Short- and long-term effects of eating on blood composition in free-feeding goats

Adult goats provided with permanent jugular and portal vein catheters were used to study the relation between spontaneous meals and blood composition in free-feeding goats in order to determine which constituents might fulfil a feedback signal function in the control of feed intake. Animals were fed ad libitum concentrates, supplemented with a small amount of hay. Blood samples were collected before, during and after spontaneous meals by means of a remote sampling system and were analysed for volatile fatty acids, glucose, insulin, glucagon and growth hormone. In a preliminary experiment the relationship between body weight, voluntary feed intake, blood glucose and plasma insulin was examined in goats fed ad libitum concentrates and hay for 18 weeks. The data indicate a regulation of energy balance through control of feed intake. Since an increased glucose and insulin concentration was observed concomitantly with a decline in feed intake, glucose and/or insulin may fulfil a signal function in the control of energy balance. Spontaneous meals were not preceded by a systematic change of any of the above blood constituents. Therefore, it is unlikely that meal initiation is governed by one of those. In contrast, spontaneous meals resulted in a rapid increase of insulin during eating (P < 0·025) whereas no clear changes were observed for the other substances. Thus, insulin might be a candidate for a feedback function in the control of meal duration.