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ORIGINAL RESEARCH COMMUNICATION |
1 From the Division of Preventive Medicine and the Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, and the Departments of Epidemiology and Nutrition, Harvard School of Public Health, Boston.
2 Supported by National Institutes of Health grant CA87969 and by Clinical Scientist Development Award K08-DK 02767 from the National Institute of Diabetes and Digestive and Kidney Diseases (to SL). In addition, for activities related to the Nurses' Health Study, we received modest resources at various times and for various periods since 1 January 1993 from the Alcoholic Beverage Medical Research Foundation, the American Cancer Society, Amgen, the California Prune Board, the Centers for Disease Control and Prevention, the Ellison Medical Foundation, the Florida Citrus Growers, the Glaucoma Medical Research Foundation, Hoffmann-La Roche, Kellogg's, General Mills Company, Lederle, the Massachusetts Department of Public Health, Mission Pharmacal, the National Dairy Council, Rhone Poulenc Rorer, the Robert Wood Johnson Foundation, Roche, Sandoz, the US Department of Defense, the US Department of Agriculture, the Wallace Genetics Fund, Wyeth-Ayerst, Merck, and private contributors. 3 Address reprint requests to S Liu, Division of Preventive Medicine, Brigham and Women's Hospital, 900 Commonwealth Avenue East, Boston, MA 02215. E-mail: siminliu{at}hsph.harvard.edu.
| ABSTRACT |
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Objective: We examined the associations between the intakes of dietary fiber and whole- or refined-grain products and weight gain over time.
Design: In a prospective cohort study, 74 091 US female nurses,
aged 38-63 y in 1984 and free of known cardiovascular disease,
cancer, and diabetes at baseline, were followed from 1984 to 1996;
their dietary habits were assessed in 1984, 1986, 1990, and 1994
with validated food-frequency questionnaires. Using multiple
models to adjust for covariates, we calculated average weight,
body mass index (BMI; in kg/m2), long-term weight changes, and
the odds ratio of developing obesity (BMI
30) according to
change in dietary intake.
Results: Women who consumed more whole grains consistently weighed less than did women who consumed less whole grains (P for trend < 0.0001). Over 12 y, those with the greatest increase in intake of dietary fiber gained an average of 1.52 kg less than did those with the smallest increase in intake of dietary fiber (P for trend < 0.0001) independent of body weight at baseline, age, and changes in covariate status. Women in the highest quintile of dietary fiber intake had a 49% lower risk of major weight gain than did women in the highest quintile (OR = 0.51; 95% CI: 0.39, 0.67; P < 0.0001 for trend).
Conclusion: Weight gain was inversely associated with the intake of high-fiber, whole-grain foods but positively related to the intake of refined-grain foods, which indicated the importance of distinguishing whole-grain products from refined-grain products to aid in weight control.
Key Words: Whole grains refined grains dietary fiber body weight prospective study obesity weight gain women
| INTRODUCTION |
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44% of women and
29% of men report that they are attempting to lose weight, and
5 million of these adults have used prescription drugs for
treatment of obesity (3, 4). Despite the public's increased
awareness of the health hazards associated with overweight and
obesity, recent reports from the Centers for Disease Control
and Prevention indicate that the prevalence of overweight and
obesity in the United States continues to increase (5, 6). At both individual and population levels, strategies that improve nutrition and increase physical activity are fundamental to the control of the epidemic of overweight and obesity (2, 7). Yet the long-term efficacy of any specific dietary approach to weight control remains to be determined (8-10). Because of the belief that diets rich in fiber are generally low in saturated fat, many national authorities have long recommended greater consumption of grain products to control weight (11-14). Whole grains may have beneficial effects on weight control through promoting satiety (15-17). The intake of whole grains may also slow starch digestion or absorption, which leads to relatively lower insulin and glucose responses that favor the oxidation and lipolysis of fat rather than its storage (15-17). However, most grain products consumed in the United States are highly refined (17, 18). Refined-grain products have a higher starch content but a lower fiber content (ie, greater energy density) than do whole grains. Concentrations of vitamins, minerals, essential fatty acids, and phytochemicals that are important in carbohydrate metabolism are also lower in refined grains (19).
