1.c. Lower-Fat Diets

Rationale: Nine RCTs testing the effects of diet on weight loss promoted diets that varied in fat and caloric content (317, 364, 369, 434, 440-444). Lower fat diets varied from 20 to 30 percent of calories from fat, and ranged from 1,200 to 2,300 calories. 

Three RCTs, all 6 months or greater in duration, promoted lower fat diets with ad libitum caloric intake. Two of the three RCTs reported that lower fat diets with ad libitum energy intake resulted in a reduction in caloric intake of 85 kcal and 300 kcal (317, 444),and all three RCTs produced greater weight loss by 1 to 3.9 kg (2.2 to 8.6 lb), compared to a higher fat diet (317, 441, 444). The third RCT had a large disparity in baseline caloric intake between the two diet groups, making the reported caloric intakes at the end of the study difficult to compare (441).

Three RCTs compared lower fat diets with targeted caloric reduction to lower fat diets alone, and all three found that weight loss is greater in the lower fat diet with caloric reduction than with the lower fat diet alone (369, 434, 442). 

When similar caloric levels occur between lower fat and higher fat diets, similar amounts of weight loss were reported in two studies (364, 443), whereas one study showed 1.8 kg (4 lb) greater weight loss on the lower fat compared with the higher fat diet, despite similar reported calorie levels (440).

Taken together, these studies show that lower fat diets ranging from 20 to 30 percent of calories can contribute to lower caloric intake even when caloric reduction is not the focus of the intervention (317, 444), but when LCDs are targeted with lower fat diets, better weight loss is achieved (369, 434, 442). However, there is little evidence that lower fat diets (per se) cause weight loss independent of caloric reduction.