Objective: To investigate the relationship between obesity and ad libitum food intake (quantity and composition) and to assess the impact of ad libitum food intake on energy expenditure and macronutrient oxidation.
Design: Male volunteers were first fed a weight maintaining diet for at least 4 days before selecting their food for the next 5 days from two computerized vending machines offering a variety of familiar, palatable foods. 24-h energy expenditure (24EE) and substrate oxidation were measured in a respiratory chamber on the last day of each weight maintenance and ad libitum intake periods.
Setting: Ten day admission on a metabolic research ward.
Subjects: Thirty-four non-diabetic Pima Indian males covering a wide range of body weight and body composition (30 +/- 8 y, 102.1 +/- 30.2 kg, 34 +/- 9% body fat, mean +/- s.d.).
Results: Weight maintenance requirements averaged 2913 +/- 342 kcal/d. Energy intake during the ad libitum period increased to 4550 +/- 921 kcal/d (12 +/- 1% protein, 40 +/- 4% fat, 48 +/- 4% carbohydrate) i.e., a spontaneous overeating by 54 +/- 32% above weight maintenance requirement, resulting in a 0.9 +/- 1.0 kg body weight gain. Neither the composition of the selected diet nor the degree of overeating was associated with physical characteristics, such as body weight and body composition. When compared with baseline, spontaneous overeating on day 5 was associated with a 396 +/- 233 kcal/d increase in 24EE, a 607 +/- 503 kcal/d increase in carbohydrate oxidation, a 214 +/- 392 kcal/d decreased in lipid oxidation (P < 0.01), and no change in protein oxidation. Increased carbohydrate oxidation correlated with the excess carbohydrate intake (r = 0.69, P = 0.0001) accounting for 68 +/- 13% (mean +/- s.e.e.) of the excess, whereas excess fat intake was not oxidized.
Conclusion: In response to spontaneous overfeeding on a mixed 'cafeteria diet', excess carbohydrate intake is oxidized, suggesting a physiological control of carbohydrate stores, whereas excess fat intake is channeled toward fat stores. None of the observed changes were related to indices of obesity.