Nocturnal Fat Oxidation and Metabolic Flexibility of Young Male Indian Adults Using Indirect Calorimetry

Indian adults tend to inappropriately accumulate body fat even at low Body Mass Index (BMI). Usually, fat that is stored in the fed state is mobilized for energy during nocturnal fasting, thus achieving daily fat balance. This is called metabolic flexibility, which may be lost in some individuals leading to body fat accumulation. Measuring fat balance requires 24h measurement of fat oxidation, but nocturnal fat oxidation could be a reasonable surrogate. The variability of nocturnal fat oxidation is also unknown. A retrospective analysis on 24h fat oxidation in adult men (n=18) was carried out to test the former hypothesis, while the variability of nocturnal fat oxidation was measured prospectively in 5 adult men, who were fed the same diet for 2 days prior to the measurement. Whole-body indirect calorimetry was used for measuring Respiratory Quotient (RQ), energy expenditure and fat oxidation. In 24h analyses, nocturnal (0.44 ± 0.21 g/kg) was significantly higher than diurnal fat oxidation (0.24 ± 0.21 g/kg) and was 64.5% of the total 24h value. Nocturnal fat oxidation was positively correlated with 24h fat oxidation (r = 0.937; p<0.01) and inversely correlated with 24h fat balance (r = -0.850; p<0.01). Metabolic flexibility, measured as the Fed: Fasted RQ ratio, was negatively correlated with BMI (r = -0.226; p=0.366). The intra- and inter-individual variability of 12h nocturnal fat oxidation was low, at 4.7% and 7.2%, respectively. Nocturnal fat oxidation has a low variability when prior diets over 2 days are constant and the Fed: Fasted RQ ratio is an index of metabolic flexibility, which relates to BMI in young adults.