A Guide to Bodybuilding with PKU and Other Metabolic Conditions

If you’re getting started in bodybuilding, weight-lifting, or just working out, there seems to be one common piece of advice: eat more protein. Protein consists of amino acids which play a role in blood clotting, immune response, vision, hormone production, and muscle and cell growth and repair. In fact, protein makes up about 80% of muscle mass. According to Health.gov guidelines, at least 10-35% of total daily consumption should consist of protein; bodybuilders and those trying to build muscle may consume even more. But what should people with phenylketonuria (PKU) or other metabolic orders like UCD and HCU, who can’t consume much protein, do to improve their physical fitness?

For those with metabolic disorders, bodybuilding or building muscle weight can be difficult. As their bodies cannot properly break down protein, they require an extremely low-protein diet, sometimes under 6g per day. But this doesn’t mean that they can’t work out or gain muscle mass!

If you have PKU, urea cycle disorders (UCD), or homocystinuria (HCU), check out these tips on how to work out, build mass, and get healthy.

Phenylketonuria (PKU) & Other Metabolic Conditions


Caused by a genetic mutation, phenylketonuria (PKU) prevents the breakdown of an amino acid called phenylalanine. This essential amino acid is not created in the body and must be digested from outside sources, such as meat, poultry, fish, peanuts, lentils, and sesame seeds. However, in PKU, phenylalanine builds up in the body, resulting in severe side effects. About 1 in every 10-15,000 people are born with PKU. However, many newborns will not show symptoms until months after birth. These symptoms include:

  • Intellectual and developmental delays
  • Hyperactivity
  • Decreased bone strength
  • Difficulty focusing
  • Crying spells
  • Microcephaly
  • Neurological problems, including seizures
  • Poor melanin production, resulting in fair skin and light (blue) eyes
  • Frequent skin rashes
  • Musty-smelling skin, breath, and urine
  • Heart defects
  • Behavioral and emotional issues

Learn more about PKU.

Urea Cycle Disorder (UCD)

Urea cycle disorders (UCDs) are genetic conditions that impact how waste from breaking down protein is removed from the body. Normally, the urea cycle consists of:

  • The liver changing nitrogen into urea using several enzymes.
  • Urea (waste) being removed from the body via urine.

However, people with UCD don’t have a specific enzyme in their liver. As a result, nitrogen cannot be converted and toxic ammonia builds up in the blood. Altogether, there are 6 types of UCDs which depend on which enzyme is missing:

  • Arginase deficiency (ARG)
  • Argininosuccinate lyase deficiency (ASL)
  • Argininosuccinate synthetase deficiency (ASS)
  • Carbamyl phosphate synthetase deficiency (CPS)
  • N-acetylglutamate synthase deficiency (NAGS)
  • Ornithine transcarbamylase deficiency (OTC)

Symptom onset varies, with most diagnoses being made in infancy to early childhood. In severe cases, symptoms appear within 24 hours of birth. These include:

  • Fatigue
  • Nausea and vomiting
  • Poor feeding
  • Irritability
  • Seizures
  • Confusion
  • Hyperactivity
  • A dislike of meat or other protein-rich foods
  • Coma

Learn more about UCD.

Homocystinuria (HCU)

Homocystinuria (HCU) is an inherited metabolic disorder which prevents the body from metabolizing amino acids like methionine found in meat, dairy, and fish. Genetic mutations often occur in the CBSMTHFRMTRMTRR, and MMADHC genes. Most commonly, HCU affects people in Ireland, Norway, Germany, and Qatar. In many times, symptoms occur within the first year following birth. Unfortunately, it is often difficult to diagnose.

When methionine and other byproducts build up in the blood, it causes damage to the heart, central nervous system, and connective tissue. Symptoms and characteristics of HCU include:

  • Long limbs
  • Intellectual and developmental delays
  • A tall, thin frame
  • Seizures
  • Nearsightedness
  • Abnormal blood clotting
  • Failure to thrive

Learn more about HCU.

How To: Bodybuilding

PKU, HCU, and UCD Exercise Tips

Says forum user Got2BHuge, in his experiences with PKU and bodybuilding:

“You have to keep your goals realistic if you have PKU. A high-protein diet is essential for putting on substantial amounts of muscle mass, and we just aren’t cut out for that.”

Additionally, PKU makes it more difficult to retain muscle. So, he recommends that those with PKU and other metabolic disorders instead work towards developing lean muscle mass. On top of that, for those looking to lose weight, PKU News suggests taking it slowly:

“Slower weight loss will minimize the loss of water and muscle, an important consideration for those with PKU. Losing weight too quickly can cause an increase in [phenylalanine] levels.”

