We investigated the effect of long-term ingestion of dietary medium-chain triacylglycerols (MCT) on body weight and fat in humans. Using a double-blind, controlled protocol, we assessed the potential health benefits of MCT compared with long-chain triacylglycerols (LCT) in 78 healthy men and women [body mass index (BMI) > or = 23 kg/m(2): n = 26 (MCT), n = 30 (LCT); BMI < 23 kg/m(2): n = 15 (MCT), n = 7 (LCT)]. Changes in anthropometric variables, body weight and body fat during the 12-wk MCT treatment period were compared with those in subjects consuming the LCT diet. The subjects were asked to consume 9218 kJ/d and 60 g/d of total fat. The energy, fat, protein and carbohydrate intakes did not differ significantly between the groups. Body weight and body fat in both groups had decreased by wk 4, 8 and 12 of the study. However, in the subjects with BMI > or = 23 kg/m(2), the extent of the decrease in body weight was significantly greater in the MCT group than in the LCT group. In subjects with BMI > or = 23 kg/m(2), the loss of body fat in the MCT group (-3.86 +/- 0.3 kg) was significantly greater than that in the LCT group (-2.75 +/- 0.2 kg) at 8 wk. In addition, in subjects with BMI > or = 23 kg/m(2), the decrease in the area of subcutaneous fat in the MCT group was significantly greater than that in the LCT group at wk 4, 8 and 12. These results suggest that the MCT diet may reduce body weight and fat in individuals (BMI > or = 23 kg/m(2)) more than the LCT diet.