Weight regain following weight loss is common although little is known regarding the associations between amount, rate, and composition of weight loss and weight regain. Forty-three studies (52 groups; n = 2379) with longitudinal body composition measurements were identified in which weight loss (≥5%) and subsequent weight regain (≥2%) occurred. Data were synthesized for changes in weight and body composition. Meta-regression models were used to investigate associations between amount, rate, and composition of weight loss and weight regain. Individuals lost 10.9% of their body weight over 13 weeks composed of 19.6% fat-free mass, followed by a regain of 5.4% body weight over 44 weeks composed of 21.6% fatfree mass. Associations between the amount (P < 0.001) and rate (P = 0.049) of weight loss and their interaction (P = 0.042) with weight regain were observed. Fatfree mass (P = 0.017) and fat mass (P < 0.001) loss both predicted weight regain although the effect of fat-free mass was attenuated following adjustment. The amount (P < 0.001), but not the rate of weight loss (P = 0.150), was associated with fat-free mass loss. The amount and rate of weight loss were significant and interacting factors associated with weight regain. Loss of fat-free mass and fat mass explained greater variance in weight regain than weight loss alone. KEYWORDS body composition, fat mass, fat-free mass, weight loss, weight regain 1 | INTRODUCTION Over half of the UK population are currently considered overweight or obese, and it is estimated that around 55% of individuals worldwide will be affected by obesity by 2050. 1 At any one time, approximately 40% of adults report that they are engaged in attempts to control their weight. 2 While these attempts show initial success, relapse is common, 3 and less than 20% of those losing 10% of their weight maintain the loss for a year or more. 4 It is likely that several factors during weight loss influence subsequent weight regain, however, although commonly measured in clinical weight loss studies, the extent to which the rate, amount, and composition (eg, fat-mass [FM] and fat-free mass [FFM]) of weight loss are associated with weight regain remains unclear. Studies of weight loss with follow-up periods in which weight regain occurs may be used as a model to identify predictive factors associated with weight regain. Factors such as a rapid rates and large amounts of weight loss have previously been associated with weight