Methionine: The Sulphur Amino Acid That Pulses Lack and Cereals Supply

Methionine is an essential amino acid that contains sulphur, and nutrition tables count it together with cysteine, which the body can make from it. ICMR-NIN's 2020 recommendation is 22 mg of the pair in every gram of dietary protein, 16 mg of that as methionine. It matters on an Indian plate because pulses are short of it: toor dal protein carries about 15 mg of the pair per gram, while rice protein carries about 44 mg. Eaten together, grain and dal make up each other's gap. More entries are in the proteins and amino acids handbook.

What is methionine, and what does it do?

Methionine is one of the nine amino acids that ICMR-NIN's Dietary Guidelines for Indians, 2024, list as essential, meaning the body cannot build it and food must supply it. Like the other eight it goes into body protein. It is also the body's source of cysteine, the second sulphur amino acid, and that is why the two are given one combined requirement.

Methionine has a second role as a carrier of methyl groups. The FAO and WHO consultation on vitamin and mineral requirements describes S-adenosylmethionine as an activated form of methionine that donates methyl groups to reactions involving lipids, hormones, DNA and proteins. Once the methyl group is given up, what remains becomes homocysteine, the subject of a later section.

How much methionine and cysteine do you need?

An adult needs about 15 mg of methionine and cysteine together for each kg of body weight a day, on the WHO/FAO/UNU 2007 figures as amended in the FAO's 2013 report on protein quality. A man of 65 kg therefore needs about 975 mg, and a woman of 55 kg about 825 mg. ICMR-NIN's 2020 recommendation, as its 2024 guidelines print it, expresses the need per gram of protein: 16 mg of methionine and 6 mg of cysteine, 22 mg in all.

The two ways of writing it match. The average adult protein requirement is 0.66 g per kg a day, and 0.66 g of protein carrying 22 mg per gram gives 14.5 mg per kg. ICMR-NIN's full 2020 chapter on amino acids could not be read for this entry, so the table below shows the international figures by age, labelled, with the Indian adult pattern beside them, and no value for pregnancy or lactation.

None of this is extra to protein. The adult allowance in ICMR-NIN's 2020 summary is 0.83 g of protein per kg a day, or 54.0 g for a man of 65 kg and 45.7 g for a woman of 55 kg, and the sulphur amino acids are part of that protein. The protein calculator works out your own figure.

The requirement measured in Indian men

At St John's Medical College in Bengaluru, seven levels of methionine, from 3 mg to 24 mg per kg a day, were tested in 21 healthy, well-nourished men on a diet with no cysteine, and amino acid balance was traced over 24 hours. The mean requirement came out at 15 mg per kg a day. A matching study in 21 undernourished Indian men gave 16 mg.

A further study from the group found that cystine in the diet may spare methionine: with 5 mg and 12 mg of cystine per kg a day, the methionine breakpoints were 20 mg and 10 mg per kg. The authors called the amount of sparing difficult to quantify.

Why are pulses short of methionine?

Because pulse protein is built with far less of the sulphur amino acids than cereal or milk protein. ICMR-NIN's 2024 guidelines name methionine as the limiting amino acid of pulses. The Indian Food Composition Tables 2017 show the scale: per 100 g of protein, toor dal has 0.87 g of methionine and 0.67 g of cystine, moong dal 1.05 g and 0.43 g, and masoor dal only 0.54 g and 0.55 g. Added together, every one falls below the 2.2 g that a pattern of 22 mg per gram amounts to.

Absorption widens the gap. In healthy Indian adults, the true digestibility of methionine was 71.8 per cent from chickpea, 56.1 per cent from yellow pea and 52.2 per cent from whole mung bean, improving to 64.3 per cent when the mung was dehulled. For whole boiled egg and chicken meat, an Indian review reports 86 and 93 per cent.

Methionine and cysteine in Indian foods

Cereals, millets, milk and sesame are the vegetarian foods that carry the sulphur amino acids. In IFCT 2017, methionine and cystine together come to 4.44 g per 100 g of protein in raw milled rice, 4.22 g in ragi, 4.01 g in whole-wheat atta, 3.73 g in cow's milk, 3.56 g in white sesame and 3.34 g in bajra. White soya bean, at 2.99 g, is the one pulse comfortably above the pattern. IFCT reports cysteine as cystine.

Egg and chicken are not in the table because their figures for these two amino acids could not be read reliably for this entry. NNWA's table of protein in Indian foods ranks every food by protein content, and the pages for toor dal and sesame seeds give those foods in full.

How do cereals and pulses cover each other in an Indian meal?

Each is rich in what the other lacks. ICMR-NIN's 2024 guidelines advise complementing pulses with cereals, millets, nuts and seeds, which are rich in methionine, just as pulses bring the lysine that grains are short of. The entry on lysine tells that half. The guidelines recommend cereals and pulses in a ratio of 3 to 1 by raw weight, and ICMR-NIN's 2020 summary sets the cereal, legume and milk composition of the diet at 3 to 1 to 2.5, against 11 to 1 to 3 in 2010.

