Adult Weight Management

AWM: Low Carbohydrate Diet (2006)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To review the 1-year outcomes between a group of severely obese adults assigned to a low-carbohydrate diet or a low-fat, low-calorie diet.
Inclusion Criteria:
18 years of age or older, BMI of 35 or greater.
Exclusion Criteria:
Serum creatinine > 1.5 mg/dl, hepatic disease, severe life-limiting medical illness, inability to self-monitor glucose levels, or active use of a weight loss program or weight loss medication. 
Description of Study Protocol:


Recruited from Philadelphia VAMC.


Randomized Controlled Trial.  Randomization performed using preestablished algorithm generated from random numbers.

Blinding used (if applicable)

Not used.

Intervention (if applicable)

Assigned to either low-carbohydrate diet or low-fat, low-calorie diet

Statistical Analysis

Primary end point was total weight loss at 1 year.  Secondary analyses included change from baseline in serum lipid levels, insulin sensitivity, and glycemic control.  100 persons (50 per group) would be needed assuming a 2-sided type I error of 5% for the study to have 80% power to detect a 5 kg difference.  Changes in weight, dietary intake, and metabolic data were compared by random-coefficient analysis.  For all analyses, covariates included an indicator variable for time, diet group, and a diet group by time interaction term.  Triglyceride, insulin and glucose levels were skewed and log-tranformed.  Baseline differences between diet groups were compared by chi-square analyses for dichotomous variables and by the unpaired t test for continuous variables.

Data Collection Summary:

Timing of Measurements

Weight, self-reported medical history, and blood pressure collected at baseline, 6 months and 1 year.

Dependent Variables

  • Weight measured through single calibrated scale
  • Fasting blood specimens obtained for glucose, hemoglobin A1c, serum lipid levels
  • LDL cholesterol calculated using Friedewald equation
  • Presence of diabetes or metabolic syndrome
  • Serum insulin through radioimmunoassay
  • Insulin resistance in nondiabetic persons estimated by the quantitative insulin sensitivity check

Independent Variables

  • Low-carbohydrate diet (<30 g carbohydrate/day) or low-fat, low-calorie diet (-500 kcal/day, 30% kcals from fat).  Diet groups met in weekly counseling sessions for 4 weeks, followed by 11 monthly sessions.

Control Variables


Description of Actual Data Sample:

Initial N: 132 subjects, 64 in low-carbohydrate, 68 in low-fat

Attrition (final N):  79 subjects at 6 months, 87 subjects at 1 year (66% completion), 44 in low-carbohydrate, 18% female, 43 in low-fat, 19% female.  45 participants had missing data for diet recall and metabolic measurements.

Age:  Low-carbohydrate diet:  mean age 55 +/- 9 years, low-fat diet:  55 +/- 9 years 

Ethnicity:  Low-carbohydrate diet:  48% White, 48% African American, 5% Hispanic.  Low-fat diet:  28% White, 70% African American, 2% Hispanic. 

Other relevant demographics:

Anthropometrics:  Participants well matched at baseline.  Dropouts were not significantly different.

Location:  Philadelphia


Summary of Results:


Baseline 1 Year (n=87) Change (n=87) Mean Difference (95% CI) P-value
Calories - low-fat 1919 +/- 940 1822 +/- 1008 -97 +/- 1067 -354 (-881 to 172) 0.183
Calories - low-carb 1972 +/- 1046 1462 +/- 776 -510 +/- 1187



Protein - low-fat 67 +/- 34 g 74 +/- 50 g 7 +/- 56 g -15 (-39 to 9) >0.2
Protein - low-carb 84 +/- 48 g 73 +/- 34 g -11 +/- 56 g
Carbs - low-fat 252 +/- 139 g 230 +/- 150 g -22 +/- 157 g -98 (-179 to -40) 0.011
Carbs - low-carb 251 +/- 155 g 120 +/- 93 g -131 +/- 168 g
Fat - low-fat 74 +/- 54 g 69 +/- 48 g -6 +/- 55 g 28 (-14 to 70) 0.194
Fat - low-carb 72 +/- 50 g 93 +/- 117 g

22 +/- 127 g



Other Findings

Caloric intake decreased more by 1 year in the low-carbohydrate group compared to the low-fat diet group, but the differences were not statistically significant.

By 1 year, mean weight change for persons on the low-carbohydrate diet was -5.1 +/- 8.7 kg compared with -3.1 +/- 8.4 kg for persons on the conventional diet.  Differences between groups were not significant (-1.9 kg [95% CI: -4.9 to 1.0 kg], P = 0.20).

For persons on the low-carbohydrate diet, triglyceride levels decreased more (P = 0.044) and HDL cholesterol levels decreased less (P = 0.025).

As seen in the small group of persons with diabetes (n=54) and after adjustment for covariates, hemoglobin A1c levels improved more for persons on the low-carbohydrate diet.

These more favorable metabolic responses to a low-carbohydrate diet remained significant after adjustment for weight loss differences.

Changes in other lipids or insulin sensitivity did not differ between groups.

Author Conclusion:
In summary, we found similar weight loss in persons randomly assigned to a low-carbohydrate diet or a conventional diet at 1 year.  Despite modest overall weight loss in both diet groups, assignment to the low-carbohydrate group had a direct and more favorable effect on triglyceride level, HDL cholesterol level and glycemic control in the smaller subgroup with diabetes.  These findings give further evidence that restriction of carbohydrates in obese persons, who may be overconsuming carbohydrates at baseline, may have favorable metabolic effects.  Caution is still needed, however, in recommending a low-carbohydrate diet, as important concerns remain.  Most important, future studies will need to evaluate whether a low-carbohydrate diet has more favorable effects on the development of diabetes and on cardiovascular outcomes.
Funding Source:
Government: Veteran Affairs Healthcare Network
Reviewer Comments:
High dropout rate of 40% with 6-month group and 34% at 1 year.  45 participants had missing data for diet recall and metabolic measurements.
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? Yes
  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? Yes
  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? 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