Dietary trials provide evidence for practice and policy guidelines, but poor adherence may confound results. Food supplementation may improve adherence to dietary interventions, but the impact of supplementation on study outcomes is not known. The aim of this review was to examine the impact of food supplementation on weight loss in dietary intervention trials. The databases Scopus, PubMed and the Cochrane Library were searched for dietary intervention trials published between January 2004 and March 2015 using the following keyword combinations: 'trial' OR 'intervention', 'food' OR 'diet', 'weight loss' and 'adherence' OR 'adherence'. Studies were included if food was provided to at least one study group and both 'weight change' and 'adherence' were reported. Random effects meta-analyses were conducted to assess weighted mean differences (WMD) in body weight (change or final mean values). The included studies formed two groups: trials involving an intervention group supplemented with a food and a control without food supplementation (food v. no food), and trials in which food was provided to all subjects (food v. food) (PROSPERO registration: CRD42015017563). In total, sixteen studies were included. Significant weight reduction was reported in the food v. no food studies (WMD -0·74 kg; 95 % CI -1·40, -0·08; P=0·03, I 2=63 %). A non-significant increase in weight was found among the food v. food studies (WMD 0·84 kg; 95 % CI -0·60, 2·27; P=0·25, I 2=0 %). Food supplementation appeared to result in greater weight loss in dietary trials. Energy restrictions and intensity of interventions were other significant factors influencing weight loss.
Keywords: Diet; Dietary compliance; Dietary trials; Food; WMD weighted mean difference; Weight loss.