Protein

Protein provides 4 calories per gram and is a key macronutrient that serves many functions in the body. Protein not only forms the major structural components of muscle, but it also forms the structural components of the bones, organs, connective tissues, the brain, nervous system, blood, skin, and hair [1, 3, 4, 5]. Furthermore, protein aids in food digestion; serves as the transport mechanism for vitamins, minerals, fats, and oxygen within the body; and protects against illness as an antibody [1, 2, 3, 4, 5].

The body’s need for dietary protein results from the constant breakdown and regeneration of the body’s cells. Unlike carbohydrates and fat, the body does not store protein. Instead, the body carefully regulates protein balance through the removal and addition of nitrogen while continuously recycling amino acids, which are the building blocks of protein [1, 3, 4]. Protein balance is measured in terms of a nitrogen balance, which is a measure of the nitrogen consumed (from dietary consumption of protein) and the nitrogen excreted (from protein breakdown) [3]. In a healthy body, the amount of protein taken in is matched by the amount of protein lost in feces, urine, and skin [3]. The muscle tissues undergo continual breakdown and resynthesis, with a fraction of muscle protein destroyed and an equal fraction rebuilt daily using amino acids from the amino acid pool [3]. A negative nitrogen balance, in which the body breaks down more protein than it can create (catabolism), occurs during times of high stress such as with severe infections and trauma [3]. An endurance athlete who consumes a high-protein, low-carbohydrate diet (and thus has minimal glycogen stores) will rely heavily on muscle protein for fuel, putting them in a negative nitrogen balance; as a result, they will experience decreased athletic performance and worsened muscular strength and endurance [3]. A positive nitrogen balance, in which the body produces more protein than it breaks down (anabolism), occurs in times of growth such as childhood, pregnancy, recovery from illness, and in response to resistance training when overloading the muscle promotes protein synthesis [3]. However, just because an athlete consumes a high-protein diet does not necessarily mean that they will be in a positive nitrogen balance and experience muscle growth [3]. Protein consumption beyond recommended amounts is unlikely to result in further muscle gains because the body has a limited capacity to use amino acids to build muscle [3]. Most studies suggest that there is a threshold effect of 1.6 to 1.7 g/kg (0.7 to 0.8 g/lb.) protein. Beyond that amount, there is no further increase in skeletal muscle protein synthesis. In fact, consumption of protein beyond 1.6 to 1.7 g/kg (0.7 to 0.8 g/lb.) promotes increased amino acid breakdown that may be converted to carbohydrate or fat [1, 3]. Excess protein intake is unlikely to result in further muscle gains because of the body’s limited capacity to utilize amino acids to build muscle [4].

Essential and Nonessential Amino Acids

Endurance and resistance training stimulate muscle protein synthesis, which is the process in which the body turns amino acids into muscle. When digested, protein breaks down into amino acids, which are used by the body to repair and rebuild muscle [1, 2, 3, 4, 5]. It is important to understand the difference between essential and nonessential amino acids. The body cannot produce essential amino acids, and there are nine of these essential amino acids that must be consumed in the diet. The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Conversely, nonessential amino acids can be produced by the human body and do not need to be consumed in the diet. The eleven nonessential amino acids are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. Generally, all nine essential amino acids can be found in animal products, which are complete protein sources. Complete protein foods include beef, poultry, fish, eggs, and dairy, soy. Most plant foods, however, are incomplete protein sources because they are low in one or more of the nine essential amino acids. Plant foods can become complete protein sources by combining complementary incomplete plant proteins that together can provide all the essential amino acids. Some excellent combinations include grains-legumes (e.g., rice and beans), grains-dairy (e.g., pasta and cheese), and legumes-seeds (e.g., falafel) [1]. There are certain plant foods that are complete proteins, such as soy, hemp seeds, peas, quinoa, and buckwheat.

