Synthesis of hepatic secretory proteins in normal adults consuming a diet marginally adequate in protein

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.