DF: Diabetes (2008)

Study Design:
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Quality Rating:
Research Purpose:

To determine the effects on glycemic control and plasma lipid concentrations of increasing the daily intake of dietary fiber in 13 subjects with type 2 diabetes from 24 g to 50 g, using unfortified foods.

To determine the effects of the fiber intervention on the intestinal absorption of cholesterol and the fecal excretion of sterols.

Inclusion Criteria:
Type 2 diabetes
Exclusion Criteria:
None given
Description of Study Protocol:


Not provided 


Comparison of glycemic control and plasma lipid concentrations in 13 subjects with type 2 diabetes after six weeks of a controlled nutrient diet with 24 g dietary fiber (8 g soluble, 16 g insoluble) vs six weeks of the same diet but with 50 g dietary fiber (25 g soluble, 25 g insoluble).

Blinding used (if applicable)


Intervention (if applicable)

Subjects were randomized to either a moderate fiber diet (24 g total dietary fiber including 8 g soluble and 16 g insoluble) or a high fiber diet (50 g total dietary fiber including 25 grams of soluble and 25 g insoluble) for six weeks, and then were crossed over to the other diet (with a washout period of 7 days). No fiber supplements were used. The two diets had the same macronutrient (55% carb, 15% protein, 30% fat), energy content (~2300 kcal), as well as saturated, cis-monounsaturated, and polyunsaturated fats and cholesterol.  All foods were prepared in a metabolic unit research kitchen.  Subjects were admitted to the General Clinical Research Center (GCRC) for 5 days (baseline) and for the last week of each dietary period; otherwise subjects ate at least one weekday meal at the GCRC and had other meals supplied by the GCRC kitchen in packages for home consumption.  Subjects were asked to maintain a constant level of physical activity throughout the study.  Fecal markers (capsules containing sitostanol, cholesterol and sitostanol) were taken on days 36 and 42.

Statistical Analysis

  • Repeated-measures analysis of variance to compare the two study periods and to assess the effect of the sequence in which the subjects received the diets.
  • Wilcoxon signed-rank test to compare the two dietary periods for skewed data.
Data Collection Summary:

Timing of Measurements

Initial hospitalization (baseline); the 6th week of each diet

Dependent Variables

  • Cholesterol (enzymatic method, kit from Boehringer Mannheim)
  • Triglycerides (enzymatic method, kit from Boehringer Mannheim)
  • VLDL (removed by ultracentrifugation and measured)
  • HDL (enzymatic method after lipoproteins containing apolipoprotein ß precipitated)
  • LDL (estimated by difference between total cholesterol and HDL)
  • Plasma glucose (glucose oxidase method)
  • Urinary glucose (glucose oxidase method)
  • Glycosylated hemoglobin (ion-exchange high-performance liquid chromatography)
  • Plasma insulin (radioimmunoassay)
  • Neutral and acidic fecal sterols (gas chromatography)
  • Fecal cholesterol (gas-liquid chromatography and mass spectrometry)

Independent Variables

  • Total and soluble and insoluble dietary fiber (estimated according to data provided in Schakel S et al. Dietary fiber values for common foods.  In Spiller GA, ed> CRC handbook of dietary fiber in human nutrition.  2nd ed.  Boca Raton, FL: CRC Press 1993:567-93)

Control Variables

  • Physical activity
Description of Actual Data Sample:

Initial N: 12 males, 1 female

Attrition (final N): none reported, 13

Age: mean of 61±9 years (range, 45-70)

Ethnicity: 9 non-Hispanic whites, 4 blacks

Other relevant demographics:

  • 3 subjects treated by diet alone; other 10 treated with 2.5-20 mg glyburide (dose not changed during study.
  • no subjects taking lipid-lowering therapy


  • mean body weight at baseline: 93.5±12.7 kg
  • mean BMI at baseline: 32.3±3.9


General Clinical Research Center, University of Texas Southwestern Medical Center, Dallas

Summary of Results:



Control Fiber Diet (25 g) during last week of study period


High-Fiber Diet (50 g) during last week of study period


P-Value (between groups, last week of study)

Weight (kg)

90.7±13.3 90.5±12.7 p=0.60

plasma glucose (mg/dl) (mean daily preprandial




Urinary glucose (g/d), median (avg of 5 daily urine collections during hospitalization) 



p= 0.008

glycosylated hemoglobin (%) 7.2±1.3 6.9±1.2 p=0.09
Plasma total cholesterol (mg/dl) 210±33 196±31 p=0.02
Plasma triglycerides (mg/dl) 205±95 184±76 p=0.02
Plasma VLDL cholesterol (mg/dl) 40±19 35±16 p=0.01
Plasma LDL cholesterol (mg/dl) 142±29 133±29 p=0.11
Plasma HDL cholesterol (mg/dl) 29±7 28±4 p=0.80


Other Findings

  • The high fiber diet lowered the area under the curve for 24-hr plasma glucose and insulins (measured every 2 hrs) by 10% (p=0.02) and 12% (p=0.05) respectively, compared with the lower fiber diet.
  • The high fiber diet
    • decreased gastrointestinal absorption of cholesterol by 10% compared to the lower fiber diet (p= 0.03)
    •  increased fecal acidic sterol excretion by 41% compared to the lower fiber diet (p=0.005)
    • did not significantly affect the excretion of neutral sterols.
Author Conclusion:
A high intake of dietary fiber, particularly of the soluble type, by patients with type 2 diabetes improves glycemic control, decreases hyperinsulinemia, and lowers plasma lipid concentrations.
Funding Source:
Reviewer Comments:
  • The GCRC diets appear to be well planned, and outpatient compliance was monitored by the dietitian; outcome measurements were done during the week of hospitalization where food consumption, physical activity and other variables could be controlled.  In addition, the two diets were the same for the macronutrients % of energy.
  • This report lacks a description of some of the essential components of research design: description of recruitment; inclusion and exclusion criteria; description of randomization. In addition, authors provided no discussion of limitations of the study.
  • Fiber values are calculated; no aliquot of a day's meal was analyzed chemically to verify the figures.
  • The study had a small number of subjects, and no power calculations were provided; however with some significant results this appears not to be a problem.
  • The analysis was comparing the 6 week data in each case, not baseline to end, so the weight loss shouldn't be a problem.
  • While subjects in both groups lost 2 kg from baseline
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? ???
  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? No
  2.2. Were criteria applied equally to all study groups? N/A
  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? ???
3. Were study groups comparable? N/A
  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? N/A
  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")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? N/A
  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%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  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? N/A
  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? Yes
  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? 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? N/A
  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)? No
  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? ???
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
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