Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates

Abstract

Background: The possible advantage for weight loss of a diet that emphasizes protein, fat, or carbohydrates has not been established, and there are few studies that extend beyond 1 year.

Methods: We randomly assigned 811 overweight adults to one of four diets; the targeted percentages of energy derived from fat, protein, and carbohydrates in the four diets were 20, 15, and 65%; 20, 25, and 55%; 40, 15, and 45%; and 40, 25, and 35%. The diets consisted of similar foods and met guidelines for cardiovascular health. The participants were offered group and individual instructional sessions for 2 years. The primary outcome was the change in body weight after 2 years in two-by-two factorial comparisons of low fat versus high fat and average protein versus high protein and in the comparison of highest and lowest carbohydrate content.

Results: At 6 months, participants assigned to each diet had lost an average of 6 kg, which represented 7% of their initial weight; they began to regain weight after 12 months. By 2 years, weight loss remained similar in those who were assigned to a diet with 15% protein and those assigned to a diet with 25% protein (3.0 and 3.6 kg, respectively); in those assigned to a diet with 20% fat and those assigned to a diet with 40% fat (3.3 kg for both groups); and in those assigned to a diet with 65% carbohydrates and those assigned to a diet with 35% carbohydrates (2.9 and 3.4 kg, respectively) (P>0.20 for all comparisons). Among the 80% of participants who completed the trial, the average weight loss was 4 kg; 14 to 15% of the participants had a reduction of at least 10% of their initial body weight. Satiety, hunger, satisfaction with the diet, and attendance at group sessions were similar for all diets; attendance was strongly associated with weight loss (0.2 kg per session attended). The diets improved lipid-related risk factors and fasting insulin levels.

Conclusions: Reduced-calorie diets result in clinically meaningful weight loss regardless of which macronutrients they emphasize. (ClinicalTrials.gov number, NCT00072995.)

Figure 1. Mean Change in Body Weight and Waist Circumference from Baseline to 2 Years According to Dietary Macronutrient Content

Solid bars represent high-protein, high-fat, or highest-carbohydrate diets. Open bars represent average-protein, low-fat, or lowest-carbohydrate diets. T bars indicate standard errors. Panels A and C show the change in body weight and the change in waist circumference, respectively, for all participants who were randomly assigned to a diet (a total of 811); missing data were imputed. A total of 403 participants were assigned to a high-protein diet and 408 to an average-protein diet, 405 were assigned to a high-fat diet and 406 to a low-fat diet, and 204 were assigned to the highest-carbohydrate diet and 201 to the lowest-carbohydrate diet. Panel B shows the change in body weight for the 645 participants who provided measurements at 2 years. Of these participants, 325 were assigned to a high-protein diet and 320 to an average-protein diet, 319 were assigned to a high-fat diet and 326 to a low-fat diet, and 169 were assigned to the highest-carbohydrate diet and 168 to the lowest-carbohydrate diet. Panel D shows the change in waist circumference for the 599 participants who provided measurements at 2 years. Of these participants, 303 were assigned to a high-protein diet and 296 to an average-protein diet, 292 were assigned to a high-fat diet and 307 to a low-fat diet, and 159 were assigned to the highest-carbohydrate diet and 155 to the lowest-carbohydrate diet.

Figure 2. Mean Changes in Body Weight and Waist Circumference at Various Time Points

Panels A and C show the mean changes in body weight and waist circumference, respectively, for all participants who were assigned to a diet (a total of 811 at every time point); missing data were imputed. Panel B shows the change in body weight for participants who provided measurements at various time points: 176 to 180 participants at 6 months, 157 to 167 at 12 months, 140 to 152 at 18 months, and 151 to 168 at 2 years. Panel D shows the change in waist circumference for participants who provided measurements at various time-points: 176 to 179 at 6 months, 154 to 166 at 12 months, 135 to 148 at 18 months, and 137 to 159 at 2 years. I bars in all panels indicate standard errors.

Figure 3. Change in Body Weight from Baseline to 2 Years According to Attendance at Counseling Sessions for Weight Loss, among the 645 Participants Who Completed the Study

Panel A shows data for the low-fat, average-protein group; Panel B, for the low-fat, high-protein group; Panel C, for the high-fat, average-protein group; and Panel D, for the high-fat, high-protein group. There were no significant differences among the regression coefficients (P>0.2 for all comparisons; R² = 0.2 for total cohort).

Figure 4. Weight Loss at 2 Years According to Adherence to Dietary Fat and Protein Goals

Intake was determined from three 24-hour diet recalls. Quintiles of fat and protein intakes are shown for the combined high-fat groups (Panel A), low-fat groups (Panel B), high-protein groups (Panel C), and average-protein groups (Panel D); there were 45 to 51 participants per quintile. Rates of attendance at group sessions (percent of total sessions attended over the 2-year period) are shown for the quintiles of fat and protein intake. I bars indicate 95% confidence intervals. P values for a trend in weight loss across quintiles are as follows: P<0.001 for fat intake in low-fat groups, P<0.001 for protein intake in high-protein groups, P = 0.36 for fat intake in high-fat groups, and P = 0.83 for protein intake in average-protein groups. The results were similar when determined within each of the four diet groups (data not shown).