Study Question:
Does lower total fat intake lead to lower body weight in adults and children?

Summary Answer:
High quality, consistent evidence shows that a reduction in total fat intake leads to a small but statistically significant and clin…ically meaningful reductions in body weight in adults, with supporting evidence for a simialr effect in children and young people.

What is Known & What This Paper Adds:
The ideal proportion of total fat in the human diet is unclear. This systematic review provides a large and consistent body of evidence that lowering total fat intake, as a proportion of energy intake, results in lower boy weight in the long term, lower body mass index, and lower waist circumference in adults, with a dose-response relation. Evidence in children and young people is more limited but supports a simialr relation.