A high-fat diet worsens metabolic control in streptozotocin-treated rats by increasing hepatic glucose production

The role of sclerostin in lipid and glucose metabolism disorders

Lipid and glucose metabolism are critical for human activities, and their disorders can cause diabetes and obesity, two prevalent metabolic diseases. Studies suggest that the bone involved in lipid and glucose metabolism is emerging as an endocrine organ that regulates systemic metabolism through bone-derived molecules. Sclerostin, a protein mainly produced by osteocytes, has been therapeutically targeted by antibodies for treating osteoporosis owing to its ability to inhibit bone formation. Moreover, recent evidence indicates that sclerostin plays a role in lipid and glucose metabolism disorders. Although the effects of sclerostin on bone have been extensively examined and reviewed, its effects on systemic metabolism have not yet been well summarized. In this paper, we provide a systemic review of the effects of sclerostin on lipid and glucose metabolism based on in vitro and in vivo evidence, summarize the research progress on sclerostin, and prospect its potential manipulation for obesity and diabetes treatment.

High-fat diet in low-dose-streptozotocin-treated heminephrectomized rats induces all features of human type 2 diabetic nephropathy: a new rat model of diabetic nephropathy

BACKGROUND AND AIM

We have developed a new rat model that mimics the natural course of diabetic nephropathy seen in type 2 diabetes.

METHODS

Nine days after intravenous injection of streptozotocin (STZ) (40 mg/kg) or vehicle to 8-week-old male Sprague-Dawley rats, the animals' right kidneys were surgically removed. Two weeks after surgery, the STZ-injected rats were fed on either a high-fat (ST+HF) or a normal (ST) diet, while the vehicle-injected rats were fed on the high-fat diet (HF).

RESULTS

Baseline biochemical markers did not differ between the three groups. Only the ST+HF group showed a significant increase in plasma glucose levels after 15 weeks, and simultaneously plasma insulin levels started to decrease, followed by an increase in plasma total cholesterol and triglyceride levels at 25 weeks and slightly later by an increase in blood pressure. In the ST+HF group, significant microalbuminuria was detected at 15 weeks followed by overt proteinuria, both of which were absent in the other two groups. Also in ST+HF, the creatinine clearance rate increased until week 15, and then gradually decreased. Histologically, ST+HF rats showed mesangial expansion at week 25, and diffuse glomerular sclerosis at the end of the experiments.

CONCLUSION

The chronological changes in biochemical, physiological and histological markers in ST+HF rats are reminiscent of human type 2 diabetes and nephropathy. Our new model of type 2 diabetic nephropathy should help us to understand the pathophysiology of the disease and serve to explore measures to prevent and treat diabetic nephropathy.

Relationship of dietary fat to glucose metabolism

The relationship between dietary fat and glucose metabolism has been recognized for at least 60 years. In experimental animals, high fat diets result in impaired glucose tolerance. This impairment is associated with decreased basal and insulin-stimulated glucose metabolism. Impaired insulin binding and/or glucose transporters has been related to changes in the fatty acid composition of the membrane induced by dietary fat modification. In humans, high-fat diets, independent of fatty acid profile, have been reported to result in decreased insulin sensitivity. Saturated fat, relative to monounsaturated and polyunsaturated fat, appears to be more deleterious with respect to fat-induced insulin insensitivity. Some of the adverse effects induced by fat feeding can be ameliorated with omega-3 fatty acid. Epidemiological data in humans suggest that subjects with higher intakes of fat are more prone to develop disturbances in glucose metabolism, type 2 diabetes or impaired glucose tolerance, than subjects with lower intakes of fat. Inconsistencies in the data may be attributable to clustering of high intakes of dietary fat (especially animal fat) with obesity and inactivity. Metabolic studies suggest that higher-fat diets containing a higher proportion of unsaturated fat result in better measures of glucose metabolism than high-carbohydrate diet. Clearly, the area of dietary fat and glucose metabolism has yet to be fully elucidated.

