Proteins are ubiquitous in cells, serving critical roles in growth, cellular repair, metabolic functions, and hormonal activities. Enzymes, crucial for biochemical reactions, and hormones, influencing metabolic activity, are examples of proteins. Muscle proteins, such as actin and myosin, store a significant portion of the body’s protein.
The body relies on 22 amino acids to create proteins, including eight essential amino acids that must be acquired through diet since the body cannot synthesize them. Protein quality is classified based on amino acid profile, with animal sources considered complete due to containing all essential amino acids. Plant sources, often incomplete, lack some essential amino acids. The process of protein digestion, facilitated by enzymes like proteases, breaks down proteins into amino acids, dipeptides, and tripeptides. Unlike carbohydrate and lipid digestion, protein digestion initiates in the stomach upon acidification.
The liver plays a pivotal role in amino acid metabolism, maintaining a free amino acid pool for various bodily functions. Amino acids contribute to structural proteins (e.g., skeletal muscle), functional proteins (e.g., enzymes), and signaling proteins (e.g., hormones). Unused amino acids undergo deamination, where the amino group is removed, and carbon skeletons are either oxidized or used for glucose or fatty acid synthesis. The liver converts the amino group into ammonia, eventually forming urea, which is excreted in urine.
The text historically references the nitrogen balance technique for assessing dietary protein adequacy, where balance indicates protein intake matching excretion. The Recommended Dietary Allowance (RDA) for protein ranges from 0.8 g/kg per day for adults, with higher needs for athletes engaged in intense training. Athletes are recommended to consume 1.5 to 2.0 g/kg body weight of protein to ensure adequate intake.
Protein quality is evaluated through the protein efficiency ratio (PER) and protein digestibility–corrected amino acid score (PDCAAS). The latter, with a scale up to 1.0, indicates protein quality, with higher scores reflecting superior quality. The text enumerates various protein sources and their classification based on PER or PDCAAS, designating milk, soy, egg, and whey as high-quality proteins.
The discussion further delves into different types of milk, highlighting its nutritional content and amino acid profile. Milk, with both casein and whey proteins, is a good source of essential amino acids. Skim milk, in particular, is noted for its favorable carbohydrate-to-protein ratio, making it beneficial for protein and carbohydrate intake. Studies suggest milk as an effective sport drink, promoting protein synthesis, and aiding in postexercise rehydration.
The subsequent sections focus on the two primary milk proteins, whey, and casein, providing a nuanced understanding of their roles and characteristics.
The text explores various protein sources, providing detailed insights into their characteristics and potential benefits. The focus in this section is on whey, casein, egg, and soy proteins.
Whey protein emerges as the predominant choice in nutritional supplements, especially in the sports nutrition market. It is available in concentrates, isolates, and hydrolysates, each with distinct processing methods impacting fat and lactose content, amino acid profiles, and glutamine  preservation. Whey protein concentrates, isolates, and hydrolysates undergo different production techniques, influencing their composition and potential health properties. The variations in subtypes and peptides within whey protein contribute to reported antioxidant, anticancer, antihypertensive, antibacterial, and other health-related properties. Whey protein’s faster digestion rate, superior mixing characteristics, and perceived higher quality make it a popular option. Studies suggest that whey protein, when consumed throughout the day, optimizes protein synthesis, potentially offering health benefits surpassing other protein sources.
In contrast, casein, derived from skim milk, presents as a relatively inexpensive protein source. It comes in various grades, differing in quality, taste, and mixing characteristics. However, casein tends to clump in acidic fluids and digests more slowly than whey, promoting a smaller but more prolonged increase in amino acids. This slower digestion rate gives casein an anticatabolic effect, reducing skeletal muscle breakdown. Research indicates that factors like calorie intake, protein quality, insulin response, and food digestibility influence protein synthesis. Casein is positioned as a valuable protein source, especially for its anticatabolic effects.
Egg protein, sourced from chicken egg whites or whole eggs, is recognized as a reference protein. Its protein efficiency ratio (PER) and protein digestibility–corrected amino acid score (PDCAAS) are similar to milk protein, slightly lower than casein, whey, and bovine colostrum. Studies suggest that egg protein is as effective as other proteins in promoting nitrogen retention, a key component of positive nitrogen balance indicative of lean tissue formation.
Soy protein, a high-quality complete protein, is suggested as an excellent dietary protein source, particularly for vegetarians. Its PER and PDCAAS are comparable to dietary meat or fish, slightly lower than egg, milk, casein, bovine colostrum, and whey protein. Beyond its protein content, soy offers potential health benefits, contributing to lower cholesterol levels and providing isoflavone phytoestrogens. Research has explored soy protein as a nutritional alternative to hormone therapy in women. In resistance training studies, soy protein demonstrated effectiveness comparable to whey protein, with no documented negative outcomes for males.
The text provides a nuanced understanding of these protein sources, considering their processing methods, amino acid profiles, digestion rates, and potential health benefits.
The text delves into the significance of bovine colostrum, gelatin, and the intricate relationship between protein intake and athletic performance, particularly in aerobic exercise.
