Amino acids, the building blocks of proteins, can be classified as essential or non-essential amino acids. Like fat, essential amino acids cannot be synthesised by humans and must be consumed through the diet. Non-essential amino acids can be synthesised from compounds already abundantly present in the body. Key sources of proteins are eggs, meats, dairy, beans, and nuts. Proteins are essential building blocks for the body, serving as structural components, enzymes, and regulators of cellular mechanisms.
Especially, muscles need protein as roughly 60-75% of all protein is found in this tissue (McArdle et al., 2016). Dietary protein is first digested into smaller strands of amino acids, called polypeptides and then into dipeptides or single amino acids. Dipeptides and single amino acids are absorbed by the small intestine and migrated to different tissues where they can be used to synthesise new proteins.
In the body there is a constant protein turnover, meaning proteins are broken down and resynthesized continuously. Protein turnover is important to replace potentially damaged proteins and to facilitate growth. Protein breakdown ensures degradation of these proteins and generates newly available amino acids. These amino acids can be used to formulate new proteins or as fuel. Under normal conditions, the body is in protein balance, meaning equal amounts of protein are broken down as synthesised. However, after exercise, protein synthesis increases to assist exercise-induced muscle adaptation. Since the body always loses some protein, proteins must be ingested through the diet. The recommended dietary intake of proteins is 0.8 g/kg/day. Athletes who exercise vigorously, require a higher protein intake due to higher protein breakdown during exercise and to facilitate the athlete’s desire to increase muscle protein synthesis. Therefore, it is recommended to increase protein intake to 1.5-2.0 g/kg/day (McArdle et al., 2016; Voedingscentrum Nederland, 2016). The rate of muscle protein synthesis is at its optimum when sufficient proteins are ingested after exercise. Especially essential amino acids, particularly leucine, will enhance the exercise-induced muscle protein synthesis.
Proteins come in different forms and each source has a distinct amino acid profile. Protein sources differ in digestion and absorption. Three supplementary protein sources are widely recognised: whey, casein, and soy protein. Whey is a by-product of the cheese-making process and has high amounts of leucine. Whey is quickly digested and absorbed in the intestine. Casein, another dairy protein, contains lower amounts of leucine but is still high in essential amino acids. Casein is slowly digested and absorbed, making it a very practical protein to consume just before bedtime. Reasoning behind this mechanism will be explained in the next paragraph. Soy protein is a common vegan option with an absorption rate between whey and casein (Jeukendrup & Gleeson, 2018; Tang et al., 2009).
Timing of protein intake is important to facilitate constant amino acids availability for muscle protein synthesis. Protein intake should be spread throughout the day, resulting in an intake of 0.4 g per kilogram bodyweight per meal (Areta et al., 2013). Furthermore, it is advised to consume slow proteins before bedtime to guarantee amino acid availability during the night. Research shows casein ingestion leads to long lasting protein synthesis, during the night (Jeukendrup & Gleeson, 2018; Kouw et al., 2017). Next to a constant amino acid availability during the day, post-exercise protein ingestion is important to promote muscle protein synthesis. After exercise, athletes enter the so-called anabolic window where muscle protein synthesis and recovery are improved if sufficient amino acids are available. Research shows increased muscle protein synthesis after consumption of roughly 20 g of protein, of which 10 g are essential amino acids containing leucine (Areta et al., 2013).
Limited research is performed investigating the effect of only protein intervention during exercise on muscle protein deposition. Some research regarding protein co-ingestion with carbohydrates is published and will be discussed. Koopman and colleagues (2004) found an improved net protein balance in ultra-endurance athletes who exercised six hours and ingested protein and carbohydrates each half hour (Koopman et al., 2004). Another study investigated the effect of carbohydrate co-ingestion with essential amino acids on muscle protein synthesis after strength exercise (Tipton et al., 2001). The researchers found an elevated muscle protein synthesis when the beverage was ingested pre-exercise, compared to post-exercise. They attributed their findings to a higher amino acid delivery, as this was considerably higher in the pre-exercise group. An important side note, only six individuals participated in the study which makes it less reliable. Another study showed promising results on the effect of carbohydrate and protein co-ingestion on exercise performance in team athletes, but the difference between co-ingestion and only carbohydrate ingestion was not significant (Williams & Rollo, 2015). In a review from Oliveira and colleagues (2017) multiple studies were compared regarding protein consumption during exercise, but no consensus has been met on the potential beneficial effects. Based on the given research and the consensus of high-rated research institutes, concise evidence to suggest that athletes should ingest proteins during exercise is still lacking (Potgieter, 2013).
• The recommended dietary intake of proteins is 0.8 g/kg/day. Athletes require a higher protein intake of 1.5-2.0 g/kg/day.
• Ingestion of proteins could enhance protein synthesis after exercise.
• Post-exercise protein consumption of high-quality proteins is sufficient to stimulate muscle protein synthesis.
• There is limited research investigating the effect of protein supplementation during exercise.
• There is no consensus on the potential beneficial effects of protein supplementation during exercise for team, strength and (ultra-)endurance athletes.