Phenylalanine: Requirement with Tyrosine, Indian Food Sources and Phenylketonuria

Phenylalanine is an essential amino acid: your body cannot build it and takes all of it from the protein you eat. The liver converts part of it into a second amino acid, tyrosine, so nutrition bodies set one requirement for the pair. ICMR-NIN's 2020 recommendation is 38 mg of phenylalanine plus tyrosine in each gram of dietary protein, and the international adult figure is 25 mg per kg of body weight a day. Cereals, dals and milk all carry far more than that, so a shortage is not a practical worry in India. This page is part of the handbook's proteins and amino acids section.

What is phenylalanine, and what does it do?

It is one of the building blocks from which every protein in your body is assembled, and it belongs to the group called aromatic amino acids because of the ring in its structure. Beyond its place in protein, its main job is to be the raw material for tyrosine. An enzyme called phenylalanine hydroxylase carries out that conversion, mostly in the liver and to a smaller extent in the kidneys, according to a 2021 review of phenylketonuria in Nature Reviews Disease Primers.

Tyrosine in turn is what the body uses to make dopamine, noradrenaline, the thyroid hormones and the skin pigment melanin, which is why the two amino acids are discussed together. The handbook's page on tyrosine follows that side of the story. When food supplies tyrosine, less phenylalanine has to be converted.

How much phenylalanine do you need?

An adult needs about 25 mg of phenylalanine and tyrosine together for each kg of body weight a day, by the WHO/FAO/UNU 2007 values as amended in the FAO's 2013 report on protein quality. That is about 1,625 mg for a 65 kg man and 1,375 mg for a 55 kg woman, and children need more for their size. The amino acid chapter of ICMR-NIN's full 2020 report could not be opened for this entry, so the table's Indian line shows only the adult pattern printed in the 2024 guidelines, with no figure for pregnancy or lactation.

The Indian requirement pattern and the plate that meets it

ICMR-NIN's Dietary Guidelines for Indians, 2024, print the 2020 recommendation for each essential amino acid beside what the institute's model day of eating supplies. For phenylalanine plus tyrosine the recommendation is 38 mg per gram of protein. The vegetarian My Plate for the Day supplies 87.9 mg and the non-vegetarian plate 89.6 mg. Lysine, by contrast, reaches 49.0 mg against a target of 45.

The pattern sits inside your protein requirement and is not an extra target. ICMR-NIN's 2020 report sets the adult protein allowance at 0.83 g per kg a day, which it rounds to 54 g for a 65 kg man and 46 g for a 55 kg woman. The protein calculator gives your own protein figure.

Phenylalanine in Indian foods

Every protein food on an Indian plate contains phenylalanine, and the Indian Food Composition Tables 2017 show how evenly it is spread. Per 100 g of protein, raw milled rice has 5.36 g, whole-wheat atta 5.03 g, ragi 5.70 g, moong dal 6.20 g, chana dal 5.97 g, cow's milk 5.09 g and paneer 5.08 g. Toor dal is the outlier at 8.76 g. Tyrosine varies more, from 5.48 g in milk protein to 2.10 g in atta.

Each food in the table carries roughly twice the 38 mg pattern or more, atta included. Household descriptions are approximate. Egg, chicken and fish are left out only because the page of IFCT 2017 that lists their phenylalanine could not be read for this entry. For protein content itself, NNWA's table of protein in Indian foods ranks the whole database, and the page for toor dal gives that food's full panel.

Is anyone in India short of phenylalanine?

No survey suggests so, and the food figures explain why. National surveys such as NFHS-5 do not measure amino acid intakes, so the judgement rests on composition: a diet would have to be very low in total protein before the aromatic amino acids ran short, and other deficits would show first. In cereal-heavy Indian diets the one to watch is lysine, the limiting amino acid. The page on essential amino acids sets the nine side by side.

What is phenylketonuria?

Phenylketonuria, or PKU, is an inherited condition in which phenylalanine hydroxylase does not work, so phenylalanine from food builds up in the blood instead of becoming tyrosine. It is autosomal recessive: a child inherits a faulty gene from each parent. Left untreated, the high phenylalanine level harms the developing brain and leads to severe intellectual disability, epilepsy and behavioural problems.

It is uncommon. A 2020 analysis in the American Journal of Human Genetics estimated a global prevalence of 1 in 23,930 live births, ranging from 1 in 4,500 in Italy to 1 in 125,000 in Japan. Treatment is a medically supervised diet that began in 1953 and has three parts: natural protein is restricted, a phenylalanine-free amino acid mixture replaces it, and special low-protein foods fill the plate. Started early and continued, it keeps intelligence within normal limits. Women with PKU need close control before and throughout pregnancy, because high maternal levels can harm the baby.

Newborn screening in India: the picture so far

PKU is found by a heel-prick blood test in the first days of life, and that test is not yet routine across India. A 2010 perspective by doctors at Maulana Azad Medical College and AIIMS noted that testing for PKU and hypothyroidism is universal in the developed world but that newborns were not screened in India, because health policy had been directed at mortality and infections. It reported an earlier pilot on about 125 thousand newborns in which PKU was among the inborn errors detected.

