Effects of macronutrient intake in obesity: a meta-analysis of low-carbohydrate and low-fat diets on markers of the metabolic syndrome

Abstract

The metabolic syndrome (MetS) comprises cardiometabolic risk factors frequently found in individuals with obesity. Guidelines to prevent or reverse MetS suggest limiting fat intake, however, lowering carbohydrate intake has gained attention too. The aim for this review was to determine to what extent either weight loss, reduction in caloric intake, or changes in macronutrient intake contribute to improvement in markers of MetS in persons with obesity without cardiometabolic disease. A meta-analysis was performed across a spectrum of studies applying low-carbohydrate (LC) and low-fat (LF) diets. PubMed searches yielded 17 articles describing 12 separate intervention studies assessing changes in MetS markers of persons with obesity assigned to LC (<40% energy from carbohydrates) or LF (<30% energy from fat) diets. Both diets could lead to weight loss and improve markers of MetS. Meta-regression revealed that weight loss most efficaciously reduced fasting glucose levels independent of macronutrient intake at the end of the study. Actual carbohydrate intake and actual fat intake at the end of the study, but not the percent changes in intake of these macronutrients, improved diastolic blood pressure and circulating triglyceride levels, without an effect of weight loss. The homeostatic model assessment of insulin resistance improved with both diets, whereas high-density lipoprotein cholesterol only improved in the LC diet, both irrespective of aforementioned factors. Remarkably, changes in caloric intake did not play a primary role in altering MetS markers. Taken together, these data suggest that, beyond the general effects of the LC and LF diet categories to improve MetS markers, there are also specific roles for weight loss, LC and HF intake, but not reduced caloric intake, that improve markers of MetS irrespective of diet categorization. On the basis of the results from this meta-analysis, guidelines to prevent MetS may need to be re-evaluated.

Keywords: low-carbohydrate diet; low-fat diet; macronutrients; metabolic syndrome; weight loss.

Figure 1

Preferred Reporting Items for  Systematic Review and Meta-Analyses flow diagram of study selection process.

Figure 2

Intake in the LC and LF groups at different time points. (A) Caloric intake (kcal); (B) carbohydrate intake (en%); (C) fat intake (en%); (D) protein intake (en%); (E) saturated fatty acid intake (en%); and (F) fiber intake (grams per day). Different letters (a, b, c, and d) show significant differences among diets and time points. Abbreviations: en%, energy percentage; LC, low-carbohydrate diet; LF, low-fat diet.

Figure 3

Bubble plot of body weight change moderator analysis. The size of the bubbles represents the precision of the effect size; larger bubbles indicate greater precision. Blue bubbles represent the meta-analysis outcome for body weight vs actual intake in the LC group. Yellow bubbles represent the meta-analysis outcome for body weight vs actual intake in the LF group. The regression line fitted to the raw data shows the slope of the relation between the body weight change and (A) caloric intake (kcal) after 6 months; (B) actual carbohydrate intake after 6 months (en%); (C) actual fat intake after 6 months (en%); and (D) actual protein intake after 6 months (en%). Abbreviations: en%, energy percentage; LC, low-carbohydrate diet; LF, low-fat diet.

Figure 4

Graphical summary of the meta-analysis and meta-regression at 6 months. In the blocks on the side, the requirements for each diet are seen in the upper half; actual intakes at 6 months are shown in the lower half. In the upper half of the figure is shown that both diets have significant effects on the markers of MetS. The numbers represent the outcomes of the meta-analysis per diet. In the lower half of the figure, the effect of actual macronutrient intake on markers of MetS and the effect of body weight on glucose levels are shown. The numbers represent the change in marker, per en% intake of macronutrient or, in the case of glucose, per kilogram of body weight. For instance, a diet consisting of 50 en% carbohydrates, 30 en% fat, and 20 en% protein results in a body weight change after 6 months of (0.093 × 50 – 0.118 × 30 – 0.345 × 20) – 5.8 kg. Abbreviations: carb, carbohydrate intake; DBP, diastolic blood pressure; EI, energy intake; en%, percentage of energy; glucose, fasting glucose levels; HDL, high-density lipoprotein; HOMA-IR, homeostatic model assessment of insulin resistance; LC, low-carbohydrate diet; LF, low-fat diet; prot, protein intake; SBP, systolic blood pressure; TAG, plasma triglyceride levels.