Different rates of glucose and fructose metabolism in rat liver tissue in vitro

High d(+)-fructose diet adversely affects testicular weight gain in weaning rats─protection by moderate d(+)-glucose diet

The use of high D(+)-fructose corn syrup has increased over the past several decades in the developed countries, while overweight and obesity rates and the related diseases have risen dramatically. However, we found that feeding a high D(+)-fructose diet (80% D(+)-fructose as part of the diet) to weaning rats for 21 days led to reduced food intake (50% less, P < 0.0001) and thus delayed the weight gains in the body (40% less, P < 0.0001) and testes (40% less, P < 0.0001) compared to the no D(+)-fructose diet. We also challenged a minimum requirement of dietary D(+)-glucose for preventing the adverse effects of D(+)-fructose, such as lower food intake and reduction of body weight and testicular weight; the minimum requirement of D(+)-glucose was ≈23% of the diet. This glucose amount may be the minimum requirement of exogenous glucose for reducing weight gain.

Health implications of fructose consumption: A review of recent data

This paper reviews evidence in the context of current research linking dietary fructose to health risk markers.Fructose intake has recently received considerable media attention, most of which has been negative. The assertion has been that dietary fructose is less satiating and more lipogenic than other sugars. However, no fully relevant data have been presented to account for a direct link between dietary fructose intake and health risk markers such as obesity, triglyceride accumulation and insulin resistance in humans. First: a re-evaluation of published epidemiological studies concerning the consumption of dietary fructose or mainly high fructose corn syrup shows that most of such studies have been cross-sectional or based on passive inaccurate surveillance, especially in children and adolescents, and thus have not established direct causal links. Second: research evidence of the short or acute term satiating power or increasing food intake after fructose consumption as compared to that resulting from normal patterns of sugar consumption, such as sucrose, remains inconclusive. Third: the results of longer-term intervention studies depend mainly on the type of sugar used for comparison. Typically aspartame, glucose, or sucrose is used and no negative effects are found when sucrose is used as a control group.Negative conclusions have been drawn from studies in rodents or in humans attempting to elucidate the mechanisms and biological pathways underlying fructose consumption by using unrealistically high fructose amounts.The issue of dietary fructose and health is linked to the quantity consumed, which is the same issue for any macro- or micro nutrients. It has been considered that moderate fructose consumption of ≤50g/day or ~10% of energy has no deleterious effect on lipid and glucose control and of ≤100g/day does not influence body weight. No fully relevant data account for a direct link between moderate dietary fructose intake and health risk markers.

Normalization of carbohydrate-induced thermogenesis by fructose in insulin-resistant states

To examine whether defects in carbohydrate oxidation and thermogenesis in aging, obesity, and diabetes are secondary to impaired insulin action or to a primary defect in intracellular metabolism, we compared substrate oxidation and energy expenditure in 9 younger, 9 older, 9 obese, and 10 non-insulin-dependent diabetic subjects after the ingestion of 75 g of glucose or fructose (a monosaccharide whose transport into the cell and subsequent metabolism are independent of insulin). In young control subjects fructose produced a significantly greater increase in carbohydrate oxidation and energy expenditure than glucose despite significantly lower plasma glucose and insulin levels. In aged, obese, and diabetic individuals the increments in carbohydrate oxidation and energy expenditure after glucose ingestion were significantly imparied versus the younger controls. After fructose ingestion the increase in carbohydrate oxidation in the three insulin-resistant groups remained below that observed in the younger volunteers, whereas carbohydrate-induced thermogenesis was enhanced to levels that were comparable with those seen in the younger group. These data suggest that 1) the stimulation of thermogenesis after fructose ingestion is related to an augmentation of intracellular metabolism rather than an increase in the plasma insulin concentration per se, 2) the insulin resistance of aging, obesity, and diabetes is associated with a defect in intracellular carbohydrate oxidation, and 3) the cellular mechanisms involved in carbohydrate-induced thermogenesis are not primarily impaired in insulin-resistant states.

