Comparative Study of the Metabolism of U‐¹⁴C‐Fructose, U‐¹⁴C‐Sorbitol and U‐¹⁴C‐Xylitol in the Normal and in the Streptozotocin‐Diabetic Rat*

Abstract

Abstract. Young male rats metabolize xylitol, sorbitol and fructose with a half‐life of 165 seconds. Within 10 min. after the intravenous injection of a load of these substrates more than 70% of the carbon‐14 in plasma is present as 14C‐glucose. The labelling of plasma glucose from these substrates appears to be insulin‐independent. A small percentage of labelled xylitol, sorbitol and fructose is retained by the liver as glycogen and total lipids. In the normal rat, this process appears to be insulin‐independent. In the chronic streptozotocin‐diabetic rat, insulin acutely enhances glycogen synthesis from all three substrates and reduces their incorporation into total lipids. Once these substrates have been converted to glucose the further storage as muscle glycogen and adipose tissue total lipids is entirely dependent on insulin, both in the normal and in the streptozotocin‐diabetic rat. Fructose makes an exception since it is metabolized by adipose tissue independently of insulin. In the light of these results in the rat a re‐evaluation of the use of various carbohydrates and polyols in parenteral nutrition of normal and diabetic subjects appears desirable.

Metabolism and oxidation of U-14C-glucose, xylitol, fructose and sorbitol in the fasted and in the streptozotocin-diabetic rat

“…It is well known that fructose and sorbitol are rapidly converted to glucose [48,49,50]. The conversion of xylitol to glucose appears to occur just as fast [37]. Our finding that similar amounts of carbon-14 are recovered in urinary glucosazones after the injection of all three substrates is a direct reflection thereof.…”

“…If the values of the 14CO2-production of the diabetic animals are multiplied by 2 89 thereby correcting for the lower specific activity of plasma glucose, the difference in the rate of ~4C02-exhalation between the two groups of animals is minimized. The hMf-life of 2 to 3 min of the polyols and of fructose [37] due to their rapid conversion to glucose, explains why their rate of oxidation in the diabetic animals is the same as that of glucose. At the time when ~C02-production was maximal none of the originally administered substrates circulated any longer in plasma, so that most of the 14C02 exhaled at that time probably originated from 14C-glucose oxidation.…”

“…Part of it, is converted to glucose in the small intestine [35]. The main portion, however, is considered to be absorbed as fructose [36,37].…”

Carbohydrates in Parenteral Nutrition

“…There is only a very small effect evoked by the sugar substitutes, especially by the sugar alcohols. On the other hand, there is no doubt that tire sugar substitutes are quickly transformed to glucose by the liver, as has been shown by liver perfusion (Forster, 1972), by animal experiments (Froesch et al, 1971), and by experiments with humans too (Keller and Froesch, 1972).…”

Glucose and Insulin Homeostasis during Postoperative Carbohydrate Infusions

“…Sorbitol is also removed rapidly from the blood (36), is converted mainly to fructose in the liver (35) and has similar protein-sparing and antiketogenic effects (5). Studies of their comparative metabolisms show that the term 'insulin independence' to describe the metabolism o f fructose and sorbitol (27) is misleading, as these substrates are rapidly converted to glucose by the liver, and further metabolism by extrahepatic tissues requires the presence of insulin (12,22).…”