The Role of Whole Grains in Body Weight Regulation

Abstract

Whole grain (WG)-rich diets are purported to have a variety of health benefits, including a favorable role in body weight regulation. Current dietary recommendations advocate substituting WG for refined grains (RG), because many of the beneficial bioactive components intrinsic to WG are lost during the refining process. Epidemiological studies consistently demonstrate that higher intakes of WG, but not RG, are associated with lower BMI and/or reduced risk of obesity. However, recent clinical trials have failed to support a role for WG in promoting weight loss or maintenance. Though the biochemical and structural characteristics of WG have been shown to modulate appetite, nutrient availability, and energy utilization, the capacity of WG foods to elicit these effects varies with the type and amount of grain consumed as well as the nature of its consumption. As such, WG foods differentially affect physiologic factors influencing body weight with the common practice of processing and reconstituting WG ingredients during food production likely mitigating the capacity for WG to benefit body weight regulation.

Full text

REVIEW
The Role of Whole Grains in
Body Weight Regulation
1,2
J. Philip Karl* and Edward Saltzman
Jean May er USDA Human Nutrition Research Center on Aging at Tufts University, Ene rgy Metabolism Laboratory, Boston, MA
ABSTRACT
Whole grain (WG)-rich diets are purported to have a variety of health benefits, including a favorable role in body weight regulation. Current
dietary recommendations advocate substituting WG for refined grains (RG), because many of the beneficial bioactive components intrinsic to
WG are lost during the refining process. Epidemiological studies consistently demonstrate that higher intakes of WG, but not RG, are associated
with lower BMI and/or reduced risk of obesity. However, recent clinical trials have failed to support a role for WG in promoting weight loss or
maintenance. Though the biochemical and structural characteristics of WG have been shown to modulate appetite, nutrient availability, and
energy utilization, the capacity of WG foods to elicit these effects varies with the type and amount of grain consumed as well as the nature of its
consumption. As such, WG foods differentially affect physiologic factors influencing body weight with the common practice of processing and
reconstituting WG ingredients during food production likely mitigating the capacity for WG to benefit body weight regulation. Adv. Nutr. 3: 697–
707, 2012.
Introduction
Obesit y is a predominant public health concern (1,2) with
related health care costs now estimated at more than $190
billion annually in the United States (3). Diet composition
is among many lifestyle factors contributing to the develop-
ment of obesity and associated chronic diseases, including
cardiovascular disease, cancer, and ty pe 2 diabetes. As
such, identifying dietary patterns, or individual foods and
nutrients that should be increased or decreased in the diet
to prevent and treat obesity, is an important public health
strategy. Consuming w hole grains (WG)
3
is postulated to
decrease chronic disease risk, in part due to beneficial effects
on body weight regulation (4–6).
The American Associa tion of Cereal Chemists (AACC)
defines a WG food ingredient as consisting of the “intact,
ground, cracked or flaked caryopsis (kernel) whose principal
components—the starchy endosperm, germ, and bran—are
present in the same relative proportions as they exist in
the intact caryopsis” (7). Slig ht derivations o f this
definition have been proposed and a recognized need
for an internationally accepted definition exists (8). Caryop-
ses are the edible seeds of cereal crops that consist of 3 pri-
mar y anatomical components: endosperm, germ, and bran.
The endosperm comprises the largest portion of the grain,
consisting primarily of storage proteins and starches needed
to feed the germ during maturation (6). The germ contains
the embryo and the bran acts as a multi-layered protective
barrier for the grain (9). Though f iber, vitamins, minerals,
and other phytochemicals are present throughout the grain,
the bran and germ are more concentrated sources than
endosperm (9).
The most commonly consumed WG worldwide are
wheat, brown and long-grain rice, maize, oats, barley, rye,
millet, sorghum, and triticale (9). Currently, these grains
are more commonly processed (e.g., milled, cracked, rolled,
ground, crushed, and flaked) and reconstituted before con-
sumption than consumed in their intact form (10). Refined
cereal grains (RG) are not fully reconstituted after process-
ing and consist primarily of endosperm retained after the
separation and removal of the bran and germ. Although
the refining process improves the texture and stability of
grain-based food products, much of the nutritive value of
the WG is lost. Importantly, the AACC definition of a WG
ingredient requires that the endosperm, bran, and germ iso-
lated from the WG during processing be reconstituted in the
1
Supported by the USDA, Agricultural Research Service under Cooperative Agreement no.
58-1950-7-707. Any opinions, findings, conclusions, or recommendations expressed in this
publication are those of the authors and do not necessarily reflect the view of the USDA.
2
Author disclosures: J. P. Karl and E. Saltzman, no conflicts of interest.
* To whom correspondence should be addressed. E-mail: james.karl@tufts.edu.
