CHANGES IN MACRONUTRIENT BALANCE DURING OVERFEEDING AND UNDERFEEDING ASSESSED BY 12-D CONTINUOUS WHOLE-BODY CALORIMETRY

Alterations in energy balance must be accommodated by adjustments in t he net storage of the major energy-yielding macronutrients: carbohydra te, protein, and fat. This study used continuous whole-body calorimetr y to measure changes in energy expenditure and substrate oxidation dur ing a 12-d imposed energy imbalance in six lean men on mixed diets (ov erfeeding: 16.5 MJ/d, +33%, n = 3; underfeeding: 3.5 MJ/d, -67%, n = 3 ). Changes in total energy expenditure (TEE) and its components were m odest; TEE changed by +6.2% (overfeeding) and -10.5% (underfeeding). I n consequence, body weight changed by +2.90 and -3.18 kg. Marked chang es in metabolic fuel selection occurred over the course of the study. Carbohydrate intake (540 and 83 g/d for overfeeding and underfeeding, respectively) exerted direct autoregulatory feedback on carbohydrate o xidation (551 and 106 g/d at day 12 for overfeeding and underfeeding, respectively). Subjects were close to balance by day 5. Changes in pro tein oxidation were small and not sufficient to prevent the oxidation of body protein mass, or its accretion, in response to energy deficit or surplus. Fat oxidation (59 and 177 g/d for overfeeding and underfee ding, respectively) was not sensitive to dietary fat intake (150 and 2 0 g/d, for overfeeding and underfeeding, respectively), rather, its ox idation was inversely related to the oxidation of other substrates. Ch anges in fat balance accounted for 74.1% and 84.0% of the energy imbal ance during overfeeding and underfeeding, respectively. This study sho ws a clear oxidative hierarchy for the macronutrients. Metabolic fuel selection is dominated by the need to maintain carbohydrate balance. T his induces inappropriate counterrgulatory alterations in fat oxidatio n during energy surplus.