Tracer methods and the metabolic disposal of a carbohydrate load in man.

Figure 18 outlines a summary of the results obtained in our laboratory and how these might be interpreted. Following a 100-g oral glucose load, about 25 g is taken up by the liver. About 5 g or 5% of this would be removed on a first-pass basis since only about a fifth of the portal vein glucose is newly absorbed. The remainder of the glucose is disposed of in peripheral tissues. This disposal is enhanced by intestinal insulinotropic factors that stimulate insulin secretion. Lactate is produced peripherally (with the red cells as one of the most important sources) by the gut and, perhaps, by hepatocytes. It is taken up by gluconeogenetic hepatocytes to form glycogen. This pathway appears to account for half to two-thirds of glycogen synthesis, the remainder being by direct uptake of glucose. The gluconeogenetic pathway of glycogen formation may be important in that it clears the obligatory production of lactate from certain tissues. The only difference between intravenous and oral glucose loading is that there is no absorbed glucose in the portal vein when glucose is infused. The glucose concentrations here are, however, almost the same as during oral glucose loading since peripheral clearance of glucose is slower in the absence of insulinotropic intestinal factors. This helps to explain why liver handling of intravenous glucose and glycogen formation are almost identical to the case of oral loading.