3
Abbreviations used: AACC, American Association of Cereal Chemists; GLP-1, glucagon-like
peptide-1; RG, refined grain; WC, waist circumference; WG, whole grain.
ã2012 American Society for Nutrition. Adv. Nutr. 3: 697–707, 2012; doi:10.3945/an.112.002782.
697
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same relative proportions as exists in the intact native grain.
Thus, WG are generally richer in dietary fiber, vitamins,
minerals, phytoestrogens, phenolic compounds, and phytic
acid relative to their refined counterparts (11). The AACC
definition, however, does not stipulate that the WG structure
remain intact, nor does it limit the degree of processing or
minimum particle size of the WG.
The superior nutritive value of WG relative to RG and as-
sociations of increased WG intake with reduced cardiovas-
cular disease (12,13), cancer (14,15), and type 2 diabetes
risk (16) underpin dietary recommendations worldwide en-
couraging WG consumption. The 2010 Dietary Guidelines
for Americans (17) recommend substituting WG for RG
to consume at least 1.5 ounce-equivalen ts WG $ 1000
kcal
21
$ d
21
($24 g WG $ 4185 kJ
21
$d
21
). Similarly, a
number of European countries (18–23), Australia (24),
and Canada (25) have recommendations encouraging intake
of WG foods. Concomitant to these recommendations, the
number of WG-containing food products introduced into
the U.S. marketplace, e.g., has considerably increased over
the past decade (26). However, WG consumption remains
well below recommended levels for many (27), especially
in the United States (28,2 9). In this review, we examine
the evidence for a role of WG in body weight regulation.
WG and body weight regulation:
observational studies
Jonnalagadda et al. (4) recently described issues associated
with estimating and comparing WG intakes in observational
studies. Briefly, definitions of WG foods have varied across
studies, with the most commonly used definitions being der-
ivations of that proposed by Jacobs et al. (30). This defini-
tion includes food items such as dark bread, brown rice,
oatmeal, popcorn, and breakfast cereals containing $25%
WG or bran by weight. Added germ and bran are frequently
included in the definition as well. This definition contrasts
with the less commonly employed U.S. FDA definition
used for health claims, which requires a WG food be
$51% WG by weight and contain germ, bran, and endo-
sperm in the same relative proportions as are found in the
intact grain. The amount of WG in a serving of a WG
food therefore varies based on the definition used in addi-
tion to the relative proportion of WG to RG in the food.
The fact that the exact relative proportion of WG and RG
grain ingredients is proprietary information and often un-
known further complicates attempts to accurately quantify
WG intake (4). Thus, WG intake may be under- or overesti-
mated, resulting in misclassification and bias.
Despite differences in the definitions and methods used
to estimate WG intake, the bulk of the epidemiological evi-
dence suggests that WG have a beneficial role in bod y weight
regulation. A 2008 meta-analysis of cross-sectional studies
examining associations between WG intake and BMI sum-
marized 15 studies including 119,829 primarily European
and American adults (31). Harland et al. (31) documented
a 0.6-kg/m
2
lower BMI in in dividuals categorized as report-
ing the highest (w3 servings/d) compared with the lowest
(<0.5 servings/d) WG intakes. The inverse association be-
tween WG intake and BMI was consistent, documented in
12 of the 15 studies reviewed (30,32–45), and is supported
by recent reports (16,29,46–50). For example, using the Bal-
timore Longitudinal Study of Aging cohort Newby et al. (49)
documented a 0.7-kg/m
2
lower BMI in individuals in the
highest (2.8 servings/d) relative to lowest (<1 serving/d)
quintile of WG intake. Likewise, McKeown et al. (47) re-
ported a 1.1-kg/m
2
lower BMI in individuals in the highest
(2.9 servings/d) relative to lowest (0.1 servings/d) quintile of
WG intake within a subset of the Framingham Heart Off-
spring and Third Generation cohorts. Nearly identical re-
sults were documented in a cohort of older adults with a
1.0-kg/m
2
lower BMI being observed in individuals in the
highest (2.9 servings/d) relative to lowest (0.2 servings /d)
quartile of WG intake (48). Of note, the inverse associations
between W G intake and BMI observed in these cross-sectional
studies do not appear to simply reflect a beneficial effect
of total grain intake on body weig ht regulation, because
positive (33,35,47) or null (30,36–38,48,49) associations be-
tween RG intake and BMI were reported.
