How to Get Less Cysteine and More Glycine

Methionine restriction fundamentally supports health, however, much of the methionine we eat is converted into another amino acid within the body called cysteine. Given that the provision of extra cysteine to methionine-restricted animals reverses some of the benefit, cysteine might be responsible for some of methionine’s deleterious effects. Indeed, epidemiological studies have found that cysteine intake correlates to a near tripling the risk of dying from diabetes. So the restriction of the sulfur-containing amino acid cysteine may do more than just reduce bad breath and malodorous rectal gas. 

Thankfully, methionine and cysteine are found concentrated in similar foods, such that even at the same overall protein intake, those eating plant-based diets consume significantly less. The RDA for methionine and cysteine together is 19 mg/kg a day, so about 1.3 grams someone who weighs 150 pounds. The average vegan gets 2.3 grams a day, the average vegetarian 3.0, the average quote-unquote “balanced” American diet 4.3 and a high-protein diet as much as 6.8. So most people may be eating three times the recommended sulfur containing amino acid intake. 

One way methionine and cysteine restriction has been studied is to put everyone on a vegan diet and then randomize half to a placebo and the other half to methionine/cysteine supplements to test low versus high intake. On the straight vegan diet, the levels of the longevity hormone FGF21 increased by 47 percent within seven days. So maybe the reduced intake of sulfur-containing amino acids accounts for some of the beneficial health effects attributed to plant-based diets. 

While cysteine may be a partner in crime, the amino acid glycine is used by the body to clear methionine from the system. Dietary glycine can be used to alleviate methionine toxicity, suggesting, perhaps, that increased glycine intake might be able to mimic some of the benefits of methionine restriction. Supplemental glycine has been shown to decelerate aging in C. elegans and prolong the lifespans of rats and mice. 

In people, while higher blood levels of cysteine correlate with obesity and are predictive of weight gain, blood levels of glycine are higher in skinnier people. Genetically determined glycine levels may… or may not correlate with heart disease risk. Glycine supplementation can decrease blood pressure in rats, but when put to the test in people one study failed to find any such effect, and the other noted a significant drop in blood pressure in men but not women. Similarly, an anti-inflammatory effect of glycine in animal models only met with mixed effects in people. However, randomized double-blind placebo controlled trials have shown extra glycine can reduce oxidative stress in men and women with metabolic syndrome and improve long-term blood sugar control in diabetics. 

How can you get more glycine in your bloodstream? The same way you get less methionine, by eating a plant-based diet, based on both cross-sectional and interventional data. At the same protein intake, vegetarians may get 23 percent more glycine in their diet.