Dietary Treatment For Metabolic Disease
Cunningham, MS, LDN, RD
None of this can happen without the help of small body proteins known as enzymes. Each time one compound is changed into another at one of thousands of steps along the body's intricate series of metabolic pathways, an enzyme is needed to make it happen. Each enzyme is made by a specific gene. When that gene is abnormal, it's enzyme isn't made properly and the chemical pathway dependent on that enzyme is blocked. Like a river that is damned, the compounds flowing into the pathway can't be processed, so they accumulate and may become very harmful. Also, on the "downstream" side of the pathway, the normal product isn't made. Often the result is metabolic disease.
An Example -
In PKU, not enough of the enzyme phenylalanine hydroxylase can be made. This enzyme is part of a metabolic pathway that changes one amino acid (phenylalanine) into another (tyrosine). The pathway is used when more food protein, containing phenylalanine, is eaten than is needed for building body protein. If there is no enzyme available, the pathway is blocked. Phenylalanine accumulates in the blood in amounts that are toxic to the brain; and tyrosine, which is also needed for healthy brain function, cannot be made.
However, phenylalanine cannot be totally taken out of the diet. There are about twenty different amino acids needed by the body. The body can make some of these for itself, usually from other amino acids, but some must be obtained from food. These are called "essential amino acids", and phenylalanine is one of them. Because the body won't build an incomplete protein, if an amino acid needed to make a protein isn't there, or can't be made (as essential amino acids can't be), the protein won't be built. It would be similar to trying to write a sentence with all the vowels removed from the alphabet.
A certain amount of phenylalanine is essential. If the diet doesn't provide enough, the body will break down muscle protein to obtain it. In a person with PKU, this will release as much phenylalanine into the blood as eating a hamburger would.
The PKU Diet
However, if a diet is restricted to the amount of food that provides the very small amount of phenylalanine that can be tolerated, the body will not have enough protein or calories to support growth and good health. A special metabolic formula must also be included in the diet. These formulas are processed to remove phenylalanine, supplement tyrosine, and provide a source of additional calories and protein that can be tolerated. Similar formulas are available for the specific needs of other metabolic diseases.
Because one person's nutritional needs are different than another's, and because everyone's needs change as they grow and mature, a PKU diet must be adjusted often to the individual needs of each person, at each stage of their life. Good dietary treatment of PKU means maintaining that perfect balance of phenylalanine, protein, and calories that supports normal growth and health but doesn't cause harm.
a Diet Prescription into Food on
When a child begins to eat solid food, the regular infant formula is gradually replaced by foods containing the same amount of phenylalanine. The metabolic formula will remain a lifelong part of the diet, and will always have to be relied on as a major source of calories and protein.
High protein foods (such as fish, chicken, eggs, milk, cheese, dried beans, nuts, and tofu) cannot be eaten. The amount of phenylalanine in a one serving of meat is more than can be tolerated by most children in a whole day. Foods containing smaller amounts of protein, such as breads and starches are also often too high to be included, and even the amount of protein in fruit and vegetables must be counted.
Foods must be chosen only from special lists and weighed or measured accurately. Milligrams of phenylalanine must be counted daily. By the end of a day the total amount of phenylalanine eaten should be no more or less than what is in the diet prescription.
The quantity and variety of food in a PKU diet can be greatly increased by using special low protein food products. For example, a slice of bread or serving of pasta will contain 200 or more milligrams of phenylalanine, while a comparable serving of low protein products will contribute only 15 to 20 milligrams. In a typical diet prescription of as little as 300 milligrams phenylalanine a day, these foods are essential.
Barriers to Success
Trying to enforce diet restrictions with a young child who is reaching for independence makes a perfect opportunity for power struggles with parents. Allowances for the child's emerging individual taste preferences, developmental stages, and behavior patterns must be made, while at the same time managing a very complicated diet.
Older children and teenagers face other challenges. Trying to follow a very restricted diet in a lifestyle that includes school cafeterias, vending machines, and fast food restaurants is very hard to do, and fear of being "different" can encourage cheating.
Young people stop following their diets most often because of these psychological issues. High blood phenylalanine, which results from poor compliance with dietary restrictions, can affect a person's ability to concentrate and make good decisions. When poor decisions are made, compliance gets even worse, and the cycle repeats.
Overcoming these barriers to good treatment requires long term commitment by the whole family. A parent must accept the importance of dietary treatment, learn to manage it, and then be able to teach their child the skills needed to assume responsibility for their own treatment as they grow up.
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