Prospective cohort studies support a role for WG in at-
tenuating weight gain (32,34,35). Liu et al. (35) used the
Nurses Health Study cohort to examine relationships be-
tween changes in WG and RG intakes, weig ht cha nge, and
obesity risk. Following 12 y of follow-up, women in the
quintile representing the largest increase in WG intake (0.9
servings $ 4185 kJ
21
$ d
21
) compared with those in the
quintile representing the largest decrease in WG intake
(20.6 servings $ 4185 kJ
21
$ d
21
) experienced a 0.4-kg
lower weight gain (4.1 vs. 4.5 kg) and 19% lower odds of de-
veloping obes ity (35). In contrast, women with the largest
increase in RG intake (0.9 servings $ 4185 kJ
21
$ d
21
) expe-
rienced a 0.4-kg greater increase in body weight (4.7 vs. 4.3
kg) and 18% higher odds of developing obesity compared
with those with the largest reduction in RG intake (20.9
servings $ 4185 kJ
21
$ d
21
) (35). Similarly, Koh-Banarjee
et al. (34) used the Health Professionals Follow-up Study co-
hort to examine relationships between WG and RG intakes
and weight gain by categorizing men into quintiles of change
in WG or RG intake. After 8 y of follow-up, a 0.5-kg lower
weight gain (0.7 vs. 1.2 kg) was observed in men reporting
the greatest increase in WG intake (1.5 servings/d) com-
pared with those with the largest decreases in WG intake
(20.9 serving/d) (34). No benefit of RG intake on prospec-
tive weight change was documented (34). Finally, in the
Physicians’ Health Study, consumption of $1 serving/d
compared with <1 serving/wk of WG breakfast cereals was
associated with a 0.4-kg lower weight gain over 13 y (1.9
vs. 2.3 kg) in age-adjusted models (32). In this study, a
0.4-kg lower weight gain (1.8 vs. 2.3 kg) was also observed
in men consuming $1 serving/d of RG breakfast cereals
compared with those who reported consuming these cereals
<1/wk (32).
In addition to beneficial effects on body weight, WG con-
sumption may be associated with reduced abdominal adi-
posity. In a subset of studies included in the meta-analysis
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of Harland et al. (31), the hig hest WG consumers were
determined to have a 2.7-cm lower waist circumference
(WC) and 0.02 lower waist:hip ratio compared with the low-
est WG consumers. McKeown et al. (47,48) recently ad-
vanced these findings by reporting associations between
WG intake and direct measures of adiposity obtained by
DXA and computed tomography. Older adults in the highest
(2.9 servings/d) relative to lowest (0.2 servings/d) quartile of
WG intake had 2.4% lower total body fat and 3.6% lower
trunk fat (48). Within a subset of the Framingham Offspring
cohort, visceral adipose tissue volume as measured by com-
puted tomography was 10% lower in individuals in the
highest (2.9 servings/d) compared with lowest (0.1 serv-
ings/d) quintiles of WG intake (47). Interestingly, no asso-
ciation between WG intake and visceral adiposity wa s
obser ved in individuals consuming $4 daily servings of
RG, suggesting that high RG intake may counter any benefits
of WG on body weight regulation (47). Few prospective co-
hort studies have directly examined associations between
WG intake and central adiposity. In 2 separate studies, Halk-
jaer et al. (51,52) repor ted no association between intake of
WG food products (51) or breads (52) and prospective WC
change in Danish adults, but they noted a positive associa-
tion between intake of RG food products or breads and
WC change in women.
In summary, observational studies strongly suggest that
consuming w3 servings/d WG is associated with lower
BMI and central adiposity relative to low or no WG con-
sumption, and higher WG intakes may attenuate weight
gain. These relationships appear to be specific to WG, be-
cause intakes of RG have not generally been associated
with lower BMI or adiposit y in these studies. Of note, the
meta-analysis of Harland et al. (31) demonstrated that
higher WG intakes were associated with both increased en-
ergy and dietary fiber intakes and lower smoking prevalence.
The greater energy intakes observed in high WG consumers
is likely associated with the trend for higher prevalence of
exercise also reported in these individuals (31). Further,
Harland et al. (31) noted trends for higher micronutrient in-
takes and supplement use and lower saturated fat intakes in
the hig hest compared with lowest WG consumers. Recent
reports are generally consistent with these findings, demon-
strating that high WG consumers tend to be more physically
active, smoke less, and consume more fruit, vegetables, and
dietary fiber than low WG consumers (46,47,49,53). Fur-
ther, cluster and factor analyses frequently link WG con-
sumption with dietary patterns that include high fruit and
vegetable intake and that are associated with other desirable
health be haviors (54,5 5). Although statistical adjustment for
these lifestyle factors is common, the potential for residual
confounding remains; therefore, associations between high
WG intake and lower BMI may be mediated by healthier
lifestyles of WG consumer s.
