’electrons from
the lipids in cell membranes, resulting in cell damage
. Prolonged oxidative stress may lead to cancer and
heart diseases . Supplementation with 10-CLA dra-
matically increased the rates of oxidative stress, to the
levels considerably higher than that observed in heavy
smokers ; it also enhanced the release of inflamma-
tory biomarkers in obese men . Long-term CLA sup-
plementation studies lasting for one and two years have
found to be well tolerated, but there was an increase in
circulatory markers of inflammation such as CRP, TNFs,
and ILs [59,102]. Changes in these markers of inflamma-
tion and oxidative stress could be related to the increase
in insulin resistance associated with the risk of cardiovas-
cular disease [79,135]. Administration of CLA (4.2 g/d) for
three month significantly induced both non-enzymatic
and enzymatic lipid peroxidation, which was suggested to
cause cell damage .
Insulin resistance is a physiological disorder, under
which the cells fail to respond to the normal actions of
the hormone insulin, though it is sufficiently produced
by the body –this impairment leads to hyperglycemia
(i.e., type 2 diabetes). Decreased sensitivity or resistance
towards insulin upon consumption of CLA was ob-
served in some studies [60,65]. Riserus et al. 
showed with obese men that10-CLA might modulate
insulin resistance in humans, and that oxidative stress
is closely related to induced insulin resistance, as evidenced
from the increased levels of the marker, 8-iso-prostaglan-
in plasma. Furthermore, insulin resistance is closely
related to the impairment (decrease) of the expression of
glucose transporter-4 (GLUT4), a membrane transporter
of glucose. It was proven beyond doubt that 10-CLA
decreases the expression of GLUT4 , which shows
that indiscriminate use of 10-CLA to treat obesity would
lead to type 2 diabetes as the immediate side effect, this
would further damage blood vessels and thereby increased
risk of CVD . Moreover, unutilized insulin (due to
resistance) in plasma can contribute to increased appetite
(especially for carbohydrates and sugary foods), which
would add to the gravity of CVD.
Irritation of intestinal tract
A few studies showed mild irritations of intestinal tract
such as irritation , laxative effects and flatulence
, gas bloating , indigestion, diarrhea and nausea
[36,48] in subjects consumed CLA. Most of these effects
Table 5 Proposed beneficiary and detrimental effects of
CLA from clinical studies
Diseases Positive effects Negative effects
Obesity Reduced body fat mass Oxidative stress
Reduced body mass index Abdominal irritations
Reduced body fat
Reduced body fat regain
Increased lean body mass
Improved muscle mass
Improved blood lipid
Enhanced production of
circulatory markers of
Reduced total cholesterol
Enhanced the levels of
Elevated levels of
inflammatory markers in
Cancer Reduced the risks of
colorectal, testicular and
Increased oxidative stress
Diabetes Enhanced insulin
Dysregulation of blood
glucose and insulin
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 14 of 20
were considered as mild to moderate and were transient;
and one may assume that these effects may be due to
the capsule material or the oily nature of the substance
or initial adaptive problem with the lipid nutrient.
Milk fat depression
Consumption of commercial CLA reduced the fat content
in cows . Since milk is the only source of nutrients for
infants, decreased milk fat in lactating humans is another
concern regarding the CLA consumption. Masters et al.
 showed that CLA consumption significantly reduced
the milk fat without affecting the total milk output.
However, another two human studies found no changes
in milk fat or protein [141,142], but in these studies,
the intervention period was too short (about a wk) to
arrive at a conclusive result.
Cross-talk on CLA consumption
General view on CLAs is that the 10-CLA exerts specific
effects on adipocytes and liver, whereas both the 9- and
10-CLAs appear to be active in inhibiting carcinogenesis
. It is likely that the inconsistent and often contra-
dictory results on the effectiveness of CLA consumption
in human health could be the outcome of a number of
factors, including differences in subject groups, age,
quantity and duration of CLA intake, composition of
CLA mixture, purity of CLA, acceptance of the CLA by
the body, food intake, gender and racial differences, gen-
etic polymorphism and also the executed measurements
(parameters studied) for assessing the effect. Moreover,
crucial factors that impact research outcomes include
the nature of control supplement (placebo), and study
design (cross-over vs. non cross-over designs), because,
the efficacy of CLA supplements remains inconsistent in
cross-over and non cross-over randomized clinical stud-
Plasma content of CLA
Determination of a normal CLA content in the blood
plasma could help in estimating if a person consumes
satisfactory amounts of CLA with the diet, and thus
takes advantage of its potential beneficial effects on
health. The only CLA isomer that appears in higher
percentage than the detection limit (0.03% of total FAs)
is 9-CLA . They arrived at this conclusion based on
the data obtained from 3 groups of individuals (n = 12
for each group), i.e., who not consumed dairy products,
individuals consumed normal amounts of dairy products
(about 50 g/d cheese) and individuals consumed 1.4 g
CLA/d as supplement (both 9- and 10-CLAs in equal
proportion). The duration of this study was 6 months,
and in the last group who consumed CLA supplement,
the average CLA content in plasma was 0.2% of the total
FAs with no untoward side effects. The blood samples
were collected for analysis in the morning (in the fasted
state) after a 12 h restriction for meal and drinks. Thus,
individuals who have 9-CLA levels in their blood
plasma within the range up to 0.09% of total FAs could
serve as ideal participants in future CLA supplementa-
CLA supplements vs. placebo
In most of the clinical studies, vegetable oils such as
sunflower oil, olive oil, safflower oil and soybean oil
(Table 2) have been used as placebo in the form of cap-
sules or pills [49,66,90]. In fact, the proportion of MUFA
and PUFA, especially LA present in these placebo oils
(for instance, the predominantly used sunflower and
olive oils) are not properly addressed by the researchers.
