Continuing Education
Is Insulin Sensitivity Improved
by Diets Rich in Whole Grains?
Nicola M. McKeown, PhD
Cindy A. Crowninshield, RD, LDN, HHC
Paul F. Jacques, ScD
Diets rich in whole-grain foods appear to protect against
the development of type 2 diabetes. On January 31, 2011,
the 2010 Dietary Guidelines for Americans were released and
the recommendations with respect to grains were for
individuals to ‘‘Consume at least half of all grains as whole
grains’’ and ‘‘Increase whole-grain intake by replacing
refined grains with whole grains.’’ Several prospective
observational studies have reported that people with
higher intake of whole grains are less likely to develop
type 2 diabetes. One potential mechanism whereby whole
grains may reduce the risk of developing diabetes is by
maintaining insulin sensitivity. The insulin-sensitizing
effect of whole grains might be partially due to dietary
fiber or magnesium, 2 components of whole grains. In
this review, we highlight the recent scientific evidence
(ie, since the release of the 2005 Dietary Guidelines) on
the role of whole grains on measures of insulin sensitivity.
Nutr Today. 2011;46(2):54–65
O
ver the past decade, most observational studies
have observed that people who consume more
whole-grain foods have a lower risk of
developing type 2 diabetes mellitus (T2DM).1Y6 A
recent meta-analysis of these data concluded that a
2-serving-per-day increment in whole-grain consumption
reduced diabetes risk by 21%.6,7 Since the release of the
last (2005) Dietary Guidelines for Americans (DGA)8,
several studies have attempted to determine whether
greater consumption of whole grains improves insulin
sensitivity (ie, that peripheral fat and skeletal muscle
cells are sensitive to the glucose-lowering effects of the
hormone insulin) and other diabetes risk factors. In this
review, we highlight some of the more recent scientific
54
evidence (ie, since 2005) relating whole grains to
measures of insulin sensitivity.
What Are Whole Grains?
Whole grains are defined as ‘‘cereal grains that consist
of the intact, ground, cracked, or flaked caryopsis
(kernel), whose principal anatomical componentsVthe
starchy endosperm, germ and branVare present in
the same relative proportions as they exist in the
intact caryopsis.’’8 Each component of the grain is
nutritionally unique, as shown in Figure. Many
different whole grains are available in the US food
supply (Table 1).
During the milling process of grains, the
nutritionally superior parts of the grain (bran and
germ layer) are discarded, and the carbohydrate-rich
endosperm is ground into white flour. As a
consequence, the more highly refined the flour,
the greater the loss in dietary fiber; the B vitamins
thiamin, riboflavin, niacin, and pyridoxine; iron;
magnesium; vitamin E; and other measured and
unmeasured dietary constituents.10 Mandatory
enrichment and fortification of refined flour with
5 nutrients (thiamin, riboflavin, niacin, folic acid,
and iron) ensure that refined grains in the United
States are not nutritionally inferior to whole grains
with respect to these nutrients. In fact, the levels
added back are much higher than naturally occurring
levels in many whole grains. Yet, despite enrichment
and fortification, diets rich in refined grains do not
appear to confer the health benefits observed with
whole grains.
Dietary fiber, both water-soluble and insoluble, is
not replaced in most refined grain products such as
breads, rolls, white rice, and some ready-to-eat
breakfast cereals. The water-soluble forms of fiber are
found in oatmeal and barley, whereas the insoluble
forms are found in whole wheat, located mainly in the
outer bran layer. Dietary fiber plays an important
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Whole Grains and Insulin Sensitivity
Continuing Education
The 2010 Dietary Guidelines for
Americans from the US Department
of Agriculture recommend that
individuals ‘‘Consume at least half
of all grains as whole grains’’ and
to ‘‘Increase whole-grain intake
by replacing refined grains
with whole grains.’’
Figure. The 3 main components of a whole grain in a generic
grain kernel.
role in regulating circulating insulin levels, and
therefore consumption of different types of
whole-grain foods ensures that different forms of dietary
fiber are being consumed by an individual. Furthermore,
whole grains deliver myriad micronutrients,
antioxidants, and phytochemicals that may
independently or synergistically influence glucose and
insulin metabolism.
