Clinical Question: In adults, are low-carbohydrate diets as effective as low-fat, low-calorie diets in producing and maintaining weight loss?

Setting: Outpatient (any)

Study Design: Meta-analysis (randomized controlled trials)

Synopsis: Although there are multiple theories on how best to help patients lose weight, most of them focus on eating. The researchers conducting this study searched seven databases for randomized controlled trials comparing low-carbohydrate diets with low-fat diets. They also checked review articles, contacted experts, and searched reference lists of articles. They found five studies that enrolled a total of 447 patients. Three of the studies monitored patients for 12 months. All studies enrolled patients who followed specified diets and bought their own foods.

The low-carbohydrate diets did not restrict calories but limited carbohydrate intake to less than 20 to 30 g per day with a gradual increase. Low-fat diets restricted fat intake to less than 30 percent of total calories, and one diet restricted fat intake to less than 10 percent of total calories. All low-fat diets restricted total calories per day.

Patients on the low-carbohydrate diets lost more weight more quickly (average of 7.3 lb [3.3 kg] more), though the low-fat dieters caught up by month 12. Triglyceride and high-density lipoprotein cholesterol levels were improved with the low-carbohydrate diets, and total cholesterol and low-density lipoprotein cholesterol levels were improved with low-fat diets. Low-fat diets resulted in an average sustained weight loss of between 7.3 and 9.7 lb (3.3 and 4.3 kg), and low-carbohydrate diets resulted in an average weight loss of 4.6 to 15.8 lb (2.1 to 7.2 kg). Blood pressure reductions were greater in the low-carbohydrate group at six months but not at 12 months. The three trials that reported glucose and insulin levels found no differences between the groups at 12 months.

Bottom Line: Patients interested in weight loss can choose a low-fat, reduced-calorie diet, or a low-carbohydrate diet without calorie restrictions to lose a small but sustained amount of weight. The effects on cardiovascular outcomes of either diet are not known, although each has different effects on lipid levels, which may or may not translate into an actual effect on patient-oriented outcomes. (Level of Evidence: 1a)