One in vivo study (10) on five healthy male volunteers found no benefit on protein synthesis measured in arm and leg tissues from oral glutamine intake. In fact the glutamine seemed to compete unfavourably with the uptake of other key amino acids such as methionine and phenylalanine, which are indispensable in muscles for protein synthesis. Another study (11) involving seven men and four women did find a small increase in protein synthesis, although this study is difficult to interpret since the glutamine was provided in a mixture of amino acids mimicking the amino acid structure of muscle of which glutamine was only a part (40g of amino acids containing 5.8g glutamine).
Regarding the next claim, that of the ability of glutamine to increase muscle glycogen concentrations after exercise the results of studies have again been disappointing. Glutamine is no more effective at replenishing glycogen than glucose nor does it have any added benefit when added to glucose. In fact re-feeding with carbohydrates can replenish glycogen to three times the level of glutamine, and no doubt three times more cheaply (12) (13).
Finally we will consider the claim that glutamine can prevent an exercise induced immuno-suppression and help athletes avoid developing minor infections post exercise.
It is commonly believed that individuals undertaking intense exercise are more prone to catching minor infections, particularly infections of the upper respiratory tract such as colds and sore throats. Work done in the 1990's by Castell et al. at Oxford University in the UK monitored the rates of infections in runners and rowers and found that the rates of such infections were higher in individuals undertaking more intense exercise (14).
Glutamine is important for the function of immune cells such as lymphocytes and macrophages which use it as an energy source. The glutamine is supplied by muscle cells which contain the largest store of glutamine in the body (15).
Different types of exercise may cause a reduction in plasma glutamine levels. Although different studies have shown varying results, on balance both prolonged exercise such as marathon running and short duration high intensity exercise such as resistance training lower plasma glutamine levels by up to 20% for between 2-4 hours (15).
On this basis a study by Castell et al. in the 1990's gave 151 athletes either a 5g glutamine drink at 30 minutes and 2 hours after exercise or a placebo. 81% of the athletes taking the glutamine reported no infections in the following 7 days while only 49% of those taking a placebo reported no infections (14).
However, subsequent studies cast doubt on this result and indicate that low glutamine levels are not responsible for the post exercise suppression of the immune system. Data from Castell et al. and other studies have shown that plasma glutamine concentration decreases by a similar degree in both glutamine and placebo-supplemented groups and that there is no significant difference in glutamine levels between subjects who do or do not develop infections (15).
Furthermore, studies which have objectively examined immune system function by measuring parameters such as lymphocyte trafficking, NK and lymphokine-activated killer cell activities and T cell proliferation have found that glutamine supplementation does not prevent post exercise immune system suppression (16) (17).