is the most abundant amino acid in the blood and in the free amino acid pool of skeletal muscle. Glutamine stimulates the synthesis and inhibits the degradation of proteins, is an important vehicle for the transport of nitrogen and carbon within the tissues, stimulates the synthesis of hepatic glycogen, and is an energy source for cell division.54 Because glutamine deficiency can occur during periods of metabolic stress, it has led to the reclassification of glutamine as a conditionally essential amino acid.55 Glutamine is also a precursor for the synthesis of amino acids, proteins, nucleotides, glutathione, and other biologically important molecules.
Glutamine is considered to have an anabolic effect on skeletal muscle. It stimulates the synthesis and inhibits the degradation of proteins. Experiments with various animal models have demonstrated glutamine supplementation can result in better nitrogen homeostasis, with conservation of skeletal muscle.55 The mechanism by which glutamine affects skeletal muscle protein turnover, and thus muscle protein balance, is unknown. However, glutamine has an anabolic effect of promoting protein synthesis and also might reduce protein breakdown. 56
Glutamine was shown to increase cell volume, while insulin and glutamine together seem to work synergistically to enhance cellular hydration. The effects of glutamine in skeletal muscle include the stimulation of protein synthesis, which occurs in the absence or presence of insulin, the response being greater with insulin.57
During various catabolic states, such as infection, surgery, burns, and trauma, glutamine homeostasis is placed under stress, and glutamine reserves, particularly in the skeletal muscle, are depleted. In these conditions, the body requirements of glutamine appear to exceed the individual's muscle deposits, resulting in a loss of muscle mass.58 In critically ill patients, parenteral glutamine reduces nitrogen loss and causes a reduction in mortality.54
With regard to glutamine metabolism, exercise stress can be viewed in a similar light to other catabolic stresses. Plasma glutamine concentrations increase during prolonged, high-intensity exercise. However, during the post-exercise recovery period, plasma concentrations decrease significantly. Several hours of recovery are required before plasma levels are restored to pre-exercise levels. If recovery between exercise bouts is inadequate, the acute effects of exercise on plasma glutamine concentrations can be cumulative. It has been observed that overtrained athletes appear to maintain low plasma glutamine levels for months or years.59 Some experts believe reduced concentration of plasma glutamine can provide a good indication of severe exercise stress.60
Results suggest, after exercise, the increased availability of glutamine promotes muscle glycogen accumulation by mechanisms possibly including diversion of glutamine carbon to glycogen. 61
Following trauma there is a loss of nitrogen, with a concomitant reduction of skeletal muscle protein synthesis. This is accompanied by a decrease in the stores of muscle free glutamine. Nutritional support with either glutamine or its carbon skeleton, alpha-ketoglutarate, has been shown to counteract the postoperative fall of muscle free glutamine and of muscle protein synthesis.62
Evidence suggests oral glutamine supplementation results in an increased release of growth hormone. An oral glutamine load (2 g) was administered to nine healthy subjects to determine the effect on plasma glutamine, bicarbonate, and circulating growth hormone concentrations. Eight of nine subjects responded with an increase in plasma glutamine at 30 and 60 minutes before returning to the control value at 90 minutes. Ninety minutes after the glutamine administration load, both plasma bicarbonate concentration and circulating plasma growth hormone concentration were elevated.63
Although some advocates recommend as much as 30 g, it is likely only marginal benefits are found at supplementary levels higher than 2-3 g per day.