12.1 MNT for Inborn Errors of Metabolism and Eating Disorders
Key Takeaways
Phenylketonuria is treated with a lifelong low-phenylalanine diet using a phenylalanine-free medical formula, with tyrosine becoming conditionally essential and aspartame avoided.
Maple syrup urine disease restricts the branched-chain amino acids leucine, isoleucine, and valine, and galactosemia eliminates lactose and galactose for life.
The Newborn Screening Act (RA 9288, 2004) makes newborn screening part of routine newborn care, ideally after the first 24 hours of life.
DSM-5-TR rates anorexia nervosa severity in adults by BMI: mild 17 or more, moderate 16-16.99, severe 15-15.99, and extreme below 15.
Nutrition therapy for bulimia nervosa and binge-eating disorder builds a regular pattern of three meals and planned snacks and avoids dieting, which drives the binge cycle.
The table of specifications places inborn errors of metabolism and eating disorders within the medical conditions examinees must manage. Both require precise diets, close monitoring, and teamwork with physicians and families.
Inborn Errors of Metabolism (IEM)
An enzyme defect blocks a metabolic pathway. The substance before the block builds up to toxic levels, and the product after the block becomes deficient. Diet therapy therefore:
- Restricts the substrate that cannot be metabolized, while giving enough for growth when it is an essential nutrient.
- Supplies the missing product, which may become conditionally essential.
- Provides adequate energy to prevent catabolism, which releases the toxic substance from body tissues.
- Uses special medical foods and monitors blood levels often.
Newborn screening in the Philippines. The Newborn Screening Act of 2004 (RA 9288) requires that parents be informed about and offered newborn screening, and it is part of routine newborn care. Blood is collected by heel prick, ideally after the first 24 hours of life. The program expanded in 2014 to a larger panel (expanded newborn screening) that includes congenital hypothyroidism, G6PD deficiency, PKU, MSUD, galactosemia, and other conditions. Early diagnosis allows diet therapy before brain damage occurs.
| Disorder | Enzyme defect | Accumulates | Diet management |
|---|---|---|---|
| Phenylketonuria (PKU) | Phenylalanine hydroxylase | Phenylalanine | Low-phenylalanine diet with phenylalanine-free formula; tyrosine supplied; avoid aspartame; limit high-protein foods; lifelong |
| Maple syrup urine disease (MSUD) | Branched-chain alpha-ketoacid dehydrogenase | Leucine, isoleucine, valine and their ketoacids | BCAA-free formula with measured natural protein; aggressive energy during illness; urine smells like maple syrup or burnt sugar |
| Classic galactosemia | Galactose-1-phosphate uridyltransferase (GALT) | Galactose-1-phosphate | Remove lactose and galactose for life; soy-based infant formula; read labels for milk solids |
| Glycogen storage disease type I | Glucose-6-phosphatase | Glycogen in liver; hypoglycemia | Frequent feedings, uncooked cornstarch overnight, limit fructose and galactose |
| Homocystinuria | Cystathionine beta-synthase (common form) | Homocysteine, methionine | Vitamin B6 for responders; low-methionine diet with cystine added; folate, B12, betaine |
| Hereditary fructose intolerance | Aldolase B | Fructose-1-phosphate | Avoid fructose, sucrose, and sorbitol |
| Urea cycle disorders | Various urea cycle enzymes | Ammonia | Protein restriction with essential amino acids; high energy; nitrogen-scavenging drugs |
| G6PD deficiency | Glucose-6-phosphate dehydrogenase | Red cell oxidative damage | Avoid fava beans, naphthalene mothballs, and specific oxidant drugs |
PKU details for the examination:
- Phenylalanine is essential, so it is restricted, not eliminated; the allowance is set by blood monitoring.
- Natural protein comes from measured amounts of foods such as vegetables, fruits, and low-protein cereals; meat, fish, eggs, milk, and legumes are mostly replaced by the medical formula.
- Maternal PKU: high maternal phenylalanine harms the fetus (microcephaly, heart defects, intellectual disability), so blood levels should be controlled before conception and throughout pregnancy.
MSUD is notably more common among Filipino newborns than in many other populations, so Filipino dietitians may encounter it in metabolic clinics. During illness, a "sick-day" plan increases energy and further limits natural protein to prevent metabolic crisis.
Eating Disorders
| Disorder | Key features (DSM-5-TR) |
|---|---|
| Anorexia nervosa | Restriction leading to significantly low body weight; intense fear of weight gain; disturbed body image. Adult severity by BMI: mild 17 or more, moderate 16-16.99, severe 15-15.99, extreme below 15 |
| Bulimia nervosa | Recurrent binge eating with compensatory behaviors (vomiting, laxatives, fasting, excessive exercise), on average at least once a week for 3 months; weight often normal |
| Binge-eating disorder | Recurrent binges with distress and loss of control, at least once a week for 3 months, without regular compensatory behavior; often linked to obesity |
| ARFID | Avoidant/restrictive food intake disorder: limited intake from sensory aversion, fear of choking, or low interest, without body image concerns |
| Pica | Persistent eating of non-food substances such as soil, chalk, or ice |
| Rumination disorder | Repeated regurgitation, rechewing, or reswallowing of food |
Clinical signs to recognize:
- Anorexia: bradycardia, hypotension, hypothermia, amenorrhea, lanugo, loss of bone density, constipation.
- Bulimia: dental enamel erosion, swollen parotid glands, calluses on the knuckles (Russell's sign), and hypokalemia with metabolic alkalosis from vomiting.
Nutrition management:
- Medical stabilization: severely malnourished patients face refeeding syndrome. Check phosphate, potassium, and magnesium before and during refeeding, and give thiamine.
- Refeeding: start feeding promptly and increase steadily under monitoring. Weight-restoration goals commonly target about 0.5-1 kg per week in inpatient care and less in outpatient care.
- Structured meal plan: three meals and two to three snacks, using exchange lists or a plate method rather than detailed calorie counting.
- Bulimia and binge-eating: regular eating breaks the restriction-binge cycle; avoid skipping meals; identify triggers; include all food groups so no food becomes "forbidden".
- Team approach: psychiatrist or psychologist (cognitive behavioral or family-based therapy), physician, and nutritionist-dietitian. The nutritionist-dietitian corrects food myths and builds normal eating but does not provide psychotherapy outside their competence.
Warning
In severe anorexia nervosa, the greatest early danger is refeeding syndrome, not the speed of weight gain. Monitor electrolytes daily during the first week of nutrition support.
A 4-year-old with PKU is on diet therapy. Which statement about the diet is correct?
Phenylalanine is restricted and supplied by a measured allowance
Tyrosine must be strictly removed from the diet
Aspartame-sweetened drinks are good sugar-free choices
The diet can safely stop once the child starts school
A newborn develops vomiting, jaundice, and cataracts after starting breastfeeding, and screening suggests a GALT enzyme deficiency. What is the appropriate feeding change?
Continue breastfeeding but add lactase drops to each feeding to digest the lactose
Switch to a cow's milk formula low in iron
Switch to a soy-based infant formula and remove all lactose and galactose
Give a low-phenylalanine medical formula
A 20-year-old woman has repeated binge eating followed by self-induced vomiting several times a week. Her BMI is 21. Which finding would you most expect?
Hyperkalemia with severe metabolic acidosis from vomiting
Lanugo with a BMI below 15
Hypercalcemia with kidney stones
Dental enamel erosion and low serum potassium
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