Whole-Grain Intake in the United States
The US Food and Drug Administration defines whole
grains as consisting of the intact, ground,
cracked, or flaked fruit of the grain whose principal
componentsVthe starchy endosperm, germ, and
branVare present in the same relative proportions as
they exist in the intact grain.8 A whole grain can be
consumed as a ‘‘single food,’’ such as oatmeal or brown
rice, or alternatively consumed as an ‘‘ingredient’’ in a
food, such as whole-wheat flour or whole-rye flour in
bread and crackers. On January 31, 2011, the 2010
DGA were released with recommendations for
individuals to ‘‘Consume at least half of all grains as
whole grains’’ and ‘‘Increase whole-grain intake by
replacing refined grains with whole grains.’’ For an
adult consuming 2000 daily kilocalories, this translates
into consuming at least three ounce-equivalent servings
per day of whole grains. In general, a 1-oz equivalent
of whole grains contains 16 g of whole grains.
Examples of 1-oz servings include 1 slice of bread or
1 cup of ready-to-eat cereal. Half-cup servings of cooked
brown rice, cooked whole-wheat pasta, or cooked
oatmeal can be considered as 1-oz equivalents from the
grains group.9
In the past, Americans were consuming, on average,
only 1 daily serving of whole grains,11 with the
major contributors being hot cereal and cold
Nutrition TodayA, Volume 46
Number 2
ready-to-eat breakfast cereals (42%), followed by
whole-grain breads (25%), popcorn (12%), and
whole-grain crackers (6%).12 After the release of the
2005 DGA, manufacturers responded by increasing the
availability of whole-grain products and foods in
the marketplace. For example, manufacturers of
refined-grain breakfast cereals have changed their
formulas to include whole grains, whereas others have
include 100% whole grains in their frozen entrees
or have made the vast majority of their breads whole
grain. In addition to new products being available
in the marketplace, consumers are more educated
about what a whole-grain food is, how to identify a
whole-grain product, and the purported health benefits
of whole grains.13
At the population level, wheat is the most common
grain consumed in the US, constituting 66% to 75%
of the total grain intake.14 However, actual levels of
specific types of whole-grain consumption by individuals
have yet to be elucidated.
Table 1. Examples of Whole-Grain Foods
and Floursa
Amaranth
Barley, whole-grain
Brown rice
Buckwheatb
Bulgur
Corn, whole-grain
Cornmeal
Millet
Oats, whole
Oatmeal
Popcorn
Quinoa
Rye, whole
Sorghum
Spelt
Triticale
Wheat berries
Wheat, cracked
Wheat, whole
Wild rice
a
When consumed in a form retaining the bran, germ, and endosperm
components. Resources to create this list include the American
Association of Cereal Chemists and MyPyramid.9
b
Buckwheat is a seed of a fruit and not strictly a whole grain, but it is
similar to wheat and thus is classified as a whole grain.
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55
Continuing Education
Whole Grains and Insulin Sensitivity
Consumers should continue to eat a
variety of whole grains daily.
Do Whole-Grain Foods Improve
Insulin Sensitivity?
Evidence From Observational Studies
During the preparation of the 2005 DGA advisory
report, only 2 cross-sectional studies relating
whole-grain intake to measures of insulin metabolism
were considered.15,16 Both studies observed that adults
who consumed 3 or more servings of whole grains
per day had significantly lower fasting insulin
concentrations compared with those who rarely
consumed whole-grain foods. Since then, 6 additional
cross-sectional studies17Y22 have provided support for
this association using various surrogate measures of
insulin sensitivity (Table 2).
In the previously mentioned studies,17Y22 food
frequency questionnaires (FFQs) were the dietary
method of choice, with the exception of 1 study that
estimated whole-grain intake based on a 7-day diet
record.18 In adults, the highest consumers of whole
grains reportedly consumed, on average, approximately
3 or more daily servings of whole grains, whereas the
lowest consumers ate less than one-half of a daily
serving of whole grains. In adolescents, reported intake
ranged from one-half to one-and-one-half daily servings
of whole grains.21
Fasting insulin is the most cost-effective method of
determining insulin resistance; higher levels reflect
hepatic insulin resistance and compensatory
hyperinsulinemia. Most of the cross-sectional studies
that considered fasting insulin as a marker of insulin
resistance reported that individuals who ate more whole
grains had lower fasting insulin concentrations.17,19Y22
However, in the Baltimore Longitudinal Study of
Aging,18 higher intake of whole grains was not associated
with fasting insulin concentrations. This may be attributed
in part to the smaller sample size (n = 460), the larger
age range (27Y88 years), or the use of a single 7-day
food record to capture dietary intake. In those studies
that observed a significant relationship between
whole-grain intake and insulin sensitivity, the observed
associations were weakened after adjustment for either
overall or central obesity22 or after accounting for
other dietary factors.19 This attenuation of the
relationship between whole-grain intake and insulin
suggests that the degree of obesity may be a mediating
56
risk factor or a confounding factor and that other
components of the diet that tend to be eaten more
frequently by high whole-grain consumers, such as
higher intake of fruits and vegetables, may also
confound the relationship between whole grains and
insulin resistance.
