Relationship between insulin resistance,insulin secretion,very low density lipoprotein kinetics,and plasma triglyceride levels in normotriglyceridemic man

We have previously postulated that resistance to insulin-mediated glucose uptake was the basic metabolic abnormality in patients with endogenous hypertriglyceridemia. In this situation, glucose tolerance would tend to deteriorate, and could only be maintained by the increased secretion of insulin. Although the ensuing hyperinsulinemia might prevent the development of glucose intolerance, we suggested that it would also lead to increased hepatic very low density (VLDL) triglyceride (TG) synthesis and secretion. In the current study we have quantified these four metabolic variables in 16 nonobese human subjects with plasma TG concentrations < 175 mg/dl. The results demonstrate the following degree of correlation: insulin resistance

insulin response to food

VLDL-TG secretion rate

plasma TG concentration. These data indicate that nonobese subjects with normal TG levels have the same relationship between degree of insulin sensitivity, insulin response to food, VLDL-TG secretion, and TG concentration previously described in patients with endogenous hypertriglyceridemia.     

Effect of fructose feeding on insulin secretion and insulin action in the rat

Insulin secretion and insulin action were studied in rats fed either a diet containing (as percent of calories) 66% fructose, 22% protein, and 12% fat, or standard rat chow (60% vegetable starch, 29% protein, 11% fat) for 7 days. Plasma glucose concentration following either an oral glucose or fructose load ($\text{180 mg}\text{100 g body weight}$) were slightly higher in the fructose-fed rats, and this was associated with a much greater elevation of plasma insulin concentrations. The ability of insulin to stimulate disposal of glucose load was determined during the continuous infusion of epinephrine, propranolol, glucose, and insulin. Under these conditions the steady state plasma insulin levels were the same in the two groups of rats, whereas the steady state plasma glucose levels were almost twice as high in the fructose fed rats. Thus, fructose feeding for 7 days resulted in an increase in the insulin response to an oral carbohydrate challenge, as well as to a loss of normal insulin sensitivity.     

Hyperinsulinemia in fructose-induced hypertriglyceridemia in the rat

Rats were fed diets containing (as percent of calories) 66% glucose or fructose, 22% casein, and 12% lard, for 1 wk. The effects of these diets on plasma triglyceride, glucose, and insulin concentrations were compared to those of control rats eating regular rat chow. Plasma triglyceride levels increased from a mean (± SE) control level of 85 ± 7 to 142 ± 9 (p < 0.001) and 380 ± 38 (p < 0.001) mg/dl in dextrose- and fructose-fed rats, respectively. Plasma insulin concentrations demonstrated a similar increase, rising from a mean (± SE) control value of 29 ± 4 μU/ml to 55 ± 10 μU/ml in dextrose-fed rats and to 85 ± 12 μU/ml in rats eating the fructose diet. Plasma glucose concentrations of the three groups were comparable. These results indicate that fructose-induced hypertriglyceridemia is associated with significant hyperinsulinemia.     

Age-related changes in very low density lipoprotein metabolism in normal rats

Plasma triglyceride (TG) concentrations rise with age, and we have carried out studies of very low density lipoprotein (VLDL) kinetics in the rat in an effort to define the cause of this phenomenon. Efficiency of VLDL-TG secretion by perfused rat liver decreases as rats age from $\text{1}\text{1}\text{2}\text{–12}\text{mo}$. However, this is compensated for by an increase in liver weight, and VLDL-TG secretion per perfused liver does not change with age. In contrast, total VLDL-TG secretion by the intact rat increases significantly as rats grow from $\text{1}\text{1}\text{2}\text{–12}\text{mo}$ of age, and this increase is proportionate to the age-related increase in liver weight. The ability of the older rat to maintain VLDL-TG secretion proportionate to liver weight is most likely due to the concomitant rise in plasma free fatty acid concentration that occurs with age. However, the efficiency with which VLDL-TG is removed from plasma is not maintained as rats age. Consequently, the age-related rise in plasma TG concentration is due to an increase in VLDL-TG secretion proportionate to secretory mass, accompanied by a relative decline in efficiency of VLDL-TG removal from plasma.     

