L-arginine is a semi-essential amino acid that increases nitric oxide levels. Nitric oxide plays a crucial role in vascular function, which means low arginine levels leads to cardiovascular problems, erectile dysfunction, and reduced physical performance.



Not surprisingly, arginine levels also affect how you respond to a COVID-19 infection.



While you do consume some arginine through your diet, you'd likely benefit from supplementing with it, or with citrulline.



In this blog post, I’ll review arginine’s health benefits and explain why supplementing with it along with citrulline is the best way to improve blood levels and gain the health, fitness, and performance benefits it offers.





What are arginine and citrulline?



L-arginine is a semi-essential amino acid synthesized from proline, glutamate, and glutamine.



It is essential for endothelial cell nitric oxide (NO) production. Endothelial cells line your blood vessels and NO levels determine the tone of and contractility of the vessels. 



Low NO levels lead to more rigid vessels. The more rigid the blood vessels, the higher your blood pressure and greater your risk of blood vessel damage and cardiovascular disease.



Nitric oxide increases blood flow and enhances muscle contraction, gas exchange, and mitochondrial biogenesis (growth of mitochondria).



Arginine also aids in creatine resynthesis and reduces lactate accumulation.



The average adult consumes 4-5 grams per day, mainly from meat, dairy, nuts, and seeds. Your kidneys synthesize another 2 grams from citrulline each day.



One of the drawbacks of arginine supplementation is that it isn’t completely absorbed into the bloodstream. The enzyme arginase metabolizes arginine. It’s found in the gut lining and liver, so some arginine that reaches either location may be metabolized before it can reach the bloodstream.



L-citrulline is an arginine precursor and is not metabolized in the gut or liver. Instead, it can pass into the bloodstream intact, where it contributes to the formation of l-arginine.



Research shows you can increase arginine levels in the blood through arginine or citrulline supplementation, but it's probably ideal to take both.



One study showed l-citrulline supplementation at a dose of 750 mg, twice daily, significantly increased blood levels of l-arginine, though taking 3 grams of l-citrulline twice daily increased levels much more.



Another study showed both arginine and citrulline increased blood levels of arginine, but you'd only need to take half as much citrulline as you’d take arginine to to reach the same blood level.



That said, it seems that combining the two is more effective than taking either individually. A 2017 study showed that taking 1 gram of each together increased blood levels of arginine more than taking 2 grams of either one individually.



I should also mention that there's more research using arginine than citrulline, which is what you might notice throughout the rest of this article.



Health Benefits



Increasing blood levels of arginine improves blood flow, prevents damage to blood vessels, stimulates hormone secretion, and enhances creatine synthesis. These effects lead to numerous health benefit, many of which I’ve summarized below.



Athletic Performance



Arginine increases blood flow throughout the body, which delivers oxygen and nutrients to working muscles and lactate and waste products away from them.



It also stimulates release of growth hormone and insulin-like growth factor 1 (IGF-1). Both hormones impact muscle growth and repair and growth hormone enhances fat metabolism.



At single doses of 5-9 grams, arginine can double resting growth hormone levels.



Exercise is the most effective way to increase growth hormone, and studies show that arginine supplementation prior to strength training may reduce the rise in growth hormone from exercise alone.



For supporting growth hormone,