Studies on the impact of dietary fat composition on proteinuria in diabetic rats

Rats with streptozotocin diabetes were pair-fed diets containing 20% beef tallow (BT), fish oil (FO), or safflower oil (SO) for up to six months. After one month, differences in glucose control were not observed but rats fed FO had more renal hypertrophy. FO reduced glomerular prostaglandin E2 and 6-keto F1 alpha, and BT increased thromboxane B2 production, but there were no differences in glomerular filtration rate (GFR) or renal plasma flow (RPF). Animals fed BT needed more insulin after two months than rats fed FO followed by SO. After six months, diabetic rats fed FO had larger relative kidney weights than SO or BT, but a similar pattern was present in non-diabetic controls fed the same diets. Diabetic rats fed BT had more proteinuria than diabetic rats fed SO but not FO. However, FO-fed controls had more proteinuria than controls fed SO and similar levels of proteinuria as diabetic rats fed FO. The composition of dietary fat alters glucose tolerance in diabetic rats after two months. BT increases glomerular thromboxane production and hastens proteinuria compared to SO. FO enhances renal growth and proteinuria, but this effect is independent of the diabetic condition.

Understanding the pathogenesis of type 2 diabetes: can we get off the metabolic merry-go-rounds?

The aetiology of non-insulin-dependent diabetes mellitus (NIDDM) is not known. The concordance of NIDDM in identical twins and differences in the prevalence rate of NIDDM between different racial groups suggest a genetic cause. Hyperglycaemia in established diabetes is caused by a combination of hepatic insulin resistance, impaired peripheral (muscle and fat) glucose uptake and a defect in glucose-mediated insulin secretion. However, it is not known if these defects are all inherited or if one can cause the others. This uncertainty is due to the fact that hyperglycaemia per se can cause defects in insulin action and insulin secretion that resemble those found in NIDDM. Furthermore the elevated free fatty acid (FFA) levels found when NIDDM is associated with obesity are known to cause both peripheral and hepatic insulin resistance. Recently we have demonstrated the mechanism by which elevated FFA levels can cause hepatic insulin resistance. However, we also have evidence that the converse holds in that genetically engineered hepatic insulin resistance in a transgenic rat model leads to obesity. Thus an understanding of the pathogenesis of NIDDM is complicated by the fact that hyperglycaemia and obesity can be both causes and consequences of insulin resistance. To overcome these difficulties, studies in young euglycaemic diabetes-prone subjects have been conducted. Results suggest that there may be different causes for NIDDM in different racial groups.

Influence of dietary curcumin and cholesterol on the progression of experimentally induced diabetes in albino rat

Effect of feeding 0.5% curcumin diet or 1% cholesterol diet was examined in albino rats rendered diabetic with streptozotocin injection. Diabetic rats maintained on curcumin diet for 8 weeks excreted comparatively less amounts of albumin, urea, creatinine and inorganic phosphorus. Urinary excretion of the electrolytes sodium and potassium were also significantly lowered under curcumin treatment. Dietary curcumin also partially reversed the abnormalities in plasma albumin, urea, creatine and inorganic phosphorus in diabetic animals. On the other hand, glucose excretion or the fasting sugar level was unaffected by dietary curcumin and so also the body weights were not improved to any significant extent. Diabetic rats fed curcumin diet had a lowered relative liver weight at the end of the study compared to other diabetic groups. Diabetic rats fed a curcumin diet also showed lowered lipid peroxidation in plasma and urine when compared to other diabetic groups. The extent of lipid peroxidation on the other hand, was still higher in cholesterol fed diabetic groups compared to diabetic rats fed with control diet. Thus, the study reveals that curcumin feeding improves the metabolic status in diabetic conditions, despite no effect on hyperglycemic status or the body weights. The mechanism by which curcumin improves this situation is probably by virtue of its hypocholesterolemic influence, antioxidant nature and free radical scavenging property.

Basal glucose turnover in Psammomys obesus. An animal model of type 2 (non-insulin-dependent) diabetes mellitus

