Effects of diet composition and fasting on glucose turnover in lean and polygenic obese mice

Effects of high beef tallow (BT), high corn oil (CO) or low-fat (LF) diets upon the outcome of genetic obesity were investigated. Diets were instituted ad libitum at the time of weaning. When mice were six months of age, blood samples were taken 1, 5, 15, 30, and 60 minutes after intravenous injection of glucose-U-¹⁴C. Within dietary treatments, obese and lean mice showed similar plasma glucose-U-¹⁴C disappearance patterns. Plasma glucose disappearance always tended to be faster in ad libitum-fed mice relative to 24-hour fasted mice. Body glucose pool sizes tended to be larger in fed obese BT and LF mice compared to their lean counterparts. This pattern was not seen in non-fasted CO mice. Fasting caused a decrease in body glucose pool sizes in all mice. In contrast to CO and LF mice, lean BT mice appeared to conserve glucose during fasting the same as the obese line. Since the glucose disappearance curves can be described by a two-exponential decay function, at least a two-component or two-pool system must be involved in plasma glucose turnover. Calculated rate constants were used to express interchanges of carbon molecules between the glucose and glycogen pool and the net movement of glucose carbon to a carbon pool representing "irreversible end products". The data indicate that differences in glucose metabolism, in part, explain the possibility that dietary energy source can overcome the genetic tendency to leanness.