Effects of a duodenal glucose infusion on the relationship between plasma concentrations of glucose and insulin in dairy cows.

The effects of duodenal infusion of glucose on the relationship between plasma concentrations of glucose and insulin and on milk composition were investigated in a crossover design. Eight dairy cows were continually infused with water (control) or glucose (1.5 kg/d). Cows received diets consisting of dehydrated whole-plant maize in restricted amounts to equalize the energy supply between treatments. Basal (before meal) plasma concentrations of glucose and insulin were increased, but concentrations of nonesterified fatty acids (NEFA) were decreased, by glucose treatment. During the first 2 h after feed distribution, plasma insulin increased, and plasma glucose and NEFA decreased, in both control and treated cows. Afterward, plasma glucose increased in treated cows but further decreased in control cows. The difference reached 8 mg/100 ml without any change in plasma insulin. During the meal, concentrations of growth hormones in plasma were inhibited to a similar extent in both groups. In response to intravenous glucose or insulin challenges, changes in plasma glucose, NEFA, and insulin stimulated by glucose were also very similar in both groups. In conclusion, duodenal infusion of glucose increased basal plasma concentrations of glucose and insulin, increased postprandial plasma glucose, and decreased NEFA without inducing insulin resistance. Glucose treatment did not change milk yield but decreased milk fat yield, mainly through a decrease in the yield of C18 fatty acids that were derived from circulating fatty acids. In the absence of insulin resistance, the decrease in the yield of C18 fatty acids might be attributed to an inhibition of adipose lipolysis or an increase in adipose lipogenesis.