L-citrulline ~ Sports drinks with fast carbohydrates that endurance athletes drink during training sessions, and also protein shakes that bodybuilders down before pumping iron, probably work better if they contain a couple of grams of L-citrulline [structural formula on the right]. Scientists at Maastricht University Medical Centre in the Netherlands discovered that L-citrulline boosts the body’s nutrient uptake during exertion.
Peri-workout-nutrition is a tried and tested performance-enhancing strategie in all areas of sport. Ingesting extra carbohydrates during a training session helps endurance athletes achieve better times, and speeds up the restoration of glycogen reserves in the muscles. Bodybuilders accumulate more muscle mass, and recover faster, if they ingest a portion of proteins or amino acids just before training.
But during intensive exertion the digestive tract becomes less active, so the uptake of amino acids and carbohydrates decreases. As a result the added value of peri-workout-nutrition is limited. For example, the optimal amount of protein that bodybuilders can take in before and after workouts is not very large: 18 g recent studies suggest. Dutch scientists reported already years ago that caffeine can have to some extent a positive effect, but little has been done with this information.
The researchers in Maastricht were curious whether supplementation with L-citrulline, an amino acid that is produced when L-arginine oxidises, might improve the functioning of the small intestine during exercise. The capillaries in the intestines become narrower during exertion because the body gives priority to supplying the muscles with oxygen and nutrients. L-Citrulline converts into L-arginine in the intestines, and enzymes in the blood vessels of the gut cells can use L-arginine to produce nitrogen monoxide [NO].
Nitrogen monoxide causes blood vessels to widen, thus improving the oxygen supply to the gut cells. It also enables more nutrients to be transported from the digestive tract to the body. At least, that’s what you’d expect.
The researchers got trained endurance athletes to cycle for an hour on two different occasions. The intensity was that of 70 percent of their maximal load.
Half an hour before starting to cycle the subjects were given a placebo on one occasion and on the other 10 g L-citrulline. The researchers chose not to experiment with L-arginine, as many people get acute diarrhoea the first time they ingest 10 g L-arginine. In addition, the body breaks L-arginine down fast.
After taking a placebo the concentration of intestinal fatty-acid binding protein [I-FABP] rose in the subjects’ blood. This protein is normally found in gut cells. If these disintegrate as a result of intensive exertion, then the concentration of I-FABP in the blood increases. The I-FABP concentration did not rise if the subjects had been given L-citrulline before starting to cycle.
While the subjects cycled the researchers used portable microscopes to examine the blood vessels under the athletes’ tongues. If these widened the blood vessels in their intestines probably did the same. The figures above show that cycling with a placebo caused the blood vessels under the tongue to become narrower, but supplementation with L-citrulline did cause them to widen.
“The current study demonstrates that a single oral dose of L-citrulline prior to exercise preserves splanchnic perfusion and reduces intestinal injury during exercise”, the Dutch write. “The mechanism probably entails increased arginine availability for NO-mediated vasodilatation. These data suggest oral L-citrulline supplementation to be a promising strategy to improve gastrointestinal blood flow and prevent intestinal injury in athletes without adverse gastrointestinal effects as observed for arginine.”
L-citrulline Improves Splanchnic Perfusion and Reduces Gut Injury during Exercise.
Splanchnic hypoperfusion is a physiological phenomenon during strenuous exercise. It has been associated with gastrointestinal symptoms, intestinal injury, and may hamper athletic performance. We hypothesized that L-citrulline supplementation improves splanchnic perfusion and decreases intestinal injury by enhancing arginine availability. The aim of this study was to determine the effect of L-citrulline intake on splanchnic perfusion, intestinal injury and barrier function during exercise.
In this randomized, double-blinded crossover study, ten men cycled for 60 min at 70% of their maximum workload after L-citrulline (10 g) or placebo (L-alanine) intake. Splanchnic perfusion was assessed using gastric air tonometry. Sublingual microcirculation was evaluated by sidestream dark-field imaging. Plasma amino acid levels and intestinal fatty-acid binding protein (I-FABP) concentrations, reflecting enterocyte damage, were assessed every 10 min. Urinary excretion of sugar probes was measured to evaluate intestinal permeability changes.
Oral L-citrulline supplementation enhanced plasma citrulline (1840.3±142.3 ?M) and arginine levels (238.5±9.1 ?M) compared to placebo (45.7±4.8 ?M and 101.5±6.1 ?M, respectively; P<0.0001), resulting in increased arginine availability. Splanchnic hypoperfusion was prevented during exercise following L-citrulline ingestion (reflected by unaltered gapg-apCO2 levels), whereas gapg-apCO2 increased with placebo treatment (P<0.01). Accordingly, L-citrulline intake resulted in an increased number of perfused small sublingual vessels compared with placebo (7.8±6.0 vs. -2.0±2.4; P=0.06). Furthermore, plasma I-FABP levels were attenuated during exercise following L-citrulline supplementation compared to placebo (AUC0-60min -185±506 vs. 1318±553 %; P<0.01). No significant differences were observed for intestinal permeability.
Pre-exercise L-citrulline intake preserves splanchnic perfusion and attenuates intestinal injury during exercise in athletes compared to placebo, probably by enhancing arginine availability. These results suggest that oral L-citrulline supplementation is a promising intervention to combat splanchnic hypoperfusion-induced intestinal compromise.
PMID: 24621960 [PubMed – as supplied by publisher]