Sarah recently attended a conference, big in the world of Phenylketonuria (PKU) – the ESPKU (European Society of PKU) – and was asked to speak about PKU and sport for a nutrition company. here she explains that sport need not be a problem for people on a low-protein diet for life.
PKU is caused by a mutation in the enzyme phenylalanine hydroxylase (PAH), which would usually break down phenylalanine (Phe) to tyrosine. Treatment continues to be a low-Phe diet, or in other words a low-protein diet. Because of this, there is a common misconception that people with PKU cannot take part in some sports due to not being able to consume enough natural protein, particularly with strength training and subsequent muscle building. This is simply not the case.
Like the general population, people with PKU need to consume enough protein and carbohydrates for sufficient recovery. Fluid is also very important but this short blog will focus on protein and carbohydrates.
PKU AND PROTEIN
When looking at PKU, sport and protein requirements, we would simply use the recommendations for the general population. Depending on the sport, duration and frequency of training we would start at 1.2-1.4g/kg. This could move up to 1.8-2g/kg for someone training most days and for longer periods of time.
For people with PKU on limited natural protein, this would usually mean an increase in their protein substitutes: the products that provide all the amino acids apart from Phe. The majority contain vitamins, minerals, trace elements and DHA, but there are some without now.
We would recommend having an extra protein substitute post training and possibly another at a later time, depending on protein requirements.
For people with PKU taking part in sport, consuming enough carbohydrates is important to replenish glycogen stores between training sessions. Under-eating carbohydrates can result in inefficient recovery, poorer performance and feeling tired and sluggish.
We would suggest having a meal (three to four hours before a training session or a snack one to two hours before. This should contain some carbohydrates as suggested below.
For training sessions less than one hour, we would recommend just consuming a snacks post training or a usual meal, if within 30 minutess of a session. This would include some carbohydrates in the form of low-protein foods, such as fruit or fruit juice, low-protein crackers, bread, pasta or rice, as examples.
For heavier sessions over an hour, such as sports tournaments, longer runs or cycling sessions, additional carbohydrates would be needed. We would suggest 1-1.2g/kg every two hours after a training session.
In our patient cohort, we look after a 23-year-old male who boxes four to six days a week. Some sessions can be an hour and others 1.5 hours.
You can see in the table below that he attended clinic on two occasions within a 12-month period. Within this period he gained 2.2kg but 4.3kg in fat-free mass. His body fat declined from 20-17.1%. This was indicative of muscle building.
Body Fat %
Fat Free Mass (kg)
We recommended 1.2-1.4g/kg = 104-122g protein daily. Our patient’s dietary regime consisted of five protein substitute drinks, each containing 20g of protein equivalent, totaling 100g of protein.
His exchanges, or natural protein allowance, is 15g a day but he admitted to having more than this some days. Therefore, we estimated he was having 115g of protein a day at least, but more likely around 120g.
Our patient was feeling tired between sessions and I realiSed he was training for over one hour on some sessions a week and likely not having enough carbohydrates. I suggested the following:
- 2 x Carb gels (60g carbohydrate) when training for over one hour
- Take rice cakes with jam and a banana one to two hours before a training session
- Increase carbohydrates post training and a carbohydrate-rich snack before bed – two hours later. This could be a small meal of low-protein pasta or rice, cereal and low-protein milk, fruit, crackers, or low-protein toast
Exercise and sport provide so many benefits for a person, not only physical but can have a huge benefit to mental health, sleeping patterns and general well-being. As dietitians working in the field of inherited metabolic disorders (IMD), we want people with PKU to know they can get involved in sport and exercise, recover well, perform and even gain muscle. We continue to learn as we go, but we are always ready to support a cohort of people to chase their goals.
Sarah Howe, RD
Sarah is an experienced NHS Dietitian specialising
in the fascinating area of Inherited Metabolic Disorders in adults.
- Júlio César Rocha*, Esther van Dam, Kirsten Ahring, Manuela Ferreira Almeida,
- Amaya Bélanger-Quintana, Katharina Dokoupil, Hülya Gökmen-Özel, Martine Robert,
- Carina Heidenborg, Emma Harbage and Anita MacDonald A series of three case reports in patients with phenylketonuria performing regular exercise: first steps in dietary adjustment.