A variety of mechanisms and genetic associations suggest that total dietary fat intake may influence body weight and risk of being overweight. Long-term RCTs suggest that reducing total fat intake (in people of normal body weight and without any intention to reduce body weight) causes a small reduction in body weight. Meta-analysis suggests that the effect of eating less than 30% of energy from fat, compared with higher fat intake, is a mean weight reduction of 1.4 kg (95% confidence interval (CI) -1.7 to -1.1 kg, in 53 875 participants from 26 RCTs, I2 = 75%). Greater fat reduction results in greater weight loss. Similar effects were seen on BMI, waist circumference and body fatness, and small improvements in lipids and blood pressure were also seen. This relationship between dietary fat and weight was confirmed in a 6-month isocaloric controlled feeding trial from China. The three trials of low-fat interventions in children suggest small reductions in BMI with a lower fat diet, but data quality was low. On the other hand, in overweight or obese adults trying to lose weight, macronutrient composition appears fairly unimportant, and weight loss depends on adherence to calorie restriction.