AWM: Low Glycemic Diets (2006)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To evaluate the efficacy of an ad libitum low-glycemic load diet, without strict limitation on carbohydrate intake, as an alternative to a conventional low-fat diet.
Inclusion Criteria:

Age between 18 - 35 years, BMI > 27, body weight < 136 kg (300 lb).

Exclusion Criteria:
No major medical illness as assessed by physical exam and laboratory screening tests (i.e. kidney and liver enzymes, thyrotropin, glycosylated hemoglobin, fasting plasma glucose, and urinalysis).
Description of Study Protocol:

Recruitment

Not described.

Design

Randomized Controlled Trial.

Blinding used (if applicable)

Not used.

Intervention (if applicable)

2 dietary treatments compared over 12 months.

Statistical Analysis

Dietary data and study outcomes were analyzed by repeated measures ANOVA.  Each variable was tested for change over time (0, 6 and 12 months) and for a difference in time course between the 2 groups by assessing the main effect of time and the group x time interaction, respectively.  To avoid increased risk of type I inferential error from multiple comparisons, we limited statistical testing of time trends to the overall change, with the exception of 2 planned comparisons for process measures.  We accounted for within-subject correlation by using a banded covariance structure, which allowed a lower correlation between the 0 and 12 month observations than between the 0 and 6 month or 6 and 12 month observations.  Primary analysis included data from 23 completers, secondary analyses included all info from 34 original subjects.  Study outcomes were log transformed for analysis, and results are expressed as percentage change.  Dietary data were analyzed without transformation.

Data Collection Summary:

Timing of Measurements

Subjects randomly assigned to experimental or conventional diet groups between August 2001 and July 2002.  The study comprised a 6-month intensive intervention with 12 dietary counseling sessions, and a 6-month follow-up with 2 dietary counseling sessions.  The duration of each counseling session was 1 hour.  Study outcomes were measured at 0, 6 and 12 months.

Dependent Variables

  • Weight assessed using an electronic scale
  • Height assessed with wall-mounted stadiometer
  • Body composition measured with DEXA
  • Blood pressure measured by using automated system while subject sat quietly
  • Blood sample drawn after overnight fast for plasma lipid concentrations.  Total cholesterol, HDL cholesterol, and triacylglycerols measured using Hitachi 911 analyzer and LDL cholesterol measured using homogenous enzymatic assay
  • Plasma concentrations of plasminogen activator inhibitor 1 (PAI-1) were measured by using ELISA
  • Plasma glucose and serum insulin concentrations were measured by Hitachi 917 analyzer and Elecsys 2010 system

Independent Variables

  • 2 dietary treatments.  Experimental treatment was not energy restricted and emphasized ad libitum consumption of low-glycemic index foods with 45 - 50% of energy from carbohydrates and 30 - 35% fat.  The conventional treatment was restricted in energy based on Harris-Benedict estimated energy requirements (250 - 500 kcal/day deficit) and fat (<30% energy), with 55 - 60% of energy from carbohydrate.  Both groups received the same behavioral therapy and physical activity recommendations.  Intervention process was evaluated on basis of attendance at dietary counseling sessions and adherence to diet prescriptions.  All subjects kept food diaries and 7-day food diaries were obtained at baseline, 3 and 6 months, and at 12 months.

Control Variables

 

Description of Actual Data Sample:

Initial N: 34 enrolled (30 females, 4 males)

Attrition (final N):  23 completed (22 females, 1 male)- 67.6%.  Of conventional diet group, 5 were lost to follow-up:  1 had unplanned pregnancy, 1 withdrew, 3 discontinued visits.  Of experimental diet group, 6 were lost to follow-up:  1 developed an illness, 1 began taking antidepressants, 1 withdrew and 3 discontinued visits.

Age:  Experimental diet (n=11):  29.8 +/- 1.7 years, Conventional diet (n=12): 27.2 +/- 1.3 years 

Ethnicity:  Not mentioned. 

Other relevant demographics:

Anthropometrics:  There were no significant differences in baseline measures between diet groups.

Location: Children's Hospital, Boston

 

Summary of Results:

 

Variable Experimental Conventional P - Group P - Time

P - Group x Time

Energy-baseline 1860 +/- 72 kcal

1802 +/- 116 kcal

0.84 < 0.001 0.76
Energy - Interim 1391 +/- 79 kcal 1409 +/- 46 kcal

 

 

 

Energy - 12 months 1494 +/- 82 kcal 1472 +/- 85 kcal
Weight - interim -8.4 (-11.4, -5.3) -7.8 (-10.7, -4.9) 0.18 <0.001 0.89
Weight - 12 months -7.8 (-13.0, -2.2) -6.1 (-11.2, -0.7)

Other Findings

Glycemic load decreased significantly in experimental diet group (0 - 6 months, P < 0.001; 0 - 12 months, P < 0.001).  Dietary fat decreased significantly in the conventional diet group (0 - 6 months, P < 0.001, 0 - 12 months, P = 0.004).

Body weight decreased significantly over a 6-month intensive intervention in both the experimental and conventional diet groups (-8.4% and -7.8%, respectively) and remained below baseline at 12 months (-7.8% and -6.1%, respectively).  Mean weight loss did not differ significantly between the groups. 

The experimental diet group showed a significantly greater mean decline in plasma triacylglycerols than did the conventional diet group (-37.2% and -19.1%, respectively, P = 0.005).

Mean plasminogen activator inhibitor 1 concentrations decreased (-39.0%) in the experimental diet group but increased 33.1% in the conventional diet group (P = 0.004).

Changes in cholesterol concentrations, blood pressure, and insulin sensitivity did not differ significantly between the groups.

Author Conclusion:
In conclusion, a low-glycemic load diet containing moderate amounts of carbohydrate from low-glycemic-index sources may be more efficacious than a conventional low-fat diet in reducing cardiovascular disease risk.  The greater benefits in response to an ad libitum diet, compared with an energy-restricted diet, are particularly noteworthy.  This pilot study provides a rationale for conducting long-term, larger scale studies comparing the effects of low-glycemic load, low-fat and very-low-carbohydrate diets on CVD risk among obese persons.
Funding Source:
Government: NIDDK, NIH
Not-for-profit
0
Foundation associated with industry:
Reviewer Comments:
Large dropout rate - 32.4%.  Predominantly female sample.  Statistical analysis unclear - whether reporting on original 34 subjects or 23 completers since both analyses were done.
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? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? ???
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
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? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  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? 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? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? No
  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.) No
  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? Yes
  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? ???
  8.1. Were statistical analyses adequately described and the results reported appropriately? ???
  8.2. Were correct statistical tests used and assumptions of test not violated? ???
  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)? ???
  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