Types of Protein

Whey. Whey is rapidly digested and absorbed and has a remarkable ability to stimulate muscle protein synthesis, even more so than other high-quality proteins [4, 6]. It takes the body 20 minutes to digest, absorb, and metabolize whey protein, and within one hour, the body will have used up the whey for either protein synthesis or oxidation. Whey contains high levels of the amino acid leucine, which plays a particularly important role in muscle hypertrophy [4, 6]. There are three varieties of whey: whey protein powder, whey protein concentrate, and whey protein isolate. All provide high levels of the essential and branched chain amino acids, vitamins, and minerals [4]. Whey powder is 11-15 percent protein and is used as an additive in many food products [4]. Whey concentrate is 25-89 percent protein, whey isolate is 90+ percent protein, and both are commonly used in dietary supplements [4]. It should be noted that while whey isolate is nearly pure whey, some of the proteins can be lost during the manufacturing process [4]. It is recommended to take whey protein around workouts.

Casein. Casein is slowly digested, resulting in a more prolonged and sustained release of amino acids into the bloodstream that can last for several hours [4, 6]. Protein synthesis peaks 3-4 hours after consuming casein. Like whey, casein also provides all the essential amino acids, but it does not raise amino acid levels as high or as quickly as whey. Even though casein does not elevate amino acid production as high as whey protein does, it significantly slows the rate of protein breakdown, protecting existing muscles. It is recommended to take casein around fasting periods or before bed. During a resistance training workout that produces microtears in the muscle tissues, a ready supply of amino acids is useful. Some studies suggest that combined casein and whey may produce the greatest muscular strength improvements after an intensive resistance training program [4].

Alternatives. Common alternatives to whey and casein include soy, egg, hemp, and pea proteins. Soy is the most widely used vegetable protein because it is one of the only vegetable proteins that contain all the essential amino acids. Like whey, soy proteins can be consumed in three types: flour (50 percent protein), which is often used in baked goods; concentrates (70 percent protein), which are commonly added to nutrition bars, cereals, and yogurts; and isolates (90 percent protein), which are highly digestible and often added to sports drinks, health beverages, and infant formulas [4]. Soy foods (e.g., tofu, soy burgers, and soy nuts) are considered heart healthy due to their low levels of saturated fats and high content of polyunsaturated fats, fiber, vitamins, and minerals [4]. In addition to soy, eggs (like all animal products) are a complete and an excellent source of high-quality protein [7, 8]. Large eggs contain about 6 grams of protein each, and egg protein is highly bioavailable, which means that the body uses more of it when it is digested [7, 8]. Hemp is another popular plant-based protein, but it is not considered a complete protein [7, 8]. Hemp protein contains a high content of omega-6 and omega-3 fatty acids, which usually come from foods like fish [7, 8]. Lastly, pea protein is very popular among vegetarians, vegans, and people with allergies or sensitivities to dairy or eggs. Pea protein contains all the essential amino acids and is considered a complete protein [7, 8]. It is comparable to whey protein in terms of its ability to increase fullness and improve body composition, strength, and performance when combined with exercise [7]. Soy, egg, hemp, and pea proteins are great alternatives for vegetarians, vegans, and people with allergies or sensitivities to dairy or eggs. Due in part to their high fiber content, plant proteins tend to digest slower than animal proteins. Although this may not pose a problem for most people, it can limit the amino acids the body can use immediately post exercise [7]. It should also be noted that most plant protein sources are incomplete proteins, so it is recommended to mix different plant proteins to provide the body with all the essential amino acids.

Benefits and Risks of a High-Protein Diet

A variety of studies have shown that higher levels of protein intake benefit muscle mass, strength, and function; bone health; maintenance of energy balance; cardiovascular function; and wound health [1]. Research indicates that high-protein diets preserve lean body mass, at least in the short-term (24 weeks), and contribute to increased satiety [4]. For the average exerciser striving to lose weight, research suggests that protein helps to preserve lean muscle mass and assure that most weight lost comes from fat [1]. While these may seem like great reasons to consume high amounts of protein, it is worth noting that protein metabolism likely becomes more efficient with exercise training, which stipulates that athletes do not have increased protein needs compared to the more sedentary population [4]. Excess protein intake beyond 2.0 g/kg (0.9 g/lb.) per day is unlikely to result in further muscle gains because of the body’s limited capacity to utilize amino acids to build muscle [4]. For example, strength/power athletes and bodybuilders consume protein-rich diets with a protein intake of 2.0 to 2.5 g/kg (0.9 to 1.1 g/lb.) of body weight per day [6]. This great consumption of protein is associated with the lipid-rich and low-carbohydrate diet, resulting in energy deficit which results in weight loss, body composition change, performance impairment, and even health damages [6].

