During the exercise in both trained and untrained groups, there were lower carbohydrate (CHO) and higher fat oxidation rates and contributions to total energy expenditure after the CAJ supplementation compared to the PLA supplementation (p<0.05). These values were greater in the trained group than the untrained group except CHO oxidation rates (p<0.05), which were not significantly different. Moreover, in both trained and untrained groups, resting plasma vitamin C concentrations were significantly higher after the CAJ supplementation compared to the PLA supplementation, without any change after the PLA supplementation. These values were greater in the trained group than the untrained group (p<0.05). There were no significant differences in glucose, insulin or lipid concentrations between the groups’ blood samples.
The findings of this study suggest that CAJ supplementation enhanced fat oxidation during exercise may enhance endurance performance, but specific studies are needed to assess this possibility.
Vitamin C; Leucine; Endurance; Fat oxidation rate; Athletes
Carbohydrate (CHO) plays a major role as an energy source for active muscle during high-intensity exercise . Moreover, the increased capacity of fat utilization is known to improve exercise capacity . Therefore, an intervention which increases fat utilization may be important for endurance of athletes. Diet and exercise training are known to increase fat utilization during exercise . It is not known whether this can be enhanced further by dietary supplement interventions which increase fat oxidation in untrained individuals.
Endurance training has been shown to improve fat utilization . Possible mechanisms proposed by a recent study involve changes in fatty acid transport protein content in whole muscle (FAT/CD36 and FABPpm), sarcolemmal (FABPpm) and mitochondrial (FAT/CD36) membranes in female human skeletal muscles .
Diets containing antioxidants and branch chain amino acids (BCAAs) are reported to have potential effects on fat utilization [6,7]. The antioxidant, vitamin C is perhaps one of the most widely used vitamins in the world today. Johnston et al.  reported that vitamin C is important for fat oxidation. This may be due to ascorbic acid (vitamin C) being a co-factor for the biosynthesis of carnitine, a molecule required for fatty acid oxidation . This may contribute to increased utilization of fatty acids in triglycerides as a fat source for muscle contraction, resulting in lower serum triglyceride levels . Leucine, the most utilized BCAA, was found to enhance fat oxidation in obese animals and overweight or obese subjects [10,11]. De Araujo et al.  showed that supplementation with BCAAs (i.e. leucine, isoleucine, or valine) increases hepatic and muscle glycogen concentrations in exercised rats, suggesting greater fat utilization during exercise . A previous study, however, reported an opposite result . This discrepancy may be due to a different nutritional status, as the rats in Cheng’s study were leucine deficient whereas animals or subjects in other studies were supplemented with leucine. Cheng’s study reported that leucine deficiency increased triglyceride lipolysis, leading to increased fat mobilization via cAMP-PKA-HSL in white adipose tissue . This was supported by the results of upregulation of AdrB3 expression, of AdrB3, the main isoform of β-adrenoceptors in the adipose tissue . Together with the effects on energy expenditure (EE) enhancement in brown adipose tissue and lipogenesis suppression, the leucine deficiency contributed to fatty acid mobilization, resulting in increased fat loss.
Cashew apple is a product of cashew nut manufacturing. It is popularly consumed in the orm of juice which comprises many nutritional components, including vitamin C and BCAAs [14,15]. For this study it was hypothesized that cashew apple juice (CAJ) would further enhance fat oxidation during high-intensity exercise, adding to the effects of training. Therefore, the effect of CAJ supplementation on substrate utilization during high-intensity exercise in trained and untrained subjects was investigated.