Homeostasis Model Assessment of Insulin Resistance
(HOMA-IR), another surrogate marker of insulin
sensitivity, takes into account a person’s underlying
glycemia status as well as his/her circulating insulin
concentration. A higher HOMA-IR is indicative of a
greater degree of insulin resistance. In the Multi-ethnic
Study of Atherosclerosis,17 after adjustment for
demographic and health behavior variables, the
HOMA-IR differences between extreme intake
categories of whole grains were approximately 8%.
This study did not consider the overall degree of
obesity in this population, and thus, the true differences
in HOMA-IR between extreme categories of intake
may, in fact, have been smaller. In the Framingham
Offspring Study,20 a cohort of healthy middle-aged
and older adults, a smaller, but significant, difference
(approximately 3%) was observed in HOMA-IR
between the highest and the lowest consumers of
whole grains after controlling for body mass index
(BMI) and waist circumference, suggestive of a
potential independent effect of whole grains on
insulin resistance.
In the Insulin Resistance Atherosclerosis Study,22 a
study of middle-aged adults with normal (67%) and
impaired (33%) glucose tolerance, estimates of
whole-grain intake derived from FFQs were related
to both direct and surrogate measures of insulin
sensitivity. The frequently sampled intravenous
glucose tolerance test (FSIVGTT) with minimal
model analysis is a direct measure of insulin sensitivity,
with higher values indicating greater insulin
sensitivity. In this study, higher whole-grain intake was
associated with significantly higher insulin sensitivity
after adjustment for BMI and waist circumference.
With respect to fasting insulin, higher intake of
whole grains was associated with lower insulin
concentrations; however, consistent with other studies,
inclusion of BMI and waist circumference attenuated
the association. Nonetheless, the authors estimated
that an increase of 1 serving of whole grains, from
0.8 to 1.8 per day, would be associated with a 13.5%
greater insulin sensitivity and a 6.3% lower fasting
insulin concentration.
Only 1 study21 conducted in a small sample of
adolescents used the hyperinsulinemic-euglycemic
glucose clamp to quantify insulin sensitivity. This
method is considered the criterion standard for insulin
sensitivity as it measures the effects of insulin in
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Whole Grains and Insulin Sensitivity
promoting glucose utilization under steady-state
conditions. After adjustment for age, sex, race,
Tanner stage, energy intake, BMI, and physical
activity, a significant positive dose-response
association was observed with whole-grain intake,
particularly for the heaviest adolescents (BMI Q27.5 kg/m2).
A similar observation was found in a larger study of
adults: a higher intake of whole grains was also
associated with better insulin sensitivity (based on
fasting insulin) in obese individuals, whereas no
relationship was observed in nonobese.16 Such findings
imply that the most significant impact of whole grains may
be among those who are on the trajectory toward
developing T2DM.
All of the aforementioned studies of whole grains
and insulin metabolism were based on servings of
whole-grain foods or food deemed to be whole grain
based on added bran and germ. More recently,
investigators implemented more quantitative estimates
of whole-grain intake, based on grams of whole grains
in individual foods.19 Quantifying whole grains in
this manner captures the actual amount of whole grains
in individual servings of foods, allowing for a more
accurate assessment of whole-grain intake and estimates
of the bran and germ. This quantitative estimate of
whole-grain intake allows for the examination of the
potential effects of the components of whole grains
(ie, the bran and germ intake) on markers of insulin
metabolism.
Using this more quantitative estimate of whole grains,
Jensen and colleagues19 examined the cross-sectional
association between whole-grain intake and several
markers of glycemic control. In this study, whole-grain
intake was inversely associated with C-peptide
concentrations, a surrogate marker reflecting the
amount of insulin that is being made by the pancreas.
Individuals with the highest intake of whole-grain
foods had C-peptide concentrations that were
approximately 17% lower compared with those with the
lowest intake of whole grains. Adjustment for dietary
factors (ie, fruit, vegetable, sucrose, and fructose
intake) only slightly attenuated these associations,
suggesting that better measurement of whole-grain
consumption may have resulted in less attenuation of
the association upon adjustment. In the same study,
an inverse association was observed between
whole-grain intake and fasting insulin, although this
finding was no longer significant after the inclusion of
other dietary factors. The authors also considered the
relations of the bran and germ on markers of risk of
insulin resistance and found that only bran intake
showed a moderate inverse association with C-peptide
concentrations (P trend = .05). The observed association
with bran suggests that fiber and magnesium, both
Nutrition TodayA, Volume 46
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Continuing Education
concentrated in the outer bran layer of the germ, may be
mediators of the association between whole grains and
insulin sensitivity.