Short term effects of fructose on blood glucose dynamics and meal initiation

We have previously shown that small (-11%) transient (approximately 18 min) declines in blood glucose which precede feeding are causally related to meal initiation under free-feeding conditions. We have also shown that IV infusions of glucose (20 mg) that blunt these declines can delay meal initiation. In order to test the specificity of exogenous glucose to delay meal initiation, the ability of another hexose, fructose, to block meal initiation was studied. Since oral and IV fructose have been reported to cause transient hypoglycemia, we also used fructose in an attempt to mimic transient declines in blood glucose and measure the latency to meal initiation. Continuous monitoring of blood glucose and meal pattern was performed in chronically cannulated female rats. When fructose (20 mg) was infused IV during transient declines in blood glucose, meal initiation occurred with a normal latency. During the early dark phase, IV fructose was followed by a slight decrease at the lowest dose or increase in blood glucose at higher doses and no feeding behavior was observed. In the light phase, however, a transient dose-dependent decrease in blood glucose was observed. Furthermore, three types of blood glucose response patterns were identified. Meal initiation occurred only following changes in the blood glucose trajectory that mimicked the spontaneous transient declines in blood glucose. The other two patterns were not followed by meal initiation. Similar effects on blood glucose were observed following oral administration of a range of fructose doses (0.25-0.75 g) in 2-hour fasted rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of radiolabeled copper from portal blood containing fructose or glucose

The present investigation was designed to study the uptake of(67)Cu when administered directly, into the portal vein, along with either functose or glucose, by the liver and extrahepatic tissues. Following weaning, male Sprague-Dawley rats were fed for 3 wk either commercial laboratory ration (chow) or semipurified diets deficient in Cu (0.6 ppm) or supplemented with Cu (6.0 ppm) and containing 62% carbohydrate as either fructuse or cornstarch. After an overnight fast, a single dose of rat plasma (0.1 mL) containing fructose or glucose extrinsically labeled with(67)Cu was injected directly into their portal vein. Although not always statistically significant, rats fed chow retained more radioactivity in the liver and several extrahepatic tissues when(67)Cu was administered with fructose than with glucose. Regardless of Cu status, rats fed diets containing fructose retained more radioactivity in extrahepatic tissues than rats fed starch. There was an increased uptake of(67)Cu by the liver, blood, muscle, and fat pad when fructose as compared to glucose was injected in combination with the isotope. These data strongly suggest that Cu requirements or utilization are greater when fructose is the main dietary carbohydrate. The results may also in part explain the reason for the increased severity of Cu deficiency in rats fed fructose.

Comparison of thermogenic effect of fructose and glucose in normal humans

After nutrient ingestion there is an increase in energy expenditure that has been referred to as dietary-induced thermogenesis. In the present study we have employed indirect calorimetry to compare the increment in energy expenditure after the ingestion of 75 g of glucose or fructose in 17 healthy volunteers. During the 4 h after glucose ingestion the plasma insulin concentration increased by 33 +/- 4 microU/ml and this was associated with a significant increase in carbohydrate oxidation and decrement in lipid oxidation. Energy expenditure increased by 0.08 +/- 0.01 kcal/min. When fructose was ingested, the plasma insulin concentration increased by only 8 +/- 2 microU/ml vs. glucose. Nonetheless, the increments in carbohydrate oxidation and decrement in lipid oxidation were significantly greater than with glucose. The increment in energy expenditure was also greater with fructose. When the mean increment in plasma insulin concentration after fructose was reproduced using the insulin clamp technique, the increase in carbohydrate oxidation and decrement in lipid oxidation were markedly reduced compared with the fructose-ingestion study; energy expenditure failed to increase above basal levels. To examine the role of the adrenergic nervous system in fructose-induced thermogenesis, fructose ingestion was also performed during beta-adrenergic blockade with propranolol. The increase in energy expenditure during fructose plus propranolol was lower than with fructose ingestion alone. These results indicate that the stimulation of thermogenesis after carbohydrate ingestion is related to an augmentation of cellular metabolism and is not dependent on an increase in the plasma insulin concentration per se.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of hepatic fructokinase to long-term sucrose diets and diabetes in spiny mice, albino mice and rats

