Anticatabolic Supplements
- By:Robert Baird Baird
Perhaps the most popular of the anticatabolic supplements, glutamine is the amide of the amino acid glutamate. Glutamine is synthesized from glutamate by the action of glutamine synthetase. Glutamate is formed from a-ketoglutarate, an intermediate of the Krebs cycle, and ammonia.
Glutamine is the most abundant amino acid in plasma and skeletal muscle and accounts for greater than 60% of the total intramuscular free amino acid pool. Furthermore, skeletal muscle is quantitatively the most important site of glutamine synthesis even though glutamine synthetase activity is relatively low per unit mass in skeletal muscle. Adipose tissue may also represent a site of glutamine synthesis similar in magnitude to skeletal muscle In addition, the lungs, liver, and brain are other sites of glutamine synthesis.
Glutamine is one of the major fuels of the gut, particularly during fasting. In fact, the GI tract accounts for approximately 40% of the total glutamine that is used by the body. Glutamine metabolism in the GI tract is similar whether one has fasted or has recently consumed a meal.
The use of glutamine (and other amino acids) by the GI tract is partly due to high turnover of intestinal mucosal cells and the need for continual provision of amino acids to sustain high protein synthetic rates. The health of these cells is critical not only for normal uptake of nutrients, but also because these cells serve as a barrier or protection against invading bacteria from the lumen of the gut. Thus, cells of the GI tract may be preferentially supplied amino acids for oxidation and protein synthesis at the expense of skeletal muscle protein. We speculate that by providing extra exogenous glutamine (via dietary supplementation), you can spare intramuscular glutamine while feeding the GI tract. Thus, you would avoid muscle proteolysis secondary to lower concentrations of glutamine.
Besides the small intestine (i.e., enterocytes), cells of the immune system (i.e., neutrophils, thymocytes, lymphocytes, and macrophages) and hair follicles use glutamine as fuel. Glutamine is used for glucose and urea synthesis in the liver, whereas the brain uses glutamine as a precursor for neurotransmitter substances. In the normal fed state, and to an even greater extent during fasting and metabolic acidosis, glutamine is used as fuel by the kidneys (i.e., kidneys use glutamine to support renal ammoniagenesis) During metabolic acidosis, glutamine is converted to a-ketoglutarate, thus generating ammonium ions . The excretion of ammonium ions helps buffer the acidotic condition.
Evidence indicates that glutamine is important for the maintenance of skeletal muscle protein levels. The reclassification of glutamine as a conditionally essential amino acid is based on the notion that under certain stressful conditions, the body's need for glutamine exceeds its ability to synthesize glutamine endogenously. But with the provision of exogenous glutamine, the loss of skeletal muscle protein during stressful states may be alleviated.
Because skeletal muscle accounts for most of the protein pool in the body, the regulation of protein metabolism in skeletal muscle is important for whole-body protein homeostasis. Skeletal muscle and adipose tissue represent the most important source of glutamine However, skeletal muscle accounts for a much larger fraction of the body's total mass and is therefore more important than adipose tissue as a source of glutamine. According to Wagenmakers, the liver can oxidize most of the amino acids, whereas skeletal muscle can oxidize amino acids (i.e., the branched-chain amino acids, aspartate, asparagine, and glutamate). This is important for the oxidation of these amino acids and for the conversion of these amino acids into glutamine and alanine.
Animal Studies
The effect of glutamine on protein synthesis and degradation in cultured rat skeletal muscle myotubes (developing skeletal muscle cell) under normal and heat-stressed conditions was assessed by Zhou and Thompson. They found that glutamine augments protein synthesis in myotubes that are under heat-stressed conditions; however, there was no effect on myotubes under normal conditions. A similar study from the University of Alberta found a positive relationship between intracellular concentrations of glutamine and the rate of muscle protein synthesis in isolated chick extensor digitorum communis muscle. That is, the greater the glutamine concentration, the greater the anabolic effect on these skeletal muscles.
The regulation of cellular volume is intimately associated with protein synthesis and degradation. An increase in cellular volume or hydration status acts as an anabolic signal, whereas a decrease in cellular volume promotes catabolic processes. Evidence suggests that glutamine may exert an anticatabolic effect by mediating increases in cellular volume. 20 Using an isolated rat skeletal muscle preparation, changes in the osmolarity of the surrounding medium affected the rates of glutamine and alanine release from skeletal muscle.