Out of the four most well-known high-quality and complete proteins in the sports nutrition market, whey protein has become the industry and athlete favorite as it can be found in most muscle mass gainer and post-workout supplements. Derived from milk protein, whey protein comprises approximately 20 % of the total protein content in commercial bovine milk, which is significantly less than the 50–90 % (depending on stage of lactation) whey fraction of total protein found in human milk [8, 29–31]. The high fraction of whey protein in human milk is a key nutritional component to child growth and development. Whey protein is commercially categorized and available as whey protein concentrate (70–80 % PRO), whey protein isolate (90 % PRO), or whey protein hydrolysate (90 % PRO). Differences between these three forms of whey protein are exhibited by the processing technique. The various methods of processing affect the concentration of whey protein subtypes and peptides, which have been determined to help counteract various health-related issues beyond enhancement of muscle hypertrophy. Other differences among the forms of whey exist between fat and lactose content (remaining protein nutrient composition), their ability to preserve glutamine residues, and their amino acid profiles [8, 24]. Elliot et al. demonstrated that the greater fat composition within the protein profile may slightly facilitate a postexercise positive net protein balance and amino acid utilization [8, 32]. The most common form of whey protein used in sports nutrition supplements is whey protein concentrate (70 % and 80 % composition) due to its lower raw material pricing and postulated better flavor profile characteristics compared to the other whey forms [8, 33].

Whey tends to be digested quicker due to its sustained solubility and rapid emptying in the stomach, has better mixing characteristics, and is commonly considered a higher-quality protein compared other complete proteins [24, 34, 35]. The digestibility of whey contributes to its more rapid onset of amino acid appearance in the blood compared to casein, resulting in a greater stimulation of muscle protein synthesis [24, 36, 37]. The higher quality of whey protein can be partly attributed to its dominance in branched-chain amino acid composition, particularly leucine [38]. A study conducted by Tipton and colleagues demonstrated peaks in arterial leucine concentration occurred sooner and with greater magnitude following postexercise whey supplementation compared to casein [34]. Additional research has demonstrated that the essential amino acid composition of whey alone, compared to whey protein as a whole, resulted in greater muscle protein balance among the latter at rest [8, 39]. This finding suggests a possible synergistic effect within the protein composition that feeds muscular hypertrophy. Despite research finding that a higher leucine content is essential to muscle protein synthesis stimulation with resistance training in older adults [40], the addition of leucine to whole proteins, such as whey, after bouts of resistance exercise had little to no effect on protein synthesis and net nitrogen balance [8, 41, 42]. Similar to the frequent meal-based protein ingestion concept, consuming whey protein throughout the day is effective for increasing protein synthesis compared to less frequent or erratic intakes of other protein types [43]. In conjunction with resistance training, supplementation with whey protein is beneficial due to its rapid absorption, high branched-chain amino acid content, and health benefits when attempting lean muscle mass accretion.

Casein, a second type of milk protein, is separated from whey protein within skim milk through resolubilizing and drying. Like whey, casein protein is found in commercial sports nutrition supplements in several variations, such as potassium caseinate, sodium caseinate, calcium caseinate, and casein hydrolysate [24]. Compared to whey, casein is a slower digesting protein. This fact is attributed to its coagulation within the stomach, which significantly reduces rate of elimination and increases transit time through the stomach [34, 44]. Tipton et al. compared the acute rate of arterial leucine appearance after post-resistance exercise protein ingestion of whey or casein [24, 34]. The results indicated that casein protein peaked later and to a lesser extent than whey protein, but the release of amino acids into the blood was sustained over a longer period of time with casein intake. A recent 8-week training study conducted by Wilborn and colleagues also assessed post-resistance exercise consumption of either whey or casein protein and demonstrated an increase in total protein synthesis, but no significant difference was exhibited between the two groups [34, 45]. Not all studies have found this response. A post-resistance training study conducted by Cribb et al. found the whey supplementation group had significantly greater gains in strength and lean body mass compared to the casein supplementation group after a 10-week training period [46]. Despite the inconclusive findings in the literature, casein has been shown to improve protein synthesis when combined with resistance training, but unlike whey, casein also has been found to have anti-catabolic properties [47]. Casein is also a moderately inexpensive form of protein, thus the reason why it is commonly found in many protein-based nutritional supplements. Due to the variation in casein formulation, a result of the specific resolubilizing processing technique affecting the amino acid profile, casein proteins are available in different grades based on protein quality (amino acid content), taste, and mixing characteristics [24, 28]. Perhaps the primary disadvantage to casein protein incorporation into a particular nutritional supplement is that it typically does not mix well in liquid, leaving clumps in suspension.

In summarizing the aforementioned protein research findings, whey was found to maximally stimulate protein synthesis, while casein was discovered to have anti-catabolic properties. Currently, most post-workout and lean mass gain supplements contain both proteins as part of their formulation. The rationale for this protein combination was confirmed by a 10-week resistance training study performed by Kerksick et al. in which three different supplement formulations were compared. The combination of both whey and casein protein supplementation groups proved to be the most effective as they demonstrated the most significant gains in lean muscle mass [48]. An interesting practical application of this study is to simply mix whey protein in bovine milk (which is ~80 % casein).