According to WHO (Codex International food stan-
dards), sunflower oil, soybean oil, olive oil and safflower
oil contained significant levels of MUFA and PUFA,
which include OA, LA and ALA (Table 6) [144,145]. It
was thought that VA is the only precursor of CLA in
humans. However, non-ruminal bacteria inhabiting human
gastro-intestinal (GI) tract like Lactobacillus acidophilus
and L. casei isolated from intestine , Bifidobacterium
bifidum and B. breve isolated from the fecal matter of
neonates , and Ruberia spp. isolated from intestine
 could efficiently produce 9-CLA from LA, probably
though the mediation of VA (in tissues) or 10-hydroxy
octadeceinoic acid (18:1) , as occurring in ruminal
biohydorgenation. In addition to 9-CLA, Lactobacillus spp.
also synthesizes 10-CLA and trans-9, trans-11-CLA .
From this, it is evident that a portion of LA in placebo oil
would be biohydogenated by the bacteria residing in GI
tract (as in rumen) into CLA through the mediation of VA.
Irrespective of this fact, most of the clinical studies use the
aforesaid vegetable oils as placebo, neglecting their effects
on human health; especially their supposed supplementary
and complementary effects. The dietary intake of the pre-
cursor VA was found to have some major effects on heart
health, blood lipid profile and immunity, and also protect-
ive against fatal ischemic heart disease [149-151]. This
would lead to the misinterpretation of the results, i.e.,
false-positive results. Therefore, during clinical studies, the
Table 6 Various vegetable oils used as placebo in clinical
studies with their polyunsaturated fatty acids (PUFA)
content including LA
Vegetable oil Total PUFA* LA*
Sunflower oil 66 66
Flax seed oil 66 13
Safflower oil 42 41
Olive oil 10 9
Soybean oil 58 51
*Units: grams fatty acids per 100 grams oil.
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 15 of 20
composition of FA in placebo and its effects on human
health need to be addressed with due respect, and inde-
pendently for getting reliable results.
Selection of the subjects
The clinical studies with CLA lack a common protocol
for selecting the subjects. Description of the subjects
including gender and age, medical treatments given
prior to intervention are the critical factors to be con-
sidered while selecting the subjects. Medical history of
the subjects should also be recorded before concluding
the safety and efficacy concerns of CLA consumption.
In most of the studies, the subjects selected were cate-
gorized and designated as normal, healthy obese, with
metabolic syndrome, with insulin resistance, etc.This
arbitrary classification for the convenience of the inves-
tigator poses a question i.e., which is the most suitable
model to study the health effects of CLA? Is it with the
designation normal, obese, immune-compromised sub-
jects with metabolic syndrome or with other diseases?
Another factor to be considered in clinical studies is the
continental, racial and gender differences among the
subjects; for instance, literature shows that most of
the clinical studies on CLA were performed in North
America and Europe. The reproducibility of such results
in racially and continently separated populations all over
the world, especially in Asia, Africa and South America is
another point of concern, which has to be verified before
accepting the nutritional status of CLA in modulating
Dosage and duration
Other factors of concern are the composition, dosage
and duration of CLA consumption. If not otherwise
stated, composition and purity of CLA are meant for 9-
and 10 CLAs. Generally, human studies use a CLA mix-
ture (about 40:40) of 9- and 10-CLAs; and proportion of
CLA isomers depends mainly on the nature of substrate,
mode of synthesis (production), physico-chemical
parameters involved in synthesis, and purification strat-
egies adopted [152,153]. Even if stated as purified, the
purity would be about 80%; and the remaining 20%
would be represented by other CLA isomers and un-
modified FAs. These ‘impurities’would also contribute
to the inconclusive results, as different isomers proved
to have different physiological actions in human body.