The main shortfall of the studies summarized above
lies within their cross-sectional design. Cross-sectional
studies, reflecting snapshots in time, do little to
illuminate the event sequence between exposures and
outcomes. As a consequence, causality cannot be
inferred; we cannot conclude based on these studies
that higher intakes of whole-grain foods improve
insulin sensitivity. In addition, the fixed food
categories on FFQs may limit our ability to adequately
capture a person’s intake of whole-grain foods, thereby
resulting in an underestimation of intake. This
misclassification could mask variability among
individuals, resulting in attenuated effect estimates for
the association between whole grains and surrogate
measures of insulin resistance. Limitations of the
cross-sectional design can be overcome in prospective
observational studies in which whole-grain intake is
related to changes in markers of insulin resistance or
sensitivity, but to date, no prospective observational
study on this relationship has been published. Despite
the limitations of data from existing observational
studies, the findings from these studies have led
scientists to generate and test hypotheses that higher
intakes of whole grains improve insulin sensitivity in
intervention studies.
A shortfall of cross-sectional studies
is that they reflect snapshots in time
and thus do not illuminate the event
sequence between exposures
and outcomes.
Evidence From Intervention Studies
The first randomized, controlled, metabolic study to test
the hypothesis that whole-grain consumption improved
insulin sensitivity was conducted in 11 overweight,
hyperinsulinemic adults. Insulin sensitivity, determined
by the euglycemic-hyperinsulinemic clamp test,
improved after 6 weeks on a high whole-grain diet
compared with a refined-grain diet.23 The authors also
observed a 10% decrease in fasting insulin concentrations
following 6 weeks on the whole-grain intervention
period. Since then, 3 additional randomized dietary
intervention studies24Y26 have been published, testing the
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57
Continuing Education
Whole Grains and Insulin Sensitivity
Table 2. Cross-sectional Studies Published Since the 2005 DGA That Examine the Association Between
Whole-Grain Intake and Measures of Insulin Sensitivity
Reference
Population
Race/
Ethnicitya
Sex
n
Mean Age
(Range), y
Mean BMI,
kg/m2
Lutsey et al17
Multi-ethnic Study of
Atherosclerosis, United States
,38% NHW,
28% AA,
23% H,
11% W
M+F
5496
,60 (45Y84)
,25.5
Newby et al18
Baltimore Longitudinal Study on
Aging, United States
,90% W
M+F
1324
53 (27Y88)
,25.5
Jensen et al19
Health Professionals Follow-up
Study and Nurses Health
Study, United States
,97% W
M
F
468
473
,60 (47Y82)
,42 (31Y48)
25.3
McKeown
et al20
Framingham Offspring Cohort,
United States
99% W
M+F
2834
54 (28Y82)
27.0
Steffen et al21
Minneapolis, Minnesota
86% W,
14% AA
M/F
155/130 13 (13Y15)
58
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Whole Grains and Insulin Sensitivity
DAM
127-item
FFQ
Whole-Grain
Intake
Measure of IS
Servings/d,
Q1 = G0.1,
Q5 = 1.4
Fasting insulin,
mU/L
HOMA-IR
7-d DR
g/d, Q1 = 2,
Q5 = 52
131-item
FFQ
g/d, Q1 = 8,
Q5 = 44
Fasting insulin,
mmol/L
(n = 460)
2-h insulin,
mmol/L
(n = 455)
Fasting insulin,
6U/mL
C-peptide,
ng/mL
HOMA-IR
126-item
FFQ
Servings/wk,
Q1 = 0.9,
Q5 = 20.4
127-item
FFQ
Fasting insulin,
Servings/d,
mU/L
T1 G0.5,
T2 = 0.5Y1.5,
T3 91.5
IS
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Continuing Education
Results
Adjusted for
Q1 = 5.37,
Q5 = 5.16,
P trend = .002
Q1 = 1.68,
Q5 = 1.53,
P trend = .002
Q1 = 71.6,
Q5 = 71.8,
P trend = .90
Q1 = 479,
Q5 = 414,
P trend = .43
Model 1:
Q1 = 13.3,
Q5 = 11.1,
P trend = .06
Model 2:
Q1 = 13.2,
Q5 = 11.3,
P = .13
Model 1:
Q1 = 2.17,
Q5 = 1.81,
P trend = .004
Model 2:
Q1 = 2.13,
Q5 = 1.84,
P trend = .03
Q1 = 6.8
Q5 = 6.6
P = .05
Age, sex, total energy Vegetable intake, refined
grain intake, dairy intake,
intake, race,
fish intake, poultry
education, survey
intake, meat intake,
center, smoking,
leisure physical activity,
alcohol use, fruit
sedentariness score
intake
Age, sex, BMI, total Education, multivitamin
use, smoking, %
energy intake,
decade of visit, race, energy from fat
oral hypoglycemic
medication, diabetes
