Glutamine is an non-essential amino acid that is involved in the production of bicarbonate in the kidney, is a major store of mobile nitrogen, is involved in protein synthesis in skeletal muscle and is also the primary fuel for enterocytes of the gut. These physiological roles mean that glutamine is a fundamentally important nutrient for the anti-inflammatory response and the immune system. Glutamine is beneficial to athletes because intense physical activity can depress the immune system and create inflammation, and this can be associated with low plasma glutamine concentrations. Even small doses of oral glutamine appear to be bioavailable, suggesting that enterocytes do not consume all glutamine before it is absorbed to the circulation. Oral glutamine may increase renal bicarbonate production, and may also raise plasma levels of other amino acids which may cause elevations to plasma growth hormone.
For example, researchers1 have administered 2g of glutamine mixed in a cola drink to 9 healthy subjects and assessed their plasma glutamine, plasma bicarbonate and plasma growth hormone levels. In 8 of the 9 subjects, plasma glutamine levels increased from baseline following the glutamine drink. The increase was 19% above the control at 30min and remained elevated at 12% above baseline at 60min, but had returned to baseline at 90 min. The glutamine drink increased plasma bicarbonate concentrations to 2.7mmol/L compared to -0.7mmol/L in the control at 90min. At 90 min, there was a 4.3-fold increase in growth hormone compared to the control (0.084 versus 0.019nmol/L). The plasma growth hormone increased despite an increase in plasma glucose due to the cola drink. These results show that even a small glutamine load can cause an increase in plasma bicarbonate and plasma growth hormone levels.
This suggests that the oral glutamine is not used as fuel by the gut, but instead is able to pass into the circulation and reach the periphery. The one subject who did not respond to supplementation had lower initial glutamine levels than other subjects. This subject also showed no increase in bicarbonate or growth hormone levels, suggesting the glutamine was the cause of the bicarbonate and growth hormone rises. The rise in growth hormone was small in this study, and it is unclear if this would be beneficial to health or cause physiological changes of significance. One possible benefit may be the shift in the utilisation of fatty acids as a fuel rather than glucose. Just how glutamine increase plasma growth hormone is not known, but conversion of glutamine to citrulline in the small intestine may increase arginine synthesis, a known stimulator for growth hormone secretion.
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