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) .