L-Arginine is a staple in many regiments due to it's ability to have vasodilative effects; the relaxation and opening of blood vessels caused by an increase in circulating Nitric Oxide(N.O.) Levels. L-Arginine and L-Citrulline are the two most effective amino acids for increasing N.O. as they are metabolized by nitric oxide synthase (the enzyme that produces N.O.) Increasing Nitric Oxide levels is one of the most common ways to improve performance as increased bloodflow to muscle tissue results in an increased amount of nutrients being transported to the tissue. This results in:
a key intermediate in the Urea cycle (excretion of ammonia from the breakdown of amino acids) and by-product of nitric oxide synthesis from arginine that is eventually recycled and used to generate more nitric oxide. Nitric oxide is a cell signaling molecule that is involved in vasodilation (opening blood vessels getting more blood and nutrients to the muscle), insulin secretion, the formation of new blood vessels, and neural development. This form is attached to the anion form of malic acid-malate, which is used in the krebs cycle to generate ATP.
Arginine is an amino acid that is directly converted to nitric oxide by nitric oxide synthase which is metabolized more rapidly than citrulline. This form is attached to alpha-ketoglutarate (AKG); a compound involved in various beneficial processes such as the Krebs cycle, nitrogen transport, and interactions with molecular oxygen
The Effects of Oral l-Arginine and l-Citrulline Supplementation on Blood Pressure.
"The current data suggest that oral Arg supplementation can lower BP by 5.39/2.66 mmHg, which is an effect that is comparable with diet changes and exercise implementation. The antihypertensive properties of Cit are more questionable, but are likely in the range of 4.1/2.08 to 7.54/3.77 mmHg. The exact mechanism by which Cit and Arg exert their effect is not fully understood, as normal plasma Arg concentration greatly exceeds the Michaelis constant (Km) of eNOS. Thus, elevated plasma Arg concentrations would not be expected to increase endogenous NO production significantly, but have nonetheless been observed in other studies. This phenomenon is known as the "l-arginine paradox"."
Citation: Khalaf, D., Krüger, M., Wehland, M., Infanger, M., & Grimm, D. (2019). The Effects of Oral l-Arginine and l-Citrulline Supplementation on Blood Pressure. Nutrients, 11(7), 1679. doi: 10.3390/nu11071679
l-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans.
"These variables were not altered by Arg supplementation (P > 0.05). In conclusion, these results suggest that short-term Cit, but not Arg, supplementation can improve blood pressure, V̇o2 kinetics, and exercise performance in healthy adults."
Citation: Bailey, S. J., Blackwell, J. R., Lord, T., Vanhatalo, A., Winyard, P. G., & Jones, A. M. (2015). l-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans. Journal of Applied Physiology, 119(4), 385–395. doi: 10.1152/japplphysiol.00192.2014
Administration of L-arginine plus L-citrulline or L-citrulline alone successfully retarded endothelial senescence.
"Therefore conclusively, L-citrulline supplementation rescued NO levels better than L-arginine supplementation by inhibiting ROS production and arginase 2 protein expression. Consequently, L-Cit and LCLA supplementation retaeded HG-induced endothelial senescence."
Citation: Tsuboi, T., Maeda, M., & Hayashi, T. (2018). Administration of L-arginine plus L-citrulline or L-citrulline alone successfully retarded endothelial senescence. Plos One, 13(2). doi: 10.1371/journal.pone.0192252
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