Weight Loss with Low-Carb or Low-Fat Diets

Obese patients lost similar amounts of weight over two years with either a low-fat or low-carbohydrate diet, but the latter had a more favorable effect on HDL cholesterol, data from a randomized trial showed....

Weight loss averaged 24 lbs. (11 kg) at one year and 15 lbs (7 kg) at two years with no significant differences between groups, according to Gary D. Foster, PhD, of Temple University in Philadelphia, PA, and co-authors. However, patients assigned to the low-carbohydrate diet had more rapid early declines in blood pressure, triglycerides, and VLDL cholesterol and greater increases in HDL throughout the study, they reported.

"This two-year multicenter study of more than 300 participants revealed that neither dietary fat nor carbohydrate intake influenced weight loss when combined with a comprehensive lifestyle intervention," they wrote. "Both diet groups achieved clinically significant and nearly identical weight loss… These long-term data suggest that a low-carbohydrate approach is a viable option for obesity treatment for obese adults."

Several randomized trials have shown that people on low-carbohydrate diets achieve greater short-term weight loss than those on low-fat, calorie-restricted diets, but long-term results have been mixed.

Nor have low-carbohydrate and low-fat diets been examined closely to determine whether they differ with respect to outcomes other than weight loss.

Foster and colleagues sought to inform on some of those issues by conducting a randomized, multicenter clinical trial that had weight loss as its primary outcome but also assessed cardiovascular risk factors, bone mineral density, and general symptoms. They hypothesized that a low-carbohydrate diet would result in greater weight loss at two years compared with a low-calorie, low-fat diet.

The study involved 307 patients with a mean age of 45, mean body mass index of 36, and mean weight of 103 kg (227 lbs). Two-thirds of the participants were women, and 70% were white. Investigators excluded patients with dyslipidemia or diabetes.

The low-carbohydrate diet limited carbohydrate intake to 20 g/d for 12 weeks and then increased by 5 g/d per week. Participants in this group could consume as much fat and protein as they wanted. Limiting carbohydrate intake was the primary behavioral focus for this group.

The low-fat diet was also calorie-restricted: 1,200-1,500 kcal/d for women and 1,500-1,800 kcal/d for men. Carbohydrates accounted for about 55% of calories, fat for 30%, and protein for 15%. Limiting total energy intake (kcal/d) was the primary behavioral target for the group.

Both groups reached maximum weight loss (11 to 12 kg) after six months. They did not differ significantly with respect to absolute weight loss at one year (about 11 kg) or at two years (7.37 kg in the low-fat group versus 6.34 kg in the low-carbohydrate group, P=0.41).

The only significant difference in weight loss occurred at three months, when the low-carbohydrate group averaged 9.49 kg versus 8.37 kg in the low-fat group (P=0.019). The trend over the entire 24 months of the trial did not differ significantly (P=0.30). The groups did differ with respect to several secondary endpoints, however.

The low-carbohydrate diet was associated with more rapid reductions in triglycerides and VLDL cholesterol, which differed significantly between groups after three months. The groups did not differ at 24 months, but the overall trend in VLDL favored the low-carbohydrate diet (P=0.027).

The low-fat diet led to greater reductions in LDL at all time points. Mean LDL increased in the low-carbohydrate group during the first six months before declining at one year. The overall trend favored the low-fat diet (P=0.0009).

HDL increased more rapidly with the low-carbohydrate diet and remained significantly different from the low-fat group at all four time points (P=0.008 to P<0.001) and in the overall analysis (P=0.0058).

The low-carbohydrate diet led to small but statistically greater reductions in total cholesterol (P=0.030 for trend).

Systolic blood pressure did not differ significantly between groups at any point in time. The low-carbohydrate diet resulted in significantly greater reductions in diastolic blood pressure at three of four intervals, but the overall trend was not significant (P=0.36).

The authors noted hypothetical concerns that a low-carbohydrate diet might lead to greater loss in bone mineral density, but the two groups did not differ with respect to changes in BMD at any time during the study, and the declines in BMD were within expected ranges.

Participants in the low-carbohydrate group reported significantly more adverse effects, particularly during the first six to 12 months of the study. The low-carbohydrate diet was associated with more reports of bad breath, hair loss, constipation, and dry mouth. However, trends over the entire trial did not differ significantly for any of these.