The eff ectiveness of low-fat diets for long-term weight loss has been debated for decades, with many randomised controlled trials (RCTs) and recent reviews giving mixed results. We aimed to summarise the large body of evidence from RCTs to determine whether low-fat diets contribute to greater weight loss than participants’ usual diet, low-carbohydrate diets, and other higher-fat dietary interventions.
We did a systematic review and random eff ects meta-analysis of RCTs comparing the long-term effect (≥1 year) of low-fat and higher-fat dietary interventions on weight loss by searching MEDLINE, Embase, Cochrane Central Register of Controlled Trials (CENTRAL), and Cochrane Database of Systematic Reviews to identify eligible trials published from database inception up until July 31, 2014. We excluded trials if one intervention group included a non-dietary weight loss component but the other did not, and trials of dietary supplements or meal replacement drink interventions. Data including the main outcome measure of mean diff erence in weight change between interventions, and whether interventions were intended to lead to weight loss, weight maintenance, or neither, were extracted from published reports. We estimated the pooled weighted mean diff erence (WMD) with a DerSimonian and Laird random eff ects method.
3517 citations were identifi ed by the search and 53 studies met our inclusion criteria, including 68 128 participants (69 comparisons). In weight loss trials, low-carbohydrate interventions led to signifi cantly greater weight loss than did low-fat interventions (18 comparisons; WMD 1·15 kg [95% CI 0·52 to 1·79]; I2=10%). Low-fat interventions did not lead to diff erences in weight change compared with other higher-fat weight loss interventions (19 comparisons; WMD 0·36 kg [–0·66 to 1·37; I2=82%), and led to a greater weight decrease only when compared with a usual diet (eight comparisons; –5·41 kg [–7·29 to –3·54]; I2=68%). Similarly, results of non-weight-loss trials and weight maintenance trials, for which no low-carbohydrate comparisons were made, showed that low-fat versus higher-fat interventions have a similar eff ect on weight loss, and that low-fat interventions led to greater weight loss only when compared with usual diet. In weight loss trials, higher-fat weight loss interventions led to signifi cantly greater weight loss than low-fat interventions when groups differed by more than 5% of calories obtained from fat at follow-up (18 comparisons; WMD 1·04 kg [95% CI 0·06 to 2·03]; I2=78%), and when the difference in serum triglycerides between the two interventions at follow-up was at least 0·06 mmol/L (17 comparisons; 1·38 kg [0·50 to 2·25]; I2=62%).
These fi ndings suggest that the long-term eff ect of low-fat diet intervention on bodyweight depends on the intensity of the intervention in the comparison group. When compared with dietary interventions of similar intensity, evidence from RCTs does not support low-fat diets over other dietary interventions for long-term weight loss.