Metabolic effects of added dietary sucrose in individuals with noninsulin-dependent diabetes mellitus (NIDDM)

This study addresses the metabolic effects of sucrose in the diets of 11 individuals with noninsulin-dependent diabetes mellitus (NIDDM). Each of two dietary periods were 15 days in length, and contained 50% of the calories as carbohydrate, 30% as fat, and 20% as protein. The only variable between the two periods was the percentage of total calories as sucrose, 16% v 1%. Fasting blood samples were analyzed for plasma glucose and insulin as well as total plasma VLDL-, LDL- and HDL-cholesterol and triglyceride concentrations. In addition, postprandial blood samples were obtained for the measurement of plasma glucose, insulin and triglyceride concentrations. Fasting plasma glucose, insulin, and day-long insulin concentrations were similar between the two diets. However, the addition of sucrose in amounts comparable to those typically consumed by the general population resulted in significantly elevated day-long glucose (P less than 0.05) and triglyceride (P less than 0.05) responses, as well as elevated fasting total plasma cholesterol (P less than 0.001), triglyceride (P less than 0.05), VLDL-cholesterol (P less than 0.01), and VLDL-triglyceride (P less than 0.05) concentrations. LDL-cholesterol and HDL-cholesterol concentrations were unchanged during the added sucrose diet. It is clear that the consumption of diets containing moderate amounts of sucrose resulted in changes to plasma lipid and postprandial glucose concentrations that have been identified as risk factors for coronary artery disease. Therefore, it seems prudent at this time to advise patients with NIDDM to avoid added dietary sucrose.     

Plasma glucose and insulin responses to oral glucose in nonobese subjects and patients with endogenous hypertriglyceridemia

Plasma glucose and insulin responses to a 50-gm oral glucose challenge were determined in 396 nonobese subjects: 220 patients with endogenous hypertriglyceridemia and 176 normal persons. These groups were further subdivided on the basis of relative body weight: 1.0-1.1 and 1.1-1.2. Patients with endogenous hypertriglyceridemia whose obesity index was between 1.0 and 1.1 had significantly increased plasma glucose (more than 25%, P less than 0.001) and insulin (more than 18%, P less than 0.01) responses. Similar findings were also observed in patients with endogenous hypertriglyceridemia whose index of obesity was between 1.1 and 1.2, ie, there was a 25% increase in the plasma glucose response (P less than 0.001) and a 37% increase in the plasma insulin response (P less than 0.001). Thus, endogenous hypertriglyceridemia can occur in nonobese individuals, and these patients have an increase in their plasma glucose and insulin responses when weight-matched with nonobese normal subjects.     

Inhibition of carbohydrate-induced hypertriglyceridemia by a disaccharidase inhibitor

The ability of an inhibitor of intestinal alpha-glucosiadase activity to prevent sucrose-induced hypertriglyceridemia was studied in nonobese rats. The results indicated that plasma triglyceride levels were approximately twice as high in untreated rats, and the reduction in plasma triglyceride levels of drug-treated rats was associated with lowered very low density lipoprotein-triglyceride secretion rates and plasma insulin levels. Since these changes could be produced with an amount of glucosidase inhibitor which did not prevent normal rate of weight gain, the possibility arises that this approach may be useful in the treatment of various hypertriglyceridemic states in man. Finally, the observation that the fall in plasma TG concentration was associated with a fall in plasma insulin concentration provides further evidence for the existence of a causal relationship between the two variables.     

Ability of exercise to inhibit carbohydrate-induced hypertriglyceridemia in rats

The ability of spontaneous running to prevent carbohydrate-induced hypertriglyceridemia was studied in young, nonobese rats. Exercise-trained and sedentary rats were fed a diet consisting of (as percent total calories) 12% fat, 22% protein, and 66% carbohydrate. The source of the carbohydrate was varied, and experiments were carried out with sucrose and glucose as the sole dietary carbohydrate. Plasma triglyceride (TG) levels rose in response to both forms of dietary carbohydrate in both sedentary and exercise-trained rats, but the magnitude of the elevation was greatly attenuated in the exercise-trained group. Plasma insulin concentrations were also significantly lower in exercise-trained rats. Measurements of hepatic very low density lipoprotein (VLDL)-TG secretion rate and adipose tissue lipoprotein lipase (LPL) activity were made in an effort to determine how exercise-training prevented the development of carbohydrate-induced hypertriglyceridemia. The results of these studies indicated that perfused livers of exercise-trained rats secreted significantly less VLDL-TG, whereas adipose tissue LPL activity of the two groups was similar. On the basis of these results, it is postulated that the ability of exercise-training to inhibit carbohydrate-induced hypertriglyceridemia is due to an increase in insulin sensitivity resulting from chronic exercise. As a result, the postprandial insulin responses to high carbohydrate diets would be relatively reduced in exercise-trained rats, leading to decreased hepatic VLDL-TG secretion, and lower plasma triglyceride concentrations.     