T1 = 16.7,
T2 = 14.4,
T3 = 13.8,
P trend = .07
T1 = 11.5,
T2 = 12.3,
T3 = 13.3,
P trend = .01a
Age, sex, energy
intake, race, Tanner
score
Model 2:
Model 1: age, sex,
Model 1 and fruit intake,
BMI, total energy
vegetable intake,
intake, alcohol
sucrose intake,
intake, smoking,
fructose intake
physical activity,
hypercholesterolemia
Age, sex, BMI,
waist-hip ratio,
smoking, total
energy intake
Alcohol intake, % SFA
and PUFA intake,
multivitamin use,
physical activity,
treatment for blood
pressure
IS remained significant
after adjustment for
physical activity and BMI
(P trend = .03) but
not fasting insulin
(P trend = .41)
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59
Continuing Education
Whole Grains and Insulin Sensitivity
Table 2. Cross-sectional Studies Published Since the 2005 DGA That Examine the Association Between
Whole-Grain Intake and Measures of Insulin Sensitivity, continued
Reference
Population
Liese et al22
Insulin Resistance
Atherosclerosis Study,
United States
Race/Ethnicitya
Sex
n
40% NHW,
34% H,
26% AA
M+F
978
Mean Age Mean BMI,
(Range), y
kg/m2
54 (40Y69)
28.4
Abbreviations: AA, African American; BMI, body mass index; DAM, dietary assessment method; DGA, 2005 Dietary Guidelines for Americans; DR, diet
record; F, female; FFQ, food frequency questionnaire; FSIVGTT, frequently sampled intravenous glucose tolerance test; H, Hispanic; HOMA-IR, homeostatic
model assessment of insulin resistance; IS, insulin sensitivity; M, male; NHW, non-Hispanic white; PUFA, polyunsaturated fatty acid; Q1, lowest quintile
category of whole-grain intake; Q5, highest quintile category of whole-grain intake; SFA, saturated fatty acid; T1, lowest tertile category of whole-grain intake;
T2, middle tertile category of whole-grain intake; T3, highest tertile category of whole-grain intake; W, white.
hypothesis that whole-grain consumption improves
insulin sensitivity. However, none of these studies
support a strong association between whole-grain
consumption and increased insulin sensitivity (Table 3).
One intervention study,24 which was designed to
compare the effects of whole wheat vs whole oat cereal
consumption, showed no effect of either whole-grain
cereal on insulin sensitivity over the course of 12 weeks.
Insulin sensitivity in this study was determined by an
intravenous glucose tolerance test, a more direct measure
of insulin sensitivity. This study included middle-aged
older adults with elevated blood pressure, and although
overweight, participants were considered healthy. It is
possible that modification of diet with whole grains in
healthy adults alters insulin sensitivity only over much
longer periods. In a randomized crossover study,
overweight adults were given either whole-grain or
refined-grain products to include in their habitual daily
diet for two 6-week periods.26 No changes in insulin
sensitivity were observed in this healthy population, in
contrast to the findings of the similarly designed study in
hyperinsulinemic adults noted above.23 The incorporation
of whole grains in diets of those with more pronounced
metabolic abnormalities, as observed in obese or
hyperinsulinemic individuals, may exert a stronger effect
than in those who are healthy. A randomized crossover
trial in postmenopausal women25 examined the effects
60
of a high-fiber rye bread compared with a low-fiber
white wheat bread on measures of glucose and insulin
metabolism, as assessed by a FSIVGTT. In this study,
no significant changes in insulin sensitivity or glucose
were observed over the 8-week intervention; however,
the acute insulin response increased significantly with
the rye-fiber breadVperhaps indicative of improved
insulin secretion by the pancreatic $ cells. More recently,
findings from a large intervention study designed to
examine the effect of supplemental whole-grain foods
on several CVD risk factors in 316 overweight adults
found no significant improvements in insulin sensitivity.28
It is possible that adding whole-grain foods while
failing to compensate by omitting other foods may
have affected the findings of this study, as discussed
elsewhere.29
The reasons for the different findings among these
interventions and between the observational and
intervention studies are complex and varied.
