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Abstract
It is unclear whether low-carbohydrate, high-protein, weight-loss diets benefit body
mass and composition beyond energy restriction alone.
The objective was to use meta-regression to determine the effects of variations in
protein and carbohydrate intakes on body mass and composition during energy restriction.
English-language studies with a dietary intervention of > or =4200 kJ/d (1000 kcal/d),
with a duration of > or =4 wk, and conducted in subjects aged > or =19 y were considered
eligible for inclusion. A self-reported intake in conjunction with a biological marker
of macronutrient intake was required as a minimum level of dietary control. A total
of 87 studies comprising 165 intervention groups met the inclusion criteria.
After control for energy intake, diets consisting of < or =35-41.4% energy from carbohydrate
were associated with a 1.74 kg greater loss of body mass, a 0.69 kg greater loss of
fat-free mass, a 1.29% greater loss in percentage body fat, and a 2.05 kg greater
loss of fat mass than were diets with a higher percentage of energy from carbohydrate.
In studies that were conducted for >12 wk, these differences increased to 6.56 kg,
1.74 kg, 3.55%, and 5.57 kg, respectively. Protein intakes of >1.05 g/kg were associated
with 0.60 kg additional fat-free mass retention compared with diets with protein intakes
< or =1.05 g/kg. In studies conducted for >12 wk, this difference increased to 1.21
kg. No significant effects of protein intake on loss of either body mass or fat mass
were observed.
Low-carbohydrate, high-protein diets favorably affect body mass and composition independent
of energy intake, which in part supports the proposed metabolic advantage of these
diets.
Many meta-analyses use a random-effects model to account for heterogeneity among study results, beyond the variation associated with fixed effects. A random-effects regression approach for the synthesis of 2 x 2 tables allows the inclusion of covariates that may explain heterogeneity. A simulation study found that the random-effects regression method performs well in the context of a meta-analysis of the efficacy of a vaccine for the prevention of tuberculosis, where certain factors are thought to modify vaccine efficacy. A smoothed estimator of the within-study variances produced less bias in the estimated regression coefficients. The method provided very good power for detecting a non-zero intercept term (representing overall treatment efficacy) but low power for detecting a weak covariate in a meta-analysis of 10 studies. We illustrate the model by exploring the relationship between vaccine efficacy and one factor thought to modify efficacy. The model also applies to the meta-analysis of continuous outcomes when covariates are present.
Untested alternative weight loss diets, such as very low carbohydrate diets, have unsubstantiated efficacy and the potential to adversely affect cardiovascular risk factors. Therefore, we designed a randomized, controlled trial to determine the effects of a very low carbohydrate diet on body composition and cardiovascular risk factors. Subjects were randomized to 6 months of either an ad libitum very low carbohydrate diet or a calorie-restricted diet with 30% of the calories as fat. Anthropometric and metabolic measures were assessed at baseline, 3 months, and 6 months. Fifty-three healthy, obese female volunteers (mean body mass index, 33.6 +/- 0.3 kg/m(2)) were randomized; 42 (79%) completed the trial. Women on both diets reduced calorie consumption by comparable amounts at 3 and 6 months. The very low carbohydrate diet group lost more weight (8.5 +/- 1.0 vs. 3.9 +/- 1.0 kg; P < 0.001) and more body fat (4.8 +/- 0.67 vs. 2.0 +/- 0.75 kg; P < 0.01) than the low fat diet group. Mean levels of blood pressure, lipids, fasting glucose, and insulin were within normal ranges in both groups at baseline. Although all of these parameters improved over the course of the study, there were no differences observed between the two diet groups at 3 or 6 months. beta- Hydroxybutyrate increased significantly in the very low carbohydrate group at 3 months (P = 0.001). Based on these data, a very low carbohydrate diet is more effective than a low fat diet for short-term weight loss and, over 6 months, is not associated with deleterious effects on important cardiovascular risk factors in healthy women.
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