WG and body weight regulation: clinical trials
A number of recent randomi zed controlled trials investigat-
ing the effects of consuming WG relative to RG on
biomarkers of health status have reported changes in appe-
tite, energy intake, body weight, or body composition as pri-
mar y or secondary outcomes following incorporatio n of
WG into diets consumed ad libitum or into energy-re-
stricted diets fo r weight loss. Ad libitum intake studies com-
monly incorporated $48 g WG/d (3 ounce-equivalents/d)
or a similar amount of RG derived from a variety of sources
(56–60) or single food items such as WG breakfast cereal
(61–63) or bread (64) into participant diets over 2–52 wk.
Only 2 of these studies reported perceived appetite or ad li-
bitum energy intake as primary outcomes (Table 1) (63,64).
Isaksson et al. (63) documented postprandial reductions in
perceived appetite following consumption of a WG rye
breakfast porridge providing 55 g WG/d compared with re-
fined wheat bread. This effect persisted after daily consump-
tion of the porridge for 3 wk but did not translate into
reduced energy intake or weight loss (63). In a pilot study
of 14 participants, Bodinham et al. (64) obser ved no differ-
ential effects on perceived appetite or ad libitum energy in-
take over 3 wk in which WG wheat rolls providing 48 g/d
WG or RG rolls were added to habitual diets. Similarly, no
evidence for a spontaneous reduction in energy intake or
body weight in response to WG interventions was observed
in any study in which these measures were reported as sec-
ondary outcomes (56–61,65 ,66). However, all but 3 of these
studies (61,65,66) appear to hav e encouraged body weight
maintenance o r a d h e re n ce to h a b i t u a l d i e ts (e xce p t i n g
the inter vention component) throughout the inter ven-
tion (Table 1). Therefore, the reported results cannot re-
liably assess whether increased WG consumption should be
expected to promote a spontaneous reducti on in energy intake
and weight loss. Two of the trials in which no recommenda-
tions regarding energy intake were given were #3wkduration
and therefore too short to infer effects on body weight out-
comes (61,66). Ross et al. (66) did, however, report energy
intake, documenting no significant effect on energy intake
while substituting 150 g/d WG for RG into a diet in which
all foods were provided over 2 wk. In another study, Brown-
lee et al. (65) attempted to investigate the effects of substitut-
ing 60 g/d or 120 g/d of WG from a variety of sources for RG
on cardiovascular risk factors to include body weight and
adiposity. The authors noted that volunteers more often ap-
peared to add rather than substitute WG into their habitual
diets, with energy intake increasing concomitant with WG
intake. No significant changes in body weight or body fat
were documented in any group (65).
Recent studies have also examined whether incorporating
WG into energy-restricted diets enhances weight loss be-
yond energy restriction alone (67– 71). Taken together, these
studies provide no evidence that a hypoenergetic diet that
includes w3–7 daily servings of WG (48–112 g/d WG) pro-
motes greater weight loss than a low-WG hypoenergetic diet
(Table 1). However, in 2 of 3 studies in which body fatness
was directly measured using DXA (67,68) or hydrostatic
weighing (71), hypoenergetic WG-containing diets were as-
sociated with a greater reduction in body fat, primarily from
the abdominal region, relative to hypoenergetic low-WG
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Table 1. Clinical studies of WG intake and energy regulation
1
Reference Design and population Intervention Fiber intake, g/d Δ Weight, kg
Δ Body
composition
Appetite/
(Δ) energy
intake, kJ/d
Energy intake ad libitum
Bodinham
et al. (64) 3 wk CO; n = 14 WG: 48 g WG/d, milled wheat bread WG: 30* Ø
BF: Ø Ø /
Nrmwt M/F, 26 y C: RG bread C: 26
WC: Ø WG: 0
C: +368
Brownlee et al. (65) 16 wk RCT; n = 266 WG1: 60 g WG/d for 8 wk WG1: +11*
WG2: +5*
WG1: +0.2 BF: Not measured/
Ovwt and Ob M/F, 18–65 y + 120 g WG/d for 8 wk
2
WG2: +0.7 WG1: +0.3% WG1: +585*
WG2: 60 g WG/d for 16 wk
2
C: 0 WG2: +1.0% WG2: +389
C: no intervention C: +0.3% C: 2677
WC: Ø
Carvalho-Wells et al. (61) 3 wk CO; n = 32 WG: 48 g WG/d, maize semolina
breakfast cereal
WG: 14*
,4
C: 3
