L-Citrulline is a natural occurring amino acid, which is not coded for by the DNA. Watermelon is the most well-known natural source for this amino acid (Fish & Bruton, 2010). Although this is a non-protein amino acid, it fulfils important functions in cellular metabolism.
Citrulline-formation is a step in the urea cycle, needed to remove toxic ammonium from the body (Curis et al., 2005). The exact function will be explained in the next paragraph. The ingredient citrulline is combined with malate. Malate is involved in the tricarboxylic acid cycle (TCA) as an intermediate (Bendahan et al. 2002), and in this way it is comparable to the function of L-citrulline. It is not an amino acid and since it is produced by the body, it is also not essential. It plays an important role in generating energy in the mitochondria, with the end product adenosine triphosphate (ATP). A second role is transport of NADH into the mitochondria, which supports energy production (Qiang 2016).
Effects of L-citrulline related to sport Before digging deeper into the investigations performed on the effects of L-citrulline related to sport, the theoretical framework will be described, to appreciate the importance of L-citrulline. During exercise, there is an increase of ammonium in the blood (Banister et al., 1983; MacLean et al., 1991), among others because of BCAA catabolism (Broberg & Sahlin, 1989) and amino acid oxidation (MacLean et al., 1991). The concentration of ammonia in the blood is sensitive to training (Yuan & Chan, 2004). Accumulation of ammonium in the body is problematic, since it is toxic. Here, the urea cycle intervenes, since it transforms ammonium via arginine to non-toxic urea, which can easily be excreted via urine, and ornithine. Ornithine is recycled to citrulline again (Atkinson, 1992). For a simplified overview of this cycle, see Figure 1.
Besides this cycle, arginine forms a substrate for an enzyme called NO synthase (NOS), which is not involved in the urea cycle. This enzyme produces nitric oxide (NO) and citrulline from arginine and O2, meaning that no urea is formed (Nagasaka et al., 2009). NO is reported to improve sport performance by a variety of human studies, because NO is an important signalling molecule in the body for a wide variety of functions (T. Suzuki et al., 2016). The most relevant function is vasodilation, the widening of blood vessels and thereby improving the blood flow through skeletal muscles (Totzeck et al., 2012). This results in a better gas and nutrient exchange and is therefore thought to improve exercise performance.
L-citrulline is the precursor for L-arginine, which is the substrate in both processes. Intake of citrulline has proven to be more effective in enhancing blood plasma levels than L-arginine, since L-arginine is cleared out in the liver (where all the blood goes through after the uptake of nutrients in the intestine) (Hickner et al., 2006; T. Suzuki et al., 2016). In contrast, L-citrulline is not processed in the liver. L-citrulline enters the body and is processed in the muscles and kidneys to L-arginine (Hartman et al., 1994), thereby increasing the plasma concentrations of L-arginine (Hickner et al., 2006). There are also indications that L-citrulline intake combined with L-arginine intake enhances the uptake efficiency and thereby the concentration of both AAs increase in the blood plasma (I. Suzuki et al., 2019).
Eight randomized, placebo-controlled, double-blind studies were analysed for the literature study on L-citrulline. Results concerning the effects of Citrulline on sport performance are mixed. Two studies found no effect on exercise performance (Ashley et al., 2018; Cutrufello et al., 2015). One study found a negative effect on the time of exhaustion upon L-citrulline intake (Hickner et al., 2006). Further, four studies did not measure a direct effect on sport performance, but indirect factors, such as arterial blood pressure and oxygen consumption (Ashley et al., 2018; Bailey et al., 2015; Hickner et al., 2006; McKinley-Barnard et al., 2015). Additionally, these factors did not show strong alterations upon intake of L-citrulline, although one study found that L-citrulline improved the sport performance compared to the placebo (Bailey et al., 2015). Three other studies found positive effects from L-citrulline on the sport performance (Chen et al., 2016; I. Suzuki et al., 2019; T. Suzuki et al., 2016). It should be taken into account that Chen et al. combined L-citrulline with other ingredients, therefore the positive effect cannot be directly linked to L-citrulline, even though there was an increase in NO concentration. Therefore, it could be hypothesized that part of the beneficial effect on the sport performance can be attributed to L-citrulline. T. Suzuki et al. (2019) also combined L-citrulline with L-arginine intake over a longer period of time. I. Suzuki et al. (2016) provided the most direct results on the impact of citrulline intake. This study was able to link a long term, high dose intake to a reduced muscle fatigue and thereby a better performance.
Effects of malate related to sport As discussed, the effects of malate supplementation are suggested to be mainly on the level of energy generation via the TCA cycle promoting aerobic and anaerobic metabolism (Bendahan et al. 2002; Glenn et al. 2016; Qiang 2016). Malate is therefore thought to improve muscle function (Wagenmakers 1998). Gibala et al. tested the hypothesis that a decrease in TCA cycle intermediates is correlated with a decrease in energy availability in muscles. Besides this autonomous pathway, malate is often supplemented in combination with a NO donor, in this case L-citrulline, to improve the bioavailability of NO (Bescós et al. 2012). Like L-citrulline, the evidence is mixed on the functionality of malate. Three studies reported sport performance enhancing capacities of citrulline malate ingestion compared to the placebo (Glenn et al. 2017; Pérez-Guisado and Jakeman 2010; Wax, Kavazis, and Luckett 2016). Glenn et al. (2016) have measured different kinds of exercise. They found a positive effect in peak power and explosive power, but no difference in cycling performance in peak and average vertical power (Glenn et al. 2016). Since L-citrulline and malate are co-ingested no conclusions can be drawn about the effect of malate itself on sport performance (Kerksick et al. 2018). Da Silva et al. performed a study on the muscle recovery, no positive effect of citrulline malate intake on muscle soreness. Another study reported decreased muscle soreness (Pérez-Guisado and Jakeman 2010).
Needs of different athletes As mentioned before, evidence for the effect of L-citrulline supplementation on sport performance is lacking. Besides, the specific needs of the different athletes are not known, since L-citrulline is a non-essential amino acid, the body can form it via the processes described before.