## 1 Introduction  
Several studies have reported that both acute and chronic supplementation with inorganic nitrate (NO\({}_{3}\)\({}^{-}\)) or betroot juice (BRJ), which is known to contain NO\({}_{3}\)\({}^{-}\), can improve exercise performance; yet, the underlying mechanisms are still not completely understood [1, 2, 3, 4, 5, 6]. NO from endogenous and exogenous sources is a signaling molecule and has many biological functions, such as dilation of arterioles [7, 8, 9], glucose uptake [10, 11], and stimulation of mitochondrial biogenesis [12]. Research also suggests that NO may improve mitochondrial efficiency by reducing the O\({}_{2}\) cost of adenosine triphosphate (ATP) resynthesis [13] and/or improve muscle contractile efficiency by reducing the ATP cost of muscle contraction and the ATP turnover rate from creatine phosphate (PCr) hydrolysis [14, 15].  
It has been shown that ingested nitrate, such as in the form of beetroot juice (BRJ), can lead to reduced oxygen cost during exercise, altered oxygen uptake kinetics [15; 16; 17; 18; 19; 20; 21], and improved exercise tolerance [15; 16; 17; 20; 22; 23]. Studies suggest that increased NO bioavailability may also improve lipid metabolism [24; 25] or reduce the utilization of muscle glycogen [26].  
Most studies have investigated the effects of nitrate supplementation on single exercise tasks, and the effects of long-term nitrate supplementation on exercise training remain unknown. Some studies with sprint interval training (SIT) have reported an increase in sprint performance upon nitrate supplementation [27; 28; 29]. Yet, the meta-analysis by [PERSON] et al. [30] revealed no changes in power during high-intensity interval training. Additionally, look at more recent review and meta-analysis papers on nitrate supplementation [31; 32; 33; 34].  
In a previous study [35], we showed a reduction in the ratio of \(\text{VO}_{2}\) to power (\(\text{VO}_{2}\)/W), up to seven days after the end of nitrate supplementation during 3-week high-intensity, high-volume intermittent training (HIIVT). These observations suggest that supplemented inorganic nitrate may have changed the physiological adaptations to HIHVT [35]. The mechanisms leading to a reduction in \(\text{VO}_{2}\) after nitrate supplementation remain unclear. In the study previously reported by [PERSON] et al. [35], nitrate supplementation was found to increase the synthesis of homoarginine (hArg) during HIHVT [36], suggesting the effects of nitrate supplementation on amino acid metabolism.  
In addition to the reduction in \(\text{VO}_{2}\)/W [35], nitrate supplementation may also reduce the substrates' turnover, thus leading to an altered energy supply, possibly resulting in changes in the plasma concentrations of related parameters. We hypothesized that nitrate supplementation would be accompanied by changes in various metabolic pathways during HIHVT [35]. If \(\text{VO}_{2}\) during exercise is decreased, muscle efficiency may be increased and therefore, a decrease in metabolites of ATP degradation, represented by xanthine (XA) and uric acid (UA), may be expected. We further hypothesized that lipid metabolism, represented by triglycerides (TG), free glycerol (Glyc), and free fatty acids (FFA), may be decreased with nitrate supplementation. Other changes may occur in amino acid metabolism, including arginine (Arg), creatinine, and ammonia (NH\({}_{3}\)). In the present study, the potential effects of short-term nitrate supplementation on healthy male recreational athletes during HIHVT were investigated.