Protein needs for the athlete on a Low Protein Diet by Sarah Adams, Senior Metabolic Dietitian

Nutrition has a key role in sports performance through its strong interaction with physical training. Nutrition supports the metabolic and physical adaptations to exercise and tailored dietary strategies can enhance an athlete's performance by reducing or delaying fatigue thereby optimising training sessions and reducing the risk of injury and illness (Beck et al, 2015, Thomas et al, 2016).

Nutritional strategies for an athlete are developed by considering the type of sport and the physical demands it has on the body. Other factors to consider in the development of tailor-made nutritional plans include the athlete's;

  • Personal performance goals
  • Training and competition programme
  • Body composition
  • Fitness level
  • Food preferences and any special dietary needs/medical condition e.g. diabetes, coeliac disease, phenylketonuria (PKU)
  • Previous response to dietary strategies.

There is a large range of dietary techniques to support individual training goals and enhance sports performance. Sporting individuals with a metabolic disorder that follow a low protein diet often ask;

  1. How can they build muscle?
  2. Should they take protein supplements?
  3. How can they best recover after exercise?
  4. Therefore, this article will focus on the nutritional strategies that support muscle growth and aid recovery post-exercise for the athlete on a low protein diet.

Protein is a key nutrient for tissue repair and muscle growth in response to physical exercise as well as for general body protein turnover. There is little evidence in the scientific literature on muscle building and sports nutrition specific to those on a low protein diet. However, what is known about the metabolic and physical response to exercise and nutrition can be applied to individuals on a low protein diet.

How can I build muscle on a Low Protein Diet?

To support muscle growth and the physical adaptations to exercise it is necessary to consume both sufficient calories to meet the energy demands of the activity and adequate protein. For most people participating in regular recreational exercise whose goal is simply to keep fit and be a healthy weight, eating a varied diet that includes a range of fruit, vegetables and complex carbohydrates plus their daily prescribed amount of natural protein (exchanges) and protein substitute should be sufficient.

How much protein does an athlete need?

Most athletes, including those participating in both strength and endurance sport, male and female (without a metabolic disorder), will typically require 1.2-2.0g of protein/kg body weight/day (Thomas et al, 2016). This is 50-250% more compared to what a sedentary person (without a metabolic condition) requires which is 0.8g protein/kg/d. For example, an adult with PKU on diet, the daily protein recommendation is 0.8g protein/kg/d plus 40% to account for the digestibility of amino acids and the effect of amino acids on phenylalanine control (van Spronsen et al, 2017). This equates to 1.12g protein/kg/d for sedentary adults with PKU.

Interestingly, it is the timing of protein consumption that appears to be important to optimise the building of muscle. Dietary protein appears to be most effective for muscle growth when consumed within 2hrs after exercise and at regular intervals across the day (Philips, 2014). Furthermore, muscle growth is enhanced for at least 24 hours after resistance, sprint and endurance exercise plus the muscle becomes more receptive to protein consumed over this period of time (Burd et al, 2011, (Thomas et al, 2016).

The optimal amount of protein to consume immediately after exercise appears to be 15-20g (or approximately 10g of EAA) (Philips, 2014, Thomas et al, 2016). The protein substitutes for metabolic disorders typically contain 20g of protein equivalent of which 10g is EAA, making them ideal for supporting muscle growth. There are no additional benefits seen from consuming larger amounts of protein and in fact this may damage kidney function. However, older individuals or those with low protein stores may require larger doses (Thomas et al, 2016).

Protein consumed before and during exercise has less of an impact than protein consumed immediately after exercise but may be of benefit to those looking to build and recondition muscle such as in resistance training and for ultra endurance (Thomas et al, 2016).

The response of muscle growth to protein ingestion stems from the essential amino acid (EAA) content of the protein (Tipton et al, 1999). The amino acid based protein substitutes specifically developed for conditions such as phenylketonuria (PKU), homocystinuria (HCU) or tyrosinaemia (TYR) are rich in essential amino acids.

Putting it into practice

For example, an 80kg athlete would need 1.2-2.0g of protein/kg body weight/day. This equates to 96g-160g/day of total protein. If they required a low protein diet for PKU and their daily allowance for natural protein is 6g (6 exchanges) to control their blood phenylalanine, then they would need 5 x 20g phenylalanine free protein substitute to meet the remainder of their protein requirements. In total, 106g/day of protein (1.32g protein/kg/d) would be consumed. 100g from the protein substitute and 6g from the protein exchange foods.

For optimal muscle repair and growth, the total daily allowance should be taken in split doses across the day and within 2hrs (ideally immediately) of finishing key training sessions. Ideally, one protein substitute would be taken at each mealtime, one immediately after exercise and at bedtime. The natural protein from the food exchanges can be divided between the 3 main meals or in a pre or post exercise snack.

These recommendations can be adapted for the more recreational athlete wishing to increase their muscle mass. The key is to eat a varied diet that provides adequate energy to maintain a healthy weight and consistently contains sufficient amount of their protein substitute taken at regular intervals across the day and immediately after a training session.

How can I improve my recovery from exercise?

Generally, sports of short duration lasting less than 1 hour do not drain muscle fuel stores and recovery needs can be met from your usual meals, protein substitute and snacks.

An effective and quick recovery is particularly important when participating in repeated high intensity, strenuous or highly skilled training sessions multiple times a day or in close succession. The goals of recovery are to:

  • Refuel energy (muscle glycogen) stores and rehydrate
  • Promote muscle repair and growth
  • Promote physiological adaptation to training
  • Support immune function

A good recovery helps you get the most out of your training sessions and ultimately leads to improved performance in competition.

Speedy refuelling requires consuming appropriate amounts of fluid, carbohydrate and protein immediately to within at least 2 hours of finishing exercise. Furthermore, taking protein and carbohydrate together during the recovery period can improve the overall body protein balance after exercise (Thomas et al, 2016).

Putting it into practice:

To support muscle glycogen refuelling, this is achieved by eating regular meals that are based on complex carbohydrate foods such as low protein pasta, rice or bread. Including a little protein from exchange foods and/or the protein substitute along with timely allocated carbohydrate rich snacks taken either before or immediately after exercise will support glycogen refuelling.


Individuals, who must follow severely restricted protein diets to maintain good health, can still achieve muscle growth on a low protein diet. The protein substitute prescribed for their metabolic disorder make an ideal alternative protein supplement to support their physical training. Carefully timed consumption of one dose of the prescribed protein substitute taken immediately after exercise and at regular intervals across the day will help to optimise muscle growth. Athletes with a metabolic disorder should seek the expert advice of a specialist metabolic dietitian with knowledge of performance nutrition for individually tailored strategies.

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  2. Burd NA, West DW, Moore DR, et al. (2011) Enhanced amino acid sensitivity of myofibrillar protein synthesis persists for up to 24hr after resistance exercise in young men. J Nutr. 141(4):568-573
  3. Phillips, SM (2014) a Brief Review of Critical Processes in Exercise-Induced Muscular Hypertrophy. Sports Med. 44 (Suppl 1):S71-S77
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  5. Tipton KD, Ferrando AA, Phillips SM, et al. Post exercise net protein synthesis in human muscle from orally administered amino acids. Am J Physiol. 1999; 276:E628-34.
  6. Van Spronsen FJ et al (2017) Key European Guidelines for the diagnosis and management of patients with phenylketonuria. Lancet Diabetes Endocrinol Jan 9