Adequate protein ingestion is essential for maximizing training-induced adaptations, particularly in strength development. Also, since protein can be metabolized for energy, adequate protein ingestion is of particular concern for athletes in energy-demanding aerobic endurance sports, such as triathlons or marathons. The next sections highlight various aspects of protein intakes for several types of athletes and physical activity preferences—aerobic, anaerobic, and strength training.
The general understanding is that protein’s contribution to prolonged aerobic exercise ranges between 5% and 15% of total energy expenditure, depending on the intensity and duration of the exercise bout (Antonio and Stout 2001; Mero 1999). For this reason, it was once thought that the dietary protein needs for aerobic endurance athletes were no greater than those for an untrained individual. However, research using advanced methods of assessing energy expenditure and protein balance has indicated that protein needs of aerobic endurance athletes are slightly higher than for the general population (e.g., 1.2-1.4 g/kg per day) (Lemon 2001). In a landmark study, Tarnopolsky and colleagues (1988) compared distance runners with sedentary controls relative to two different protein intakes to determine their effects on nitrogen balance. A 10-day period of normal protein intake was followed by a 10-day period of altered protein intake in both groups of male subjects. The nitrogen balance data revealed that the aerobic endurance athletes required 1.67 times more daily protein than sedentary controls. Aerobic endurance athletes excreted more total daily urea than either bodybuilders or controls. The authors concluded that the aerobic endurance athletes required greater daily protein intakes than sedentary individuals to meet the needs of protein catabolism during aerobic exercise.
Using this factor of protein intake, Friedman and Lemon (1989) instructed five well-trained distance runners to consume two different diets for a period of six days each. In one of the six-day intervention periods, the runners consumed the Recommended Dietary Allowance of protein (~0.8 g/kg body weight per day). For the other six days, the runners consumed a protein intake that was 1.7 times higher (~1.5 g/kg body weight per day). During each trial, the runners followed their regular training program (7-10 miles of running daily). Estimates of whole-body nitrogen retention (an indicator of protein synthesis and protein breakdown) were obtained from urinary and sweat nitrogen losses. The differences in protein intake combined with the nitrogen excretion measures showed significant differences in estimated whole-body nitrogen retention between the two protein intakes. Specifically, nitrogen retention remained positive during the high-protein trial but was significantly reduced in the lower-protein trial. The authors stated that the current protein RDA (0.8 g/kg body weight per day) may be inadequate for athletes engaging in chronic high-intensity aerobic endurance exercise. Since approximately 1.5/kg body weight per day resulted in a positive nitrogen retention in this group of athletes, athletes engaged in this mode of training should strive to ingest this amount of protein in their diets.
While it is easy to obtain this amount of protein in the diet, it may be advantageous for aerobic endurance athletes to time protein intake in order to optimize training adaptations (Kerksick et al. 2008). For example, studies have shown that ingesting protein (0.5 g/kg) with carbohydrate (1.5 g/kg) after exercise is more effective in promoting glycogen retention than ingesting carbohydrate alone (Zawadzki, Yaspelkis, and Ivy 1992). Moreover, ingesting creatine (a combination of three amino acids) with carbohydrate reportedly promotes greater glycogen storage than ingesting carbohydrate alone (Green et al. 1996). Also, creatine loading before carbohydrate loading has been reported to promote glycogen supercompensation (Nelson et al. 2001).
Ingestion of EAA and protein with carbohydrate after exercise has also reportedly enhanced protein synthesis (Borsheim et al. 2002; Tipton et al. 1999) and help mediate the immunosuppressive effects of intense exercise (Gleeson, Lancaster, and Bishop 2001). Finally, some evidence suggests that ingestion of branched-chain amino acids with carbohydrate during exercise may help lessen the catabolic effects of exercise (Mero 1999; Coombes and McNaughton 2000; Bigard et al. 1996; Carli et al. 1992; Rowlands et al. 2008). Consequently, for aerobic endurance athletes, it is important to ingest enough protein in the diet to maintain nitrogen balance. There may be some advantage to ingesting a small amount of protein or amino acids before, during, and after exercise in order to help athletes better tolerate training (Kerksick et al. 2008). For more on the timing of nutrient intake, see chapter 9.
