To assess the effect of exercise on body fat balance

To assess the effect of exercise on body fat balance, the measurement of accumulated fat oxidation over 24h (24-h fat oxidation) is required. Persistent increases in fat oxidation during the post-exercise period require long-term calorimetry to evaluate the effect of exercise on body fat balance (Bielinski et al., 1985; Melanson et al., 2009a; Gaesser and Brooks, 1984; Henderson and Alderman, 2014). One of the earliest studies that reported 24-h fat oxidation was increased in response to exercise (Bielinski et al., 1985). However, the effect of exercise on 24-h fat oxidation is likely attributable to a state of negative crm1 balance on the day of exercise (Henderson and Alderman, 2014). If exercise is accompanied by an increase in energy intake to achieve energy balance, i.e., intake and expenditure of energy over 24h are matched, 24-h fat oxidation remains similar to that observed on sedentary days (Melanson et al., 2009a, 2009b; Dionne et al., 1999). Thus, exercise per se apparently has little effect on 24-h fat oxidation. Of note, the consensus among previous literature is derived from studies in which exercise was performed in the post-prandial state. Our previous studies indicate exercise increases 24-h fat oxidation if performed in a post-absorptive state. In experiments under energy-balanced conditions, greater 24-h fat oxidation is observed when exercise is performed before breakfast than that after breakfast (Shimada et al., 2013) or lunch (Iwayama et al., 2015). However, these studies did not include evaluations of the sedentary condition as a control. The aim of the present study was to examine the effect of exercise performed before breakfast on 24h fat oxidation compared to sedentary day. Accordingly, 24-h indirect calorimetry was performed on four occasions with a session of 60-min exercise performed before breakfast, after lunch, or after dinner. Identical measurements were performed on sedentary day. All experimental conditions were designed to be energy balanced over 24h. In order to assess the mechanisms underlying a potential effect of exercise on 24-h fat oxidation, the association between exercise-induced transient energy deficits and 24-h fat oxidation was examined. The results of the present study contribute to our understanding of the effect of time of day on the beneficial effects of exercise on fat oxidation.
Methods
Results
Discussion Of the macronutrients stored in the body, the pool size of carbohydrate is the smallest, and metabolic responses to changes in carbohydrate storage are more sensitive than those for fat and protein (Flatt, 1988). Glycogen storage reached its nadir after overnight fasting and was further reduced by early morning exercise. Carbohydrate depletion after morning exercise in the present study was roughly equivalent to 18% of whole-body glycogen storage, assuming whole-body glycogen stores in the post-absorptive state are approximately 2300kcal (Hargreaves, 2000). The magnitude of transient carbohydrate deficit, i.e., the lowest value of relative carbohydrate balance, was found to be negatively correlated with 24-h fat oxidation. Recent studies have provided evidence supporting a potential molecular link between glycogen depletion and activation of fat oxidation (Philp et al., 2012; Izumida et al., 2013). Tissue glycogen functions not only as carbohydrate reserve but also as molecular sensor capable of activating signaling pathways in response to exercise including the nuclear translocation of AMPK and up-regulation of genes responsible for fat oxidation (Philp et al., 2012). Analysis of combined data sets further strengthened the observed association between transient energy and carbohydrate deficits and 24-h fat oxidation. Thus, transient energy deficits, particularly with regards to carbohydrate, appear to contribute to increased 24-h fat oxidation, even when energy intake and expenditure is matched over 24-h periods.