Indirect evidence from both epidemiologic and short-term experimental studies suggests a beneficial role of a high-fiber diet in weight control (19-21). However, few trials have examined directly the effects of whole grains, as opposed to those of refined grains, on body weight and weight changes. Nor is there any large prospective epidemiologic study directly linking the intake of whole grains to changes in weight over time. To examine further the associations between grain intake and body weight and weight changes, we analyzed prospective data from the Nurses' Health Study (NHS) from 1984 to 1996 using repeated measurements of grain intake and body weight.
| SUBJECTS AND METHODS |
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Assessment of whole- and refined-grain consumption
Measurements of whole-grain and refined-grain foods, along
with other aspects of diet, were repeated in 1986, 1990, and
1994 using FFQs nearly identical to the questionnaire used in
1984. For each food, a commonly used unit or portion or
serving size (eg, 1 slice of bread) was specified, and participants were asked how often, on average, during the previous
year they had consumed that amount. Nine responses were
possible, ranging from "never" to "
6 times/d." On the basis
of the responses to these questions, we calculated the average
grain intake (servings/d) for each participant. The type and
brand of breakfast cereal also were ascertained. We used a
procedure developed by Jacobs et al (23) and Liu et al (24) to
classify foods into whole and refined grains. This classification
scheme was previously used in this cohort for studying the
relations of grain intake to the risk of type 2 diabetes (25) and
CVD (24, 26). Specifically, whole-grain foods included dark
bread, whole-grain breakfast cereal, popcorn, cooked oatmeal,
wheat germ, brown rice, bran, and other grains (eg, bulgur,
kasha, couscous). Refined-grain foods included sweet rolls and
cakes or desserts, white bread, pasta, English muffins, muffins
or biscuits, refined-grain breakfast cereal, white rice, pancakes
or waffles, and pizza. The list of breakfast cereals reported in
the semiquantitative FFQ was evaluated for whole-grain and
bran content; breakfast cereals with
25% whole grain or bran
content by weight were classified as whole grain. A full description of the semiquantitative FFQs and data on reproducibility and validity in this cohort were published previously
(22, 27). The performance of the semiquantitative FFQ in
assessing the individual grain products was documented to be
high (28). For example, between the FFQ and detailed diet
records in a sample of the participants, correlation coefficients
were 0.75 for cold breakfast cereal, 0.71 for white bread, and
0.77 for dark bread. Overall, these data indicate that the semiquantitative FFQ provides a reasonably valid measure of average long-term dietary intake.
Measurements of body weight, body mass index, and
changes in weight
Participants self-reported their body weight every 2 y from
1984 to 1996. Height was reported in 1976. In this cohort, body
mass index (BMI; in kg/m2) is minimally correlated with
height (r = -0.03) and highly correlated with height-adjusted
fat mass measured by underwater weighing of young and
middle-aged women (r = 0.8-0.9) (29). We calculated weight
changes from 2-4-y intervals between 1984 and 1996 as well
as from 1984 to 1996. In a sample of 184 participants described
previously (30), the correlation between self-reported and directly measured weight was 0.96.
Statistical analysis
We first conducted a cross-sectional analysis of the associations between baseline covariates and the intakes of whole grains
and refined grains in 1984. Unless otherwise indicated, intake of
whole or refined grains was categorized into quintiles, and we
calculated mean body weight and BMI according to these quintiles. We then plotted the mean attained BMI according to the
intake of whole or refined grains at each 2-y interval from 1984
to 1996. Second, we used generalized estimating equations to
examine the direct relation between changes in intake of whole
or refined grains in relation to changes in weight in the same
period with adjustment for changes in covariates. Changes in
weight were defined as the difference between weight assessed
in each dietary period (1984-1986, 1986-1990, and 1990-1994). Covariates, including age; years of follow-up; change in
exercise (ie, differences in metabolic score); change in smoking status (ie, increase, decrease, no change); change in hormone replacement status status (ie, from current use to no use,
same use, from no use to current use); and changes in intakes
of alcohol, caffeine, and total energy were defined by taking
the differences between 1984 and 1986, between 1986 and
1990, and between 1990 and 1994. Because exercise status was
not assessed in 1984, we carried forward information from
1982. Tests of linear trend across increasing categories of
changes in grain consumption were conducted by treating the
median of intake in categories (servings/d) as a continuous
variable. The model has the following form:
![]() | (1) |
In some analyses, we adjusted for baseline covariate status
rather than for changes in covariate status, and the results were
similar to those with the adjustment for covariate status. To
examine whether the relations between grain intake and BMI
differ by BMI at baseline, we conducted stratified analyses
using the BMI in 1984 (BMI
25 compared with BMI < 25).
