Sulfur amino acid restriction for longevity?
Methionine or Cysteine? Pulling the amino acid levers for weight loss and healthspan.
Historically, animal studies have shown that calorie restriction significantly increases longevity. My mental picture: hungry mice that live an extra couple of hungry months. It turns out that it is the restriction of specific amino acids that drives increased longevity, rather than calorie restriction alone.[ref]
Amino acids are the building blocks of protein, and there are 20 amino acids that we need to survive. We can synthesize some of the amino acids endogenously, but others - essential amino acids - we need to get from foods.
Researchers have found that two of these essential amino acids, methionine and/or cysteine, are key to both longevity and weight loss.
I wrote last year about the studies on methionine restriction for longevity. You can read it in full here, or here’s a TL;DR for you:
Methionine restriction (MR)—limiting the essential amino acid methionine in the diet —has been shown in animal studies to extend lifespan and improve health, by reducing cancer, heart disease, and neurodegenerative risk. Traditional MR diets are hard to follow long-term and can lead to muscle loss and frailty, since methionine is found in most protein-rich foods. A 2024 study found that intermittent MR (eating a low-methionine diet just 3 days a week) offers similar longevity and health benefits as full-time MR, with fewer negative effects on muscle and bone health. Intermittent MR lowers IGF-1, a hormone linked to aging and disease, without the drawbacks of chronic protein restriction.
Methionine and cysteine: A little background
Methionine and cysteine are both sulfur-containing amino acids. Methionine is a precursor for cystine through the transsulfuration pathway, which converts methionine to homocysteine. Homocysteine can then be converted back to methionine with the addition of a methyl group, or it can be converted to cystathione and then cysteine via the transulfuration pathway and CBS enzyme. (Genetic Lifehacks article on CBS if you want more details)
Dietary cysteine reduces the demand on the methionine/methylation cycle.
Beyond the methylation cycle, both methionine and cysteine have antioxidant roles. Cysteine is essential for glutathione formation.
Thus, it may seem illogical that there would be benefits from restricting either methionine and/or cysteine. So let’s take a look at what’s going on…
New study on cysteine restriction:
A new study in mice shows that cysteine restriction alone causes significant weight loss - 30% in 7 days - in mice that lacked the ability to synthesize cysteine. Adding cysteine back into the diet reversed the weight loss. The study is interesting because the researchers dug into the reason why cysteine restriction causes significant and quick weight loss in conjunction with manipulation of the gene (CSE, cystathionine γ-lyase) for endogenous cysteine production.
The restriction of cysteine causes the animals to switch over to actively burning fat as fuel, even though calories were the same on the cysteine-deprived diet compared to the control diet. Importantly, the mice lost fat mass and not muscle mass. The researchers found that cysteine depletion shifted the metabolic response in the liver significantly, and it also upregulates a bunch of stress response genes, including FGF21. The metabolic changes included reduced lipogenesis and a “decline in adipose tissue lipid content”.
I’m pointing out the link to increased FGF21 since it’s a metabolic regulator with multiple studies showing that increasing it can increase healthspan and extend lifespan.[ref][ref]
Multiple other studies similarly show that cysteine is key to the weight loss and lipid benefits from sulfur amino acid restriction.[ref]
Life extension through restricting sulfur amino acids:
A sulfur amino acid restriction diet limits cysteine, methionine, homocysteine, and taurine. It’s been known since the 1980s that restricting methionine increased lifespan and healthspan, but many of the early studies on this also included the restriction of cysteine. A 2011 study teased out that adding cysteine to a methionine-restricted diet reversed many of the positive effects.[ref]
Studies in animal models show that cysteine restriction as part of sulfur amino acid restriction (SAAR) increases lifespan.[ref][ref]
This new study on complete cysteine restriction concludes that cysteine is the key amino acid in producing the longevity and metabolic benefits from SAAR.[ref]
Cysteine by itself is fairly rapidly converted in the liver to glutathione or taurine because it has cytotoxic properties. Cellular levels of cysteine are the lowest of all the amino acids. Studies in humans show that higher cysteine levels are linked to obesity, Alzheimer’s, Parkinson’s, and cardiovascular disease.[ref][ref][ref]
A 2024 clinical trial in adults (n=20) looked at the differences between a methionine restriction diet (4 weeks) and sulfur-amino acid restricted diets. The results showed that the SAAR restricted diet for four weeks led to decreased body weight, cholesterol, uric acid, leptin, insulin, and IGF-1, as well as increased plasma FGF-21. There were fewer changes noted from methionine restriction alone.[ref]
Another clinical trial involving an SAAR diet in healthy adults found that it reduced weight by 20% more than a similar diet (equal calories) that was high in sulfur amino acids.[ref] While weight loss alone doesn’t necessarily increase healthspan, being obese is a risk factor for many chronic diseases of aging.
In mice, SAAR restriction lowers protein synthesis rates, which may be beneficial in aging. Interestingly, the lower glutathione levels didn’t cause cellular stress but instead increased the Nrf2 pathway for the antioxidant response.[ref] In addition, cysteine restriction specifically increased serine biosynthesis and decreased triglyceride synthesis.[ref]
What does a low-cysteine diet look like?
A simplistic view is that any low-protein diet that avoids meat, eggs, and whole grains will be low in cysteine. However, the studies on low-cysteine diets all involve getting the other essential amino acids in the right amount. One of the clinical trials used a diet including low-protein foods, such as fruits and vegetables (13% protein), along with an amino acid nutrient shake that included amino acids other than cysteine.
According to the USDA database, foods highest in cysteine (per 100g) include whey protein powder, soy flour, sesame, peanut butter, eggs, almonds, flaxseed, walnuts, pecans, turkey, cheese, and beef.
Which protein sources are available that are low in cysteine? Collagen and gelatin are low in cysteine.
Where is this headed? Drug development…
Another recent study (April 2025) explored the role of low glutathione, along with sulfur amino acid restriction or a drug that inhibited glutathione. The study found that an SAAR diet lowered glutathione levels in the liver, but increased Nrf2 (antioxidants) and serine. Adding N-acetylcysteine to the SAAR diet abrogated the positive effects on lipids and weight loss. However, the study also included a group given a glutathione synthesis inhibitor drug. Even without the SAAR diet, inhibiting glutathione alone also reduced fat mass and had positive effects on lipids, albeit at a smaller effect size.[ref]
A 2023 study also looked at the effect of a cysteine lowering medication. The study showed that it worked for weight loss in mice and lowered cysteine levels in men.[ref]
Questions that remain:
For me, I don’t have a full picture of whether long-term cysteine deficiency is a good idea. On the one hand, low-protein diets such as vegetarianism have been shown to be beneficial for some aspects of healthspan. On the other hand, preserving muscle mass is important.
Other questions that come to mind for me:
Why do studies show that supplemental NAC and increased glutathione are good for aging (per this study)?
At what point does a sulfur amino acid restriction diet cause the breakdown of muscle mass? What is the effect on bone health?
Is intermittent cysteine restriction beneficial, similar to intermittent methionine restriction? Animal studies show that it is likely beneficial, but human studies are needed.[ref]
Conclusion:
Restricting cysteine and/or glutathione depletion has significant positive effects on weight and healthspan. To be honest, I have a hard time wrapping my brain around it, although the studies are extensive and clear. Perhaps it is cognitive bias on my part — I like to eat meat think of glutathione as the master antioxidant with positive benefits.
However, the increase in FGF21 fits with a lot of longevity research, as do the beneficial changes to lipid levels, weight, and overall lifespan. I’m going to give it a try with eating low protein - avoiding the sulfur amino acids - for a couple of days a week.