Effect of habitual dietary carbohydrate on maximal fat oxidation in active but untrained humans

Changes in macronutrient intake has been linked to specific changes in rates of fat oxidation. However previous work has typically employed short duration interventions ranging from a couple of days to weeks. The aim of this study was to investigate the effect of habitual dietary macronutrient intake on maximal fat oxidation (MFO) during exercise. One hundred and seventeen healthy individuals were recruited and participated in an incremental exercise test to exhaustion for determination of MFO and crossover point. Basic anthropometry, 24 hour food recalls and resting blood sampling was also completed to determine cardiovascular disease risk factors and blood glucose control. A hierarchical multiple regression attributed 53% of variability in MFO to sex, age, fat free mass, fat mass, and VO 2 max. Increased fat free mass, VO 2 max and female sex were positively correlated with MFO. The addition of protein, carbohydrate and fat intake (g/day), explained a further 4.0% of variability, predominately due to a negative correlation of CHO intake. with severe CHO restriction (<20% total energy intake (TEI) as CHO) exhibited higher MFO than participants consuming >45% TEI as CHO (0.52 ± 0.24 g/min, respectively, 0.38 ± 0.13 g/min, respectively, p = 0.020). No change was observed when stratifying the population into groups consuming <130 g and >130 g of CHO/day. The change in MFO in the extreme groups persisted when a univariate analysis was conducted, accounting for fat free mass, age, sex and aerobic fitness (p = 0.035). Reductions of dietary CHO intake resulted in higher MFO in a study recruiting general population free living participants. This relationship was more evident with greater CHO restriction.