Soy protein is the only plant-based protein source that is considered high quality due to its high concentration of all essential amino acids except methionine. Soy protein is made from legume and soybeans through a water- or water-ethanol-based extraction method [49]. Variations in this extraction method produce a soy protein concentrate or isolate. When compared to the consumption of milk proteins, soy protein was able to induce significant changes in muscle protein synthesis; however, greater net protein balance, fractional synthesis rate (of mixed muscle proteins), and protein synthesis in skeletal muscle were found with milk protein ingestion postexercise [38, 50, 51]. There are currently several studies that have attempted to explain the difference in muscle protein synthesis response between milk and soy proteins. In general, they have concluded that milk protein tends to support peripheral (e.g., skeletal muscle) protein synthesis, while soy protein tends to support greater splanchnic protein synthesis [38, 52, 53].

In addition to promoting muscle protein synthesis, soy protein sources, such as soy milk, contain the omega-3 fat alpha linolenic acid (ALA) and isoflavone phytoestrogens. Soy milk also has less saturated fat and cholesterol and tends to have a lower glycemic index than most bovine milk products [24]. Isoflavone phytoestrogens have been reported to stimulate estrogen receptors and promote nonhormonal estrogen-like properties that could benefit (pre- and post-) menopausal women [24, 54, 55]. More specifically, soy protein isolate contains a myriad of antioxidant compounds such as isoflavones, saponins, and copper, a primary component of many antioxidant enzymes [51]. Recent research has attributed soy isoflavones to improving markers of bone (i.e., protection against osteoporosis), lipid, and thyroid function while decreasing the risk of heart disease and promoting a chemo-protective effect against breast and prostate cancer [24, 56]. Despite the purported beneficial health effects of soy protein with no long-term side effects, there are several studies that question the long-term impact of diets containing high levels of phytoestrogens. Additional research needs to be conducted on several soy-related questions, such as whether soy protein ingestion effects hormone levels or semen quality in men, as it could potentially have a large impact on athletic performance [24].

The variable effects that soy protein supplementation can have on different genders leads to a difference in supplementation recommendation values. With the nonhormonal estrogen-like properties, soy protein can be an excellent source of high-quality protein for both pre- and postmenopausal female athletes. For male athletes, the lower muscle protein fractional synthesis rate of soy protein compared to whey may be less advantageous, but for female athletes, the lower saturated fat and cholesterol content may help to promote more nutrient-dense caloric intake when used as a supplement. Finally, due to the lower glycemic index of soy protein and its moderate rate of digestion between casein and whey, supplementation may be beneficial in isolated situations or in conjunction with the primary milk proteins.

Egg protein has been a staple protein source in many nutritional supplements as it historically was considered the best source of protein due to its wide consumer acceptance as a high-quality protein. When processed for incorporation into a nutritional supplement, egg protein is extracted from chicken egg whites (ovalbumin) or whole eggs using a range of different techniques [24, 49]. Unlike casein protein, egg protein is highly miscible and able to form a homogeneous mixture when incorporated in liquids. Despite their decrease in popularity and regular incorporation into newer nutritional supplements due to its higher cost compared to other protein forms, egg white protein is labeled as the reference standard for comparing protein quality [24, 57]. Due to higher cost and poor taste of egg protein supplements, most athletes choose other sources of complete protein. Several studies compared post-resistance training effects of egg protein supplementation to casein and whey sources and concluded that there is no difference between the three forms of complete protein when assessing promotion of nitrogen retention. Due to its effectiveness and place as a reference standard, egg protein or ovalbumin is still incorporated in some meal replacement and protein power supplements on a minor scale [49].

Evidence from the Academy of Nutrition and Dietetics (AND) and other associated research confirms the nutritional needs of athletes can be met through consumption of a vegetarian or vegan diet. Despite this, doubt continues to linger among many nutrition professionals who counsel athletes. There are a growing number of athletes, especially endurance athletes, who have adopted a vegetarian- or vegan-based diet to help control body weight and meet increased carbohydrate needs [58, 59]. In comparison to omnivorous diets, vegetarian and vegan diets contain significantly higher quantities of complex carbohydrates, fiber, antioxidants, and phytochemicals. The high ratio of carbohydrates can help many athletes, particularly endurance or strength-endurance athletes, maintain adequate levels of glycogen storage in the muscle and liver as required fuel for muscular contraction [60]. Maintenance of adequate glycogen stores sustains a physiological state of higher energy availability, directly correlated to an environment primed for muscle mass development. Secondly, the inclusion of various antioxidants (e.g., vitamin C/ascorbic acid, vitamin E/tocopherol, and β-carotene) and phytochemicals from plant sources may positively affect athletic performance, recovery, and lean mass accretion by mitigating exercise-induced oxidative stress [59]. The potential to optimize glycogen stores from increased carbohydrate intake has been shown to have negative effects on the bioavailability of key nutrients such as iron, zinc, and other trace minerals among the general active population. The impaired nutrient bioavailability, particularly suboptimal iron levels, has been speculated in the vegetarian/vegan athletic population, but no connection to impaired muscle mass growth has been demonstrated [60, 61].

