Cysteine: The Sulphur Amino Acid Counted With Methionine

Cysteine is one of the two sulphur amino acids in protein; the other is methionine. Your body can make cysteine, but only out of methionine, so nutrition guidelines set one requirement for the pair. ICMR-NIN's 2020 recommendation is 6 mg of cysteine in every gram of dietary protein and 22 mg of methionine plus cysteine together. Wheat and rice protein carry several times as much cystine as dal protein does. This entry is part of the proteins and amino acids section of the handbook.

What is cysteine, and why is it counted with methionine?

Cysteine is an amino acid with a sulphur atom in it, and it is counted with methionine because methionine is its only raw material in the body. A review of glutathione synthesis in Biochimica et Biophysica Acta gives three origins for the cysteine in your cells: the diet, the breakdown of body protein, and methionine converted in the liver by a route called transsulfuration. Plenty of methionine can therefore cover both needs.

That is why cysteine is usually listed among the amino acids the body can make, while methionine is one of the nine essential ones in ICMR-NIN's Dietary Guidelines for Indians, 2024. Food tables print the figure as cystine, two cysteine molecules joined through their sulphur atoms; for a reader the two words describe the same nutrient. The other half of the pair has its own entry on methionine and its requirement.

How much cysteine do you need?

An adult needs about 15 mg of methionine plus cysteine per kg of body weight a day, by the WHO/FAO/UNU 2007 requirement as amended in the FAO's 2013 report on protein quality. For a 65 kg man that is about 975 mg a day, and for a 55 kg woman about 825 mg. ICMR-NIN's 2020 recommendation, printed in its 2024 guidelines, writes the same need as a pattern: 22 mg of the pair in each gram of protein, of which 16 mg is methionine and 6 mg is cysteine.

The two forms agree. Both bodies use an average protein requirement of 0.66 g per kg a day, and 0.66 g of protein carrying 22 mg per gram gives about 15 mg per kg. On the same arithmetic the cysteine share works out at about 4 mg per kg a day, a figure calculated here and not printed in either document. The amino acid chapter of ICMR-NIN's full 2020 report could not be opened for this entry, so the table gives the international values by age and no figure for pregnancy or lactation.

None of this is an extra target to chase. The sulphur amino acids sit inside the protein allowance, which ICMR-NIN's guidelines give as 0.83 g per kg a day, or 54 g for a person of 65 kg. NNWA's protein calculator gives the figure for your own weight, and the pattern above tells you what that protein should contain.

Cysteine in Indian foods

The Indian Food Composition Tables 2017 list cystine for each food as grams per 100 g of protein. Cereals lead: 2.24 for whole-wheat atta and 1.84 for raw milled rice. Dals trail: 0.84 for chana dal, 0.67 for toor dal, 0.55 for masoor dal and 0.43 for moong dal. White soya bean, at 1.44, is the exception among pulses. Cow's milk has 1.22 and paneer 0.72, though both are rich in methionine.

Read the middle column against the pattern of 22 mg per gram: every cereal, milk, paneer, soya bean and goat meat clears it, and chana, toor and moong dal do not. The copy of IFCT 2017 read for this entry lacked the page with these values for hen's egg and broiler chicken, so they are left out. For protein content across the database see NNWA's table of protein in Indian foods; the wheat flour page gives atta in full.

Why do dals fall short on sulphur amino acids while cereals do not?

The two seed families store different proteins, and the result is that each covers the other's gap. ICMR-NIN's 2024 guidelines state that cereals have lower levels of lysine but higher levels of the sulphur-containing amino acids, methionine and cysteine, and that in pulses the reverse is true. The same document names methionine as the limiting amino acid of pulses, so a plate of dal alone scores below the pattern for the sulphur pair.

The remedy is the combination Indian kitchens already use. ICMR-NIN advises cereals and pulses in a ratio of 3 to 1 by raw weight, with 250 ml of milk a day. Its vegetarian My Plate for the Day supplies 11.2 mg of cysteine and 28.4 mg of methionine plus cysteine per gram of protein, against recommendations of 6 and 22. The lysine side of the same bargain is in the entry on lysine and the cereal and pulse ratio.

What does cysteine have to do with glutathione?

Cysteine is the building block that decides how fast your cells can make glutathione. Glutathione is a small molecule built from three amino acids, glutamate, cysteine and glycine, and the review in Biochimica et Biophysica Acta calls it the most abundant non-protein thiol in mammalian tissue and a defence against oxidative stress. The same review names two things that govern its synthesis: the availability of cysteine and the activity of one rate-limiting enzyme. The third partner has its own entry on glycine and what it builds.

That cysteine is required for glutathione is established biochemistry. That a healthy adult eating enough protein gains anything by adding extra cysteine is a different claim, and the human trials behind it are small. Cysteine has other fates too: a review in the Journal of Inherited Metabolic Disease describes its breakdown to taurine and to sulphate, the form in which surplus sulphur leaves in urine.

