## 4 Discussion
### Lipid Metabolism  
To further assess energy supply, metabolites of lipid metabolism were examined. HIHVT [41] as well as other endurance exercises have been shown to increase lipolysis capacity, fatty acid uptake, and fatty acid oxidation [76; 77; 78]. [PERSON] et al. [79] showed an increase in fat oxidation of approximately 36% during exercise in women after seven units of high-intensity interval training [79]. Nitrate supplementation has also been shown to increase fatty acid oxidation in the muscle tissue of rodents [80] and decrease TG concentrations in plasma in mice [69]. Therefore, significant effects on lipid metabolism through HIHVT under nitrate supplementation were expected. After the 3-week training period with HIHVT and nitrate supplementation, as published elsewhere, no changes in RER were found during ECT in either the supplementation or placebo group [35]. Thus, in this study, there did not appear to be any adaptations in fat metabolism with respect to energy supply. However, other studies using the same training protocol have found a reduced RER after 3 weeks [41; 81]. Therefore, the lack of training effects in the present study was not related to nitrate supplementation. Since the relVO\({}_{2\text{peak}}\) during IT in the present study was approximately 12% higher than that reported by [PERSON] [41] and [PERSON] [81], the missing adaptation to training could be due to a higher training level. Further research is needed to investigate possible changes in lipid metabolism with nitrate supplementation. Although in the present study, no differences were found between the groups in RER, Glyc, and FFA, mean plasma TG concentrations at rest and during intermittent exercise decreased by approximately 37% in the N group compared to the P group. Plasma TG concentrations are influenced by dietary habits, and large inter- and intra-participant differences have been observed [76; 82; 83]. As no changes in dietary behavior were observed in the present study and no differences in baseline measurements were found between the two groups, it is suggested that chronic NO3- supplementation may reduce plasma TG concentration. During exercise, plasma TG may serve as an energy substrate but is thought to be less important [77]. However, in trained participants, intramuscular triglycerides (IMTG) account for a significant proportion of energy requirements [62; 77]. In particular, type IIa muscle fibers have a lower capacity for plasma FFA consumption. They are more likely to derive FFA from IMTG. If IMTG is hydrolyzed, it is possible that FFA is immediately oxidized in muscle mitochondria and does not enter the bloodstream [78]. Something similar may happen with Glyc. [PERSON] et al. [84] discussed that whole-body lipolysis assessed by Glyc may remain underestimated, as Glyc released from intramuscular lipolysis may be directly oxidized and reutilized in muscle [84; 85]. This could be an explanation for changes in lipid metabolism in the absence of changes in plasma concentrations of Glyc and FFA. If plasma TG serves to replenish IMTG under the present circumstances, then nitrate supplementation may enhance this effect on muscle lipid metabolism. This seems plausible, as studies have shown that NO3- induces an increase in lipoprotein lipase (LPL) activity, which may serve to replenish IMTG stores and intramuscular glycogen [86; 87]. LPL is active in muscle cells during and after exercise and is inactive in adipose tissue [88]. This would be consistent with the lower plasma TG concentrations during exercise in the present study. However, it does not fully explain why baseline TG levels were already lower in the nitrate group. Studies have shown that LPL overexpression in transgenic mice reduces plasma TG and increases high-density lipoprotein cholesterol (HDL-C) concentrations [89; 90]. Reduced plasma TG concentrations may have implications for health and the prevention of cardiovascular disease [91; 92], although the effect of chronic nitrate supplementation on lipid metabolism is not yet fully understood.