Diet composition and energy content modulate free fatty acid (FFA) release. The aim of this study was to evaluate the dose–response effects of euenergetic variations in dietary carbohydrate and fat content on postabsorptive FFA release. The rate of appearance (Ra) of palmitate was measured by infusion of [2,2-2H2]palmitate after an overnight fast in six healthy men on three separate occasions, i.e. after 7 d on euenergetic control, high-carbohydrate and high-fat diets. The protein content and composition was identical for each diet. Postabsorptive plasma fatty acid concentrations were not different between the high-carbohydrate and control diets (0·36 (SE 0·07) V. 0·43 (se 0·04) mmol/l), but were increased after the high-fat diet (0·75 (se 0·09) mmol/l, (P<0·01 compared with the other diets). Ra palmitate was not different between the high-carbohydrate and control diets (1·36 (se 0·20) v. 1·47 (se 0·15) μmol/kg per min). However, Ra palmitate was increased to 2·36 (se 0·26) μmol/kg per min after the high-fat diet (P<0·01 compared with the other diets). The fatty acid flux and whole-body fat oxidation were not affected by the high-carbohydrate diet compared with the control diet, but were increased by 67 and 47 % respectively, on the high-fat diet (P<0·01 compared with the other diets). A euenergetic high-fat diet results in increased postabsorptive FFA release and fat oxidation, whereas a euenergetic high-carbohydrate diet does not affect these variables of fat metabolism.

The effect of dietary intake level on in vivo plasma leucine and plasma palmitate flux rates and on the response to a bolus injection of bovine growth hormone (GH) was investigated in six young steers. Animals were fed on a pelleted diet of dried grass–barley (0.7:0.3, w/w) in quantities sufficient to supply 0.8, 1.2, 1.6, 2.0, 2.4 or 2.65 × maintenance energy requirement, offered in hourly portions. Continuous intravenous infusions of [1-13C]leucine or [1-13C]palmi-tate were used to determine the flux of amino acid and fatty acid through the plasma pool before, immediately (1–3 h) after and 22–24 h after a subcutaneous injection of bovine GH (0.55 mg/kg body weight). Hourly blood samples were taken for 27 h to monitor the temporal responses of circulating hormones and metabolites following GH administration. The animal on the lowest plane of nutrition had elevated plasma GH and reduced insulin-like growth factor-1 concentrations compared with those fed on higher intake levels. Plasma leucine flux and leucine concentration increased with intake while palmitate flux and plasma non-esterified fatty acid (NEFA) concentrations were inversely related to intake. Leucine flux rate decreased in the animals fed on the two highest intake levels in response to GH 22–24 h after administration, but plasma leucine concentrations were reduced in all animals at this time. Only the animal fed on the lowest intake level showed an immediate response to GH (within 3 h of administration) with increased palmitate flux and plasma NEFA concentrations but a lipolytic response was apparent in other animals 22–24 h post-administration although the magnitude of the response was markedly reduced at high intakes. We conclude that lipid and protein metabolism are differentially responsive to GH and nutritional status.