What it is
Cysteine is a conditionally essential amino acid.
What it does
Amino acids are the building blocks of proteins. You don’t really need to think about what each amino acid does – your body just needs enough of each amino acid to build the proteins it needs to build.
Nonessential amino acids are amino acids your body can synthesize from other raw materials. You don’t need to consume nonessential amino acids in your diet, but most normal diets will contain plenty of nonessential amino acids.
Essential amino acids are amino acids your body can’t synthesize from other raw materials. So, you do need to consume essential amino acids in your diet.
Conditionally essential amino acids are amino acids your body can generally synthesize under most circumstances, though there are instances where it may not be able to (due to infancy, advanced age, liver disease, or certain other disease states).
As a conditionally essential amino acid, cysteine can generally be synthesized in adequate quantities, but there are circumstances where it must be derived from dietary sources.
For more on the general effects of protein, refer to the article on protein.
The recommended combined intake of cysteine and methionine is 25mg per kilogram of body mass. A normal diet generally has about twice as much methionine as cysteine, so we set the cysteine target at 8.5mg/kg.
Likelihood of tracking completeness: Very low
Cysteine is not a nutrient food manufacturers are required to disclose on nutrition labels. The vast majority of food manufacturers do not voluntarily list cysteine content on nutrition labels, so most branded products in the MacroFactor database lack information on cysteine. So, if you’d like to accurately track your cysteine intake, you’ll need to make a point of mostly tracking “common foods,” which come from research-grade databases that have full nutrient reporting.
For more on when you can track using branded foods versus common foods when you’re trying to accurately monitor your intake of particular nutrients, you should check out this article.
Likelihood of insufficient intake: Low
Insufficient intake of a single essential amino acid can have health consequences. The essential amino acid with the lowest intake relative to physiological needs is referred to as the limiting amino acid. The four most common limiting amino acids are lysine, methionine, threonine, and tryptophan. Lysine or threonine is likely to be the limiting amino acid for individuals who consume most of their total energy from cereal grains (like rice and wheat). Methionine is likely to be the limiting amino acid for individuals who consume most of their total energy from legumes (like beans, chickpeas, and lentils). Tryptophan is likely to be the limiting amino acid for individuals who consume most of their total energy from corn/maize.
As a conditionally essential amino acid, it’s uncommon to need to derive cysteine from dietary sources. When it is essential, it can still be synthesized from methionine, assuming methionine intake is adequate.
For more on nutrients with a greater likelihood of insufficient or excessive intake, you should check out this article.
Signs of deficiency/insufficiency
The most common signs of a cysteine deficiency are impaired immune function (more frequent illnesses) and impaired antioxidant levels, since cysteine is required to produce glutathione – one of your most important endogenous antioxidants.
Omnivores and vegetarians rarely need to worry about consuming adequate cysteine, assuming their total protein intake is sufficient. Meat, eggs, and dairy products are generally rich in all of the essential and conditionally essential amino acids.
Among vegan sources of protein, mycoprotein (like Quorn) is particularly rich in cysteine. Lentil sprouts, dried seaweed (nori), and soy-based products are also rich sources of cysteine per unit of energy.
If you’d like to learn more about micronutrients generally, there’s a five-part series on the MacroFactor website you might enjoy.