Dietary Treatment For Metabolic Disease
Amy
Cunningham, MS, LDN, RD
Metabolism is
the word which describes how the
body breaks down the nutrients found
in food and how the body uses nutrients for energy,
growth, healing, and all other body
functions. An "inborn error
of metabolism," or metabolic
disease, occurs when a person inherits
abnormal (mutated) genes that stop
or change the flow of normal metabolism.
There are hundreds of metabolic
diseases that vary widely in severity
and rarity. In many of these diseases
diet is the primary - and often
the only - treatment.
The
Basics
In order to grow,
a child has to obtain enough protein
and calories from the food he eats
to build a body that is continually
stretching and maturing. An
adult needs protein and calories
to keep the body working and in
good repair. The protein that makes
up all the body's cells and tissues,
as well as the protein in the food
we eat consists of individual building
blocks called amino acids. These
amino acids are linked together
into long protein chains. When food
protein is digested, the chains
are split apart into individual
amino acids. These are absorbed
and then put together again in different
combinations to make protein structures
the body needs. When more food protein
is eaten than is needed to make
body protein, excess amino acids
are either burned as fuel for energy
or changed into other compounds.
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.
The Theory
Diets used to
treat metabolic diseases vary with
the specific disorder, but the principles
are the same. Food providing the
substance that accumulates at the
blocked pathway is restricted so
that harmful amounts won't build
up. This substance may be a sugar,
or a fat, but it is most often particular
amino acids found in protein. In
theory this seems very easy, but
translating theory into food on
the table is often very complicated.
Using the treatment for one metabolic
disease as an example can make it
easier to understand.
An Example -
PKU
The most common
metabolic disease, and the first
one to be treated by diet, is Phenylketonuria,
or PKU. Before the 1960's, untreated
PKU was one of the most common causes
of mental retardation in the United
States. Now all newborns in the
US are given a blood test at birth
that can diagnose PKU. If dietary
treatment is begun early and continued
throughout life, children with PKU
can expect to reach adulthood with
normal health and intelligence.
In earlier years it was believed
that dietary treatment could be
stopped at the age when brain growth
was complete. However it is now
known that children and adults who
stop following dietary restrictions
can experience learning disabilities,
hyperactivity, shortened attention
span, and other problems. For this
reason, the PKU diet - and any other
metabolic diet - is considered to
be a lifelong treatment.
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.
Treatment for
PKU
Dietary treatment
of PKU begins with limiting the
amount of phenylalanine going into
the blocked pathway. Because the
amount of protein found in a normal
diet contains much more phenylalanine
than can be tolerated by a person
with PKU, foods containing protein
must be severely limited.
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
The PKU diet
is based on providing enough phenylalanine
to build body protein and provide
for growth in children, but not
so much that the blocked pathway
is used beyond it's capabilities.
Not too much - and yet not too little.
In addition, tyrosine must be added
to replace that which can't be made
by the blocked pathway.
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.
Translating
a Diet Prescription into Food on
the Table
For babies, following
a PKU diet prescription is easy.
The proper amount of phenylalanine
is provided by normal baby formula.
The addition of phenylalanine-free
metabolic formula provides the rest
of what is needed for growth.
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.
Monitoring Control
The success of
treatment can be monitored by regularly
measuring phenylalanine levels in
the blood. If plasma levels fall
within a recommended treatment range
(2-8 mg%) the brain is thought to
be protected from the toxic effects
of phenylalanine. It is important
to monitor blood levels frequently,
especially in a rapidly growing
child, so that dietary changes can
be made as often as they are needed.
Regular clinic visits are also important
so that growth and development can
be measured.
Compliance and
Barriers to Success
Compliance with
the restrictions of metabolic diets
is often a struggle. The diet is
complicated. It requires careful
measurement, calculation, and record
keeping. It requires buying and
preparing foods that are different
from what the rest of the family
eats, as well as more planning time
and more food preparation time.
Low protein specialty foods that
make the diet more "normal"
are very expensive, and are not
commonly available in retail stores.
The metabolic formula has a very
strong taste, and may not be accepted
well. It is also important to note
that in metabolic diseases other
than PKU, the consequences of poor
dietary compliance can be life threatening.
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.
Summary
Dietary treatment
is successful for many metabolic
diseases that could not be treated
even 10 years ago, and can make
it possible for people with metabolic
disease to live normal, healthy
lives. However, success depends
on compliance with significant restrictions
that are complicated and often difficult
to follow. New products that make
these diets easier to live with
are continually introduced, and
research is ongoing to find new
therapies. For now, diet is the
medical treatment and should be
followed with care throughout life.
To Learn More:
Try these links
to learn more about dietary treatment
of metabolic disease.
www.pkunews.org
www.msud-support.org
www.nucdf.org
www.oaanews.org
www.tmc.tulane.edu/departments/human-genetics/main.htm
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