Quick, name some items that can be recycled. Glass, plastic, paper, aluminum, cardboard, steel…amino acids L-arginine and L-citrulline? It may not be quite the same process, but the idea is similar. While used materials like paper and plastic go to a recycling plant, our bodies have an internal recycling system, known as the L-citrulline/L-arginine recycling pathway. Together, the two amino acids, along with several other nutrients, work together to produce Nitric Oxide (NO) within the body.

NO is a critical signaling molecule in the cardiovascular system; the molecule plays an important role in the cardiovascular system through its production in the lining of the blood vessels and enhances the delivery of oxygen and nutrients to the cells. As a vasodilator, NO is  a compound responsible for controlling blood flow and pressure, while helping keep the vessels young, elastic and healthy. You can’t supplement NO, but you can give your body more of the nutrients it needs to produce it, namely L-arginine and L-citrulline.


How the L-arginine/L-citrulline Recycling Pathway Works

The pathway works basically like this: Within an endothelial cell, L-arginine is converted to NO. A byproduct of that process is L-citrulline. L-citrulline is recycled back to L-arginine. And the process repeats.


Of course, the path from L-arginine to NO isn’t quite so straightforward. L-arginine is converted to NO through the action of an enzyme. For that specific enzyme to work, it needs folic acid.


Keeping the Process Going

You might be thinking this process sounds self-sustaining. If our bodies can recycle the byproduct of NO (L-citrulline) into L-arginine (which is converted into NO), then why would we need to add those amino acids into our bodies? Won’t they just keep going round and round the recycling process, keeping the endothelial cells healthy and NO plentiful?

Unfortunately, NO levels naturally decline with age, and there are other processes that can diminish endothelial health and NO supply, such as free radical damage. That’s why supplementing with L-citrulline and L-arginine is so important. There are other nutrients I personally supplement with, but we’ll focus on just those two here.

Adding L-citrulline and L-arginine to the body is just simple science: When your endothelial cells have more of the nutrients that are involved in producing NO, the endothelium is likely to produce more NO. It’s also important that you are getting plenty of both amino acids. It’s true that L-arginine is directly converted into NO, but L-citrulline can more easily cross the cell membrane. Since L-citrulline is converted to L-arginine, which is converted to NO, you end up with more of both amino acids within the cell, giving the endothelial cells even more to work with.


Sources of L-arginine and L-citrulline

An amino acid is a protein building block. When protein is consumed, it’s broken down by the digestive system into the simplest forms, including L-arginine and L-citrulline. Certain protein sources like red meat, fish, chicken, beans, soy and nuts are sources of L-arginine. L-citrulline is found in many of the same sources, and it’s also found in melons.

Most people just don’t consume enough of the two amino acids—and the other nutrients needed to support endothelial health—to produce optimal levels of NO. In many cases, supplementation may be a good option.


Your body has a powerful internal recycling system. Why not give it the materials it needs to function at its best—especially L-arginine and L-citrulline? Please share your strategies for getting plenty of these amino acids in the comments.


[1]Ignarro, Louis J. 2005. No More Heart Disease: How nitric oxide can prevent—even reverse—heart disease and stroke. New York: St. Martin’s Press.