Ø WC: Ø Not measured/
Not measuredNrmwt, Ovwt, and Ob
C: RG breakfast cerealM/F, 20–51 y
Isaksson et al. (63) 3 wk CO; n = 24 WG: 55 g WG/d, cut and rolled rye porridge WG: 21 Ø Not measured YH, YDTE,
Nrmwt M/F, 18 –60 y C: RG wheat bread C: 15 [S after WG
breakfast; Ø
after lunch
3
/
WG: 24
C: 2184
Ross et al. (66) 2 wk CO; n = 17 WG: 150 g WG/d
2
WG: 32* WG: 20.5 Not measured Not measured/
Nrmwt M/F, 20 –50 y C: RG foods C: 19 C: 0 WG: 8414
C: 8628
Energy intake restricted
Katcher et al. (67) 12 wk RCT; n = 47 WG: 4– 7 oz-eq WG foods/d
2
WG: 21* WG: 23.7 BF: Not measured/
Ob M/F with MetS, 20–65 y C: Avoid WG foods C: 15 C: 25.3 WG: 21.2% WG: 21488
All: 2090 kJ/d (500 kcal/d) energy
deficit + DG 2005 + activity
C: 21.0% C: 22884
Abdominal BF:
WG: 22.2%*
C: 20.9%
WC:
WG: 22.5 cm
C: 24.7 cm
Kristensen et al. (68) 12 wk RCT; n = 72 WG: 105 g WG/d
2
WG: 11*
,4
WG: 23.6 Total FM: Not measured/
Ovwt and Ob postmenopausal
F, 45–70 y
C: RG foods C: 4 C: 22.7 WG: 23.0%*
,5
WG: 6329
All: 1254 kJ/d (300 kcal/d) energy deficit C: 22.1% C: 6061
Central FM:
WG: 23.8%**
C: 22.7%
WC:
WG: 24.1 cm
C: 24.1 cm
(Continued)
700 Karl and Saltzman
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diets despite conferring no additional weight loss benefit
(67,68). Katcher et al. (67) randomly assigned 50 obese
adults with metabolic syndrome to receive dietary advice to ob-
tain all recommended daily grain servings from either W G or
non-WG sources while reducing energy intake 2090 kJ/d (500
kcal/d) from weight maintenance requirements. The group re-
ceiving the advic e to increase WG intake averaged 5 servings/d
WG compared with <0.25 servings/d in controls. Although re-
ductions in total body fat, abdominal fat, and W C were ob-
served in both gr oups after 12 wk, the abdominal fat loss
was 1.3% greater in the WG group (67). In another trial,
Kristensen et al. (68) instructed overweight and obese post-
menopausal women to reduce energy intake by $1255 kJ/d
(300 kcal/d) from weight mainten ance requirements and to
consume w1985 kJ/d (475 kcal/d) of either WG or RG foods
that were provided to participants. Total body and centra l fat
mass and WC were reduc ed in both groups; however, decreases
in total fat mass were significantly greater and decreases in cen-
tral fat mass tended to be gr eater in women consuming the
WG-rich diet (68). In contrast, substituting 84 g/d oats for
RG wheat in a h ypoenergetic diet did not differentially affect
body fat loss over 6 wk in healthy adults (71). The inclusion
of normal-weight p articipants, a g reater energy deficit
of 4185 kJ/d (1000 kcal/d), type of grain used, or shorter du-
ration of this study may account for the discrepant results.
In summary, recent clinical trials provide little evidence for
an effect, beneficial or detrimental, of WG intake on body
weight regulation. When W G are added to or substituted for
RG in ad libitum diets, no spontaneous reduction in energy in-
take or body weight is observed. Likewise, on the background
of a h ypoenergetic diet for weight loss, substitu ting WG for RG
does not appear to promote body weight loss beyond energy
restriction alone. In 2 separate studies, W G-rich hypoenergetic
diets appear ed to be associated with gre ater losses of body fat
or central body fat. Howev er, the observed effect sizes were
small, within the measurement error of the methods used to
assess fat mass (72–74), and not always consistent with
changesinWC(67,68).Thesefindingsshouldthereforebe
consider ed preliminary but warrant further inv estigation.
Factors mediating physiological effects of WG
relevant to the regulation of body weight and
composition
The structural and physicochemical properties of WG are
diverse and made more so when incorporated into foods.
This diversity is the result of the varied chemical composi-
tions and quantities of indigestible carbohydrates within dif-
ferent WG, the deg ree to which grains are processed prior
to consumption, the methods by which WG foods are pre-
pared, and interactions with the food matrix (75). Together,
these factors mediate the physiological effects of WG and
may in turn differentially influence short-term appetite
regulation and nutrient utilization, thereby altering WG-
mediated effects on the regulation of body weight and com-
position (Fig. 1).
