Treatment is therefore based on a specific diet, which can be combined with medicines (cofactors or scavenger agents).
- Ensuring normal growth in terms of weight, height and cranial perimeter, as well as good nutritional status;
- Achieving normal psychomotor development;
- Ensuring proper metabolic balance in the long term;
- Preventing acute decompensation (not applicable in phenylketonuria) by preventing catabolic states (when the body breaks down its own protein and releases amino acids (see Fig. 1).
Principle of the diet therapy
The principle of dietary treatment is to adhere to a diet with small amounts of the amino acid involved in the metabolic disorder, while eating other nutrients in sufficient quantities; i.e. other amino acids, carbohydrates, fats, vitamins and minerals. Treatment is therefore based on:
- Drastic limits on protein intake (i.e. exogenous amino acids) to prevent toxicity. This is about restricting, but not completely eliminating protein, as the intake of limited amounts of amino acids from natural foods is necessary to meet the minimum requirements for growth and renewal of proteins in the body (Fig. 4). A diet completely devoid of protein would hinder normal growth and cause a deficit of precursor amino acids. These nutrients are necessary because our body is incapable of producing them. There must be enough to allow for normal growth;
- Supplementation with a mixture of essential amino acids without the toxic amino acid(s) (depending on the disorder and clinical condition) to meet the requirements for other essential amino acids (whose intake is restricted by the limits on protein) and to facilitate protein synthesis;
- Normal caloric intake (carbohydrates and lipids), which needs to be rebalanced and/or compensated to take into account the elimination of numerous foods;
- Normal intake of non-caloric nutrients (minerals, vitamins, trace elements) in accordance with your child’s requirements at any given age.
This requires knowing the composition of foods and choosing the right ones. Foods are usually categorised into groups. Foods in the same group have the same nutritional properties.
Children with enzyme deficiencies cannot eat foods from all categories on a day-to-day basis without leading to toxicity. Foods are therefore categorised into:
However, they can sometimes be eaten in controlled amounts and under certain circumstances. For example, natural protein intake during the first few months of a baby’s life comes in the shape of milk. They are sometimes permitted in controlled amounts in certain mild forms of the disease.
Some of these foods have artificial substitutes that increase the variety of the diet without boosting its protein content. An explanation about these foods will be provided later.
You will be provided with lists of vegetables (including potatoes) and fruits, which set out the weights of fruits and vegetables corresponding to a given amount of the restricted amino acid: this set amount is known as a PART or EQUIVALENT. Although this amount was chosen arbitrarily, it is now in widespread use.
For phenylketonuria, 1 part = 20 mg of phenylalanine. There is no such consensus for other disorders, and the equivalence of a part in milligrams of an amino acid or in grams of natural protein may vary from one medical team to the next.
Little by little, you will learn to work with these parts to prepare the right amounts for your child without exceeding his or her tolerance.
Amino acids mixes: Almost all foods of animal origin are forbidden due to their high protein content, while some foods of plant origin are restricted. As a result, the diet provides small amounts of the restricted amino acids, but also of the other amino acids (those whose metabolism works properly). This may lead to a deficit of these essential amino acids, limiting protein synthesis and, therefore, growth. The requirements for these other amino acids need to be covered with supplementation of non-toxic amino acids.
This supplement is given in the form of a specific amino acids mixes(powder or liquid) without the toxic amino acid. The mixture often contains the minerals, vitamins and trace elements your child needs. It must be taken every day throughout the person’s lifetime. This supplementation can be considered for certain disorders. “Low protein” food substitutes: some companies make or import protein-poor foods that can “replicate” and/or “substitute” foods we eat often but which contain too much protein. They are part of the “Foods for special medical purposes” (see table below).
All these foods are essential for two reasons:
- for their caloric content,
- but also, to help create meals that appear as close to normal as possible when your child grows.
These foods will be introduced gradually, as with any other child, and will help you to cook varied recipes. Variety makes it easier to accept the diet and, therefore, to achieve better long-term outcomes.
|Food groups||Could be “replaced” by low protein foods|
|Meat, fish, eggs||Egg substitute|
|Milk, dairy products||
|Sugar, sweet products||
In practice, this is a VEGAN-like diet consisting mainly of weighed fruits and vegetables, together with low protein foods and, depending on the disorder, supplementation in the form of amino acids mixes with or without the toxic amino acids. This mixture usually contains minerals, vitamins and trace elements to replace those in forbidden foods. If the diet does not contain enough of them, they will have to come in the form of medicines.
Cofactors and/or scavenger agents
Medicinal products can be considered for certain forms of these diseases. Cofactors, which are usually vitamins, boost the residual activity of the defective enzyme and mitigate the impact of the metabolic block (e.g. vitamin B12 in certain forms of methylmalonic acid and BH4 in phenylketonuria). Scavenger agents accelerate the clearance of the accumulated toxic products (e.g. carnitine in organic aciduria). The aim of these treatments is to improve metabolic balance and/or broaden the diet of your child. Unfortunately, these treatments only work very well in certain rare forms of the disease (cofactor-sensitive).