Does insulin have an anabolic effect?

Fråga: Does insulin have an anabolic effect?

Sammanfattning: Insulin shows an anabolic effect. It is an important regulator of protein metabolism, but some questions still exist concerning its mechanisms and sites of action. It is also unclear to what extent the different mechanisms contribute to the total anabolic effect.

There is no indication for the use of insulin by healthy people. Use of insulin is associated with great risks. One case of seizures and unconsciousness in a male body builder caused by self-administration of insulin has recently been reported.

Svar: An anabolic hormone can exert its effect by stimulating protein synthesis and/or inhibiting protein breakdown. Insulin affects protein metabolism in many ways.

Proteins continuously undergo demolition and rebuilding. Amino acids for use in protein synthesis may be derived from plasma or from the intracellular free pool supplied by protein breakdown.

Insulin stimulates amino acid transport into the cells. Some amino acids are affected more than others and insulin may act synergisticly with growth hormone in stimulating transport of the same or different amino acids into the cells. Also, by stimulating glucose uptake insulin reduces the need for amino acids as an energy source. These two factors in combination would make larger quantities of amino acids available to the tissues for protein synthesis (1).

Insulin also regulates protein synthesis by increasing the rate of transcription of DNA in the cell nuclei, thus forming increased quantities of RNA. It also increases the translation of mRNA forming new proteins. Insulin deficiency has been shown to result in reduction of protein synthetic capacity and increased degradation of ribosomes (2).

Insulin inhibits the catabolism of proteins. The effect is probably due to its stabilising effect on the lysozomes (3).

In the liver, insulin depresses the rate of gluconeogenesis by decreasing the activity of the enzymes that promote gluconeogenesis. Since the substrates most used are the plasma amino acids this would also contribute, by making larger quantities of amino acids available for protein synthesis (1).

It is unclear whether the above-mentioned mechanisms are important or not. In most circumstances insulin is anabolic. Lack of insulin in diabetic patients is associated with protein loss and the catabolic processes are reversed by insulin therapy (2).

Insulin should only be used by diabetic patients. There is no documentation concerning any effect of the drug in increasing muscle mass in healthy people. Uncontrolled administration of insulin can result in hypoglycaemia, a severe condition that may cause brain damage, confusion, epileptic seizures, unconsciousness and even death (4). A case of seizures and unconsciousness in a male body builder caused by self-administration of insulin has recently been reported (5).