The effect of covertly manipulating the energy density of mixed diets on ad libitum food intake in 'pseudo free-living' humans

OBJECTIVE: This study examined the effects of covert alterations in the energy density (ED) of mixed, medium fat (MF) diets on ad libitum food and energy intake (EI), subjective hunger and body weight in humans.

DESIGN: Randomised cross-over design. Subjects were each studied three times (factorial design), during 14 d, throughout which they had ad libitum access to one of three covertly-manipulated MF diets.

SUBJECTS: Six healthy men, mean age (s.e.m.) = 30.0 y (12.76 y), mean weight = 71.67 kg (19.80 kg); mean height = 1.79 m (0.22 ml, body mass index (BMI) = 22.35 (2.60) kg/m², were studied. The fat, carbohydrate (CHO) and protein in each diet (as a proportion of the total energy) and energy density (ED) were, low-ED (LED), 38:49:13%; 373 kJ/100 g; medium-ED (MED), 40:47:13%; 549 kJ/100 g; high-ED (HED), 39:48:13%; 737 kJ/100 g. Subjects could alter the amount but not the composition of foods eaten. They were resident in (but not confined to) a metabolic suite throughout the study.

RESULTS: Solid food intake decreased as ED increased, giving mean values of 2.84, 2.51 and 2.31 kg/d, respectively. This was insufficient to defend energy balance, since energy intake increased with increasing ED (F(2,10) 16.08; P < 0.001) giving mean intakes of 10.12, 12.80 and 16.17 MJ/d, respectively. Rated pleasantness of food (measured on visual analogue scales) was not significantly different between diets nor was subjective hunger different between the LED, MED and HED diets, respectively. Diet significantly affected body weight (F(2,10) = 4.62; P = 0.038), producing changes of - 1.20, 0.02 and 0.95 kg, respectively, by day 14.

CONCLUSION: Dietary ED can influence El and body weight, since changes in amount eaten alone are insufficient to defend energy balance, when subjects feed on unfamiliar diets and diet selection is precluded. Comparison with our previous studies suggest that there was compensation in solid food intake when ED was altered using mixed diets (as in this study) compared to previous studies which primarily used fat or CHO to alter dietary ED.