What is arginine?

In one line

Arginine is one of the amino acids that make up protein, and the body uses it as raw material to make nitric oxide, which widens blood vessels, and creatine, among other things.

What is arginine?
The name you see on 'nitric oxide booster' labels
'Arginine' often appears on the labels of exercise supplements that claim to widen blood vessels. This post explains what arginine is, where the body gets it, what it uses it for, and where it goes when you swallow it, step by step.
In one line
A building material for protein and for a vessel-widening signal
Arginine is one of the building materials of the body's proteins. The body uses arginine to make a signal substance that widens blood vessels, and also to make creatine, which muscles use when they produce force.
First, what is an amino acid?
The 20 kinds of pieces that make up protein
Amino acids are the small pieces that make up protein. The body builds its proteins by linking 20 kinds of amino acids together in chains, and arginine is one of those 20. So when you eat food that contains protein, you also take in arginine.
The body makes it too
Usually enough, but sometimes short
The body makes arginine on its own. For example, the small intestine makes a similar amino acid called 'citrulline', and the kidneys turn it into arginine. Usually this is enough, but during serious injury or illness, or fast growth, it falls short. Because only at such times must it come from food, it is called a 'conditionally essential amino acid'.
Raw material for nitric oxide
An enzyme turns arginine into a vessel-widening gas
An enzyme is a protein that helps chemical reactions happen in the body. An enzyme in blood vessel walls turns arginine into a gas called 'nitric oxide'. Nitric oxide signals the muscle in vessel walls to relax, which widens the vessels, and what is left of the arginine becomes citrulline.
Other things it makes
It is also raw material for urea and creatine
Arginine is raw material for many substances besides nitric oxide. In the liver, it is used in the process that turns waste left over from using protein into 'urea', which is passed out in urine. In the liver and kidneys, it is used together with another amino acid, glycine, to make creatine.
Where does swallowed arginine go?
A large part is broken down first in the gut and liver
When you swallow arginine, enzymes that are plentiful in the gut and liver break down a large part of it first. Breaking down means cutting it into smaller pieces and turning it into other substances. So less reaches the blood than you swallowed, and blood arginine actually rose more after taking citrulline.
Taking a lot at once
Stomach upset came mostly above 9 g at once
When people took 3-6 g of arginine at once, problems were rare. Most side effects appeared when more than 9 g was taken at once, and they were nausea, vomiting, and diarrhea. When the same amount was split into several smaller doses, these symptoms went away.
A common misconception
Does taking arginine quickly raise nitric oxide?
No. When healthy people took 6 g of arginine once, the blood substances that show how much nitric oxide was made did not differ from a fake pill over 2 hours. After taking it for 1 week, how well blood vessels widened did not improve from the start either.
Summary
Raw material for protein, nitric oxide, and creatine
Arginine is an amino acid that makes up protein and raw material the body uses to make nitric oxide, creatine, and more. But swallowed arginine is largely broken down in the gut and liver, and when healthy people took it, nitric oxide did not quickly rise. Related terms: citrulline, nitric oxide, creatine.