Components of whole-grain foods vary between the
intervention foods and may have a differential impact on
glucose and insulin metabolism. Thus, the interpretation
of these intervention studies may be complicated by the
variations in the dietary interventions themselves. For
instance, the form of the food and the botanical structure
have been found to modify postprandial glucose and
insulin response in healthy adults.30 Probably more
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Whole Grains and Insulin Sensitivity
DAM
114-item
FFQ
Whole-Grain
Intake
Measure of IS
Servings/d, 0.8
Results
Fasting insulin,
2U/mL
Model 1:
" coefficient =
j.065, P = .02
Model 2:
" coefficient =
j.031, P = .32
IS (FSIVGTT)
Model 1:
" coefficient =
j.082, P = .001
Model 2:
" coefficient =
.043, P = .03
Model 3:
" coefficient =
.041, P = .11
important are the varied study populations (eg, obese,
nondiabetic subjects, hyperinsulinemic subjects, age
range) and the relatively short-term nature of most of
these interventions.
Observational studies have linked
higher whole-grain intake with
better insulin sensitivity, but the
evidence from dietary intervention
studies has yielded mixed results.
Role of Dietary Fiber and Other
Mediating Nutrients of Whole Grains
‘‘The whole is greater than the sum of its parts’’ is an
aphorism that appears to be particularly apropos for the
potential health benefits of whole grains. By including
whole grains in their diets, consumers automatically take
in higher amounts of dietary fiber (varying degrees of
soluble and insoluble fiber, depending on the
whole-grain source), magnesium, vitamins B6 and E,
and other unmeasured nutrients and nonnutrients. In
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Continuing Education
Adjusted for
Model 1: age, sex, Model 2: Model 1 and
BMI, waist
ethnicity, total
energy intake, total circumference
Model 3: Model 2 and
energy
dietary fiber,
expenditure,
magnesium
smoking, family
history of diabetes
addition, there is great variability among the various
whole grains with respect to phytochemicals such as
phenolic compounds, lignan, and sterols, and there
remains a great deal of inquiry into the potential mediating
nutrients that may be eliciting the observed beneficial
effects.31 Magnesium is an essential cofactor for enzymes
involved in glucose and insulin metabolism. In clinical
studies, magnesium supplementation improved insulin
action among normal patients and patients with
T2DM.32,33 Magnesium is an integral micronutrient found
primarily in the outer bran layer of whole grains and so
may contribute to the improved insulin sensitivity
observed in some studies. Indeed, the relationship
between whole-grain intake and fasting insulin
concentrations was no longer statistically significant after
adjustment for magnesium and dietary fiber in at least
one observational study,22 suggesting perhaps that
the apparent insulin-sensitizing effect of whole grains
might be partially mediated by this nutrient.
Dietary fiber, in particular soluble fiber, can blunt
the postprandial rise in blood glucose by delaying
gastric emptying and macronutrient absorption from
the gut.34 Alternatively, colonic fermentation of
nondigestible carbohydrates may increase peripheral
insulin sensitivity,35,36 perhaps due to the increased
production of short-chain fatty acids during the
fermentation process in the colon.37 Other attributes
of whole-grain foodsVsuch as particle size, type of
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61
62
Study
Design
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Test: WG products,
Overweight,
Control: RG products
hyperinsulinemic
adults (n= 11)
6 wk
Randomized,
nonblinded,
crossover,
controlled
feeding trial
Pereira
et al23
Abbreviations: BMI, body mass index; DGA, 2005 Dietary Guidelines for Americans; IS = insulin sensitivity; RB, rye bread; RG, refined-grain; T2DM, type 2 diabetes mellitus; WG, whole-grain.
a
mg glucose I kg body wtj1 I minj1 per unit plasma insulin (mU/L) x 100.
b
This study compared whole wheat vs oat consumption (data results reflect the change from baseline).
Middle-aged and
older men
(n = 36) with
elevated blood
pressure
Oat cereals (5.5 g
$-glucan, n = 18) or
wheat cereals (no
$-glucan, n = 18)
High-fiber RB appears to
No significant changes in
enhance insulin
fasting insulin or insulin
secretion, possibly
sensitivity during the study;
indicating improvement
the proportional change in
of $-cell function
acute insulin response at
the end of the RB period
was greater than that at the
end of the white-wheat
bread period
Fasting blood glucose: oat:
No significant changes in
5.4 to 5.6 mmol/L; wheat:
fasting glucose, insulin,
5.3 to 5.3 mmol/L; insulin
or insulin sensitivity
concentrations: oat: 93.8 to
were observed over
88.4 pmol/L; wheat: 55.3 to
time or across groups
58.8 pmol/L. IS: oat: 1.8 to
1.8 10j4 minj1 I 2Uj1 I
mLj1; wheat: 2.6 to 2.2
10j4 minj1 I 2Uj1 I mLj1
Fasting insulin: mean
IS is improved after 6
difference in WG vs RG is
weeks of consuming
j14.0 T 5.5 pmol/L
whole-grain foods.