The activity of hepatic fructokinase increased about 2-fold in desert-derived spiny mice (Acomys cahirinus) and laboratory bred albino mice and rats, maintained on a 50% sucrose diet for 3 months. The role of fructose as the specific inducer was apparent, as 25% fructose diet produced activity increases similar to those of sucrose in contrast to 25% glucose diet. The activity of hexokinase was not affected by the sucrose diet, that of glucokinase rose marginally but those of pyruvate kinase and NADP-malate dehydrogenase rose pronouncedly, especially in the spiny mice. Fructokinase activity increased significantly only after 2 weeks on the diet and continued to rise gradually. The activities of other gycolytic enzymes rose markedly already after 3 days and peaked at about 14 days. Fasting for 48 hr did not influence fructokinase activity while markedly reducing that of glucokinase, pyruvate kinase and NADP-malate dehydrogenase. Streptozotocin diabetes in rats resulted in a 40% reduction in fructokinase activity after 14 days which was restored after 6 days of insulin treatment. The activity increases of other glycolytic enzymes were more marked. However, the fructokinase induction on the sucrose diet was evident also in diabetic rats, suggesting that the insulin and substrate effects are independent. The preference of fructose over glucose phosphorylation capacity was clearly demonstrable in the non-diabetic and diabetic rats and became enhanced on sucrose feeding. The activity of triokinase also increased on the sucrose diet in the 3 rodent species, suggesting a coordinative substrate effect on the induction of these two rate-limiting fructolysis enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary carbohydrate decreases 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and cholesterol synthesis in rat liver

Diets high in carbohydrate and low in fat led to a decrease in the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase in livers of healthy, reverse light-cycled rats. The effect on reductase was due both to a decline in expressed enzyme activity and to a decrease in total number of enzyme molecules. Inhibition of reductase activity was paralleled by a fall in hepatic cholesterolgenesis and by an increase in triglyceride formation. Thus, a reduction in endogenous cholesterol synthesis may explain the serum cholesterol-lowering effect of high carbohydrate, low fat diets in man.

Effects of glucose and fructose loading on glycogenesis in chicks infected with avian tuberculosis

The chick was used as a rapid metabolic model to determine the fate of ingested fructose vs. glucose in noninfected chicks and in those subjected to the stress of avian tuberculosis. The chicks were crop-loaded with either a 72% fructose or glucose solution 21 and 28 days post TB inoculation and killed 2 and 4 hr after loading. In noninfected chicks, both sugars were rapidly converted to glycogen, and there was an interaction with time and the amount of glycogen formed from each sugar. Infection depressed glycogen formation from both fructose and glucose. While the total amount of glycogen formed from glucose could be directly correlated to increased liver size in the TB chicks loaded with glucose, in the chicks loaded with fructose less glycogen was formed even though liver size was increased as a result of the TB infection. The depression in glycogen formation was not related to the severity of the infection since the TB involvement was not the same in the two experiments conducted; but in both cases chicks loaded with fructose showed a greater reduction in the capacity of the liver to synthesize glycogen.

[The fructose induces "glycogenosis". III. Histochemical and morphometrical studies of glycogen metabolism in mouse liver after fructose overload (author's transl)]

INTRODUCTION

After feeding fructose for 7 days rat liver cells show an accumulation of glycogen, a high activity of glucose-6-phosphatase combined with a SER- and RER-reduction. This result was reviewed by mouse liver cells using histochemical and morphometrical methods.

MATERIAL AND METHODS

60% fructose in drinking water was given mice as only nutritional source. Controls had free access to Altromin-R-standard diet and drinking water. Glycogen and glycogen metabolizing enzymes are demonstrated in the course of an 1-14 days fructose diet. After a 7 days diet liver tissue was analysed morphometrically.

RESULTS AND DISCUSSION

Feeding of fructose leads to a high glycogen content, combined with a high activity of glycogen-phosphorylase and glucose-6-phosphatase in the liver parenchyma of mouse. Glycogen-synthetase activity falls to a low level. The SER and RER and the peroxisomes are reduced. The single volume of the hepatic nucleus is decreased and the hepatocellular chondrioma is transformed in a smaller number of larger mitochondria. Compared with the rate the analysed organelles and enzymes of mouse liver show only slight quantitative differences. The increase of glucose-6-phosphatase and simultaneous reduction of endoplasmic reticulum-membranes is illustrated by the dynamic structure of endoplasmic reticulum-membranes, which adapt to metabolic changes. The variable turnover of different parts of endoplasmic reticulum-membranes seems to be very important.

