In intensively milk-fed calves post-prandial glucose (G) and insulin (I) concentrations, but not preprandial G concentrations, increased or failed to decrease during the growth period, compared with data from calves that were progressively weaned. This study was, therefore, designed to investigate G and I metabolism in veal calves. Euglycemic-hyperinsulinemic and hyperglycemic clamps in the unfed state demonstrated mutual responsiveness of I and G, but revealed a relative I resistance. After feed consumption, I resistance was exaggerated, as seen by decreased G clearance rates after i.v. G and I administration in fed compared with unfed calves. Milk replacer is a source of readily available lactose, fat, and protein, the intake of which, on a kilogram.75 basis, gradually increased with age. Increased substrate availability and effects of nutrients themselves were probably responsible for elevated plasma concentrations of G and I and led to I resistance. Additionally, hyperglycemia > 1.5 g/L was followed by urinary excretion of G.

Based on studies in Fe-deficient calves demonstrating enhanced blood lactate concentrations during treadmill exercise, the hypothesis was advanced that glucose metabolism is also disturbed at rest. Insulin-dependent glucose metabolism was therefore investigated in calves fed milk replacer containing 20 or 50 mg Fe/kg. Calves receiving only 20 mg Fe/kg of milk replacer developed moderate Fe deficiency anemia and had lower average daily gain than calves fed milk replacer containing 50 mg Fe/kg, but feed intake and feed refusals did not differ between groups. In Fe-deficient calves, insulin responses to glucose, based on intravenous and oral glucose tolerance and hyperglycemic clamp tests, were normal. Using hyperglycemic and euglycemic hyperinsulinemic clamps combined with [13C6]glucose infusions, glucose utilization and tissue sensitivity to insulin were greater in Fe-deficient calves than in Fe-adequate calves. Observed reductions in growth performance may be explained by decreased activity of Fe-dependent enzymes, increased anaerobic glycolysis and lactate-glucose cycling, adaptations that are expected to be energy expensive.