Too little cysteine: who is at risk in India?

No deficiency disease is tied to cysteine by name, and this entry found no Indian survey that measures it. Because the body makes cysteine from methionine, a shortfall appears only when the pair is short, which in practice means a diet low in total protein or one built almost wholly on pulses. Indian plates are usually the opposite, heavy in cereal, and on a typical dal, rice and roti diet the weak point is lysine, not the sulphur pair.

The people worth a second look are those who have cut grains sharply while relying on dal, sprouts and besan for protein. Digestion matters as well. A 2018 review in the Indian Journal of Medical Research reports mean methionine digestibility of 80 per cent from rice, 60 per cent from ragi, 54 per cent from mung bean and 86 per cent from whole boiled egg, from Indian measurements that were then unpublished.

What does the evidence say?

First, the requirement rests partly on Indian experiments. At St John's Medical College in Bengaluru, 21 healthy men were fed seven levels of methionine on a diet with no cysteine, and the result was a mean requirement of 15 mg per kg a day, similar to that of Western adults. A companion study in chronically undernourished Indian men proposed 16 mg per kg, not significantly different.

Second, cysteine in the diet appears to spare methionine, but by how much is unsettled. A Canadian study in six men found that an excess of dietary cysteine lowered the mean methionine requirement from 12.6 to 4.5 mg per kg a day, a sparing of 64 per cent. The Bengaluru group, feeding cystine at 5 and 12 mg per kg, found the amount of sparing difficult to quantify, and a later review in the Journal of Nutrition called the question controversial.

Third, the glutathione trials. In one American study eight elderly people had lower red blood cell glutathione than eight younger ones, and two weeks of cysteine and glycine raised their concentration by 94.6 per cent. A later placebo-controlled trial by the same group gave glycine with N-acetylcysteine to 12 older adults for 16 weeks and reported improvements in glutathione and several other measures.

A worked example: a dal-only day and a mixed day

This is an illustrative case, not a real person. Take a 34-year-old vegetarian teacher in Chennai who weighs 55 kg. Her requirement for methionine plus cysteine is about 825 mg a day. To raise her protein she drops rice and roti for a week and eats 200 g of raw dal a day instead: 100 g of moong dal, 60 g of toor dal and 40 g of masoor dal. Using IFCT 2017, that supplies 46.6 g of protein but only about 660 mg of the sulphur pair, 14.2 mg per gram against a pattern of 22.

Now a mixed day: 100 g of rice, 100 g of atta, 35 g each of toor and moong dal, and 250 g of milk or curd. Protein is a little lower at 42.6 g, yet the sulphur pair rises to about 1,320 mg, or 31.0 mg per gram, and cystine alone to about 570 mg, 13.4 mg per gram against the recommended 6. The sums ignore digestibility, so read them as a comparison between two days and not as a measured intake.

Common mistakes and myths about cysteine

The usual mistake is treating dal as a complete answer to protein. Dal is the right partner for cereal because of its lysine, and the wrong food to eat alone because of its sulphur amino acids. A second is reading a cystine figure in isolation: paneer looks poor at 7.2 mg per gram, yet its methionine lifts the pair to 30.0 mg, well above the pattern.

Is N-acetylcysteine a food supplement or a medicine?

It is both, and the two uses should not be confused. N-acetylcysteine, often shortened to NAC, is a modified form of cysteine. As a medicine it is the hospital antidote for paracetamol overdose: a Cochrane review concludes that acetylcysteine should be given to people at risk of toxicity, while grading the randomised evidence as low quality. That is emergency care decided by doctors.

On the food side, Schedule II of FSSAI's health supplements regulations lists L-cysteine and N-acetyl-L-cysteine among the amino acids that may be used. The pack must carry the words HEALTH SUPPLEMENT, the amount of each nutrient, the advisory NOT FOR MEDICINAL USE, a warning not to exceed the recommended daily usage, and a statement that the product is not a substitute for a varied diet. No label may claim to prevent, treat or cure a specific disease unless FSSAI has approved it.

Safety: too much, and when to see a doctor

Cysteine from food arrives in modest amounts inside protein, and this entry found no report of harm from it. Supplements are a different matter and no dose is given here. ICMR-NIN's 2024 guidelines advise against high intakes of protein supplement powders, tell readers not to overdo any single nutrient in supplement form, and warn that high protein intakes may worsen kidney function in people who already have kidney disease.

A suspected overdose of paracetamol is an emergency: go to a hospital at once and do not try to manage it at home with a supplement. See a doctor also for weight or muscle loss you cannot explain. Do not stop or change a prescribed medicine because of something you have read about an amino acid.

Learning amino acids at NNWA

Protein quality, the amino acid pattern and the reading of supplement labels are taught at NNWA, a private skill academy in Kolkata that holds its classes online in English and Hindi. Its courses, among them the Diploma in Nutrition and Sports Nutrition, train people to coach healthy clients and to refer medical cases to a doctor. None of them is a degree or a dietitian qualification.