Comparative effects of fructose and glucose on the lipid and carbohydrate metabolism of perfused rat liver

1. Livers from rats fed on a standard diet were perfused with whole blood, and infused continuously with glucose and fructose at equimolar rates. 2. Infusion of fructose increased both the secretion of very-low-density-lipoprotein (VLDL)-triglycerides and the incorporation of free fatty acids (FFA) from the perfusate into VLDL-lipids, but neither of these two processes was affected by infusion of glucose. 3. Infusion of fructose decreased the oxidation and increased the esterification of FFA, but glucose infusion had no effect on these processes. When fructose and glucose were infused together was a further decrease in oxidation. 4. When fructose was infused alone or together with glucose, blood concentrations rapidly became stabilized at those found in the hepatic portal vein in vivo, with similar rates of hepatic uptake to those found in the intact animal. Infusion of glucose alone resulted in continuously increasing perfusate glucose concentrations, and rates of uptake which were only 20% of those for fructose. Blood glucose concentrations were reduced, and lactate concentrations were increased by fructose infusion, and when glucose and fructose were infused together there was a greatly increased rate of glucose uptake. 5. Liver glycogen was not affected by the infusion of fructose or glucose alone; however, their combined addition significantly increased its concentration. 6. Uptake of perfusate FFA was not affected by either fructose or glucose infusions. 7. The results are discussed in terms of the differences in nutrition and metabolism between glucose and fructose, with particular reference to the development of hypertriglyceridaemia.

The effect of high-carbohydrate diets on liver triglyceride formation in the rat

The effect of feeding diets containing 75% glucose or fructose on liver triglyceride formation in the rat was studied by both in vivo and in vitro techniques. The results were compared with those from control rats fed laboratory chow. Both high-sugar diets increased the capacity for triglyceride formation from sn-glycerol-3-P by rat liver homogenates and correspondingly increased incorporation of [1,3-(14)C]glycerol into hepatic triglyceride by the intact animal. These independent measures of hepatic triglyceride production changed with a similar time-course characteristic for each diet. The 75% fructose diet produced a greater increase in both determinations, reaching a maximum after 11 days.Despite the increase in hepatic triglyceride formation by both high-sugar diets, only the 75% fructose diet resulted in a consistent and sustained increase in serum triglyceride. This results most probably from differences in the fractional rate of serum triglyceride removal between the two groups.When serum triglyceride removal was inhibited by administration of Triton WR-1339, both high-sugar diets increased incorporation of [1,3-(14)C]glycerol in serum triglyceride in vivo and increased serum triglyceride level above that in control rats.

The immediate effects of insulin and fructose on the metabolism of the perfused liver. Changes in lipoprotein secretion, fatty acid oxidation and esterification, lipogenesis and carbohydrate metabolism

1. When livers from fed rats were perfused with blood containing elevated concentrations of rat insulin or blood to which fructose was added, the oxidation of free fatty acids was depressed and their esterification was increased. 2. Raised concentrations of insulin or addition of fructose increased secretion of triglyceride in very-low-density lipoproteins, but only insulin caused more of the free fatty acids taken up by the liver to be incorporated into very-low-density lipoproteins. 3. When insulin and fructose were added together the combined effect on oxidation and esterification of free fatty acids and on secretion of very-low-density lipoproteins was equal to the sum of the effects of either alone. No statistically significant interaction between the effects of fructose and insulin was found for any of the parameters investigated. 4. Bovine insulin had similar effects, in most respects, to comparable studies with raised concentrations of rat insulin. 5. Lipogenesis was increased in the livers treated with fructose plus bovine insulin. 6. A significant proportion of the fatty acids in very-low-density lipoproteins were derived either from the liver triglyceride pool or from lipogenesis. This fraction was increased both by treatment with insulin or fructose, and was augmented further when both insulin and fructose were present together. 7. The uptake of fructose by the perfused liver was similar to that found in vivo. It was unaffected by the presence of insulin. 8. Addition of fructose to the perfused liver caused perfusate lactate concentrations to increase, as a result of diminished hepatic uptake of lactate. 9. The uptake of free fatty acids by the perfused liver was unaffected by the addition of either insulin or fructose. 10. The distribution among the various lipid classes in plasma lipoproteins of label arising from the hepatic uptake of [(14)C]oleate was unaltered by the addition of either fructose or insulin. 11. It is suggested that the effects described are due principally to control of the balance between esterification of fatty acids and lipolysis of the ensuing triglyceride, fructose enhancing esterification and insulin inhibiting lipolysis.