Protein and Athletic Performance

Protein metabolism becomes more efficient with exercise training, which supports the Institute of Medicine’s assertion that athletes do not have increased protein needs compared to the more sedentary population [4]. Muscle protein synthesis is further enhanced if protein is consumed around the time of exercise training [4]. Research indicates meals and snacks consumed throughout the day, particularly foods consumed before, during, and after exercise, should be a combination of carbohydrates and protein at approximately a 3:1 ratio to encourage a positive nitrogen balance, which ensures muscle synthesis, hydration, and adequate energy to sustain exercise [4]. Consumption of protein immediately after exercise (within 15 to 30 minutes) helps in the repair and synthesis of muscle proteins [4]. Furthermore, it is recommended that 6 to 20 grams of protein be consumed with 30 to 40 grams of carbohydrates within 3 hours’ post-exercise as well as immediately before exercise to encourage muscle resynthesis. Research indicates that as little as 5 to 10 grams of protein consumed immediately after exercise can promote optimal muscle repair [4, 6]. Protein consumption with the intake of water post-exercise also helps to restore hydration. Carbohydrates are important to pair with protein. If only protein is consumed without sufficient carbs to provide the body’s energy needs, then muscle synthesis may be compromised [4].

Protein Recommendation

• Per the Acceptable Macronutrient Distribution Range (AMDR), which was developed by the Institute of Medicine, protein should account for 10-35 percent of total daily calories [1, 3, 4].

• The daily minimum for most healthy individuals is 0.8 g/kg (0.36 g/lb.) of body weight; however, athletes may need anywhere from 1.2 to 1.7 g/kg (0.5 to 0.8 g/lb.) of body weight per day [1, 3, 4, 6].

• Recommended protein intakes are best met through diet, though many athletes do turn to whey- or casein-based protein powders and other supplements to boost protein intake [3].

Sources

[1] Bryant, C. X., & Green, D. J. (2017). Ace Essentials of Exercise Science for Fitness Professionals. American Council on Exercise.

[2] Efferding, S., & McCune, D. (2021). The vertical diet. Victory Belt Publishing.

[3] Muth, N. D., & Tanaka, M. S. (2013). Ace Fitness Nutrition Manual. American Council on Exercise.

[4] Muth, N. D., & Zive, M. M. (2020). Sports nutrition for health professionals (2nd ed.). F.A. Davis.

[5] Zinczenko, D., & Spiker, T. (2010). The new ABS diet: The six-week plan to flatten your belly and firm up your body for life. Rodale Press.

[6] Bertucci D R Ferraresi C 2016 Strength Training: Methods, Health Benefits and DopingBertucci, D. R., & Ferraresi, C. (2016). Strength Training: Methods, Health Benefits and Doping. Nova Science Publishers. https://eds-s-ebscohost-com.ezproxy.umgc.edu/eds/ebookviewer/ebook/bmxlYmtfXzExMzQ0NTZfX0FO0?sid=b5e6955f-8aa1-4fff-906c-5bba28f3d5d2@redis&vid=1&format=EB&rid=1 202201071347301767052055

[7] Spritzler, F. (2023, January 26). The 7 best types of protein powder. Healthline. https://www.healthline.com/nutrition/best-protein-powder#3.-Egg-protein

[8] There’s more than one whey to build muscle. MYPROTEIN. (2021, November 5). https://us.myprotein.com/thezone/supplements/9-best-whey-protein-alternatives/