Glutamine
syn. L-glutamine
syn. 2-amino-4-carbamoyl-butanoic acid
Description:
Glutamine is a common dietary amino acid. In humans, it is the most abundant amino acid in the extracellular pool. Furthermore, over 90% of the whole-body glutamine content is found in the muscles.365 Glutamine is classified as a conditionally essential amino acid, which means that while the body is normally capable of manufacturing enough to meet its metabolic needs, under certain conditions (such as catabolic or immune-compromised states) it must be supplied in the diet.366 This amino acid is active beyond its role as a component of tissue proteins, and is involved in a variety of metabolic functions including the support of immunity, gastrointestinal integrity, insulin secretion, neurological activity, and muscle protein synthesis.367
Glutamine has been shown to exert several biological activities that, at least in theory, might be of benefit to an exercising individual. To begin with, glutamine supports the synthesis of proteins in muscle cells.368 It is also involved in the osmotic regulation of cell volume. Increasing glutamine pools may result in cell swelling.369 This effect increases cell size and potentially also the rate of protein synthesis.370 Some studies also suggest an ability for glutamine supplementation to manipulate tissue insulin sensitivity, which may help to partition nutrients away from fat storage and towards the muscles and liver.371 Glutamine may also reduce muscle protein breakdown rates, an effect that may be mediated in part via the inhibition of myostatin, a protein that negatively influences muscle protein retention.372
Cellular metabolism and energy production are also influenced by glutamine in several other ways. For example, glutamine is an intermediary in the citric acid (TCA) cycle, and is utilized for energy by some cells.373 Glutamine is also involved in glucose metabolism, and its supplementation has been shown to increase the storage of whole-body glycogen.374 An increase in the cellular glutamine pool may also increase the exercise-induced release of IL-6 (Interleukin-6), a hormone-like cytokine involved in inflammation.375 While low resting IL-6 levels are desirable for health, brief IL-6 elevations immediately following training may increase glucose disposal and fatty acid oxidation.376 Glutamine may also help combat the buildup of ammonia, a waste product of anaerobic energy metabolism.377 It can also increase antioxidant activity through conversion to glutathione.378
Glutamine is also very important to human digestion and immunity. Glutamine depletion after prolonged endurance exercise has been associated with suppression of the immune system. In some studies, the supplementation of glutamine has been shown to stimulate the immune system and reduce the incidence of illness following prolonged endurance exercise.379,380 Glutamine supplementation may also support the health and integrity of the intestinal lining.381 It is often used in hospitals for its ability to support gastrointestinal health and nutritional absorption in patients with gastrointestinal disease, burn-trauma, or HIV/AIDS.382 Because glutamine is so readily utilized by the intestines, liver, and kidneys, its bioavailability in serum is fairly low. It is estimated that as little as 10% of the administered dose may reach the muscles intact.383
Supplements containing glutamine are widely used in sports nutrition to enhance muscle growth, immunity, and recovery following intense exercise. Glutamine levels do decline with intense exercise,384 and supplementation of this amino acid has been shown to increase muscle glutamine pools.385 The potential anabolic and performance-enhancing benefits to this, however, remain the subject of much debate. An ergogenic effect has been demonstrated with glutamine supplementation, but such benefits have been very inconsistent. In spite of this inconsistency, glutamine appears to be a supplement with significant potential, especially for those undergoing intense weight training programs or high-level endurance activities. Glutamine is available in both free amino acid form, and in the form of glutamine peptides (protein bound). Studies suggest that the bioavailability of free glutamine is considerably higher than that of glutamine peptides.386 Most clinical studies have also been conducted with free L-glutamine, supporting the preferred use of this type of glutamine supplement. Benefits have also been noted in clinical studies with glutamine peptides, however, so both are likely to offer some effect. Further research is needed to betterunderstand the therapeutic advantages and disadvantages to both types of glutamine, as well as the overall ergogenic value of this conditionally essential amino acid.
Promoted Benefits:
Glutamine is promoted to increase muscle mass and strength, and improve athletic performance.
Clinical Studies:
This ingredient has been shown to improve performance in placebo-controlled studies with trained adults. Its Clinical Support Rating is 5 (5/5).
One placebo-controlled study examined the effects of glutamine on exercise performance in a group of competitive soccer players.387 The glutamine was added to a carbohydrate drink in the form of peptides, which was taken 30 minutes prior to exercise evaluations. This consisted of motorized treadmill running, along with measurements of oxygen consumption. The glutamine content of the drink was 3.5 grams, which was combined with 50 grams of maltodextrin. A drink containing only 50 grams of maltodextrin was used as a control group. The athletes taking the glutamine enhanced drink noticed a statistically significant improvement in total distance covered (15571 m versus 12750 m) and exercise duration before exhaustion (88 min versus 73 min).
Another study with positive findings examined the effects of glutamine supplementation on protein balance and glycogen storage following exercise in a group of healthy men.388 The subjects exercised on a stationary cycle ergometer for 90 minutes at 65% peak oxygen consumption (VO2peak). After exercise, they consumed a carbohydrate (1 g/kg of body weight) and essential amino acid recovery drink (9.25 grams). In one group, glutamine was added to the drink at a dose of .3 g/kg of body weight. The study found that while the addition of glutamine failed to improve protein or glycogen synthesis rates, it did result in less muscle tissue breakdown (an anti-catabolic effect) during a three-hour monitored recovery period. In theory, this should result in a greater net protein gain from exercise.
Studies with glutamine have had conflicting results, with some investigations (often with short-term supplementation protocols) failing to find statistically significant improvements in exercise performance.389,390,391,392 Whether these inconsistent results are due to individual factors, study methodological difficulties, or specific circumstances in which glutamine can exert tangible ergogenic effects remains unclear. Further research is needed to better understand the potential role of glutamine in improving exercise performance.
Empirical Evidence:
Glutamine is widely available as a stand-alone nutritional supplement. The feedback on glutamine as an ergogenic aid has been mixed. In fact, glutamine may be defined as a “love it or hate it” type of supplement. Many bodybuilders find it to be a regular and important component of their exercise support and recovery drinks. Among this group, glutamine is said to help preserve muscle mass during periods of high activity, and sometimes even produce visible improvements in muscle size and strength during bulking phases of training. Glutamine also appears to be popular with many prolonged endurance and high intensity athletes, helping to both support optimal metabolism and reduce the incidence of infections and illness. For many athletes of all types, it is also commonly used to protect intestinal health, with the understanding that optimal absorption of nutrients is key to maximizing recovery, growth, and performance. It seems that an equally strong and vocal percentage of users, however, believe glutamine to be completely without noticeable effect and merit as an ergogenic supplement.
It is difficult to speculate as to why these opinions tend to vary so strongly. Given the low bioavailability of oral glutamine, insufficient dosage may be an issue, at least in some of these reports. Glutamine has an Empirical Evidence Rating of 3 (3/5).
Effective Dosage:
Based on clinical studies, a dosage of 10-30 g per day is recommended.
Side Effects / Safety:
Glutamine was well tolerated during clinical studies, with no significant side effects reported.393