(P = .01); rate of glucose
infusion (insulin clamp):
higher with WG diet
indicating greater IS
(0.07 10j4 mmol kgj1 I
minj1 I per pmol/L)
Test: high-fiber RB,
Control: white-wheat
bread
12 wk
8 wk
Randomized,
nonblinded,
parallel
trialb
Randomized,
nonblinded,
crossover
study
Conclusions
Fasting insulin: no significant
Substitution of WG for RG
effect of WG (56.2 T 22.9
products in the habitual
pmol/L vs 57.6 T 24.3) or RG
daily diet of healthy
(60.4 T 30.6 vs 57.6 T 25.7
moderately overweight
pmol/L) (P = .47) over study
adults for 6 wk did not
period; Peripheral ISa: no
affect IS or markers of
lipid peroxidation and
significant effect of WG (6.8 T
inflammation
3.0 vs 6.5 T 2.7) or RG (6.4 T
2.9 vs 6.9 T 3.2) (P = .79)
Results
Test: WG products
(112 g/d),
Control: RG products.
Daily test and control
portions included 3
slices bread, 2 slices
crisp bread, 1 portion
muesli, and 1 portion
pasta.
Test and Control Diets
Davy
et al24
Juntunen
et al27
Healthy,
moderately
overweight,
and/or
abdominally
obese subjects
(n = 30) with
1 or more
metabolic
abnormalities
Healthy,
overweight,
postmenopausal
women
(n = 20)
Duration Study Population
Andersson Randomized, 6Y8 wk
nonblinded,
et al26
crossover
study
Reference
Table 3. Intervention Studies Published Since the 2005 DGA on Whole-Grain Intakes and Markers of Insulin Sensitivity
Continuing Education
Whole Grains and Insulin Sensitivity
March/April, 2011
Whole Grains and Insulin Sensitivity
grain (eg, rye compared with wheat), ratio of bran or
germ to endosperm, or presence of insoluble dietary
fiberVmay also be responsible for eliciting the favorable
glycemic response. More research is needed on the
impact of structural properties and the type of grain
on insulin response.
When compared with consumption of refined-grain foods,
consumption of whole grains may exert different
physiological effects, resulting in lower insulinemic and
glycemic responses. This may be, in part, due to the foods
structure, the glycemic index, or the dietary fiber content of
the whole-grain food.30,34,38 In 1 study,27,39 whole-meal
rye bread and rye bread baked with whole kernels produce
a lower insulin response compared with a refined wheat
bread, yet no differences were observed in glucose
responses. These findings suggest that less insulin is
required for the regulation of postprandial glucose
excursions after the consumption of whole-grain rye breads.
The early (acute) postprandial response of glucose after a
meal is offset by the pancreatic $-cell release of insulin, the
stimulation of gut hormones (glucagon-like peptide 1 and
glucose-dependent insulinotropic polypeptide), and the
inhibition of the glucagon release from the pancreatic "
cells. Over the long term as a consequence of the elevated
demand for insulin, higher intakes of refined grains may
lead to pancreatic exhaustion. Once the pancreas is no
longer able to secrete sufficient insulin to compensate for
higher glucose levels, mild hyperglycemia and impaired
glucose tolerance arise. Coupled with this physiological
change, there is a decrease in insulin clearance by
the liver, and the peripheral fat and skeletal muscle cells
become insensitive to the glucose-lowering effects of
insulin, and insulin resistance arises.40
Given that there are several potential mediating
attributes of whole grains that could play a critical role in
glucose and insulin metabolism, it is difficult to separate
the independent effects of nutrients from the structural
properties of different types of whole grains. Furthermore,
the health benefits of whole grains observed with higher
whole-grain intake in observational studies may not be
due to the whole grains per se, but rather due to ‘‘the
company they keep.’’ For example, people who eat more
whole-grain foods tend to live healthier lifestyles and
choose healthier foods.
