Together these factors explained 58% of variance between studies, using the equation: weight change (kg)=-5.77 kg+0.16 kg/1% energy from total fat in control group -0.19 kg/1% decrease in energy from total fat in intervention group+0.01 kg/months' duration. In all cases the results consistently suggested that reducing total fat intake results in a small but statistically significant reduction in weight compared with usual fat intake.

A feasible explanation for the relative weight reduction in lower fat arms might be that the participants in the intervention (low fat) groups received more time, attention, and support than those in the control arms, so that the weight loss was due to factors other than the fat content of the diet. Meta-analysis of the nine randomised controlled trials with data on body mass index found a significantly lower body mass index in the low fat arms compared with usual fat arms (-0.51, 95% confidence interval -0.76 to -0.26, I2=77%).