Whole-body tyrosine flux in relation to energy expenditure in weight-losing cancer patients

Whole-body tyrosine flux was measured in seven weight-losing cancer patients and compared with that of seven noncancer patients with malnutrition. L[U-14C] tyrosine was infused intravenously (IV) after an overnight fast under resting conditions and flux rates, oxidation, and incorporation into proteins of tyrosine were calculated from plateau values of specific activity of tyrosine in plasma and of labeled expired carbon dioxide. Rates of protein synthesis were calculated from the flux rate of tyrosine after subtracting the proportion oxidized. Simultaneous measurements of whole-body carbon dioxide production and oxygen uptake were also performed in each subject. Cancer patients had elevated whole-body tyrosine flux, protein synthesis, and energy expenditure when expressed in relation to body weight and whole-body potassium while the differences in tyrosine kinetics became of borderline significance when expressed in relation to energy expenditure. Tyrosine incorporation into whole-body proteins corresponded to a synthesis rate of 2.70 +/- 0.17 protein/kg/d in cancer patients and 2.18 +/- 0.17 in control patients (P less than 0.025). The results show that elevated protein synthesis in weight-losing cancer patients may explain not more than one third of the elevated energy expenditure. Therefore, other systems that utilize energy must increase in activity.