Conclusion
Is insulin sensitivity improved by consumption
of whole-grain foods? Based on the current state of
knowledge, the evidence linking higher intake of whole
grains to better insulin sensitivity is inconclusive. The
2010 DGA advisory committee also reported that the
scientific evidence on the relationship between whole
grains and T2DM was limited. An apparent contradiction
Nutrition TodayA, Volume 46
Number 2
Continuing Education
exists between observational studies on insulin sensitivity
and whole-grain intake on the one hand, and short
clinical trials on the other. One possibility is that
long-term quality of a high whole-grain diet reduces
insulin demand and consequently reduces pancreatic
$-cell exhaustion, insulin resistance, glucose intolerance,
and risk of T2DM. To date, there is a lack of data from
prospective studies on the impact of whether high
whole-grain intake will slow the progression of insulin
resistance over time. Fortunately, although understanding
the science behind the benefits of whole grains on
insulin sensitivity is important, their public health benefit
exists largely independent of that knowledge. The 2010
DGA committee concluded there was moderate evidence
that intake of whole grains and grain fiber was associated
with lower body weight and that whole-grain intake,
which includes cereal fiber, protects against cardiovascular
disease (http://www.nutritionevidencelibrary.com/
conclusion.cfm?conclusion_statement_id=250211&
highlight=whole%20grain&home=1). Several studies have
observed that higher intake of whole grains is associated
with lower weight and less weight gain,41 and 1 recent
study found that older adults consuming approximately
3 daily servings of whole grains had significantly lower
abdominal fat.42 Dietary fiber is a key mediator of the
health benefits of whole grains, but it would be remiss to
not consider the myriad micronutrients, antioxidants,
phenolic compounds, and unmeasured compounds in
whole grains that positively impact metabolic risk factors.
Health professionals should continue to promote
including a variety of whole-grain foods as a part of an
overall healthy lifestyle, with the goal of increasing intake of
whole grains to at least 3 servings per day and whenever
possible, emphasizing the need to replace refined grains
with whole grains.
Acknowledgments
The authors thank Adela Hruby for her assistance in
proofreading and editing this article.
Nicola M. McKeown, PhD, is a scientist in the Nutritional
Epidemiology Program at the Jean Mayer USDA Human Nutrition Research
Center on Aging at Tufts University. She is Assistant Professor and Program
Director of Nutritional Epidemiology at the Gerald J. and Dorothy R.
Friedman School of Nutrition Science Policy at Tufts University.
Dr McKeown’s research interests include the role of dietary carbohydrates
on diabetes risk factors, in particular the influence of whole grains and
cereal fiber.
Cindy A. Crowninshield, RD, LDN, HHC, Butler Hospital/Sodexo,
Providence, Rhode Island. She leads a unique parallel career in business
and science as a licensed registered dietitian and conference program
developer/project manager of scientific conferences. She divides her time
being a clinical dietitian at Butler Hospital/Sodexo, a registered dietitian
at Body Therapeutics, and a conference program developer/team leader
March/April, 2011
Copyright @ 2011 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
63
Continuing Education
Whole Grains and Insulin Sensitivity
at Cambridge Healthtech Institute. Ms Crowninshield is also the
owner/founder of Eat2BeWell. Her career and research interests include
nutrition and genetics, functional nutrition therapy, food allergies, and public
health/community nutrition.
Paul F. Jacques, ScD, is the director of the Nutritional Epidemiology
Program and senior scientist at the Jean Mayer USDA Human Nutrition
Research Center on Aging at Tufts University and Professor at the
Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy
at Tufts University. Dr Jacques is the author of more than 200 original
research articles and dozens of editorials, book chapters, and reviews
on the role of nutrition in age-related disorders such as atherosclerosis,
diabetes, cataract, osteoarthritis, and dementia. His current research
interests include the role of dietary factors such as whole grains and
flavonoids on metabolic markers of diabetes and cardiovascular disease
risk; the impact of Food and Drug Administration-mandated folic acid
fortification of enriched cereal-grain products on folate status and health
of Americans; and the relation between dietary patterns, diet quality, and
risk of chronic disease.
Any opinions, findings, conclusions, or recommendations expressed in this
publication are those of the authors and do not necessarily reflect the views
of the US Department of Agriculture.
US Department of Agriculture (agreement 58-1950-7-707) and in part by a
research grant from General Mills Bell Institute of Health and Nutrition,
Minneapolis, Minnesota.
Correspondence: Nicola M. McKeown, PhD, The US Department of
Agriculture, Human Nutrition Research Center on Aging, 711 Washington St,
Tufts University, Boston, MA 02111 ([email protected]).
DOI: 10.1097/NT.0b013e31821189cf
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Whole Grains and Insulin Sensitivity
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