Glutamine and Endurance Competition: Does it Help Prevent Infections?
The search for ways to avoid infections while competing in endurance sports is always a hot topic. Various supplement ingredients have been bandied about for this purpose, including vitamin C, probiotics, anti-oxidants, etc. But, as with everything in sports and in life, the strong desire to have a remedy can often tend to skew one’s take on the data, so that separating the science from the snake oil becomes all the more challenging. With that in mind, today I take on an ingredient I am increasingly seeing on the shelf marketed for reducing infections in endurance athletes: glutamine.
Exercise and immune function is thought to follow a “j curve”, where regular moderate exercise will improve your ability to fight infections, but prolonged intense exercise may prove detrimental. And catching an upper respiratory tract infection (URI, or a “cold” in layman’s terms) after an endurance event is more than just a simple nuisance. It can be a very significant concern for those competing regularly, as it impairs one’s ability to recover and prepare for the next event. Clearly, any means of improving resistance to infection during one’s training and competition is beneficial. Could glutamine supplementation have such an effect?
Glutamine is an amino acid that is considered by many to be “conditionally essential”, meaning supplementation is only necessary in certain circumstances, such as when the body is severely stressed (6). An example of this occurs when the body is fighting a serious illness (4, 5, 6). In such a situation, the body’s glutamine stores, although normally adequate, may suddenly become insufficient to meet the increased demands placed on the body.
A similar scenario may arise during prolonged endurance training. Plasma glutamine levels have been demonstrated to fall during prolonged exercise activity (9, 10). Much consideration has been given to the possibility that this may have an effect on immune function following exercise, since glutamine is used extensively by the immune cells of your body.
A 1996 study by Castell et al examined the incidence of URIs and glutamine supplementation on marathoners and ultra-marathoners, and found a positive benefit (Castell et al 1996). Certainly a promising start, but one small study in isolation shouldn’t be enough to draw definitive conclusions. Especially when an understanding of the specific mechanisms of benefit remain elusive. And as with all forms of supplementation, understanding the effects throughout the body is essential before declaring that a supplement is either safe or effective.
Glutamine is very important for the proper functioning of your white blood cells, particularly the subset known as lymphocytes (11), which are unable to synthesize their own glutamine and must therefore depend on the glutamine synthesized by your muscles. The thought, therefore, is that through supplementation of glutamine one can avoid the fall in white blood cells which often occurs following prolonged intense exercise, and in doing so decrease the risk of infection.
However, this has not been supported in clinical trials. In several studies, endurance athletes supplemented with glutamine avoided the drop in blood glutamine levels post-exercise, but this had no impact on the decrease in lymphocyte count or activity (12, 13) or on the activity of salivary IgA (an important defense mechanism against URIs) (14). This would seem to contradict the notion that supplementing glutamine will help prevent infections post-endurance activity.
And, at least in the medical world, significant safety concerns have come to the forefront in recent years. Two large studies in intensive care unit populations demonstrated an increased mortality for critically ill patients receiving glutamine supplementation (15, 16). Of course, these were patients who were very very sick, so this obviously does not compare directly to endurance competition. But it’s an important point to make that anything you put in your body beyond it’s usual amounts always has the possibility of causing harm. Potential benefits always have to be weighed against potential risks.
Glutamine supplementation may well find a role in the future for helping endurance athletes avoid infections. But, thus far at least, the evidence isn’t enough to make a recommendation. And given theoretical safety concerns of over-supplementation, I am going to have to rate it as “not yet ready for prime time”.
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