Effect of Low-Fat vs. Other Diet Interventions on Long-Term Weight Change in Adults: A Systematic Review and Meta-Analysis

BACKGROUND: The effectiveness of low-fat diets for long-term weight loss has been debated for decades, with dozens of randomized trials (RCTs) and recent reviews giving mixed results. METHODS: We conducted a random effects meta-analysis of RCTs to estimate the long-term effect of low-fat vs. higher fat dietary interventions on weight loss. Our search included RCTs conducted in adult populations reporting weight change outcomes at ≥1 year, comparing low-fat with higher fat interventions, published through July 2014. The primary outcome measure was mean difference in weight change between interventions. FINDINGS: Fifty-three studies met inclusion criteria representing 68,128 participants. In the setting of weight loss trials, low-carbohydrate interventions led to significantly greater weight loss than low-fat interventions (n comparisons=18; weighted mean difference [WMD]=1.15 kg, 95% CI=0.52 to 1.79; I(2)=10%). Low-fat did not lead to differences in weight change compared with other moderate fat weight loss interventions (n=19; WMD=0.36, 95% CI=-0.66 to 1.37; I(2)=82%), and were superior only when compared with “usual diet” (n=8; WMD=-5.41, 95% CI=-7.29 to −3.54; I(2)=68%). Similarly, non-weight loss trials and weight maintenance trials, for which there were no low-carbohydrate comparisons, had similar effects for low-fat vs moderate fat interventions, and were superior compared with “usual diet”. Weight loss trials achieving a greater difference in fat intake at follow-up significantly favored the higher fat dietary interventions, as indicated by difference of ≥5% of calories from fat (n=18; WMD=1.04, 95% CI=0.06 to 2.03; I(2)=78%) or by difference in change serum triglycerides of ≥5 mg/dL (n=17; WMD=1.38, 95% CI=0.50 to 2.25; I(2)=62%). INTERPRETATION: These findings suggest that the long-term effect of low-fat diets on body weight depends on the intensity of intervention in the comparison group. When compared to dietary interventions of similar intensity, evidence from RCTs does not support low-fat diets over other dietary interventions.