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DF: Obesity (2008)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To examine the associations between the intakes of dietary fiber and whole or refined grain products and weight gain over time.

Inclusion Criteria:

Women in the Nurses Health Study completing the 1980 dietary questionnaire. 

 

Exclusion Criteria:
  • Reported daily energy not between 2,514KJ (600kcal) and 14,665KJ (3,500kcal)
  • Previously diagnosed diabetes
  • Previously diagnosed cardiovascular disease including angina, myocardial infarction, stroke and other CVDs
  • Previously diagnosed cancers.
Description of Study Protocol:

Recruitment

Recruitment to Nurses Health Study participants. 81,757 female nurses, who completed the 1984 FFQ that included 126 items, were reviewed for inclusion, based on the above criteria. After exclusion, the final baseline population was 74,091 women.

Design

Women were followed from 1984 to 1996. Nearly identical FFQs to that utilized in 1984 were completed in 1986, 1990 and 1994. For each food, a commonly used unit or portion or serving size was specified. Participants were asked how often, on average during the previous year, they had consumed that amount. Nine responses were possible, ranging from "never" to "six or more times per day." The average daily grain intake was calculated for each participant, based on their responses. The type and brand of breakfast cereal was ascertained and foods were classified as whole and refined grains.

  • Whole grain foods included dark breads, whole-grain breakfast cereal, popcorn, cooked oatmeal, wheat germ, brown rice, bran and other grains
  • 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
  • Breakfast cereals with at least 25% whole grain or bran content by weight were classified as whole grain.

Participants self-reported their body weight every two years from 1984 to 1996. Height was reported in 1976. Weight changes were calculated from two- to four-year intervals between 1984 and 1996.

Outcomes such as weight, BMI and weight change from 1984 to 1996 were analyzed according to:

  • Fiber intake in 1984
  • Change in fiber intake from 1984 to 1994.

Statistical Analysis

  • Cross-sectional analysis of association between baseline covariates and the intakes of whole grains and refined grains in 1984. Intake of whole and refined grains was categorized into quintiles.
  • Generalized estimating equations to examine the direct relationship between changes in intake of whole or refined grains with changes in weight in the same period, with adjustment for changes in covariates
  • Tests of linear trend across increasing categories of changes in grain consumption were conducted by treating the median of intake in categories (servings per day) as a continuous variable
  • Robust variance used in estimating the population; average change in body weight associated with change in grain intake
  • Calculated multivariate-adjusted mean change in BMI to understand the weight changes over time, in relation to changes in grain intake
  • Regression calibration to calculate an estimate of deattenuated effect of change in dietary fiber intake on weight change
  • Multiple logistic regression (odds ratio)
  • Significance, P≤0.05.
Data Collection Summary:
  • Timing of measurements: Data was collected in 1984, 1986, 1990 and 1994
  • Dependent variables: Weight change.

Independent Variables

  • Dietary fiber intake
  • Dietary whole grain intake
  • Dietary refined grain intake.

Control Variables

 

  • Age
  • Smoker
  • Pre-menopausal
  • Hormone replacement therapy user
  • Hypertension
  • Hypercholesterolemia
  • BMI
  • Metabolic score
  • Total calorie intake
  • Carbohydrate intake
  • Glycemic load
  • Glycemic index
  • Protein intake
  • Polyunsaturated fat intake
  • Monounsaturated fat intake
  • Saturated fat intake
  • Cholesterol intake
  • Trans fatty acid intake
  • Fruit and vegetable intake
  • Caffeine intake
  • Alcohol intake.
Description of Actual Data Sample:
  • Initial N: 74,091 female nurses
  • Attrition: Not applicable
  • Age: 50 years (range 38-63 years)
  • Ethnicity: Not described.

Anthropometrics

At baseline, the highest quintile intake of whole grain was associated with women who

  • Smoked less
  • Exercised more
  • Used post-menopausal hormones
  • Had higher intakes of carbohydrate, protein, dietary fiber and fruits and vegetables
  • Had lower intakes of fats, cholesterol and alcohol
  • Weighed 0.9kg less than women in the lowest quintile of intake (BMI of 24.5, compared with a BMI of 24.9; P<0.0001 for trend).

At baseline, the highest quintile of intake of refined grain was associated with women who weighed 1.2kg more than women in the lowest quintile of intake (BMI of 25.2, compared with BMI of 24.6; P<0.0001 for trend)

Location

National: Data collected as part of the Nurses Health Study.

Summary of Results:

Whole Grains

  • Increases in intake associated with less weight gain (mean weight gain of 1.58kg in two to four years in the lowest quintile and of 1.07kg in the highest quintile; P<0.0001 for trend)
  • Increases in intake associated with less weight gain over the 12-year follow-up (P<0.0001 for trend)
  • Women who consumed a larger amount weighed less in each interval than those who consumed a smaller amount (P<0.0001)
  • During the 10 years from 1984 to 1994, mean daily intake increased from 0.64 servings per 1,000kcal to 0.77 servings per 1,000kcal
  • BMI was inversely associated with intake but increased in time, regardless of level of intake.

Refined Grains

  • Increases in intake related to greater weight gain (mean weight gain of 0.99kg in two to four years, in the lowest quintile and of 1.65kg in the highest quintile; P<0.0001 for trend)
  • Increases in intake related to more weight gain over the 12-year follow-up (P<0.0001 for trend)
  • Women who consumed a larger amount in each interval weighed more than those who consumed a smaller amount of refined grains (P<0.0001)
  • BMI positively associated with intake and increased in time, regardless of level of intake.

Dietary Fiber

  • Increases in intake were associated with less weight gain every two to four years
  • A weight gain of 1.52kg less than those with the smallest increase in intake (P<0.0001 for trend) during the 12 years of follow-up, independent of body weight at baseline, age, changes in physical activity, smoking status, alcohol consumption, caffeine intake, hormone replacement therapy use and intakes of different types of fat, protein and total energy
  • An increase of 12g in intake associated with 3.5kg (eight lb) less weight gain in 12 years
  • Among the overweight women, those with the highest quintile of intake had about one-half (2.65kg) as much weight gain as those in the lowest quintile of intake (5.10kg) during the 12-year follow-up (P<0.0001 for trend)
  • Women who increased their intake from the lowest to highest quintile reduced their risk of major weight gain (≥25kg) by 49% (OR=0.51; 95% CI, 0.39, 0.67; P<0.0001 for trend)
  • BMI inversely associated with intake, but increased in time, regardless of the level of intake
  • Beneficial effects of increased fiber intake were greater in those that were overweight at baseline.
Author Conclusion:

Weight gain was inversely associated with the intake of high-fiber whole-grain foods, but posiively 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.

Funding Source:
Government: NIH
Reviewer Comments:

Cross-sectional and longitudinal analysis of FFQ data.

Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? No
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) Yes
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? Yes
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  6.6. Were extra or unplanned treatments described? No
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? Yes
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? No
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes