Regulations of catabolic and anabolic mechanisms; the interactions between exercise, carbohydrates and an excessive intake of amino acids : A review of some of the metabolic pathways that affects the homeostasis of the body, as well as β-oxidation and protein synthesis

Insulin as well as glucagon are important hormones in maintaining glucose homeostasis and regulating the metabolism in the body. Insulin receptors (IR) are transmembrane receptors that promote a signal transduction when activated by insulin. This can for example cause an increased influx of glucose into the cell performed by so called glucose transporters (GLUTs). These membrane proteins facilitate the transport of glucose from the blood into the cells, so the cell always has a constant supply of energy. Peroxisome proliferator-activated receptors (PPAR) are nuclear fatty acid receptors. They are activated by lipids and regulate fatty acid transcription. PPARδ/β is located in skeletal muscle and can promote fatty acid catabolism as well as cause a switch in fuel preference from glucose to fatty acids. It has been suggested that ligands for PPARδ could act as insulin sensitizers. The PPARγ coactivator-1α can increase mitochondrial content in skeletal muscle if over expressed. The same is true for endurance exercise. Hormones released from adipose tissue can cause hyperphagia and obesity if over- or under expressed. They can also work in the opposite way by decreasing appetite with weight loss as an effect. Impaired signalling or dysfunctional receptor can cause insulin resistance, obesity and diabetes. Lipolysis occurs in adipose tissues and is conducted by three enzymes, namely adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL). There are some factors that can increase lipolysis such as caffeine, a low glycemic index, high protein intake and training. The enzyme PEPCK is involved in the gluconeogensis in the liver and kidney cortex, and also in the glyceroneogenesis in the liver, as well as in brown and white adipose tissue. When overexpressed in skeletal muscle the enzyme increases the muscle activity. The overexpression of the enzyme did promote the β-oxidation as energy source for the muscles during exercise, instead of muscle glycogen as fuel. The processes of protein synthesis and breakdown are together called protein turnover. Muscle grows when synthesis is greater than breakdown, and withers if breakdown exceeds the level of synthesis. Acute effects of training is catabolic, but long time exercise causes however an increased protein synthesis. Leucine, an essential amino acid, has an important role in the initiation phase of translation. Glutamine is probably important in the regulation of muscle protein synthesis and breakdown. Together with glutamate, aspartate and asparagine, these are responsible for the amino acid metabolism that occurs in the muscles. Protein synthesis reaches its maximum in the recovery phase after intense training