Effect of fasting on plasma metabolites and hormones in lactating dairy cows.

Metabolic and pathophysiologic changes of secondary ketosis were studied. Plasma samples were obtained from a group of 8 mature, lactating dairy cows before, during, and after a 48-hour fast. These samples were analyzed for glucose, free fatty acids (FFA), lactic acid, ketone bodies, glucocorticoids, insulin, and growth hormone concentrations. In the prefasting period, metabolites and hormones remained constant. Lactic acid and glucocorticoids also remained stable during the entire experiment, except on the 1st day after termination of the fast, when glucocorticoids increased significantly (P less than 0.05). Glucose values decreased significantly (P less than 0.01) with the onset of fasting, began to rise halfway through the fast, and were elevated following termination of the fast. Plasma glucose concentration returned to normal by the end of the experiment. Plasma FFA concentrations increased during the early portions of the fast and decreased thereafter. Plasma ketone body concentration responded similarly, but the change occurred approximately 12 hours after the plasma FFA changes. A value determined as ketotic was reached during the fast (14.6 +/- 1.3 mg/dl). Insulin concentrations remained low during the fast and increased after fasting. Plasma growth hormone concentration increased to a new plateau in response to fasting and then decreased somewhate after fasting was ended.