by NSCA’s Guide to Sport and Exercise Nutrition
Kinetic Select December 2021
The following is an exclusive excerpt from the book NSCA’s Guide to Sport and Exercise Nutrition, published by Human Kinetics. All text and images provided by Human Kinetics.
Amino acids are the building blocks of protein and can serve as an energy source for skeletal muscle (Ohtani, Sugita, and Maryuma 2006). Protein synthesis can also occur at rest even when essential amino acids are consumed in relatively small amounts, for example 15 g (Paddon-Jones et al. 2004). Branched-chain amino acids are becoming more popular in aerobic endurance exercise due to their potential performance benefits. The BCAAs include leucine, isoleucine, and valine. They can be oxidized by the skeletal muscle to provide the muscles with energy, can enhance postexercise muscle protein synthesis, and can reduce exercise-induced muscle damage (Koopman et al. 2004). The average BCAA content of food proteins is roughly 15% of the total amino acid content (Gleeson 2005); therefore individuals regularly consuming good-quality protein-rich foods are probably consuming adequate amounts of BCAAs to support daily body protein needs.
During aerobic endurance exercise, the BCAA pool, in particular, is maintained through muscle protein breakdown, which makes it even more important that the body remain in protein balance. In longer-duration aerobic endurance exercise, the oxidation of BCAAs in skeletal muscle usually exceeds their supply from protein. This causes a decline of BCAAs in the blood and may facilitate the progression of “central fatigue.” According to the central fatigue hypothesis, central fatigue occurs when tryptophan crosses the blood–brain barrier and increases the amount of serotonin forming in the brain (Ohtani, Sugita, and Maryuma 2006). The hypothesis predicts that during exercise, free fatty acids (FFAs) are mobilized from fat tissue and transported to the muscles to be used as energy. Because the rate of FFAs being mobilized is greater than their uptake in the muscles, the concentration of FFAs in the blood increases. Free fatty acids and the amino acid tryptophan compete for the same binding sites on albumin. Because FFAs are present in high amounts in the blood, they bind to albumin first, thus preventing tryptophan from binding and leading to an increase in free tryptophan concentration in the blood. This increases the free tryptophan–to–BCAA ratio in the blood, resulting in an increased free tryptophan transport across the blood–brain barrier. Once free tryptophan is inside the brain, it is converted to serotonin, which plays a role in mood and the onset of sleep (Banister et al. 1983). Thus, an end result of more serotonin production in the brain may be central fatigue, forcing individuals to either stop exercise or reduce the intensity.
➤➤ albumin—A water-soluble protein found in many animal tissues.
Although several studies show a drop in BCAA concentration in the blood and although BCAAs may help with mental performance during or after exhaustive exercise, supplementation may have little impact on actual aerobic endurance performance. A few studies have examined changes in amino acid concentration after exhaustive exercise. Researchers in one study examined these changes in 22 subjects participating in a marathon and eight subjects participating in a 1.5-hour army training program. Both groups experienced a significant decline in their plasma concentration of BCAAs. No change was noted in the concentration of total tryptophan in either group, though the marathon subjects showed a significant increase in free tryptophan leading to a decrease in the free tryptophan/BCAA ratio (Blomstrand, Celsing, and Newsholme 1988). Other studies also show a decrease in BCAA concentration and increase in free or total tryptophan after exhaustive exercise (Blomstrand et al. 1997; Struder et al. 1997). In a double-blind examination of the direct effect of BCAAs on tryptophan, 10 aerobic endurance-trained males cycled at 70% to 75% maximal power output while ingesting drinks that contained 6% sucrose (control) or 6% sucrose + one of the following: 3 g tryptophan, 6 g BCAAs, or 18 g BCAAs. Tryptophan ingestion resulted in a 7- to 20-fold increase in brain tryptophan levels, whereas BCAA supplementation resulted in an 8% to 12% decrease in brain tryptophan levels at exhaustion. No differences were noted in exercise time to exhaustion, indicating that the changes in amino acid concentrations did not affect aerobic endurance exercise performance (Van Hall et al. 1995).
In addition to studying changes in amino acid concentration, it makes sense to examine whether there is a subsequent change in cognitive performance. In a study examining the central fatigue hypothesis and changes in cognition, subjects received either a mixture of BCAAs in carbohydrate or a placebo drink. The investigators measured cognitive performance before and after a 30 km cross country race. Subjects given BCAAs showed an improvement from before to after the run in certain parts of a color–word test; the placebo group showed no change. The BCAA-supplemented group also maintained their performance in shape rotation and figure identification tasks, whereas the placebo group showed a significant decline in performance in both tests after the run. The authors noted that BCAA supplementation had a greater effect on performance in more complex tasks (Hassmen et al. 1994). In another study of cognitive functioning after exhaustive exercise, participants ran a 42.2 km cross country race during which they were supplemented with either BCAAs or placebo. The BCAA supplementation improved running performance in the slower runners only. What is more interesting is that the BCAAs positively affected mental performance. The BCAA-supplemented group showed significant improvement in the Stroop color and word test postexercise compared to preexercise (Blomstrand et al. 1991).
Branched-chain amino acid supplementation can also influence recovery from exercise. Feeding BCAAs before aerobic exercise increases the concentration of human growth hormone and helps prevent a decrease in testosterone, which results in a more anabolic environment (Carli et al. 1992). Prolonged exercise will reduce the body’s amino acid pool; thus it is important to maintain higher levels, specifically BCAA levels, to suppress cell signaling cascades that promote muscle protein breakdown (Tipton and Wolfe 1998). Creating an anabolic environment through BCAA use may assist in faster recovery from exercise. Branched-chain amino acids also have a positive effect on lessening the degree of muscle damage. In one study, untrained men performed three 90-minute cycling bouts at 55% intensity and consumed a 200-calorie beverage consisting of carbohydrate, BCAA, or placebo before and at 60 minutes during exercise. The BCAA-supplemented beverage trial lessened the amount of muscle damage resulting from the exercise session compared to the placebo trial at 4, 24, and 48 hours after exercise and the carbohydrate trial at 24 hours after exercise (Greer et al. 2007).
NSCA’s Guide to Sport and Exercise Nutrition will lead you through the key concepts of sport and exercise nutrition so that you can assess an individual’s nutrition status and—if it falls within your scope of practice—develop customized nutrition plans. The book is available in bookstores everywhere, as well as online at the NSCA Store.