Finally, to understand further the relation of grain intake to
the risk of obesity and major weight gain, we categorized
participants into 2 categories of BMI (BMI
30 and BMI
< 30) or 2 categories of weight gain (changes in weight
25
kg and changes in weight < 25 kg) and estimated the odds
ratios for each quintile of changes in grain intake (from the
smallest increase in intake to the largest increase in intake). All
analyses were conducted with the use of SAS software (version
8; SAS Institute Inc, Cary, NC). All P values were two-sided.
| RESULTS |
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0.9 kg less than did women in the
lowest quintile of intake (BMI of 24.5 compared with BMI of
24.9; P for trend < 0.0001) (Figure 1
1.2 kg
more than did women in the lowest quintile of intake (BMI of
25.2 compared with BMI of 24.6; P for trend < 0.0001)
(Figure 1B
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3.5 kg (8 lb) less
weight gain in 12 y. Overall, women who consumed a larger
amount of whole grains consistently weighed less in each
interval than did those who consumed a smaller amount of
whole grains; in contrast, women who consumed a larger
amount of refined grains consistently weighed more than did
those who consumed a smaller amount of refined grains (P <
0.0001).
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30) and for major weight
gain (
25 kg) in 12 y according to changes in intake in the
same period. During the 12-y follow-up, 6400 women became
obese and 657 had a major weight gain (Table 4
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| DISCUSSION |
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These findings should be interpreted in the context of the study's strengths and limitations. The NHS data are unique in that multiple measurements of diet and body weight are available, which makes it possible to observe the temporal relation between the intake of whole or refined grains and changes in weight. By using each woman as her own control and directly defining changes in grain intake through multiple assessments of diet, we can estimate the amount of weight change associated with changes in the intake of whole or refined grains.
The observational nature of our study design, however, still hampers straightforward interpretation. First, because both intake of whole grain and body weight are time-dependent variables, we cannot determine with complete certainty that changes in grain intake preceded changes in weight. For example, those who had recently gained weight might, in an attempt to lose weight, increase their intake of grain products. Thus, it is not clear whether increased intake of whole grains prevented weight gain, or whether it is just a marker for attempted weight loss among people with recent weight gain. In addition, the general health consciousness that might be associated with both increased whole-grain intake and caloric restriction remains a potential threat to the validity of our findings. To address this issue, we defined changes in weight as well as changes in grain intake by taking the difference between 2 measurements from 1984 to 1996 and with adjustment for previously attained weight in the model so that any systematic biases due to attained weight could be minimized. However, calculating the differences between 2 dietary assessments would increase measurement error by incorporating errors of both assessments, which suggests that our estimates of the potential effects of dietary fiber or whole grains on weight changes might be conservative.
Further complicating the picture is the possibility not only that BMI can change in response to diet but also that attained BMI may lead both to real changes in diet and to changes in self-reporting of diet (ie, a misclassification of dietary intake that is dependent on body weight). For example, it has been reported that those who are obese are more likely to underreport their energy intake, which makes the study of diet and obesity in an observational setting difficult (33-35). However, assessment of whole- grain foods with the questionnaires used in this study had relatively high validity (r > 0.70 compared with dietary record for most whole-grain foods). In multivariate analyses, we adjusted for many potential predictors of underreporting, including age, smoking, physical inactivity, and other known predictors of weight gain, that might minimize this bias. Moreover, results from these multivariate analyses showed that weight gain over time was inversely associated with increases in whole-grain intake but positively related to increases in refined-grain intake. Other dietary factors may have accounted for the observed relations between grain intake and body weight. However, the association between the intake of grain products and weight changes remains almost identical when we conducted a series of sensitivity analyses in which the intakes of red meats, fruit and vegetables, and dairy products were simultaneously included in the same model along with whole grains or refined grains (data not shown). The similar magnitudes of associations compared with the age-adjusted analyses and the specific findings regarding the different effects of whole grain versus refined grain intake on weight gain argue against confounding as a full explanation for our findings.
