Health and Nutrition
Glutamine and infection
Most people expect to get the occasional cold or flu at some time over the winter and these are accepted as a normal part of life. For athletes training hard, however, any minor illness or infection that interrupts training can have a devastating effect. There are also many reports of an increased risk of minor illness after a hard race, leading to delayed recovery. Susceptibility to minor infectious illnesses is often taken to be a sign of a weakened immune system, and many of the supplements now being targeted at athletes claim to stimulate the immune system and increase resistance to infections. One of these is glutamine.
Evaluation of the value of these supplements requires several lines of evidence to be examined carefully. The questions that have to be asked include:
- Is it true that athletes in hard training are more likely to suffer from illness and infections at a greater rate than the general population?
- If the above can be shown to be true, is there any evidence as to what causes this?
- If a dietary supplement is proposed as a way of reducing the risk of infection, can a plausible mechanism of action, involving what is known of the function of the body's immune system, be established.
- Does administration of the supplement in question reduce the number and severity of episodes of infectious illness suffered by athletes during hard training?
- Lastly, and most important from the athlete's point of view, is there any effect on performance?
Exercise, Glutamine and Infection
It is now generally accepted that modest levels of regular exercise are associated with an increased sensation of physical wellbeing and a decreased risk of upper respiratory tract infections (URTI). Hard training, or indeed any unusually severe stress, however, is often associated with an increased susceptibility to colds and flu. David Nieman, one of the prominent research workers in this field, was the first to describe a J-shaped curve linking the risk of illness to the amount and intensity of training. The consequences of minor URTI symptoms are usually minimal, but for the athlete in hard training or preparing for a major competition, they may be sufficiently serious to cause training to be reduced or suspended or for a race to be missed.
It has been suggested that severe exercise results in a temporary reduction in the body's ability to respond to a challenge to its immune system and that an inflammatory response similar to that occurring with sepsis and trauma is invoked. It is not clear, however, that the various changes in parameters of the immune system that have been reported will result in a reduced ability to deal with opportunistic infective agents. Several studies have shown a reduced circulating glutamine level in the hours after hard exercise. In view of the role of glutamine as a fuel for the cells of the immune system, this has been proposed as a mechanism that would compromise the ability to respond to infection. Other studies have shown that athletes suffering from chronic fatigue symptoms attributed to overtraining also have low circulating glutamine concentrations.
Modest exercise stimulates immune function and protects against minor infections, but hard exercise can depress the immune system and may increase the risk of illness.
Glutamine is a fuel for the immune system, and plasma glutamine concentrations fall after hard exercise, in infection and after trauma.