All essential and nonessential amino acids can be obtained from a diet strictly fueled from plant sources if enough calories are consumed to match the activity level of the individual and if a variety of sources are used to obtain those calories [59, 60, 62, 63]. Vegetarians (lacto-ovo-vegetarians) are more likely to meet the necessary protein macronutrient requirements due to their consumption of dairy products, eggs, and complementary mixtures of high-quality plant proteins (e.g., soy protein) resulting in approximately 12–14 % of their calories coming from protein [59]. Quality protein consumption can be a large concern for vegans (those who avoid consumption of all animal proteins) because plant-based proteins, making up only 10–12 % of caloric consumption, can be limiting in quantity of lysine, threonine, tryptophan, and/or sulfur-containing amino acids (methionine, cysteine, homocysteine, and taurine) as seen in Table 7.2 [59, 61]. Vegetarian athletes now have a wide variety of high-quality protein food and beverage sources (egg, dairy, and soy) containing all of the essential amino acids necessary to drive protein synthesis. However, vegan athletes may need to consider specific meal planning strategies that focus on plant sources rich in protein such as legumes, nuts and seeds, rice, and other whole-grain products in order to achieve proper nitrogen balance during periods of muscular growth [59, 62]. Additionally, well-planned vegan diets for competitive athletes may meet or exceed the RDA for protein in general, but may be deficient in lysine as most plant protein sources do not provide sufficient quantities [61]. As previously mentioned, recent protein and amino acid research demonstrates that leucine is the primary driving force behind the positive effects of branched-chain amino acids in the development of new muscle. In order to provide sufficient quantities of leucine in the diet of vegan athletes, a focus must be directed toward overall higher protein quantity of which a greater percentage should be provided by beans and legumes [61].

Compared to omnivores, vegetarians and vegans typically have a lower creatine pool, measured by significant decreases in total and free creatine concentration in addition to muscle creatine content [60, 61]. This decreased creatine pool demonstrates that the diminished dietary creatine intake from meat sources is not completely offset by increases in endogenous creatine production within vegetarian and vegan athletes [60, 64]. Creatine supplementation has been shown to have large ergogenic effects in active and athletic populations, but with diminished endogenous creatine stores, vegetarian and vegan athletes experience greater metabolic, performance, and body composition changes through increases in lean mass when supplementing with creatine [59]. Refer to Sect. 6 of this chapter for additional information concerning creatine supplementation.

High-quality protein through whole foods and supplements, in adequate quantities addressing the intensity, volume, and duration of exercise, is the foundation for muscle hypertrophy. All four sources of high-quality protein, whey, casein, soy, and egg, are viable options for inducing lean muscle mass gains in strength-power athletes. While soy and egg proteins are less common protein sources found in today’s muscle-building supplements, they are great options for lactose-intolerant athletes. In order to maximize the benefits of protein ingestion on lean muscle gains, both quantity and quality have to be considered in addition to proper protein nutrient timing.

It has been well established that protein ingestion is necessary for muscle hypertrophy. To optimize muscle hypertrophy, carbohydrate-protein combinations and nutrient timing strategies can be used. In the absence of nutrient intake pre- and postexercise, the body will remain in a negative net protein balance state, therefore stimulating muscle catabolism [2, 4, 65]. This section will explain the importance of the carbohydrate-protein combination for muscle hypertrophy, nutrient timing optimization, as well as what happens when nutrition is neglected.

Creatine is one of the most extensively researched and popular nutrition supplements. Over the last two decades, creatine has been examined in over 500 research studies that have assessed its viability on muscle physiology, body composition, and/or exercise performance in trained, healthy, and various diseased populations. The vast quantity of research conducted supports the claim that creatine supplementation during training, particularly high-intensity exercise, results in promotion of significant strength, fat-free mass, and performance improvements. The majority of scientific evidence available in the literature demonstrates that creatine monohydrate supplementation in athletic, active, and clinical populations is a generally effective nutritional ergogenic aid when paired with various forms of exercise [106]. The laundry list of proven benefits as a result of creatine supplementation coupled with solidified safety, when the recommended dosages are consumed, cements the reason why creatine is considered the gold standard of nutritional supplements.

Serving as key messengers in the body, hormones play an essential role to building and maintaining muscles. Food, stress, and activity all heavily influence the concentration of hormone levels in the body. Although there are hundreds of hormones in the body, the ones discussed in this chapter are specifically related to muscle building. The first set of hormones reviewed is anabolic hormones, better known as the hormones that directly stimulate muscle growth. The next set of hormones is concerned with preventing catabolism of muscle (Sect. 9). Understanding the role of hormones in the muscular growth process is important for optimizing body composition as well as realizing the positive and negative effects of supplementation. The key anabolic hormones found in the body are testosterone, human growth hormone, insulin, and insulin-like growth factor 1 (IGF-1).