Can a high-protein diet cause a glycine deficiency?

In people who are overweight or obese, the body appears to handle protein-rich meals differently than in people of a healthy weight. After a high-protein meal, blood glycine levels rise less sharply in people who are overweight than in leaner people. This applied not only to glycine but also to other amino acids such as alanine, glutamine, and threonine. This finding comes from a small study of 30 people, so it should be considered preliminary.

Exactly how this works has not yet been explained. One possible explanation is that the body in people who are overweight distributes or uses up glycine differently, possibly because their metabolism functions in a different way. The study only measured the immediate response after the meal, not what happens to blood glycine levels over the longer term. Whether a high-protein diet causes a glycine deficiency in everyone cannot be concluded from this.

As for the link with blood pressure: a higher intake of certain amino acids that are common in high-protein diets, namely branched-chain amino acids such as leucine, isoleucine, and valine, is associated in a body of research covering nearly 58,000 participants with an increased risk of high blood pressure. Glycine points in the opposite direction: people who get more glycine through their diet had a 25% lower risk of high blood pressure in that same research. Whether this is a cause-and-effect relationship has not been proven; these are associations measured in people followed for years.

In practical terms, this means that a high-protein diet influences the balance of amino acids in your blood, and that people who are overweight appear to respond differently than people of a healthy weight. Whether you should therefore consciously eat more glycine through sources such as gelatine, bone broth, or collagen does not follow from these studies. That is a step beyond what the research supports.