Table 1. (Continued )
Reference Design and population Intervention Fiber intake, g/d Δ Weight, kg
Δ Body
composition
Appetite/
(Δ) energy
intake, kJ/d
Maki et al. (69) 12 wk RCT; n =
144 WG: 3 c/d oat cereal WG: 22* WG: 22.2 WC: Not measured/
Ovwt and Ob M/F, 20–65 y C: Low-fiber foods C: 13 C: 21.7 WG: 23.3 cm* WG: 21714
All: 2090 kJ/d (500 kcal/d) energy deficit C: 21.9 cm C: 21714
Melanson et al. (70) 24 wk RCT; n = 92 WG: 40 –80 g/d WG breakfast cereal WG: 23* WG: 24.7 Not measured Not measured/
Ovwt and Ob M/F, 18–70 y C: Avoid breakfast cereals C: 17 C: 25.0 WG: 22901
All: 2090 kJ/d (500 kcal/d) energy deficit
from diet + exercise
C: 21793
Saltzman et al. (71) 6 wk RCT; n = 41 WG: 45 g $ 4185 kJ
-1
$ d
21
rolled oats WG: 17* WG: 24.4 FM: Trend for YH
Nrmwt, Ovwt, and Ob C: 45 g $ 4185 kJ
-1
$ d
21
RG wheat C: 13 C: 24.3 WG: 22.6 kg on WG diet**/
M/F, 18–78 y All: 4185 kJ/d (1000 kcal/d) energy deficit C: 23.0 kg WG: 23678
C: 23816
1
**P # 0.1, *P , 0.05 compared with control. Ø, no difference (actual values not reported); BF, body fat percentage; C, control; CO, crossover; DG, Dietary Guidelines for Americans; DTE, desire to eat; F, female; FM, fat mass; H, hunger; M, male;
MetS, metabolic syndrome; Nrmwt, normal-weight (18.5 kg/m
2
# BMI ,25 kg/m
2
); Ovwt, overweight (25 kg/m
2
# BMI ,30 kg/m
2
); Ob, obese (BMI $30 kg/m
2
); oz-eq, ounce-equivalents; RG, refined grain; s/d, servings per day; S, satiety; WC,
waist circumference; WG, whole grain; wt, body weight.
2
Various sources of WG consumed.
3
Measured over 12 h following WG or RG test meal.
4
From intervention foods.
5
Percent change.
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Chewing
The fiber content, particle size, and structural integrity of
WG alter the amount of chewing required for ingestion of
WG foods (76,77). Increased chewing may promote satia-
tion by enhancing gastric distention (78), augmenting gut
hormone responses (79,80), prolonging orosensory stimula-
tion (81), or slowing eating rate (82,83).
Energy density and availability
WG foods generally have a lo wer energy density, define d as di-
gestible energy per unit weight, than comparable RG foods.
This effect derives from the low digestible energy per unit
mass (84) and water-holding capacities of dietary fibers intrin-
sic to many WG. Short-term studies have demonstrated that
humans have a tendency to eat a consistent weight of food ir-
respectiv e of energy conte nt, indicating that from meal to
meal, appetite is influenced more by the mass of food than
the amount of energy consumed (85–87). Consequently , de-
creasing dietary energy density results in a reduction in energy
intake without a concomitant increase in hunger (88,89).
The magnitude of the reduction in energy density that
can be achieved using WG, however, varies with the amount
and type of fiber present in the grain. WG contain both non-
fermentable fibers, which provide no digestible energy, and
fibers that are fermented to varying degrees by colonic mi-
crobes salvaging otherwise unavailable energy in the form
of SCFA. Further, viscous fibers present in varying amounts
within different WG decrease digestible energy by impairing
macronutrient digestion and absor ption, leading to energy
excretion (90,91).
Energy digestibility has been shown to decrease by 3–4%
following 20- to 25-g/d increases in dietary fiber intake, a re-
duction equivalent to w418 kJ/d (100 kcal/d) in these stud-
ies (92,93). This relationship appears to be dose dependent,
as evidenced by manipulations of dietary fiber content lead-
ing to 150-kJ/d (36 kcal/d) increases in fecal energy content
for each additional 5 g/d fiber consumed (94). Similar effects
on energy availability appear to be achieved when dietary
substitutions of WG for RG are made, but only if dietary fi-
ber intake increases substantially. Substituting $350 g/d of
coarse or finely ground wholemeal bread for mixed wheat
bread resulted in a 21-g/d increase in dietary fiber intake
and a 3% reduction in digestible energy (95). However,
substituting 105 g/d milled WG for RG did not affect fecal
energy excretion when dietary fiber intake increased by
only 7 g/d (68). Similarly, an 84-g/d rolled oats intervention
did not affect the fecal energy content of healthy adults de-
spite a 4-g/d increase in soluble fiber intake (71). Taken to-
gether, these findings suggest that consuming WG-r ich
foods may contribute to body weight regulation, at least in
the short term, by reducing energy availability, but only if
substantial increases in dietary fiber intake are concurrent.
Glycemic response
By combination of reduced nutrient availability and delayed
gastric emptying, viscous fibers reduce the postprandial gly-
cemic response (90). Foods that elicit high glycemic re-
sponses are postu lated to promote fat storage and hunger
by augmenting postprandial insulin secretion and altering
counter-regulatory hormone responses (96,9 7). However,
the glycemic response associated with consuming WG foods
is less dependent on the fiber content of the grain. Rather,
factors such as the structural integrity, grain particle size af-
ter processing, and the food matrix appear to be the primary
determinants of glycemic responses to WG foods (98).
