Dietary protein intake is an independent and dose‐dependent regulator of hepatic anabolism in rats fed high protein diets

Abstract

We previously observed that drastic increases in protein consumption greatly modify hepatic protein anabolism in rats, yet the confounding effects of the other macronutrient changes and the ability of a moderate protein increase to produce the same effects are not established. This study examined the metabolic and hormonal responses of rats submitted to 14‐d isoenergetic diets with normal, intermediary or high protein levels (NP:14% of energy, IP: 33%, HP: 50%) and different carbohydrate (CHO) to fat ratios within each protein level. Fasted or fed rats (n=96) were killed after injection of a flooding dose of 13C‐valine. Hepatic protein content increased with the dietary protein level (P<0.05). The hepatic fractional synthesis rates (FSR) of protein were significantly influenced by both the protein level and the nutritional state (fasted vs. fed) (P<0.0001) but not by the CHO/fat ratio and reached on average 110%/d, 92%/d and 83%/d in rats fed the NP, IP and HP diets, respectively. Muscle FSR did not differ among diets. Insulin levels significantly decreased with both increasing protein level and decreasing CHO/fat ratio. Our results reveal that excess dietary protein inhibits hepatic protein synthesis rates independently of the other macronutrients and the related changes in insulin levels. This response is observed at moderate levels of protein intake (33%) that are plausible in human consumption.