# The Effects of L-Citrulline and Malic Acid on Substrate Utilisation and Lactate Elimination
## 4 Discussion  
The present study aimed to answer the question of whether the separate and acute administration of L-citrulline and malic acid could improve aerobic endurance. Our results show that L-citrulline significantly lowered RER values at 70, 105 and 140 W, while malic acid significantly decreased RPExHR values at 175 and 210 W. On the other hand, neither of the supplements affected lactate levels or increased maximal power at RER 1.1 significantly. There are multiple explanations for the findings presented above. Although [PERSON] et al., (2011) who studied a swimming until exhaustion test in mice [10], and [PERSON] et al., (2017) who monitored a group of handball players during a 4-week training block [15], found lower lactate levels after citrulline supplementation, in the present study, neither citrulline nor malic acid had a meaningful impact on lactate levels. As we used similar dosages to other studies and malic acid also did not impact lactate levels on its own, there may be an interaction between L-citrulline and malic acid which led to lower blood lactate levels in the research where they used a citrulline-malate blend [15]. Therefore, further research is needed to identify the pathway through which malic acid may highlight L-citrulline and its impact on blood lactate levels [15].  
[PERSON] et al. (2016) observed an increase in performance during a cycling time trial after supplementing with citrulline for 7 days [10], while [PERSON] et al. (2007) administered malic acid for 30 days to mice which significantly increased their swimming time until exhaustion [12]. Nonetheless, we expected to see a higher power output at RER 1.1 compared to the placebo in our participants. Although the differences in maximal power did not reach statistical significance, supplementing with these two compounds may actually be worth considering for professional athletes. For competitions at the highest level, where placings are often decided by a few seconds or even less, a 4-5 Watt increase in power is certainly beneficial to every athlete. Considering the low cost and accessibility of these supplements, athletes who wish to improve their aerobic endurance may consider using these supplements.  
According to [PERSON] (2011), the most common reasons for fatigue during long-distance events are dehydration, hyponatremia and the depletion of glycogen stores [26]. One way to prevent glycogen depletion during exercise is by utilising higher amounts of fatty acids. [PERSON] et al. (2018) identified the range for maximal fat oxidation (MFO) to be 45-65% of one's VO2 max [27]. When further increasing intensity, the 70:30 ratio of fatty acid/carbohydrate metabolism starts shifting more towards the breakdown of additional glucose [28]. Our results seem to suggest that supplementing with L-citrulline as well as malic acid may decrease RER values at low-to-medium intensities which would enable  
Figure 6: RER and lactate mean values during active recovery.  
the athlete to supply more of their energy demands from fatty acids and be less reliant on glucose metabolism, thus sparing glycogen stores. This may apply mostly to long-distance events, where the depletion of glycogen can be a major factor in a drop in performance (long-distance triathlons, marathons, ultramarathons, etc.).  
[PERSON] et al. (2018) made an observation that there may be a discrepancy between RPE, HRand lactate values at given intensities ([PERSON] et al., 2018). They concluded that, in order to determine an athlete's actual state of fatigue, it is necessary to incorporate RPE besides HRand/or lactate as people often perceive a given load differently. The significantly lower RPExHRvalues observed following malic acid supplementation can be useful to reduce perceived fatigue during training and competition. Furthermore, we encourage coaches and self-coached athletes to use this metric as it interlinks objective and subjective parameters which helps to estimate the athlete's current state of fatigue and how they perceive a certain training load.