Holt et al. (77) demonstrated progressive reductions in
postprandial glycemic and insulinemic responses with in-
creasing particle size following consumption of WG wheat
provided in intact, cracked, coarse, and finely ground forms
and similar findings have been reported for maize, barley,
and rice (99–102). Appetite may also be affected by grain
processing, as Holt et al. (77) documented higher satiety rat-
ings following consumption of the intact relative to the
finely ground grain. More recently, Kristensen et al. (103)
measured glycemic responses and appetite following con-
sumption of 4 separate meals composed of either bread or
pasta made using milled WG wheat or refined wheat flours.
No differences in energy intake at the subsequent meal were
obser ved, though the WG bread was rated as more satiating
that the RG wheat bread. Consuming WG foods had no ef-
fect on glycemic responses despite being higher in fiber, sug-
gesting the relative importance of particle size and structural
integrit y. Further, glycemic responses were attenuated
following consumption of the pastas compared with the
Figure 1 Structural and physicochemical properties of WG
foods mediate the effect of WG on physiologic factors
influencing body weight and composition. GLP-1, glucagon-like
peptide-1; PYY, peptide-YY; WG, whole grain.
702 Karl and Saltzman
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breads, demonstrating the importance of the food matrix on
glycemia (103).
In addition to postprandial modulation of glycemic re-
sponses, WG-rich meals have also been shown to favorably
affect glucose metabolism following the subsequent meal
(104), an occurrence termed the “second meal effect.” Nilsson
et al. (105,106) recently e xtended this finding by demon-
strating that, relative to RG wheat bread, consuming an
equivalent amount of available carbohydrate from barley
kernels prepared using various methods at evening meals
depressed the glycemic response following a standardized
breakfast the next morning. Colonic fermentation of indi-
gestible carbohydrate was postulated to be an underlying
mechanism as evidenced by inverse associations between
glycemic responses and breath hydrogen, plasma SCFA,
and plasma glucagon-like peptide-1 (GLP-1) concentrations
(106,107). Similar effects, however, were not observed when
wheat kernels were substituted for barley (105) or when the
barley meal was consumed at breakfast and glycemic re-
sponses measured following the subsequent dinner 9.5 h
later (108). In a separate study, compared with RG wheat
bread, an evening meal consisting of barley kernels de -
pressed the postprandial glycemic response, enhanced pe-
ripheral insulin sensitivity, and increased breath hydrogen
and plasma butyrate concentrations the following morning
(109). Of note, the fiber contents of the barley meals con-
sumed in these studies exceeded the average daily amount
consumed by Americans (110) and reached 81 g/meal
(106). Thus, whether these findings are applicable to the
types and amounts of WG commonly consumed is unclear.
Fermentation and the gut microbiota
The studies of Nilsson et al. (105–108) and Priebe et al.
(109) draw attention to the potential influence of the gut mi-
crobiota in mediating relationships between WG intake and
body we ight regulation. SCFA produced durin g the fer-
mentation of certain fibers within WG contribute to the
regulation of body weig ht and composition by serving as
metabolizable energ y sources, d irectly mediating hepatic
and peripheral glucose and lipid oxidation and stimulating
secretion of the gut hormones peptide-YY and GLP-1, which
act to suppress appetite, slow gastrointestinal transit, and
modulate glucose metabolism (111). SCFA production is
influenced by a number of factors, including the availability
of fermentable substrate and the composition of the gut mi-
crobiota (112). Substrate availability and gut microbiota
composition are interrelated, as evidenced by the prebiotic
effect, a symbiotic relationship between the gut microbiota
and human host whereby specific fermentable carbohydrate
selectively promote the proliferation of colonic bacteria ben-
eficial to host health (113). Emerg ing evidence has demon-
strated that the composition of the gut microbiota may be
linked to human obesity (114–120) and is sensitive to mul-
tiple dietary factors (116,121 – 124), thus suggesting a poss i-
ble role for WG in body weight regulation via modulation of
the gut microbiome.
Few studies to date have investigated interrelationships
between WG-based diet interventions, gut microbiota com-
position, and energy regulation. Consuming 48 g/d of WG
wheat (125) or maize (61) cereals over 3 wk was shown to
have a prebiotic effect in 2 separate trials; however, similar
effects were not observed when 150 g WG/d from a variety
of food sources was consumed over 2 wk (66). The short du -
ration of these studies precludes any meaningful conclusions
with regard to body weight regulation from being reached,
and only one study quantified energy intake, reporting no
effect of the WG intervention (66). No changes in fasting in-
sulin, fasting glucose, or fecal SCFA concentrations were re-
ported in any of these studies, nor was breath hydrogen
excretion or endocrine mediators of appetite such as pep-
tide-YY or GLP-1 measured.
