Metabolism of valine and isoleucine in growing rats at various dietary protein levels.

Abstract

The metabolic fate of the carbon skeletons of L-[U-14C]valine and L-[U-14C]isoleucine was investigated in growing rats fed with diets containing different percentages of protein calories (0, 5, 10, 15 and 30 PC%) at 4, 100 kcal of metabolizable energy per kg of diet. The nutritional significance of the metabolism of the branched chain amino acids is discussed. The incorporation of 14C into the body protein from 14C-valine or isoleucine was about 80% of the injected dose in the 0 and 5 PC% groups, but it decreased with the increasing level of dietary protein from 10 to 30 PC%. The expired 14CO2 production from 14C-isoleucine was depressed with reducing PC%, and then increased with higher PC% in the diets, showing a break point at 5 PC%, whereas a linear increase in 14CO2 production was observed for labeled valine with increasing of the dietary protein level. The conversion of the carbon skeleton of 14C-valine into the body lipid was the lowest among the branched chain amino acids, reflecting its glycogenic property. These results indicate that the metabolic response of the carbon skeleton of valine and isoleucine to dietary protein level changes at around 10 PC%, at which the growth rate and body protein retention reached approximate maxima.