The vegetarian plate clears the combined pattern easily, but on methionine alone it supplies 17.2 mg against 16, a narrow margin that depends on the grain staying on the plate. Rajma with rice, idli with sambar and vegetable khichdi all pair a pulse with a cereal.

Methionine, homocysteine, folate and vitamin B12

Homocysteine is what methionine becomes after it has given away its methyl group. The FAO and WHO consultation explains that it is then either broken down or remethylated to methionine, and that the remethylating enzyme, methionine synthase, needs both folate and vitamin B12. When either vitamin is short the recycling slows and homocysteine in the blood rises; the same report lists vitamin B6 among the nutrients involved. See the entries on folate and folic acid, vitamin B12 and vitamin B6. Doctors measure it as plasma total homocysteine, and reading that result is their job.

Is high homocysteine in Indians caused by too much methionine?

The Indian evidence points to too little vitamin B12, not too much methionine. A study of 441 middle-aged men in and around Pune found raised homocysteine in 58 per cent and low B12 in 67 per cent; among the urban middle class the figures were 79 and 81 per cent. Low B12 accounted for 28 per cent of the risk of raised homocysteine and low red cell folate for 2 per cent. Vegetarians had 3.0 times the risk of raised homocysteine, although pulse-based diets are the ones lower in methionine. The article on vitamin B12 for vegetarians deals with that gap.

What does the evidence say about methionine and health?

First, the requirement is well founded: the tracer studies in Indian men agree with the international figure. Second, large amounts do raise homocysteine. A review of methionine toxicity in the Journal of Nutrition found no adverse effects from moderate swings in dietary intake, but intakes above five times normal raised homocysteine, and in infants intakes of two to five times normal impaired growth.

Third, lowering homocysteine with vitamins has not delivered the heart benefit once hoped for. A Cochrane review of 15 randomised trials with 71,422 participants found that folic acid, B6 and B12 supplements made no difference to heart attacks or deaths compared with placebo. Stroke was slightly less frequent, at 4.3 per cent against 5.1 per cent. Those trials tested vitamins, not changes in methionine intake.

A worked example: a high-dal, low-grain vegetarian day

This is an illustration, not a real case. A 24-year-old vegetarian in Kolkata weighs 65 kg and needs about 975 mg of methionine and cysteine a day. To cut carbohydrate he has dropped roti and now eats, in raw weights, 100 g of toor dal, 100 g of moong dal and 60 g of rice. By IFCT 2017 that is 50.3 g of protein with about 898 mg of the sulphur amino acids: 17.8 mg per gram against a pattern of 22, and about 77 mg short before digestibility is counted.

Now put the grain back in ICMR-NIN's proportions: 125 g of atta, 125 g of rice, 45 g of toor dal, 40 g of moong dal and 300 g of milk or curd. Protein is almost unchanged at 52.2 g, but the sulphur amino acids rise to about 1,626 mg, or 31.1 mg per gram. Methionine alone goes from 11.2 mg to 17.7 mg per gram of protein. The sums leave out digestibility, so they compare two days and do not measure an intake.

Common mistakes and myths about methionine

The commonest mistake is the one in the example: treating dal as a complete answer and the grain as empty carbohydrate. The grain is what supplies the methionine. A related mistake is calling pulse protein poor. It is short of one amino acid and rich in lysine, which cereals need; the article on vegetarian protein sources shows the range available.

Too much methionine: supplements, safety and when to see a doctor

Methionine from food has not been shown to harm healthy adults, but large isolated amounts are a different matter. The Journal of Nutrition review notes that a single research test dose raises homocysteine sharply, that a tenfold larger amount given by mistake caused a death, and that methionine is known to worsen symptoms in people with schizophrenia. ICMR-NIN's 2020 table of upper limits carries no figure for methionine or any single amino acid. No supplement dose is suggested here.

FSSAI's health supplement regulations list DL-methionine in Schedule II among the essential amino acids, without an amount. For formula foods used in weight control, the same regulations allow only the L forms of essential amino acids to be added to improve protein quality, except methionine, where the DL form is allowed. A supplement label must say NOT FOR MEDICINAL USE.

See a doctor if you have numbness or tingling in the hands and feet, unusual tiredness or a sore tongue, which can go with low vitamin B12, or if a report shows raised homocysteine. Never stop or alter a prescribed medicine or vitamin on the basis of a diet change.

Protein quality in NNWA's courses

Protein quality, complementary proteins and the use of the Indian food tables are part of the teaching at NNWA, a private skill academy based in Kolkata whose classes run online in English and Hindi. Courses such as the Certification in Nutrition and the Diploma in Nutrition prepare people to coach healthy clients and to send medical cases to a doctor. None of these courses is a degree or leads to a dietitian's qualification.