Submitted to: American Journal of Physiology - Gastrointestinal and Liver Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2001
Publication Date: 11/8/2001
Citation: JACKSON, A.A., PHILLIPS, G., MCCLELLAND, I., JAHOOR, F. SYNTHESIS OF HEPATIC SECRETORY PROTEINS IN NORMAL ADULTS CONSUMING A DIET MARGINALLY ADEQUATE IN PROTEIN. AMERICAN JOURNAL OF PHYSIOLOGY GASTROINTESTINAL AND LIVER PHYSIOLOGY. 2001. v. 281. p. G1179-G1187. Interpretive Summary: Although a protein intake of 3/4 gram per kilogram body weight per day has been recommended as being adequate for most populations in the world, some nutritionists have suggested that this intake of protein is just marginally adequate and may become inadequate when someone becomes sick with say a chronic disease or an infection. They argue that under these circumstances the body breaks down and loses more protein at the same time that the the liver requires more protein to make special proteins to transport nutrients and drugs and to assist the immune system to fight the disease. Hence, the recommended protein intake becomes inadequate to maintain these functions. In this study we wanted to see whether 6 adult individuals consuming a diet of adequate calories and a low protein of 2/3 gram per kilogram body weight per day will be able to make sufficient quantities of these special liver proteins. We found that the rate at which the liver made these proteins decreased markedly, especially 3 proteins that are responsible for transporting fats in the blood. This change in the amount of these proteins following the low-protein diet may be important for fat transport and may explain why some people eating a low protein diet have high blood fat levels.
Technical Abstract: The plasma concentration and hepatic synthesis rates of albumin, transthyretin, very low-density lipoprotein apolipoprotein B-100 (VLDL-apoB-100), high-density lipoprotein apolipoprotein A-1, fibrinogen, alpha1-antitrypsin, and haptoglobin were measured in six normal adults before and after consuming a protein intake of 0.6 g. kg body wt(-1). day(-1) for 7 days. The synthesis of hepatic proteins was measured from the incorporation of [(2)H(5)]- phenylalanine, following prime/continuous infusion, using plasma VLDL-apoB-100 isotopic enrichment to represent the precursor pool. Synthesis of albumin declined by 50% (P < 0.001) following the lower-protein diet, VLDL-apoB-100 declined by 20% (P < 0.001), and apoA-1 declined by 16% (P < 0.05). By contrast, synthesis increased for fibrinogen (50%, P < 0.05) and haptoglobin (90%, P < 0.001). This pattern of change, with decreased synthesis of nutrient transport proteins and increased formation of acute-phase proteins, suggestive of a low-grade inflammatory response, was accompanied by increased plasma concentration of the inflammatory cytokine interleukin 6 (30%, P < 0.05). The pattern of change in the synthesis of hepatic secretory proteins following 7 days on the low-protein diet may be of functional relevance for lipid transport and the capacity to cope with stress.