# The Effects of L-Citrulline and Malic Acid on Substrate Utilisation and Lactate Elimination
## 1 Introduction  
Endurance athletes often aim to achieve a more efficient aerobic metabolism and lactate-eliminating mechanisms to maintain exercise performance at higher intensities. Both citruline and malic acid can be found in their natural form in foods we eat every day. Watermelon is known for its high citruline content, whereas apples and cherries contain large amounts of malic acid [1, 2, 3]. They are also popular supplements that are taken by people suffering from kidney diseases with the goal of increasing blood flow thus treating hypertension and erectile dysfunction [4]. With regard to sporting performance, these supplements are mostly used blended together as a citruline-malate 2:1 powder which resembles the basis of many pre-workout supplements, used to increase blood flow to the working muscle, thus enhancing muscle contraction [5, 6]. The citric acid cycle plays an important role in aerobic metabolism, as every aerobic pathway produces acetyl-CoA, which enters this cycle and becomes further metabolised. Malate is an intermediate product of the citric acid cycle which is created from fumarate by the enzyme fumarase and is further converted to oxalacetate by the enzyme malate dehydrogenase [7]. Malate can also be produced directly from pyruvate via an anaplerotic reaction [8]. Exogenous malate may improve citric acid cycle function, which might lead to a more efficient aerobic metabolism [8]. L-citrulline has many important roles in the human body, two of which are worth highlighting. Firstly, citrulline is a urea cycle intermediate that is responsible for the excretion of ammonia, which is a byproduct of amino-acid breakdown [7]. Secondly, citrulline plays an important role in vasodilation where l-arginine is converted to N-hydroxyl-arginine and then into citrulline by the nitric-oxide enzyme in the endothelium of the vascular wall. This process is accompanied by nitrogen monoxide exiting into the smooth muscle tissue which leads to muscle relaxation [7]. Therefore, citrulline may provide better blood flow, possibly enhancing lactate elimination [7]. There is a substantial amount of research on this topic. However, in most of these studies, citrulline or malic acid is not given in isolation but in the form of citrulline-malate, which contains citrulline and malic acid in a 2:1 ratio [5; 6]. Therefore, these studies are unable to answer the question of whether the isolated administration of these supplements can benefit sporting performance. Furthermore, most of these studies examine the effects of citrulline-malate on resistance training, not aerobic endurance [5; 6]. Nonetheless, these studies show significant performance-enhancing benefits, including an increase in repetitions performed until failure, lower blood lactate levels and a decreased rate of perceived exertion (RPE) [5; 6]. Far more studies have been conducted involving only citrulline [9; 10; 11] than malic acid [12] supplementation regarding sporting performance. However, research on L-citrulline supplementation provides contradictory results. Most research papers confirm the theory that citrulline supplementation increases plasma nitrate levels [13]; however, this does not always lead to an increase in performance. [PERSON] et al. (2016) observed that although plasma citrulline and arginine levels were higher after supplementation, this was accompanied by higher resting blood pressure and did not improve exercise time until exhaustion [14]. In those studies, which show benefits for aerobic performance, the effects were comparable to the effects on resistance training: increased time until exhaustion (e.g., swimming until exhaustion), lower lactate and ammonia levels and decreased RPE at the same intensity [9; 10]. These benefits occurred in response to long-term supplementation rather than acute ingestion [9; 15]. The research examining the possible sporting performance-enhancing effects of isolated, acute malic acid ingestion is even more uncertain.