Inulin and oligofructose are prebiotic fibers found in a
variet y of cereal grains but predominantly obtained from
wheat products in American diets (126,127). Daily oligo-
fructose supplementation has been shown to alter glucose
metabolism, enhance postprandial gut hormone responses,
increase breath hydrogen levels, and promote weight and
body fat loss, suggesting inter relationships between prebi-
otic fiber intake, gut microbiota activity, and energy regula-
tion (128,129). However, these effects may be dose
dependent, because similar effects are not always observed
at lower doses (130). Though these studies provide some
support for a beneficial effect of intrinsic WG components
on body weight regulation, the amount of oligofructose
shown to be effective (>16 g/d) would be difficult to con-
sume from WG products alone (9), and more work is
needed to determine if smaller doses achieved through con-
sumption of WG would have similar beneficial effects.
Conclusion
Evidence for beneficial effects of WG intake on body weight
regulation should derive from randomized controlled trials
demonstrating spontaneous weight loss, enhanced weight
loss during energy restriction, or prevention of weight (re)
gain in response to increased WG intake. Recent trials exam-
ining the effects of substituting WG for RG on body weight
regulation do not provide evidence for a benefit of WG con-
sumption, which is counter to epidemiologic observations.
However, the nature of WG interventions employed may
have dampened the ability to detect effects of WG intake on
body weight regulation. In particular, the <10-g/d differences
in dietary fiber intake achieved by substituting WG for RG in
these studies is less than what has been reported to induce
even modest weight loss (131,132) and may have been insuf-
ficient to modulate short- or long-term mechanisms involved
in body weight regulation. The differences in fiber intake
achieved, however, are consistent with the increase in total
daily fiber intake expected for an average individual adopting
the 2010 Dietary Guidelines for Americans recommendation
to replace RG with WG. Currently consumed forms of WG
may also mitigate the potential benefit of WG consumption
on body weight or composition. Processing clearly influences
WG-mediated postprandial physiologic responses, with less
Whole grains and body weight regulation 703
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processing associated with metabolic effects most likely to
benefit body weight and composition, specifically the blunt-
ing of postprandial glycemic and insulinemic responses. In-
tervention trials commonly used commercially available or
similar food products in which WG ing redients are more
often pro cessed and reconstituted than consumed intact.
Although using these products reflects the current food en-
vironment, the null findings may suggest that many WG
products, as commonly consumed, are ineffective for facili-
tating weight control or triggering the underlying physio-
logic mechanisms.
Re pr oducing rec ent findings of beneficial effects of WG in-
take on body composition and elucidating the underlying
mechanisms should be targeted in future trials because central
adiposity is a more clinically relevant predictor of metabolic
risk than body weight or BMI (133). It is intriguing to speculate
that the mechanisms derive from interactions between WG in-
take, gut microbiota composition, and host physiology. Direct
effects of the gut microbiota on fatty acid metabolism and the
regulation of total body and fat mass have been demonstrated
(134,135); however, much of this work is restricted to animal
and in vitro models, with the relevance to humans undeter-
mined. Owing to recent advances in molecular biology tech-
niques, the study of dietary influences on the human gut
microbiome has become more accessible. Long-term random-
ized trials applying these techniques to explore interrela tion-
ships between WG-based diet interventions, gut micr obiota
composition, and the regulation of adiposity are needed.
In summary , intervention trials conducted to date hav e
failed to demonstrate beneficial effects of WG intake on body
weight r egulation despite obs ervational studies consistently
demonstrating that high intakes of W G are associated with
lower BMI, and the existence of a variety of mechanisms that
could result in WG-mediated effects on body weight. Nonethe-
less, recent findings suggesting possible beneficial effects of WG
intake on body composition deserve further attention. Investi-
gations of sufficient duration to capture changes in body com-
position, providing less process ed forms of W G, comparing
different varieties of WG, and discriminating between effects
of fiber and WG are needed. The continued development of
biomark ers of W G intak e is also needed for monitoring com-
pliance in these studies. Plasma and urine alkylresorcinol con-
centrations are promising in this respect (66,68), though large
interindividual variation; exclusivity to W G wheat, rye, barley,
and triticale; short half-life; and possible nonlinear increases at
high levels of WG intake remain limitations (136).
At present, insufficient evidence exists to demonstrate
clear beneficial effects of WG intake on body weight regula-
tion. However, given the nutritive superiority of WG relative
to RG, WG consumption should continue to be encouraged
as part of a health-promoting diet.
Acknowledgments
Both authors read and approved the final manuscript.
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