Influence of diet and prolonged fasting on blood lipids, ketone bodies, glucose and insulin in adult sheep.

In order to study changes in blood composition during feeding and prolonged fasting, six adult Suffolk ewes received three different diets during three periods of four weeks. Two intake levels (one and three times maintenance) and two roughage/concentrate ratios (100 and 14 p. 100 roughage) were compared according to a twofold latin square. Afterwards the animals were fasted for an 11-day period. The concentration of free fatty acids in the blood decreased when concentrate was added to the diet and when energy intake level, insufficient to meet maintenance requirements, was increased to just sufficient and then to excessive. Blood lipid and lipoprotein composition was not influenced by a change in diet. Supplying concentrate instead of roughage produced increases in the proportions of stearic acid in the triglycerides and free fatty acids and of linoleic acid in the phospholipids and cholesteryl esters and decreases in the proportions of oleic acid in the free fatty acids and of oleic and palmitic acids in the triglycerides and cholesteryl esters. When increasing intake level, the proportion of linoleic acid in the phospholipids and cholesteryl esters was further increased. Increased insulinemia was also observed with concentrate intake, particularly when the concentrate was fed ad libitum. Differences in blood composition resulting from the dietary treatments rapidly disappeared with fasting. After an initial decrease, blood ketone bodies increased and the beta-hydroxybutyrate/acetoacetate (BHB/AA) ratio decreased. Glycemia decreased initially and then stabilized; the concentration of free fatty acids increased from the first day onwards and the proportion of free fatty acids in total blood lipids also increased. An increased proportion of oleic acid in the free fatty acids and triglycerides was compensated for by a decrease of saturated acids. A decrease in the level of polyunsaturated fatty acids in the phospholipids and cholesteryl esters was balanced by increased levels of saturated and oleic acids. The high density lipoprotein/low density lipoprotein ratio (HDL/LDL) decreased and insulinemia showed a strong decrease during the first day. Blood composition seemed to stabilize after-5-6 days of fasting, except for the ketone bodies which increased again after 10 days of the fast and for glycemia which increased after 5-6 days of the fast. The influence of previous dietary treatment on changes in blood composition during fasting clearly depended on the body conformation of the animal. When animals showed similar body conformation, no differences in the blood changes were observed during fasting. But when lean and fat animals were compared, extensive differences were found in the blood composition after the food was removed.