During the three hours required for absorption of most of a U-C-14-glucose load (1 gm./kg.) given orally to an intact unanesthetized dog in the postabsorptive state, about three quarters of the load reaches the peripheral circulation. The absence of randomization of the label, seen when an equal oral load of 6-C-14-glucose is given, indicates that the glucose reaching the peripheral circulation has not been fragmented, then resynthesized into glucose. During the three-hour period, only 0.34 gm. per kilogram of new (unlabeled) glucose is released by liver instead of the 0.63 gm. per kilogram which would have been produced in the absence of oral glucose; the uptake of the glucose of peripheral circulating blood is 1.03 gm. per kilogram instead of 0.63 gm. per kilogram. Endogenous insulin secreted in response to the oral load exerts a somewhat larger effect to decrease hepatic glucose release (relative to the increase in glucose uptake from peripheral blood) than is the case when insulin is given by peripheral vein.

Steele, R., Bjerknes, C., Rathgeb, I., & Altszuler, N. (1968). Glucose Uptake and Production During the Oral Glucose Tolerance Test. Diabetes, 17(7), 415–421.