Research indicates that individuals who are physically active require higher levels of protein than 0.8 g/kg body weight per day, which is the current recommended daily allowance (RDA) for healthy adults (Trumbo et al., 2002). Physically active individuals require more protein regardless of the exercise type or the state of training (Campbell et al., 2007). When consuming insufficient amounts of protein, a negative nitrogen balance occurs leading to impaired recovery (Kreider et al., 2004). Therefore, the International Society of Sport Nutrition advises exercising individuals to have a protein intake ranging from 1.4 to 2.0 g/kg/day (Campbell et al., 2007). For individuals on a vegan diet this is especially relevant, since this diet is often lowest in protein as compared to vegetarian, pesco-vegetarians and omnivorous diets (Clarys et al., 2014). Recent research estimating protein requirements based on amino acid oxidation in endurance (Bandegan et al., 2019) and resistance training (Bandegan et al., 2017) even concluded the estimated average requirement to be between 1.7 and 2.6 g/kg/day. Although it would be possible to reach protein requirements through a varied healthy diet, a supplement can give more certainty and convenience of a sufficient amount of protein intake for exercising individuals. Along with the appropriate amount of daily intake, the timing, portion size and source of the protein are relevant as well. Levenhagen et al. (2001) stated that whole body and leg protein synthesis and net protein deposition are increased when protein is consumed shortly (< 1hour) after exercise when compared to three hours later. Additionally, a meta-analysis by Schoenfeld et al. (2013) investigated the effects of protein timing on strength and hypertrophy after prolonged resistance training and also concluded the existence of a post-workout anabolic window. However, they do emphasise that consuming an adequate amount of protein is more important than timing when aiming to increase MPS. It is often proposed that MPS reaches a maximum at a protein intake of 20-25 g/portion, above which oxidation of amino acids occurs (Areta et al., 2013; Moore et al., 2009; Moore et al., 2012), as can be seen in Figure 4. However, it should be kept in mind that this amount is based on results of whey protein, which is a high quality and fast-acting protein with an absorption rate of ~10 g/hour (Bilsborough & Mann, 2006). A protein source that is absorbed at a slower rate or has a lower digestibility would potentially attenuate protein digestion and absorption and influence the nutritional value or ‘quality’ score of the protein. The FAO/WHO (2013) concluded that quality of a protein can best be measured by a digestible indispensable amino acid score (DIAAS) which is based on individual essential amino acids (EAAs) digestibility in the small intestine. The quality of the protein could thus be influencing the maximum amount for optimal MPS. A more personal approach by Morton et al. (2015) proposed that 0.4 g/kg/meal in four meals per day would maximally stimulate MPS. Quantification of a maximum protein amount per meal for muscle anabolism has shown to be a challenge due to individual variables and goals but also different protein sources and quality but especially EAAs composition (Brad Jon Schoenfeld & Aragon, 2018).
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).