Living well with the diet

Phenylketonuria (PKU)

A rare genetic disease, phenylketonuria prevents the body from breaking down phenylalanine normally. It was the first inherited metabolic disorder for which a dietary treatment was developed.

The disease

An enzyme, phenylalanine hydroxylase, does not work or works poorly: it no longer converts phenylalanine into tyrosine, and phenylalanine builds up. Depending on how much activity this enzyme has left, a distinction is made between classic PKU, atypical PKU, which tolerates more phenylalanine, and permanent mild hyperphenylalaninaemia, which does not usually require a diet. At birth, the enzyme may still be immature, and tolerance may improve during the first few years.

BH4-responsive forms

To work, phenylalanine hydroxylase needs a cofactor, BH4 (tetrahydrobiopterin). In some forms, mainly atypical and very rarely classic, a BH4-based medicine taken by mouth increases the enzyme’s activity and allows the diet to be broadened, sometimes to the point of stopping the amino acid mixture. This responsiveness must first be tested, and your doctor will tell you whether this treatment is right for you, particularly depending on age or in pregnancy. BH4 synthesis deficiencies are a different disease, with different treatment and follow-up.

Your child’s diet

As for any child, the introduction of solid foods starts between 4 and 6 months, but you count the phenylalanine in each fruit and vegetable introduced, one at a time, to stay within the prescribed daily total. Every day, your child takes all of their amino acid mixture, without being forced and without the rule being relaxed. If they refuse it from the bottle, ask your dietitian whether it can be given more concentrated, mixed into baby cereal or fruit, or replaced by another mixture. This is not a slimming diet: avoid “light” products, and be aware that aspartame, used to sweeten some light fizzy drinks, juices and fruit compotes, is a precursor of phenylalanine and is one of the forbidden foods.

In adolescence

The diet is often partly relaxed after the age of 10, and again in adolescence, because the brain copes better with higher phenylalanine levels. This relaxation is decided with your team and must not go too far. An excess does not show straight away, but it can lead to difficulty concentrating at school, tiredness, irritability or a change in behaviour.

Planning a pregnancy

If you have PKU, it is very unlikely that your baby will have the disease, but the baby’s liver will only be able to eliminate phenylalanine at the end of pregnancy: until then, the baby is exposed to your own levels. To protect the baby from an intoxication that can cause malformations, the strict diet must be resumed and well balanced before conception, without weight loss, with blood tests at least once a week. As soon as you have made your decision, see the team with your partner: they will check your vitamins and minerals and help you choose mixtures that suit you. It is your doctor who will tell you when to stop contraception, once your weight is stable, your levels are within the set targets and you yourself are ready to keep to the diet for several months.

During pregnancy

Spread your phenylalanine over the day, cook varied meals and top up your energy with reduced-protein foods and small snacks. Take your mixture in 3 or 4 doses, preferably with meals that contain natural protein, and if you feel sick, drink it very cold, in small amounts at regular intervals. In the last trimester, the baby’s liver starts to eliminate phenylalanine, your tolerance increases markedly and your team can broaden the diet step by step. From the day you give birth, you return to your usual diet, and you can breastfeed without controlling phenylalanine.

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Taranis know-how.

Since 1979, Taranis has been creating varied, enjoyable foods designed for everyday life despite the constraints of a restricted diet under medical supervision. Its research and development team creates reduced-protein recipes whose taste and texture come close to those of ordinary products, working with specialist hospital departments, prescribers and the people who follow the diet. The products are made to precise specifications, with checks at every stage of production, and most of them are foods for special medical purposes.

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Reviewed by Steeve Roland, physiotherapist, over 20 years in clinical nutrition, founder of BSNUTRITION · Updated on

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