Evidence of an age-related decline in mitochondrial glycerophosphate dehydrogenase activity of isolated rat islets

The activities of three enzymes--two mitochondrial and one microsomal--were measured in isolated islets of Langerhans from 2-month-old and 12-month-old rats. Mitochondrial glycerophosphate dehydrogenase activity (expressed as nanomoles of iodonitrotetrazolium reduced per minute per milligram of protein), decreased (P less than 0.01) from a mean (+/- SEM) of 73.2 +/- 11.2 (2-month-old) to 34.7 +/- 5.9 (12-month-old). In contrast, activities of neither mitochondrial monoamine oxidase nor microsomal NADH cytochrome-c reductase changed with age. These results demonstrate that the activity of the glycerophosphate shuttle decreases as rats grow older, and it raises the possibility that the consequent difficulty in regenerating cytosolic NAD+ may play a role in the insulin secretory defect associated with aging.     

Effect of acute uremia on triglyceride kinetics in the rat

Plasma triglyceride (TG) levesl were elevated 24 hr after the production of acute uremia in rats. The effect of acute uremia on TG production rate was estimated by determining the rate of TG accumulation following Triton WR 1339 inhibition of lipoprotein removal, by measuring hepatic TG secretion rate during in situ liver perfusion, and by quantifying hepatocyte very low density lipoprotein content with the electron microscope. The results of all three of these approaches indicated that TG synthesis and secretion were decreased in acute uremia, suggesting that the associated increase in plasma TG levels had to result from a removal defect. This hypothesis was tested directly by injecting pre-labeled very low density lipoprotein TG into acutely uremic and control rats, and measuring its rate of disappearance from plasma. The t1/2 of removal in acutely uremic rats was found to be approximately twice that of control, confirming the hypothesis that the rise in plasma TG levels in acute uremia is due to a defect in removal of TG from plasma.     

Does day-long absolute hypoinsulinemia characterize the patient with non-insulin-dependent diabetes mellitus?

Plasma glucose and insulin responses to a standard oral glucose tolerance test (75 g of glucose) and to mixed meals were compared in 15 normal subjects and 15 patients with non-insulin-dependent diabetes mellitus (NIDDM). Fasting plasma glucose levels were above 140 mg/dL in all patients with NIDDM, and the two groups were weight matched. Plasma glucose levels were significantly higher in patients with NIDDM throughout the glucose tolerance test, and this was associated with a marked reduction in plasma insulin response. Plasma glucose levels were also higher in patients with NIDDM when measured hourly from 8 AM to 5 PM (mixed meals were consumed at 8 AM and 12 PM), but the plasma insulin concentration of the two groups were similar. Thus, the day-long circulating insulin levels of patients with NIDDM are not reduced. Consequently, these patients cannot be considered to be absolutely insulin deficient.     

Effect of high-carbohydrate-low-fat diets on plasma glucose,insulin and lipid responses in hypertriglyceridemic humans

Two levels of dietary carbohydrate (40% and 60% of calories) were incorporated into typical US diets and fed for 15 days each to eight patients with endogenous hypertriglyceridemia. Fasting blood samples were drawn on days 13, 14, and 15 of each dietary period, and analyzed for glucose, insulin, cholesterol, and triglyceride concentrations, as well as for triglyceride and cholesterol content of the various lipoprotein classes. In addition, these same measurements were made before and for three hours after the noon meal on days 14 and 15. Fasting plasma triglyceride (TG) and very-low-density lipoprotein (VLDL)-TG concentrations were significantly increased (P < 0.005) on the low-fat-high-carbohydrate diet. In addition, integrated postprandial insulin, TG, and VLDL-TG responses to the noon meal were significantly (P < 0.01?0.001) elevated on the low-fat-high-carbohydrate diet. No dietary-induced changes were noted in either the fasting or postprandial values of glucose cholesterol, chylomicron-TG, low-density lipoprotein-cholesterol, high-density lipoprotein (HDL)-cholesterol, HDL₂-cholesterol, or HDL₃-cholesterol. These results indicate that low-fat-high-carbohydrate diets accentuate the metabolic risk factors for coronary artery disease that are already present in patients with endogenous hypertriglyceridemia.     

