The pattern of intestinal substrate oxidation is altered by protein restriction in pigs

Previous studies indicate that amino acids and glucose are the major oxidative substrates for intestinal energy generation. We hypothesized that low protein feeding would lower the contribution of amino acids to energy metabolism, thereby increasing the contribution of glucose.

Piglets, implanted with portal, arterial, and duodenal catheters and a portal flow probe, were fed isocaloric diets of either a high protein (0.9 g/[kg/h] protein, 1.8 g/[kg/h] carbohydrate, and 0.4 g/[kg/h] lipid) or a low protein (0.4 g/[kg/h] protein, 2.2 g/[kg/h] carbohydrate, and 0.5 g/[kg/h] lipid) content. They received enteral or intravenous infusions of [1-13C]leucine (n = 17), [U-13C]glucose (n = 15), or enteral [U-13C]glutamate (n = 8).

CO2 production by the splanchnic bed was not affected by the diet. The oxidation of leucine, glutamate, and glucose accounted for 82% of the total CO2 production in high protein-fed pigs. Visceral amino acid oxidation was substantially suppressed during a low protein intake. Although glucose oxidation increased to 50% of the total visceral CO2 production during a low protein diet, this increase did not compensate entirely for the fall in amino acid oxidation.

Although low protein feeding increases the contribution of enteral glucose oxidation to total CO2 production, this adaptation is insufficient. To compensate for the fall in amino acid oxidation, other substrates become increasingly important to intestinal energy generation.