Most of clinical studies evaluated the effects of CLA
consumption for a short period, usually of 4–12 wk. But
Gaullier et al. [66,79] conducted comparatively long-
term study spanning for 1–2 yr. Generally, in these stud-
ies, CLA (isomer or mixture) dosages varied from 0.7 g/d
to 6.8 g/d per human and administered mostly in the form
of TAGs or free FAs. The dosage of CLA administration
in humans is also very low, compared to animal studies
(in terms of body weight); thus the results in pre-
clinical animal studies (high dose) may not be compar-
able with the real clinical studies. Therefore, CLA dose
(intake) may be considered based on energy percentage.
Two people with the same body weight may have a very
different body composition (e.g., women vs. men; body
builder vs. obese person), which in turn impacts the
Another crucial question is the retention of the so-
called good effects for a long time; of course, one might
expect that CLA should be consumed as if drugs are
taken for chronic diseases. Unlike in mechanistic in vitro
studies, the criss-crossed signaling pathway through
which CLA induce its effects has to be elucidated clearly
in clinical studies. Moreover, the biological effects of
individual CLA isomers, mainly 9- and 10-CLA, their
synergistic interactions and even the possible opposition
between the isomers have to be unveiled.
CLA and adjuncts
Effect of CLA consumption along with various adjuncts
is another area of clinical research that has to be studied
evidently. Some studies showed the positive health bene-
fits of CLA are related to heart health and body fat
reduction on consumption along with calcium, VA, whey
proteins and oryzanol [49,68,103]. CLA (6 g/d) supple-
mentation along with creatine and whey protein resulted
in enhanced strength improvements and LBM with
high-volume strength training in well-trained young
adults . CLA consumption along with other PUFA
was found to have protective effect against renal carcin-
oma . Therefore, an effective combination of CLA
along with other supplements or with ω-3 FAs has to be
addressed to reveal the possible real effects of CLA
consumption on human health.
As far as the voluminous literature on CLA is concerned,
only a few studies to date examined the effects of CLA
in humans in vivo.However,resultsofthesestudiesdo
not reflect the dramatic and consistent data demon-
strated in animal studies. Thus, these disappointing re-
sults in humans demand more precise experimentations
with humans. The interest in CLA research still persists,
and hence, many questions related to the safety and
efficacy on the consumption CLA have to be answered
scientifically. Hence, it is imperative to critically evaluate
and consolidate prominent findings on human con-
sumption of CLA, i.e., the principal actions of this minor
lipid nutrient exerting on human system so that future
investigations would focus on specific CLA isomers and
the most reasonable mechanism of action due to them.
One of the major limitations in human studies is that
most of the studies depend only on the blood cells or
Benjamin et al. Nutrition & Metabolism 2015, 12:4 Page 16 of 20
plasma, and fat deposition. Thus, majority of the clinical
studies failed to provide conclusive evidences for the
effectiveness of CLA on human health, except for anti-
obesitic properties which offered a little hope to prevent
body weight regain though fat deposition, nevertheless
increased oxidative stress and insulin resistance due to
such over-consumption of CLA poses contradictory
concerns. Moreover, age, gender, genetic polymorphism
and immune status of the subject, role of other nutrients
present in the diet, and extend of absorption of individ-
ual isomers to different tissues have to be well addressed
during the intervention period –so as to evaluate the
safety and efficacy of CLA consumption on human
health. As far as human consumption of CLA is con-
cerned, a definite conclusion for safety and efficacy has
not been reached yet. At this context, we strongly rec-
ommend the need for more precise and well-designed
long-term intervention studies with controlled food
intake and activity level to assess the effectiveness of
CLA on human health. Moreover, such studies need to
be duplicated in other laboratories giving emphasis to
men and women, age group, ethnic background, food
style, continental and even national uniqueness, cul-
tural and geographic barriers, etc. without comparing
data from animal studies –i.e., a real double-blind
clinical study. In toto, clinical evidences indicate a possible
link of supplemental CLA per se toward negative or incon-
clusive outcomes; thus, inclusion of CLA in the Codex
Alimentarius (Book of Food) –which describes internation-
ally recognized standards of food –may be considered.
The authors declare that there exists no conflict of interest.
SB designed and wrote the manuscript, and contributed substantially to
discussion, PP and SS collected literature and structured the reference, FS
and AGW edited it with interpretation. All authors read and approved the
The authors gratefully acknowledge the Department of Biotechnology (DBT),
Ministry of Science and Technology, Government of India, for a research
grant (No. BT/PR 12714/FNS/20/411/2009).
Biotechnology Division, Department of Botany, Enzyme Technology
Laboratory, University of Calicut, Kerala 673 635, India.
School of Animal and
Comparative Biomedical Sciences, University of Arizona, Tucson, AZ 85721,
Department of Molecular Biosciences, Heinrichstrasse 31, University of
Graz, 8010 Graz, Austria.
Received: 20 November 2014 Accepted: 21 January 2015
Published: 3 February 2015
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Cite this article as: Benjamin et al.:Pros and cons of CLA consumption:
an insight from clinical evidences. Nutrition & Metabolism 2015 12:4.
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