L-glutamine is a non-essential amino acid which is present in the human body in the largest amount – it makes up the most abundant share of free amino acids both in skeletal muscle and in blood plasma. The supplement is credited with anabolic and immune-stimulating properties. But how does modern sports science see it?
The biological functions of glutamine
In the body, glutamine performs a range of irreplaceable roles that go far beyond simply building muscle:
The manufacturers' promises vs. the scientific reality
A normal diet rich in protein provides roughly 3 to 6 grams of glutamine a day, which under normal circumstances is enough. After demanding physical exercise, however, its level in blood plasma falls. It is precisely the depletion of glutamine stores that tends to be linked with weakened immunity in endurance athletes (Parry-Billings et al., 1992).
If we wanted to prevent a marked drop in its level after demanding training, we would have to take in roughly 0.1 g per kilogram of body weight every 30 minutes for 2 to 3 hours (Nieman and Pedersen, 2000). Let us look at the specific claims in more detail:
After long physical activity (50–70 % VO2max, e.g. a half marathon or marathon) the level of glutamine in the blood falls by 10 to 30 % and this reduced level lasts for several hours (Castell et al., 1997; Walsh et al., 1998). In theory this could create a temporary window of a weakened immune system.
The study by Castell, Poortmans and Newsholme (1996) did indeed find a positive correlation between supplementation with 5 g of glutamine immediately after a marathon race and a lower incidence of infections. This study, however, did not measure glutamine levels, which is a methodological failing. It is not possible to determine exactly what effect this dose had on the level of glutamine. Very probably, though, such a small amount would hardly prevent a drop in glutamine in the blood.
The absorption of water can be speeded up by adding sugar and salt, because water naturally follows the movement of these substances. Glutamine works in a similar way – studies confirm that in rehydration solutions it helps the body draw in fluids faster than plain water (Silva et al., 1998).
In the study by Wilkinson et al. (2006) there was a hint that taking glutamine together with carbohydrates and essential amino acids slightly limited muscle breakdown in the later phases of recovery – the practical significance, however, is so far unclear.
The study by Bowtell et al. (1999) suggested that glutamine may support glycogen synthesis in the first phase of recovery. The methodologically better study by Wilkinson et al. (2006), however, did not confirm this idea.
There are certain scientific hints that glutamine may help prevent muscle damage and soreness after endurance exercise. However, muscle damage caused by physical activity does not in itself reduce the level of glutamine in the blood.
Conclusion for athletes
Even though L-glutamine is widely promoted as a supplement for immunity, for reducing muscle damage and catabolism or for muscle growth, there is not enough evidence that this is really the case. It is far more sensible to focus on a sufficient intake of good-quality protein (meat, eggs, whey protein) with the full spectrum of essential amino acids.
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