Glutamine: What It Is and How It Works
Glutamine is the most abundant free amino acid in the human body and one of the longest-standing products on sports nutrition shelves. It has been sold for muscle growth, recovery and immunity, yet research paints a more nuanced picture. Our editors explain what glutamine does, where the evidence is solid, and why it is rarely a priority for healthy gym-goers.
What glutamine is
Glutamine is an amino acid that the body can make for itself, so it is classed as non-essential. Under certain conditions, such as severe illness, major surgery or burns, demand can exceed production, and it is then described as "conditionally essential".
Most glutamine is produced in skeletal muscle, which acts as the body's main reservoir. From there it is released into the blood and used by many tissues, especially the gut lining, immune cells and the kidneys.
It is also present in ordinary food. Meat, fish, eggs, dairy, beans and tofu all supply glutamine as part of their protein. A diet providing around 1.6 g of protein per kilogram of body weight already contains several grams of glutamine every day.
Supplements usually contain free L-glutamine powder, sometimes in capsules or as part of recovery blends. Glutamine peptides from hydrolysed wheat protein are another form, mostly used in clinical nutrition.
How glutamine works in the body
Glutamine has an unusually wide range of jobs. It carries nitrogen between tissues safely, which helps the body handle the ammonia produced when amino acids are broken down. It also donates nitrogen for building other amino acids and the components of DNA.
Rapidly dividing cells rely heavily on it as fuel. Cells lining the small intestine and certain white blood cells use glutamine as a major energy source, which is why researchers became interested in its role in gut integrity and immunity.
In the kidneys, glutamine is central to acid–base balance, helping to remove excess acid. It also feeds into glutathione, one of the body's key antioxidants, and can be converted to glucose in the liver and kidneys when needed.
During prolonged hard exercise, blood glutamine levels may fall, and some studies found lower levels in overtrained athletes. That observation led to the hypothesis that supplements might support recovery and immunity, which we examine below.
Glutamine and muscle: the evidence
The marketing idea is simple: muscle stores a lot of glutamine, so more glutamine should mean more muscle. Research has not confirmed this. A large share of oral glutamine is used by the gut and liver before reaching the muscles at all.
In a study by Candow and colleagues in 2001, healthy young adults followed a six-week resistance training programme with either glutamine or placebo. Both groups gained strength and lean mass, with no meaningful difference between them.
Reviews, including one by Gleeson in 2008, concluded that there is little evidence glutamine improves muscle growth, strength or performance in healthy people who eat enough protein. Glutamine also does not appear to add anything beyond what complete proteins such as whey already provide.
Some smaller studies suggest it might reduce soreness or speed recovery of strength after damaging exercise, but these findings are inconsistent and the effects modest. They are not strong enough to make glutamine a core supplement for lifters.
| Claim | What research suggests | Editorial rating |
|---|---|---|
| Increases muscle growth | No benefit in trained or untrained adults with adequate protein | Not supported |
| Improves strength | No consistent effect | Not supported |
| Reduces muscle soreness | Some small positive studies | Uncertain |
| Supports immunity in endurance athletes | Early positive data, later not confirmed | Weak |
| Supports gut barrier during heat stress | Promising small studies | Emerging |
Immunity, the gut and endurance sport
In the 1990s, Castell and Newsholme reported fewer self-reported infections in marathon and ultramarathon runners given glutamine after races. That work sparked interest in glutamine as an immune supplement for endurance athletes.
Later research did not confirm a clear effect on immune cell function, even when blood glutamine levels were maintained. The International Olympic Committee's consensus statement on supplements does not list glutamine among those with good evidence for immune support.
The gut is a more promising area. Hard exercise in the heat reduces blood flow to the intestines and can make the gut lining more permeable. A few small studies suggest glutamine taken before such exercise may reduce markers of this "leaky gut" effect.
In hospitals, glutamine has been used in clinical nutrition for critically ill patients. However, a large trial published in the New England Journal of Medicine in 2013 found that high-dose glutamine did not improve survival in critically ill patients with organ failure and may have been harmful. That is a reminder that more is not always better.
Doses, safety and practical use
Studies in athletes have typically used single doses of about 0.1–0.3 g per kilogram of body weight, or fixed amounts of 5–20 g per day. Daily amounts up to around 14 g appear well tolerated in healthy adults over the short to medium term.
Side effects at these intakes are usually limited to bloating or mild digestive discomfort. Very large doses are poorly studied in healthy people and are unnecessary.
Some groups should be cautious or avoid supplements unless advised by a doctor:
- people with liver disease, particularly cirrhosis or a history of hepatic encephalopathy;
- those with kidney disease;
- people with certain neurological conditions or on anticonvulsant medication;
- pregnant and breastfeeding women, due to limited data.
For most healthy people who train, the money spent on glutamine is better directed towards adequate protein, creatine or simply good food. Where it may be worth trying is in endurance athletes training in heat or those with frequent digestive symptoms during exercise, ideally with advice from a sports dietitian.
Conclusions of the editorial team
Glutamine is a versatile amino acid with important roles in nitrogen transport, the gut, immunity and acid–base balance. The body produces plenty of it, and a protein-rich diet supplies more.
For healthy people with adequate protein intake, research does not support glutamine as a way to build muscle or strength. Claims about immune protection are weaker than marketing suggests.
The most interesting emerging use is protecting the gut during prolonged exercise in the heat, but evidence is still based on small studies.
Readers may also want to explore our articles on BCAAs versus complete protein, on supplements for endurance athletes, and on gut problems during running.
References
- Gleeson M. Dosing and efficacy of glutamine supplementation in human exercise and sport training. J Nutr. 2008;138(10):2045S–2049S.
- Candow DG, Chilibeck PD, Burke DG, et al. Effect of glutamine supplementation combined with resistance training in young adults. Eur J Appl Physiol. 2001;86(2):142–149.
- Castell LM, Newsholme EA. The effects of oral glutamine supplementation on athletes after prolonged, exhaustive exercise. Nutrition. 1997;13(7–8):738–742.
- Heyland D, Muscedere J, Wischmeyer PE, et al. A randomized trial of glutamine and antioxidants in critically ill patients. N Engl J Med. 2013;368(16):1489–1497.
- Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med. 2018;52(7):439–455.
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