PROTEIN REQUIREMENTS FOR ATHLETES

The metabolism of protein and carbohydrates is intertwined and impacts athletic performance and recovery.  Protein synthesis and oxidation is dependent on carbohydrates, while dietary protein impacts carbohydrate metabolism (Tipton et al., 2001).  During exercise, when blood glucose levels plummet, protein is supplemented through oxidation and protein synthesis remains either unchanged or decreases.  This results in a net protein loss and in order to enhance recovery and stimulate synthesis, adequate dietary intake of protein and carbohydrate is essential.  Research has shown that additional protein may be beneficial for active adults and athletes to improve performance, enhance recovery, and stimulate gains in lean body mass.  In order to optimize performance, various factors need to be considered when prescribing protein intake, such as: energy intake, genetics, training status, exercise duration and intensity (ADA & ACSM 2000; ADA 2009).   

During high intensity exercise, catabolic hormones are released to supply fuel by degrading energy stores and amino acids account for less than 5% of the total energy (ADA & ACSM, 2000; ADA 2009).  These exercise bouts are usually associated with microscopic tears in the muscle, tissue damage, suppression of the immune system due to high levels of cortisol, and glycogen depletion, which is directly related to fatigue (Baty et al., 2007).  To facilitate rapid glycogen replenishment, dietary carbohydrate (1.0-1.5 g/kg) consumption must be introduced within 30 minutes and then continued at two-hour increments for approximately six hours (ADA 2009).  However, in order to blunt the catabolic hormonal response after exercise supplementation of dietary protein rich in amino acids may help to promote an anabolic response by regulating synthesis in skeletal muscles, enhancing recovery, and altering hormonal release (ADA 2009; Tipton et al., 2001).  According to a recent study, muscle glycogen storage is significantly improved within the initial minutes (0-40 minutes) and further enhanced within four hours with the combination of carbohydrate and protein (80 g CHO, 28 g PRO, 6 g FAT) than the other recovery methods of carbohydrate (80 g CHO, 6 g FAT) or isocaloric high carbohydrate (108 g CHO, 6 g FAT) (Ivy et al., 2002).  All subjects cycled until exhaustion to deplete glycogen and then received supplementation during recovery at 10-minutes and again at 2-hours.  The findings showed the carbohydrate and protein group had faster glycogen recovery rates by at least two to four times and significantly lower glucose levels.   Similarly, in another study, the combination of carbohydrates and protein was more effective in replenishing muscle glycogen following cycling exercise when compared to carbohydrate (Morifuji, Kanda, Koga, Kawanaka, & Higuchi, 2010).  While another study has found that the addition of protein to a carbohydrate supplement increased insulin levels but did not enhance muscle glycogen resynthesis rate any differently than carbohydrate or protein alone (Jentjens, van Loon, Mann, Wagenmakers, & Jeukendrup, 2001).     

The anabolic hormone, insulin, regulates blood glucose levels and is stimulated to be released from the pancreas by specific amino acids and proteins in combination with a carbohydrate load.   Insulin functions to promote glucose uptake into the cell through the GLUT-4 protein and certain BCAA, including leucine, activates glucose uptake through PI3-kinase.  After exercise, increased insulin levels optimize recovery by promoting glucose uptake, glycogen synthesis through glycogen synthase activation, and tissue repair (Morifuji et al., 2010).  In a recent study, after two hour cycling bouts, the combination of carbohydrate (1.2 g/kg/h) and protein supplementation (0.4 g/kg/h) did not further enhance the glycogen synthesis rate than the other carbohydrate only group (Howarth, Moreau, Phillips, & Gibala, 2009).  However, the coingestion shifted the rate of protein breakdown and significantly increase the mixed muscle protein synthesis rate.  As compared to the carbohydrate alone, the combination of protein and carbohydrate helped to improve recovery by enhancing the anabolic response, promoting muscle protein synthesis, and shifting net protein balance to positive.  As a result, I believe that endurance athletes can benefit from intertwined carbohydrate and protein metabolism by co-ingesting both macronutrients immediately and continuously for up to four to six hours after exercise and thereby altering hormonal responses.  Even though findings from previous studies do not support the claim, protein ingestion during recovery may maximize the training response of the muscle and achieve protein balance (Pendergast, Meksawan, Limprasertkul, & Fisher, 2011)

Due to the specific needs associated with training, the current recommended dietary allowance (RDA) for dietary protein (0.8 g/kg/d) is inadequate and additional protein, ranging between 1.2 to 1.7 g/kg/d, may be beneficial for active adults and athletes.  Protein is a macronutrient that functions to repair muscle damage, provide energy, and increase lean tissue mass (ADA & ACSM, 2000; ADA 2009).  In order to maintain energy and nitrogen balance, active individuals may need to consume additional energy, including protein than the general population (Butterfield & Calloway, 1984).  Leucine is a branched chain amino acid (BCAA) that is broken down and oxidized in the mitochondria by the enzyme branched-chain 2-oxoacid dehydrogenase (BCOAD) (Lamont, McCullough, & Kalhan, 1999).  Endurance exercise increases protein degradation and exercise intensity is directly related to the oxidation of leucine.  As a result, endurance athletes should consume between 1.2 to 1.4 g/kg/d of dietary protein and BCAA may delay exhaustion by the central nervous system (ADA 2009; McKenzie et al., 2000).  Repetitive training has shown to improve oxidative and aerobic capacity by enhancing mitochondrial and enzymatic activity.  These training adaptations enhance protein utilization and nitrogen retention; less dietary protein may be needed to maintain lean body mass and nitrogen equilibrium (McKenzie et al., 2000; Butterfield & Calloway, 1984).

Strength training athletes leads to muscle damage and these athletes require almost double the dietary protein than the general population; dietary protein should range between 1.2 to 1.7 g/kg/d (Burke et al., 2001).  Additional energy intake between 500-1,000 kcal/d and up to 1.6-1.7 g/kg/d of dietary protein for trained male bodybuilders may be helpful to promote the hypertrophic training response and maintain muscle mass (ADA & ACSM, 2000).  Resistance training leads to muscle damage, fiber tears, swelling and soreness.  After strength training, muscle protein synthesis is reduced and the inflammatory response peaks within 24-hours that involves catabolic hormones and swelling.  The supplementation of the appropriate carbohydrate and protein ratio will stimulate muscle synthesis by increasing insulin levels in the blood and maintain protein breakdown by reducing cortisol levels (Pendergast et al., 2011).  As a result, reducing muscle damage may improve recovery rates and athletic performance.  In a study by Tipton et al., after 45 minutes of lower body resistance training, the ingestion of essential amino acid (6 g) and carbohydrate (35g) stimulated the anabolic hormones and the net protein balance shifted to positive (2001).  According to a study by Baty et al., male participants (n= 34) consumed either a placebo or carbohydrate-protein (CHO-PRO) supplement before, during, and after resistance training (2000).   Within 24-hours, cortisol and creatine kinase (CK) was significantly elevated in the placebo group.  The CK levels were an indicator of muscle damage and may be linked to the elevated cortisol levels in the placebo group.  In contrast, significantly increased insulin levels were found in the CHO-PRO group, which enhanced recovery by acting to increase protein synthesis and inhibit protein breakdown. 

Overall, protein metabolism is affected by various components and resistance training requires additional protein to optimize muscle gains and maintain the hypertrophic response to training.  In order to facilitate glycogen resynthesis following endurance exercise, protein and carbohydrate co-ingestion has been found to collectively promote anabolic response by increasing insulin, promoting glucose uptake, stimulating glycogen resynthesis, decreasing cortisol, and maintaining net muscle loss. 

 

References

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