Bovine colostrum, derived from cows in the first 24 to 48 hours postpartum, stands out for its superior nutrient density and protein quality compared to regular dairy milk. With a protein efficiency ratio (PER) of approximately 3.0, bovine colostrum surpasses other protein sources like beef, fish, poultry, soy, egg, milk, casein, and whey. Rich in insulin-like growth factor (IGF)-I, growth factors (IGF-II, TGF-β), immunoglobulins (IgG, IgA, IgM), and antibacterial, bovine colostrum is positioned as a unique source with potential immune-boosting and growth-promoting properties. Despite its expense, it finds application in various food products, such as infant formulas and protein supplements.
Gelatin, derived from boiling animal skin, tendons, and ligaments, contains protein, collagen, and various amino acids. Although classified as an incomplete protein due to the absence of tryptophan, gelatin is commonly used in foods and pharmaceuticals. Marketed as a supplement supporting bone and joint health, gelatin contains amino acids like proline, hydroxyproline, and glycine, commonly found in collagen.
The text acknowledges the complexity of determining the best protein type for nutritional products, emphasizing that different proteins may offer specific advantages based on the target population or desired outcomes. For instance, soy protein may be ideal for vegetarians, egg protein for lacto-ovo vegetarians, milk protein for those not lactose intolerant, casein for minimizing catabolism during extended periods between meals, whey protein for optimizing protein synthesis and immune function, and bovine colostrum for its potential to enhance training adaptations, albeit at a higher cost.
The narrative then transitions to the crucial role of protein in maximizing training-induced adaptations, particularly in strength development and energy-demanding aerobic endurance sports. Protein’s contribution to prolonged aerobic exercise is estimated to range between 5% and 15% of total energy expenditure, with slightly higher protein needs for aerobic endurance athletes compared to the general population. Research, such as the study by Tarnopolsky and colleagues, underscores the increased daily protein requirements for aerobic endurance athletes to meet the demands of protein catabolism during exercise. This heightened need, approximately 1.2-1.4 g/kg per day, challenges the notion that protein needs for endurance athletes are equivalent to those for untrained individuals.
The text further highlights the significance of timing protein intake to optimize training adaptations, citing studies that demonstrate the effectiveness of ingesting protein with carbohydrate after exercise in promoting glycogen retention and protein synthesis. Additionally, the ingestion of branched-chain amino acids with carbohydrate during exercise may help mitigate the catabolic effects of exercise. For aerobic endurance athletes, maintaining nitrogen balance through adequate protein intake is crucial, and there may be advantages to strategically timing protein intake before, during, and after exercise.
The section explores the role of protein in anaerobic exercise, particularly its contribution to energy metabolism and the impact of training on protein metabolism. Contrary to previous beliefs that protein played a minimal role in the energy demands of anaerobic exercise, contemporary literature suggests that proteins are indeed degraded and contribute to metabolism even during high-intensity exercise. Notably, a single bout of resistance exercise stimulates gene expression related to protein synthesis, underlining the dynamic nature of protein metabolism during anaerobic activities.
While carbohydrates remain the primary fuel for high-intensity exercise, the text emphasizes that proteins can serve as a fuel source during various types of intense exercise, including intermittent and prolonged bouts. Acknowledging the importance of protein in this context, the recommendation is to ingest carbohydrates along with protein or amino acids before, during, and after exercise. This strategy aims to replenish amino acids used during exercise and optimize recovery, aligning with the suggested protein intake range of 1.5 to 2.0 g/kg per day for athletes engaged in anaerobic exercise.
Moving to the realm of strength training, the text delves into the protein requirements for resistance-trained athletes. The emphasis is on ingesting a sufficient amount of protein to maintain a positive nitrogen balance and support anabolic processes. Studies indicate that protein or amino acids, when consumed before, during, or after intense exercise, can influence protein synthesis pathways. Key questions are raised, including whether protein supplementation promotes muscle hypertrophy during training, if different types of protein yield varied training adaptations, and the influence of nutrient timing on training responses.
Addressing the first question, the text references numerous studies demonstrating that supplementing the diet with protein fosters greater training adaptations during resistance training compared to isoenergetic amounts of carbohydrates. Moreover, the type of protein, when combined with other ergogenic nutrients, may offer additional benefits. The emerging consensus suggests that strength athletes should consider protein intake at the upper end of the recommended range (1.5 to 2.0 g/kg per day) and strategically consume protein or amino acids around exercise sessions to optimize training adaptations.
The term “anabolism” is introduced, referring to the building of body cells and substances from nutrients, particularly the synthesis of proteins and muscle mass. The discussion also touches on studies examining the combination of amino acid–carbohydrate supplements in the context of resistance exercise, with a focus on the effects of intact proteins like whey and casein on nitrogen balance.
One notable study by Tipton and colleagues is highlighted, emphasizing that ingestion of whey and casein after resistance exercise led to similar increases in muscle protein net balance and net muscle protein synthesis, despite different patterns of blood amino acid responses. . This section provides a comprehensive overview of the intricate relationship between protein, anaerobic exercise, and strength training, offering insights backed by contemporary literature and research findings.