Integration of carbohydrate and lipid metabolism in skeletal muscle during postnatal development.

In the adult, muscle metabolism represents a large drain of energetic substrates. The newborn has to provide additional energy to its muscles in order to ensure a rapid growth. However, since during the neonatal period the newborn is fed with a high-fat low-carbohydrate diet, i.e., milk, the newborn must also spare glucose for organs which are obligatory glucose consumers such as the brain. Thus, regulation of energetic substrate utilisation by muscle is of upmost importance for postnatal metabolic homeotasis. In the human, fibre types at birth can already be histochemically classified as adult types and the proportional distributions of each fibre-type approximate those seen in the adult. On the other hand, glycolytic and oxidative maximal enzyme activities are lower than adult levels. In the rat, the capacity for glucose utilisation in muscles is low at birth, reaches a peak at weaning and subsequently decreases. During the suckling period, the concentration of lipid-derived substrates is high as well as the muscle capacity for their oxidation; moreover, insulin concentration is low and insulin sensitivity of muscle glucose utilization is also reduced. Thus, during the suckling period, fuel availability and insulin concentrations, as well as tissue sensitivity towards the hormone, favour the limitation of glucose utilisation by skeletal muscles.