High-protein diets differentially modulate protein content and protein synthesis in visceral and peripheral tissues in rats

Energy restriction only slightly influences protein metabolism in obese rats, whatever the level of protein and its source in the diet

“…We had previously shown that in rats fed ad libitum, a high level of protein in the diet (30 to 50% of energy) decreased protein FSR in the liver for as long as the protein content remained high. 16,17 The lack of any effect of diet composition during the present study suggests that this modulation in the liver could not occur in a situation of energy deficit. Because proteolysis gene expression was not decreased in the liver, contrasting with what had previously been observed with HP diets, 16,17 we therefore concluded that there is no regulation because the overall protein intake was not sufficient to trigger these mechanisms (trough AA signaling in proteolysis pathways).…”

“…16,17 The lack of any effect of diet composition during the present study suggests that this modulation in the liver could not occur in a situation of energy deficit. Because proteolysis gene expression was not decreased in the liver, contrasting with what had previously been observed with HP diets, 16,17 we therefore concluded that there is no regulation because the overall protein intake was not sufficient to trigger these mechanisms (trough AA signaling in proteolysis pathways). Few data are available on energy restriction and protein synthesis rates, and they all concern muscle and the findings have been controversial, depending on the muscle type as well as physiological and nutritional status.…”

“…17 Briefly, the protein fraction was precipitated with 5-sulfosalicylic acid and the free amino acids were extracted from the supernatant on a cation exchange resin. The pellet containing the tissue proteins was rinsed, freeze-dried and then hydrolyzed for 48 h with HCl 6N under N 2, and the amino acids released from the proteins were extracted on cation exchange resin following the same procedure as for the free amino-acid fraction.…”

“…13 --15 In addition, protein anabolism has not been investigated in other organs such as the liver or kidney, whereas it has been observed under ad libitum conditions that liver and kidney protein synthesis were modulated by the protein level in the diet, unlike synthesis in muscle. 16,17 Studies of the effect of energy restriction on proteolysis are scarce and, unlike those on protein synthesis, concerned the liver 18,19 but neither the muscle nor kidney. It thus seems necessary to clarify the consequences of energy restriction on protein fluxes as a function of the composition of the diet and particularly its protein content.…”

Scientific Opinion on Dietary Reference Values for protein

“…In vivo, amino acids display an anabolic effect (Giordano et al, 1996;Volpi et al, 1996) and were shown to stimulate muscle protein synthesis (Bohe et al, 2003;Liu et al, 2002;Nair and Short, 2005;Nygren and Nair, 2003). There was no effect of a dietary protein level above the PRI on muscle mass and protein content, and a high protein diet of around 2 g/kg body weight per day has not been demonstrated to modulate skeletal protein synthesis in both exercising and non-exercising human subjects (Bolster et al, 2005;IoM, 2005;Juillet et al, 2008) or animals (Almurshed and Grunewald, 2000;Chevalier et al, 2009;Masanés et al, 1999;Morens et al, 2001;Taillandier et al, 2003). However, increasing protein intake above the individual requirement increases amino acid oxidation and modifies protein turnover.…”