The aim of this study was to examine whole-body glucose turnover and glucose uptake into individual tissues in Psammomys obesus. The animals were classified according to the level of circulating glucose and insulin in the fed state: group A was normoglycaemic and normoinsulinaemic (glucose < 8.0 mmol/l), insulin < 150 mU/l), group B was normoglycaemic and hyperinsulinaemic (glucose < 8.0 mmol/l, insulin > or = 150 mU/l), and group C was hyperglycaemic and hyperinsulinaemic (glucose > or = 8.0 mmol/l, insulin 150 mU/l). The animals were deprived of food for 6 h, after which they were anaesthetized and cannulated, using the jugular vein for infusions and the carotid artery for blood sampling. Whole-body glucose turnover was measured using a primed-continuous infusion of 6-[3H]-glucose and saline to quantitatively assess hepatic glucose production (HGP), glucose disposal (Rd), and the metabolic clearance rate of glucose (MCR). Following the 2-h infusion period, the glucose metabolic index (Rg') of individual tissues was measured using a fixed-dose bolus of 2-deoxy-[14C] glucose. Under the steady-state conditions of the experiment, HGP was assumed to be equal to Rd, and both variables were found to be significantly correlated to the fasting glucose concentration (r=0.534, P<0.05, n=19). On the other hand, MCR was found to be inversely correlated to the fasting plasma glucose concentration (r=0.670 P < 0.01, n=19). When the animals were divided into three groups as described above, HGP in group C animals was significantly elevated compared with group A (20.8 +/- 2.6 vs 12.7 +/- 0.6 mg.kg-1.min-1; P < 0.05), and MCR showed a tendency to be lower in group C than group A, although the difference was not statistically significant. HGP and MCR were not significantly different between groups A and B. Measurement of the glucose metabolic index in individual tissues showed that group C animals had significantly higher Rg' values in muscles and adipose tissues compared with those in group A (P < 0.05). In addition, Rg' in group B white gastrocnemius and soleus were significantly higher than in group A despite similar rates of HGP and levels of glycaemia. These findings suggest that an early increase in skeletal muscle glucose uptake and hyperinsulinaemia can be demonstrated in group B Psammomysobesus before significant hyperglycaemia.

Nutritional therapy for diabetes mellitus

Dietary therapy affects diabetes management in the dog and cat directly through control of blood glucose and indirectly through control of obesity and lipid abnormalities. Caloric intake, the feeding schedule, food form, macronutrient composition of the diet, and the presence of any concurrent problems must all be considered when planning the dietary regime. Generally, the healthy diabetic dog or cat should be fed a diet with increased fiber and moderate carbohydrate in a quantity sufficient to attain and maintain optimal body weight; whenever possible, the daily food allotment should be divided into multiple small meals that are fed through the day and evening when the physiologic effects of administered insulin are present. Once established, the dietary regime should be kept constant from day to day. Following these guidelines will help minimize postprandial hyperglycemia and may lead to a decreased exogenous insulin requirement. However, if a concurrent disorder has dietary requirements that conflict with those for the diabetic pet, nutritional management of the other disorder should usually take precedence.

Effect of propionate on in vivo carbohydrate metabolism in streptozocin-induced diabetic rats

Undigested carbohydrates and some dietary fibers are fermented in the large intestine to form short-chain fatty acids (SCFA), including acetate, propionate, and butyrate. It has been suggested that some of the beneficial effects of high-carbohydrate, high-fiber diets on carbohydrate and lipid metabolism are mediated by the metabolism of SCFA in the liver. Propionate has been shown in vitro to decrease glucose production in rat hepatocytes. The aim of the present study was to investigate the effects of propionate on carbohydrate metabolism in normal and streptozocin (STZ)-induced diabetic male Sprague-Dawley rats. Rats were fed a high-fat diet with or without sodium propionate supplementation (either 0.5% or 5% wt/wt) for 4 weeks. At the completion of the feeding period, body weight and liver glycogen concentrations were significantly decreased in STZ-diabetic rats and were unaffected by propionate supplementation. Although STZ-diabetic animals had elevated fasting plasma glucose, cholesterol, and triglyceride levels relative to nondiabetic rats, propionate supplementation had no significant effect on these parameters in either group. Basal and insulin-stimulated carbohydrate metabolism were assessed using the euglycemic clamp technique in overnight-fasted animals with 3(H)-6-glucose infusion. As expected, basal hepatic glucose production (HGP) was higher and the metabolic clearance rate of glucose (MCR) was lower in STZ-diabetic rats. High-dose insulin infusion (3 mU.kg-1.min-1) suppressed HGP in nondiabetic and diabetic animals and increased the MCR in nondiabetic animals. However, propionate supplementation did not alter basal or insulin-stimulated HGP or the MCR in either nondiabetic or diabetic animals.(ABSTRACT TRUNCATED AT 250 WORDS)