Although there are limited studies on exercise or bodybuilding routines specifically for people with metabolic disorders, there are a few recommendations available:

  • Kate Buckland recommends exercising 3-5 times per week with an exercise that greatly raises the heart rate. This can be cardio, weight training, or both. However, if weight training, rest one full day between body parts to avoid injury, over-use, or fatigue.
  • The National PKU Alliance (NPKUA) offers different suggestions for children and adults:
    • For adults: 150 minutes (weekly) of moderate intensity aerobic activity or 75 minutes of vigorous physical activity. 2+ days of muscle strengthening. Balancing exercises.
    • For children: 60 minutes (daily) of moderate or vigorous physical activity. 3+ days (weekly) of vigorous activity. 3+ days (weekly) muscle and bone strengthening. Body weight exercises like squats, push-ups, lunges. Impact exercises like running.
  • Finally, the Children’s Hospital Boston published a guide on ways to increase exercise in your life. These are great for those starting their fitness journey before getting into more intense bodybuilding regimens.

PKU, HCU, and UCD Diet

Proper nutrition is crucial to health and wellness. When well-fueled, you are better able to perform, support brain health, and strengthen and recover your muscles. Additionally, proper nutrition reduces your risk of injury. Further, shares metabolic dietitian Stephanie Hacker, MS, RD, LDN, CNSC, maintaining a healthy weight prevents heart disease, diabetes, arthritis, and high blood pressure.

But with PKU or other metabolic disorders, it can sometimes be difficult to understand what your diet should look like. As a result, some people struggle with gaining or maintaining weight, while others have problems with losing weight. According to Hacker, female patients with PKU are significantly more likely to be obese than the general population. While the rates are somewhat more comparable with male patients, there are still heightened levels of obesity.

Additionally, failure to follow a PKU-friendly diet can result in extreme harm or even death. One example is 25-year-old bodybuilder Meegan Hefford, whose undiagnosed UCD, and protein ingestion, led to her death.

However, following a PKU/HCU/UCD-friendly diet will help kickstart your fitness journey.

Diet and Nutrition Tips

  • Leverage your dietitian. Your dietitian will guide you in creating a nutritional plan; understanding what phenylalanine-free foods to eat; and monitoring any health problems. Speak to your dietitian before making any dietary changes or taking any new supplements to understand the potential risks. 
  • Stay hydrated. Hydration helps break down fat, burn calories, and promote blood flow to muscles. If you’re dehydrated, you won’t only hit a problem with muscle maintenance or weight gain/loss; you’ll also experience dizziness, cramps, headache, and even organ damage. Personal trainer Kate Buckland, who has PKU, recommends drinking 2-3 liters of water daily. 
  • Don’t maintain a caloric deficit. As explained by Abby Hall, a caloric deficit can actually increase levels of phenylalanine. Additionally, it promotes catabolism, or breaking down lean muscle mass. Consuming enough calories will fuel your body while moving you towards your fitness goals. Easy snacks for a quick caloric boost include fruit or veggie chips. 
  • Eat at the right times. National PKU News recommends eating a meal 3-4 hours prior to exercising, or a snack 1-2 hours prior. Meals should be high in carbohydrates but low in fat and fiber. Additionally, consuming a carbohydrate-heavy meal, or formula, within 30 minutes of exercise will restore glycogen stores and promote energy.

Formula and Nutritional Foods

According to trainer Kate Buckland, she is most often asked how to build muscle and lose fat by clients with PKU. To start, she notes that your focus should be on taking your measurements and body fat percentage. Don’t focus too much on the number on the scale. For diet, she recommends eating nutrient-rich food like vegetables; cooking with healthy fats like coconut or olive oil; and lowering sugar levels in the diet.

Additionally, people with metabolic conditions should drink formula every day. The exact amount will differ and depends on dietitian recommendation. Some formulas, like RESTORE, even contain beneficial electrolytes. Other options include LoPhlex, BetterMilk, Easiphen, Aminogran, and Maxi 10. 

To increase safe protein consumption and calories, you could potentially consume more formula. However, please speak to your doctor first. Additionally, if formula consumption increases, so should fluid intake.


In addition to formula, you may be able to take supplements to help build muscle and lose fat. But what supplements should you take? I decided to get the answers straight from patients with PKU.

According to toby_w, one supplement to consider is leucine, which helps with protein synthesis and muscle building. Leucine also regulates blood sugar levels, repairs muscle, and assists with wound healing.

One bodybuilding forum recommends:

    • Multivitamins
    • Fish oil
    • BCAAs
    • Creatine
    • Testosterone boosters
    • Nitric oxide
    • L-glutamine
    • L-carnitine

Jessica Lynn

Jessica Lynn

Jessica Lynn has an educational background in writing and marketing. She firmly believes in the power of writing in amplifying voices, and looks forward to doing so for the rare disease community.

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