Why the Most Abundant Amino Acid in Your Body Might Be the One You're Short On

Why the Most Abundant Amino Acid in Your Body Might Be the One You're Short On

Glycine is one of the most abundant amino acids in the body, and yet it rarely makes the conversation. It shows up in proteins, neurotransmitters, and hormones, and while we get some from animal meats, the amount pales in comparison to what's in bone broth and connective tissue. In this blog, we'll cover what glycine actually does, why the standard "non-essential" label is misleading, and how to get more of it from food.

The "Non-Essential" Misunderstanding

In nutrition textbooks, glycine is classified as a non-essential amino acid. That means the body can synthesize it on its own, so you technically don't need to get it from food.

This is one of those biochemistry terms that turns out to mean something different than it sounds. "Non-essential" doesn't mean the body makes enough. It means the body can make some. The actual production capacity, when measured against demand, falls short by a meaningful margin for most people.

A 2009 paper by Meléndez-Hevia and colleagues calculated the gap and put it at around 10 grams per day. That's the amount of glycine the body uses but doesn't produce on its own. To make up the difference, you have to eat it.

The problem is that most people aren't eating the foods that contain it.

Where Glycine Actually Comes From

Glycine is concentrated in connective tissue: skin, tendons, cartilage, ligaments, and bones. The parts of the animal that get slow-cooked, broken down, and turned into broth, gelatin, and stew. Muscle meat, the boneless skinless chicken breasts and lean ground beef that dominate modern eating, is comparatively low in glycine.

This matters because for most of human history, eating an animal meant eating the whole animal. Connective tissue, organs, marrow, and bones were part of the package, either eaten directly or simmered into broth that extracted what couldn't be chewed. Nose-to-tail wasn't a wellness trend. It was the default.

The modern Western diet broke that pattern. Muscle meat became cheap and ubiquitous, while connective tissue, organs, and bones got separated from the food supply and sold off as pet food or rendered down for industrial uses. The result is a diet that delivers plenty of protein but is shaped very differently from what humans evolved eating.

What Glycine Does

Once you actually have enough of it, glycine ends up doing a surprising amount of work. A few of the most important roles:

Collagen synthesis. Roughly a third of collagen, by residue, is glycine. Collagen is the most abundant protein in the body and forms the structural framework of skin, tendons, ligaments, bones, cartilage, and the gut lining. Every time you build or repair connective tissue, you're spending glycine to do it.

Glutathione synthesis. Glycine is one of three amino acids that make up glutathione, the body's primary intracellular antioxidant. Glutathione is central to detoxification, immune function, and protecting cells from oxidative stress. Adequate glycine is a prerequisite for adequate glutathione.

Sleep. Glycine acts as an inhibitory neurotransmitter in the central nervous system. Research on glycine supplementation before bed has shown improvements in subjective sleep quality and measurable changes in sleep architecture, particularly slow-wave sleep. The mechanism appears to involve a drop in core body temperature mediated by glycine's effect on circulation.

Methylation balance. This is where the story gets interesting. Methionine, abundant in muscle meat, raises homocysteine when consumed in excess. Glycine acts as a buffer, helping the body clear excess methyl groups through a pathway called glycine N-methyltransferase. The implication is that the ratio of methionine to glycine in the diet may matter as much as either one in isolation, and that traditional whole-animal eating naturally balanced this ratio in a way modern muscle-meat-heavy eating does not.

Digestion. Glycine is required for bile acid conjugation, which is part of how the body processes dietary fats. It also plays a role in maintaining the integrity of the gut lining, where collagen is a major structural component.

That's not an exhaustive list, but it's enough to make the point. Glycine is structural, metabolic, neurological, and digestive all at once. Falling short on it doesn't show up as a single deficiency symptom. It shows up as a slow accumulation of suboptimal function across multiple systems.

The Connective Tissue Catch

There's a specific consequence of low glycine intake that anyone who trains hard should pay attention to.

Tendons and ligaments turn over much more slowly than muscle. When you lift, climb, or run, your muscle adapts to the load relatively quickly. Your connective tissue lags behind, sometimes by months. This adaptation gap is one of the most common drivers of overuse injuries in athletes, particularly the kind that show up after a period of rapid strength gain. The muscle is ready for the load; the tendon isn't.

Collagen synthesis is rate-limited by a handful of inputs: glycine, proline, vitamin C, and mechanical loading. If any one of those is short, the body can't repair connective tissue as fast as it's being stressed.

Keith Baar's lab at UC Davis has done some of the most interesting work in this area, showing that consuming gelatin (essentially concentrated glycine and proline) with vitamin C about an hour before intermittent loading increases collagen synthesis. It's one of the more practical applications of glycine research, especially for anyone training through or coming back from a connective tissue injury.

Cofactors Matter

This is also why isolated collagen peptide supplements have limits. Building collagen in the body requires more than just the amino acids. Vitamin C, copper, zinc, and adequate overall protein are all part of the equation. Pulling glycine out of its food context and selling it as a powder misses the matrix that makes it work.

Bone broth, by contrast, brings the full picture: glycine, proline, hydroxyproline, the minerals dissolved out of the bones during simmering, and the trace cofactors that come along with whole-food processing. It's a more complete delivery system, which is part of why traditional cultures got results from broth without knowing the biochemistry behind it.

Practical Takeaways

If you want to actually move the needle on glycine intake, the foods that matter are:

• Bone broth, especially long-simmered or properly processed powdered versions
• Slow-cooked tougher cuts (short ribs, oxtail, shanks, shoulder)
• Skin-on poultry
• Gelatin (the powdered kind for cooking, not flavored Jell-O)
• Organ meats, which contribute alongside their other benefits
• Connective-tissue-heavy preparations like stews, braises, and confits

A diet built around lean chicken breast, egg whites, and protein powder will hit your total protein numbers but leave you short on glycine specifically. The fix isn't more protein. It's different protein.

The Bigger Pattern

Glycine is one example of a broader pattern in modern nutrition: the foods that traditionally provided certain nutrients have been edited out of the diet, and the resulting gaps don't show up as classic deficiency diseases. They show up as low-grade dysfunction across systems that depend on those nutrients to run well.

The body knew what it was doing. We just stopped feeding it the full animal.