Hormonal effects on partitioning of nutrients for tissue growth: role of insulin.

Understanding factors that potentially regulate pathways of nutrient utilization is essential to the development of means of manipulating tissue growth to optimize the type and quality of product produced. Insulin is one hormone that has pronounced effects on carbohydrate and protein metabolism. Relative to most monogastric animals, ruminants absorb little glucose directly from the gastrointestinal tract. Therefore, it might be expected that insulin would have a less important role in regulating glucose and carbohydrate metabolism in the ruminant animal. Effects of insulin on lipogenesis and lipolysis both in vitro and in vivo in the ruminant appear to be small. Insulin may promote lipid deposition by increasing adipocyte membrane permeability to glucose with subsequent metabolism to alpha-glycerolphosphoric acid and thereby stimulating fatty acid esterification. Insulin also stimulates adipose lipoprotein lipase, which would increase the supply of fatty acids for esterification in adipose tissue. Insulin appears to indirectly alter hepatic glucose production by decreasing the release of gluconeogenic precursors from peripheral tissues. The known effects of insulin on lipid and carbohydrate metabolism have tended to direct attention away from protein metabolism, a process on which insulin may have a more significant role in the ruminant. Insulin or insulin and glucose have marked effects on regulating plasma levels of branched-chain amino acids, presumably by promoting their uptake or by decreasing their catabolism by muscle tissue. More information is needed to fully understand the role of insulin in regulating muscle protein metabolism.