Later work is from single centres. An ICMR task force screened 104,094 babies at five metropolitan centres for congenital hypothyroidism and congenital adrenal hyperplasia, not PKU, and called for a national programme. A hospital in Mysuru screened 5,157 newborns for seven conditions between 2020 and 2022 and confirmed one case of PKU; only 26.7 per cent of babies with a positive screen came back for retesting. No national incidence of PKU for India could be verified for this entry, so none is quoted.

Why does a sweetener packet carry a warning for phenylketonurics?

Because the sweetener aspartame is built from phenylalanine, and a person with PKU has to count every source. Aspartame is the methyl ester of a dipeptide, and digestion releases its phenylalanine into the blood like that from any protein. FSSAI's Labelling and Display Regulations, 2020, require a package of food containing aspartame, and aspartame sold as a table-top sweetener, to carry a boxed statement. The first wording was "Not for phenylketonurics"; an amendment in force from September 2021 changed it to say the product is not recommended for phenylketonurics, for children with seizure disorders, or for pregnant and lactating mothers.

For someone without PKU the warning is not about phenylalanine at all. A safety review in Critical Reviews in Toxicology found that the rise in blood phenylalanine after aspartame, at intakes up to 50 mg per kg of body weight, does not exceed the rise seen after a meal, and that typical consumption stays well below the acceptable daily intake of 40 mg per kg set by the European Food Safety Authority. See the article on whether sugar-free sweeteners are safe.

What does the evidence say?

First, the requirement has been measured in Indians. In a study at St John's in Bengaluru, published in the American Journal of Clinical Nutrition in 2006, 32 healthy men ate diets with no tyrosine and eight graded levels of phenylalanine. The two tracer methods used put the requirement at 37 and 38 mg per kg a day, and the authors proposed 38 mg, similar to the figure for Western adults. The confidence interval ran from 31 to above 47 mg.

Second, tyrosine spares phenylalanine. That 38 mg was measured with no tyrosine in the diet; the international requirement of 25 mg per kg assumes a normal diet that supplies both. Third, giving extra tyrosine has not been shown to help people with PKU. A Cochrane review updated in 2021 found three suitable trials with 56 participants in total: blood tyrosine rose, no other outcome differed, and the authors could make no recommendation for routine practice.

A worked example: a vegetarian teacher's day

This is an illustration, not a real person. Picture a 29-year-old vegetarian staff nurse in Coimbatore whose weight is 55 kg. At 25 mg per kg she needs about 1,375 mg of phenylalanine and tyrosine together in a day. Her food on an ordinary day is cooked from 150 g of rice, 100 g of atta and 60 g of toor dal, raw weight, with 250 g of milk taken as curd and in coffee.

From IFCT 2017, the rice gives 635 mg of phenylalanine and 517 mg of tyrosine, the atta 533 and 223 mg, the toor dal 1,141 and 302 mg, and the milk 415 and 447 mg. The day comes to 43.6 g of protein, about 2,724 mg of phenylalanine and 1,488 mg of tyrosine: 4,212 mg together, or 96.6 mg per gram of protein against the pattern of 38. That is roughly three times her requirement, before any allowance for digestibility. Her protein is a little under the 46 g allowance, which is the thing worth fixing.

Common mistakes and myths about phenylalanine

The commonest mistake is reading the label statement as a general warning. It is addressed to people with one inherited condition, plus the groups the 2021 amendment names, and says nothing about phenylalanine being harmful in ordinary food. The opposite mistake is buying it as a supplement when three ordinary meals already deliver several grams. And a baby with PKU looks well at birth, which is why screening uses a blood spot.

Phenylalanine supplements: safety, and when to see a doctor

A healthy adult has no nutritional reason to take phenylalanine on its own, and this entry gives no dose. FSSAI's regulations on health supplements and nutraceuticals list L-phenylalanine among the amino acids that may be used as an ingredient, which makes such a product legal, not necessary. Some medicines also raise specific concerns. The prescribing information for the antidepressant phenelzine, a monoamine oxidase inhibitor, lists phenylalanine and L-tyrosine among the compounds a patient on the drug should not take.

Levodopa, used in Parkinson's disease, is absorbed from the gut and carried into the brain by a transport system it shares with the large neutral amino acids, a group that includes phenylalanine. A review of levodopa's pharmacokinetics lists timing doses in relation to meals and a controlled dietary protein intake among the ways of keeping its effect steady. That is for the treating neurologist to decide; do not change your protein intake or your medicine on your own.

See a doctor promptly if a newborn's screening test is reported positive, or if a baby or young child shows delayed development, seizures, or skin and hair much lighter than the rest of the family. A woman with PKU should speak to her metabolic clinic before planning a pregnancy. Nothing in food or in a supplement cures PKU; the diet manages it for life.

Learning amino acids at NNWA

Amino acid requirements, protein quality and the reading of food labels are part of what NNWA teaches. NNWA is a private skill academy in Kolkata that runs its classes online in English and Hindi. Its courses, among them the Certification in Nutrition and the Diploma in Nutrition, train people to coach healthy clients and to refer medical cases, PKU among them, to a doctor. None is a degree or a dietitian qualification.