Diet may affect body weight through multiple pathways,
including control of satiety and metabolic efficiency, or
through modulation of insulin secretion and action. Total and
saturated fats have been the focus of intense scientific scrutiny
as potential causes for overweight and obesity, but as yet there
is no conclusive evidence directly linking dietary fats to body
fat. Whereas there is no doubt that overfeeding animals with a
large percentage of energy from fat can cause obesity and
insulin resistance (36, 37), the magnitude and long-term significance of the effect of a low-fat diet on weight control in
humans remain uncertain (9, 38). In contrast, energy-restricted
low-fat diets have been shown to achieve
10% reduction in
weight in many feeding trials (39, 40). But much of the weight
thus lost is regained within 12 mo, and no long-term efficacy
has ever been convincingly shown (41). Moreover, the effects
of changes in dietary composition (percentage of energy from
fat rather than from carbohydrates) alone on weight loss appeared minimal (41).
Reports based on NHANES III and a recent time-trend analysis indicate that intakes of both total and saturated fat, in terms of the percentage of total energy intake, have been declining in the United States since 1960s (42, 43). In contrast, in the same period, an increase in the intake of refined carbohydrates in the form of processed grains, soft drinks, sugars, and refined flours in the US food supply has been reported to parallel the increased prevalence of obesity and diabetes (44). Consistent with these reports, our data indicate that the intake of refined grains is directly associated with dietary glycemic load and index, which suggests that an overemphasis on a low-fat diet might have contributed to an increase in intake of high-energy and high-glycemic-load foods. Long-term intervention trials may be necessary if we are to better estimate the relation between changes in the intake of refined grains and weight control, but the feasibility of such long-term trials remain unclear.
Experimental data indicate that refined-grain products, unlike whole-grain products, can induce an increase in fat synthesis in animal feeding trials even when the total energy intake
is unchanged and body weight remains constant (45). Results
from several short-term (a single meal or a single day) feeding
trials in humans suggest that the consumption of whole-grain
products with a low glycemic index might increase satiety and
reduce energy consumption and thus contribute to weight loss
(20). Our data are consistent with results from these animal and
human experiments. In addition, attained body weight may
function as an important modifier for the effects of whole
grains, as indicated by the observation that the relation between
dietary fiber or whole-grain food intake and weight change also
appeared to differ by BMI status at baseline (Figure 2
). The
beneficial effects of increased intake of dietary fiber on weight
gain were most significant among subjects who were overweight at baseline. Several epidemiologic studies of dietary
fiber also suggest that the intake of whole grains but not of
refined grains is inversely associated with body weight and fat
distribution (21, 46). The inherent high-fiber content of most
whole-grain foods may help prevent weight gain by increasing
appetite control through producing a delay in carbohydrate
absorption (47). Moreover, multiple enzyme inhibitors that
exist in a whole-grains-fiber complex might directly affect
metabolic efficiency (19). This mechanism might serve as yet
another way whereby whole grains could have beneficial effects on body weight. Finally, high concentrations of insulin
associated with low-fiber refined grains may over the long term
lead to weight gain by directing metabolic fuels from oxidation
to storage.
In conclusion, in this prospective study of apparently healthy middle-aged women, weight gain was inversely associated with increases in the intake of whole grains but positively associated with increases in the intake of refined grains. This suggests the potential importance of increasing the intake of dietary fiber as well as of distinguishing whole-grain from refined-grain products to control weight.
| ACKNOWLEDGMENTS |
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