DLM: Fiber (2001)

Citation:

Ludwig DS, Pereira MA, Kroenke CH, Hilner JE, Van Horn L, Slattery ML, Jacobs DR Jr. Dietary fiber, weight gain, and cardiovascular disease risk factors in young adults. JAMA 1999;99:1539-1546.

Worksheet created prior to Spring 2004 using earlier ADA research analysis template.
PubMed ID: 10546693
 
Study Design:
Cohort Study
Class:
B - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To test the hypothesis that fiber consumption is independently and inversely associated with insulin levels, weight gain, and other CVD risk factors among adults, and to compare fiber with fat and other major dietary components.

Inclusion Criteria:
  • 18-30 years of age
  • healthy adults
Exclusion Criteria:
  • Extreme values of dietary intake for men (<800 and >8000 kcal/day) and women (<600 and >6000 kcal/day)
  • pregnant/lactating women at baseline exam or within 180 days of year 10 exam
  • Diabetes: medications or a fasting blood glucose concentration of >140 mg/dl at baseline or year 10. 3
  • On meds for HTN or lipid control
  • Individuals who had not fasted for at least 8 hours prior to clinic visit.
Description of Study Protocol:

Recruitment:  recruitment not specified, but stratification was used to obtain nearly equal numbers of blacks and whites, younger and older (up to age 30) adults and those with more and less education.

Design:  Coronary Artery Risk Development in Young Adults (CARDIA) Multicenter (Birmingham AL, Chicago, Minneapolis, Oakland) population based cohort study over 10 years 1985-86 to 1995-96 in healthy black & white adults, 18-30 years of age.

Blinding (if applicable):  not applicable

Intervention (if applicable): not applicable

Statistical Analysis: Race-specific general linear models were computed to adjust least squares means of CVD risk factors according to quintiles of dietary factors. Quintile cutpoints for dietary factors were based on distributions of the entire cohort. To express insulin and triglyceride concentrations according to their natural scale, geometric means were computed by exponentiating the adjusted least squares means.

Data Collection Summary:

Timing of Measurements

  • Clinic visits and blood work at 0, 7 & 10 years
  • CARDIA interviewer administered FFQ including 700 foods at years 0 and 7.
  • Ht & wt measurements at years 0, 7 & 10.

Dependent Variables:

  • body weight measured with calibrated balance beam scale 
  • waist-to-hip ratio
  • systolic and diastolic blood pressure obtained with standardized random-zero sphygmomanometer 
  • insulin levels measured through radioimmunoassay
  • blood lipids estimated using enzymatic procedures
  • fibrinogen analysis measured by clot formation rate 

Independent Variables:

  • dietary fat and fiber intake through CARDIA diet history

Control Variables:

  • age
  • sex
  • CARDIA field center
  • education (high school graduate vs. <high school graduate at year 7)
  • energy intake
  • vitamin supplementation
  • cigarette smoking
  • alcohol intake
  • total physical activity

 

Description of Actual Data Sample:

Initial N:  3609 healthy black & white adults before application of exclusion criteria.

Attrition (Final N):  2909 black & white adults

Age:  18-30 years of age

Ethnicity: Black & white adults

Other relevant demographics: none

Anthropometrics: No anthropometrics given in this article for the final n.

Location: Birmingham, AL; Chicago, IL; Minneapolis, MN; Oakland, CA

Summary of Results:

Other Findings:

Dietary Fiber: After adjustment for potential confounding factors, dietary fiber showed linear associations from lowest to highest quintiles of intake with the following body weights:  Whites: 78.3-75.0 kg, P<0.001; Blacks: 83.5-79.9 kg. P<0.001

Waist to Hip Ratios (WHR): Whites: 0.813-0.801, P=0.004; Blacks: 0.809-0.799, P=0.05

Fasting insulin adjusted for BMI: Whites: 77.8-72.2 pmol/L , P=0.007 Blacks: 92.4-82.6 pmol/L , P=0.01

2-Hour postglucose insulin adjusted for BMI: Whites: 261.1-237.7 pmol/L , P=0.03 Blacks: 370.2-259.7 pmol/L, P<0.001

Fiber was also associated (inverse relationship) with blood pressure and levels of TG, HDL, LDL and fibrinogen; these associations were substantially attenuated by adjustment for fasting insulin level. Fiber consumption predicted insulin levels, weight gain, and other CVD risk factors more strongly than did total or saturated fat consumption. At all levels of fat intake, individual eating the most fiber gained less weight than those eating the least fiber. 

Author Conclusion:

Dietary fiber was inversely associated with insulin levels, weight gain, and other risk factors for CVD in young adults.  High fiber diets may protect against obesity and CVD by lowering insulin levels and preventing body weight gain.

Funding Source:
Government: NHLBI
Reviewer Comments:

Well designed study. The study did not examine the type of fiber (soluble and insoluble) or the source (whole grain, refined grain, vegetable, fruit, etc.). 

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) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  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.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? Yes
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%.) Yes
  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? Yes
5. Was blinding used to prevent introduction of bias? ???
  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.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? ???
  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? Yes
  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? N/A
  6.6. Were extra or unplanned treatments described? N/A
  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)? N/A
  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? N/A
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