As previously discussed, for many years the conventional thought was that protein did not contribute significantly to energy metabolism during prolonged exercise. For this reason, the contribution of protein or amino acids to the energy demands of anaerobic exercise was thought to be minimal. Current literature now supports that proteins are degraded and that they contribute to metabolism even during a single bout of high-intensity exercise (Bloomer et al. 2007, 2005) and that training influences the content of enzymes involved in protein metabolism (Howarth et al. 2007). A single bout of resistance exercise also stimulates gene expression related to protein synthesis (Hulmi et al. 2009). Performing a number of sprints or successive bouts of intense exercise promoted protein degradation and oxidation (De Feo et al. 2003). Moreover, performing exercise bouts in glycogen-depleted conditions promoted a greater degradation and utilization of protein as a metabolic fuel (Wagenmakers 1998).
While carbohydrate remains the primary fuel needed for high-intensity exercise, protein can serve as a fuel source during high-intensity, intermittent, and prolonged exercise bouts. For this reason, it is important to ingest carbohydrate along with protein or amino acids (or both) before, during, and after exercise in order to replenish amino acids used during exercise and optimize recovery (Kerksick et al. 2008). In general, athletes participating in anaerobic exercise should consume 1.5 to 2.0 g/kg of protein per day.
Research has established that resistance-trained athletes need to ingest a sufficient amount of protein in the diet to maintain a positive nitrogen balance and anabolism (Lemon 2001). Studies also indicate that ingesting protein or amino acids before, during, or after intense exercise (or at more than one of these time points) can influence protein synthesis pathways (Willoughby, Stout, and Wilborn 2007; Esmarck et al. 2001; Tipton and Ferrando 2008; Tipton et al. 2001). Several questions remain:
- Does protein supplementation promote muscle hypertrophy during training?
- Do different types of protein promote greater training adaptations?
- Does nutrient timing influence training responses?
Concerning the first question, a number of studies have shown that supplementing the diet with protein promotes greater training adaptations during resistance training than ingesting an isoenergetic amount of carbohydrate (Andersen et al. 2005; Hulmi et al. 2008; Kalman et al. 2007; Hayes and Cribb 2008; Kerksick et al. 2007, 2006; Kraemer et al. 2006). Moreover, different types of protein (combined with carbohydrate or other ergogenic nutrients like creatine and β-hydroxy–β-methylbutyric acid [HMB]) may have additional benefits (Willoughby, Stout, and Wilborn 2007; Rowlands et al. 2008; Hulmi et al. 2008; Kalman et al. 2007; Solerte et al. 2008; Kendrick et al. 2008; Candow et al. 2008; Cribb, Williams, and Hayes 2007). Consequently, growing evidence indicates that strength athletes should ingest quantities of protein at the upper end of the range of 1.5 to 2.0 g/kg per day, as well as ingest protein or amino acids either before, during, or after exercise (or at more than one of these times) in order to optimize training adaptations (Campbell et al. 2007; Kerksick et al. 2008; Lemon 2001).
- anabolism—The building of body cells and substances from nutrients, especially the building of proteins and muscle mass in the body.
Multiple studies have examined the combination of amino acid–carbohydrate supplements in the time frame that encompasses a resistance exercise session, but fewer have addressed supplementation with intact protein (such as whey and casein) after resistance exercise and its effects on nitrogen balance. Tipton and colleagues (2004) studied the ingestion of casein and whey protein and their effects on muscle anabolism after resistance exercise. They concluded that ingestion of whey and casein after resistance exercise resulted in similar increases in muscle protein net balance and net muscle protein synthesis, despite different patterns of blood amino acid responses (a quicker response of plasma amino acids for whey protein and a more sustained response for casein protein). In a similar study, Tipton and coworkers (2007) looked at whether ingestion of whole proteins before exercise would stimulate a superior response compared with after exercise. They reported that net amino acid balance switched from negative to positive after ingestion of the whey protein at both time points. For more specific information on the importance of the timing of protein ingestion and resistance exercise, refer to chapter 9.