CRD summary

The review assessed low-carbohydrate versus low-fat diets. The authors concluded that low-carbohydrate non-energy-restricted diets appear at least as effective as low-fat energy-restricted diets in inducing weight loss for up to one year. Potential favourable effects on triglyceride and high-density lipoprotein cholesterol should be weighed against potential unfavourable effects on low-density lipoprotein cholesterol when considering low-carbohydrate diets. The conclusions appear reliable.

Authors' objectives

To compare the effects of low-carbohydrate diets without energy restriction versus energy restricted low-fat diets on weight loss, blood-pressure, and lipid values in randomised controlled trials (RCTs) with dietary interventions for at least 6 months.

Searching

The authors searched MEDLINE, EMBASE, Pascal, Global Health, HEALTH, Web of Science and the Cochrane Library from January 1980 to the end of February 2005. They also reviewed UptoDate (version 2005) and Clinical Evidence Concise (2004), contacted experts and checked the reference lists of identified publications.

Study selection

Assessment of study quality

Allocation concealment, blinded outcome assessment, loss to follow-up, and full descriptions of losses to follow-up and withdrawals were assessed. Two reviewers independently assessed the quality of the studies.

Data extraction

The data were extracted in duplicate. The reviewers extracted the mean difference in weight loss between the two groups from baseline to 6 or 12 months as the main outcome, and further mean differences in percentage change of body weight, systolic and diastolic blood-pressure, blood lipid levels, fasting glucose level, fasting insulin level and quality of life.

Methods of synthesis

Results of the review

Five trials (n=447) met the inclusion criteria.

All trials used concealed treatment allocation, patient blinding was not possible, and an independent blinded outcome assessment was not used.

The funnel plot did not indicate publication bias.

All arms of the 5 trials showed weight loss after the diet: this ranged from -12 to -3.2 kg in the low-carbohydrate group and from -1.9 to 6.5 kg in the low-fat intake group after 6 months. After 12 months, the low-carbohydrate groups showed weight losses between -7.2 and -2.1 kg. Weight losses in the low-fat group ranged from -4.4 to -3.1 kg in the 3 studies that provided these data.

The participants on low-carbohydrate diets were more likely to complete the trial (statistically significant for 6 months' follow-up).

After 6 months, individuals on low-carbohydrate diets had lost significantly more weight than individuals randomised to low-fat diets (WMD -3.3 kg, 95% CI: -5.3, -1.4; 5 trials). The results showed statistical heterogeneity (P=0.02), which was traced to one trial allowing 10% energy intake from fat compared with 30% in the other trials. After 12 months, the difference was reduced to a mean of -1.0 kg (95% CI: -3.5, 1.5; not significant; based on 3 trials).

No differences in blood-pressure were found. A low-carbohydrate diet had favourable effect on triglyceride levels (WMD -22.1 mg/dL, 95% CI: -38.1, -5.3; 4 trials) and high-density lipoprotein cholesterol values (WMD 4.6 mg/dL, 95% CI: 1.5, 8.1; 4 trials). A low-fat diet showed favourable results on total cholesterol (WMD 8.9 mg/dL, 95% CI: 3.1, 14.3; 4 trials) and low-density lipoprotein cholesterol values (WMD 5.4 mg/dL, 95% CI 1.2, 10.1; 4 trials) .

Authors' conclusions

Low-carbohydrate, non-energy restricted diets appear at least as effective as low-fat, energy-restricted diets in inducing weight loss for up to one year. Potential favourable changes in triglyceride and high-density lipoprotein cholesterol values should be weighed against potential unfavourable changes in low-density lipoprotein cholesterol values when low-carbohydrate diets are considered.

CRD commentary

This was a well reported systematic review with a clear question and clear inclusion criteria. The search encompassed several databases. It is, however, unclear whether any language restrictions were applied or whether a systematic approach to finding unpublished studies would have located more studies; the authors inspected a funnel plot for publication bias. Measures were taken to reduce errors and bias in the selection of trials and the extraction of data. The included studies appear to have had little control over adherence to the diet restrictions, and drop-outs and losses to follow-up were substantial. The quality of the included studies was assessed with appropriate criteria and the results were reported. Details of the included studies were reported and systematically assessed (e.g. drop-outs), and the results were critically discussed. The study results were pooled in a meta-analysis, identified heterogeneity between study results was successfully traced for the main outcome, and sensitivity analyses were performed. The conclusions appear to be reliable, although the small number of included studies and participants should be kept in mind.

The authors stated that the funding source played no role in the review process.

Implications of the review for practice and research

Practice: The authors stated that there was insufficient evidence to make recommendations about the use of low-carbohydrate diets to induce weight loss (especially for long-term weight loss).

Research: The authors stated that future trials should obtain complete evaluations of body weight and cardiovascular risk factors for all participants, regardless of their adherence to the allocated diet.

Funding

Swissmilk.

Bibliographic details

Nordmann A J, Nordmann A, Briel M, Keller U, Yancy W S, Brehm B J, Bucher H C. Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials. Archives of Internal Medicine 2006; 166(3): 285-293. [PubMed: 16476868]

Original Paper URL

http://archinte.ama-assn.org

Indexing Status

Subject indexing assigned by NLM

MeSH

Blood Glucose /analysis; Blood Pressure; Cardiovascular Diseases /blood /prevention & control; Cholesterol /blood; Diet, Fat-Restricted; Dietary Carbohydrates /administration & dosage; Humans; Insulin /blood; Randomized Controlled Trials as Topic; Risk Factors; Triglycerides /blood; Weight Loss

AccessionNumber

12006008119

Database entry date

30/06/2006

Record Status

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.