Studies of the mechanism of fructose-induced hypertriglyceridemia in the rat

Carbohydrate-induced hypertriglyceridemia is easily produced in the rat, and fructose has been shown to be particularly potent in this regard. In this study we have compared the effects of feeding rats diets high (66% of total calories) in fructose or glucose on various aspects of carbohydrate and lipid metabolism. The results confirmed previous observations that fructose (456 +/- 276 mg/dl) was more potent (p less than 0.001) in raising plasma TG concentration than was glucose (242 +/- 13 mg/dl), and indicated that the difference in magnitude of hypertriglyceridemia produced by the two carbohydrates was closely related to the ability of the test diets to increase VLDL-TG secretion (r = 0.85, p less than 0.001). Both glucose and fructose feeding led to comparable degrees of hyperinsulinemia, and plasma TG concentrations increased before hyperinsulinemia evolved in fructose-fed rats. Therefore, it was concluded that fructose can act directly on the liver to increase VLDL-TG secretion, and that fructose-induced hypertriglyceridemia can occur in the absence of hyperinsulinemia. On the other hand, the rise in plasma TG concentration produced by fructose was reduced dramatically in exercise-trained rats, and this was associated with a decrease in plasma insulin concentration. Based upon these observations, we suggest that fructose feeding produces hypertriglyceridemia by directly stimulating hepatic VLDL-TG secretion, as well as by producing insulin resistance and hyperinsulinemia, and that it is the combined effect of these two separate actions which accounts for the magnitude of fructose-induced hypertriglyceridemia.     

Effect of obesity on the relationship between very low density lipoprotein production rate and plasma triglyceride concentration in normal and hypertriglyceridemic subjects.

The effect of obesity on the relationship between very low density lipoprotein (VLDL)-triglyceride (TG) production rate and concentration was studied in 80 subjects whose TG concentrations ranged from 41-1315 mg/100 ml and whose relative weight varied from 0.74-1.46. There was a positive correlation between VLDL-TG production rate and plasma TG concentration in all 80 patients (r = 0.78), which was highly statistically significant (p less than 0.001). Both male and female subjects were subdivided into quartiles on the basis of relative weight; the relationship between VLDL-TG production rate and plasma TG concentration was found to be comparable in all subgroups. These observations suggest that moderate degrees of obesity do not affect VLDL-TG kinetics in patients whose plasma TG levels vary over an extremely wide range.     

Functional homogeneity of pancreatic islets of aging rats

Islets from different regions of pancreases of aging rats were compared for size and variations in response to glucose stimulation. The results show that pancreatic islets from the ventral-duodenal and splenic regions of 12-mo-old retired breeder Spraque-Dawley rats are comparable in all respects measured: thus, pancreatic regional differences cannot explain the age-associated reduction in beta cell secretory response noted in previous studies.     

The relationship of dietary calcium to the maintenance of skeletal integrity in man—an interface of endocrinology and nutrition

Calcium restriction reliably produces osteoporosis in animals, but the impact of dietary calcium on age-related bone loss in man is controversial. The opinion that calcium intake is not clearly related to bone mass or to osteoporosis is well-entrenched in the nutrition literature, and has been influential in setting recommended intakes for the American public. This position rests on the poor correlation between habitual calcium intake and the incidence of osteoporosis in some non-Western countries, and on demonstrations that young men can achieve calcium balance on low intakes. However, such data neglect a variety of factors which determine calcium nutriture. These include the efficiency of mineral utilization, gonadal status, and the effect of dietary protein. It has recently been demonstrated that calcium requirements increase during adult life in women. This increase is attributable in large measure, but not completely, to estrogen deprivation associated with the menopause. In addition, the American diet is abundant in protein, a situation which may be deleterious to bone. Based on the evidence currently available, it is reasonable to propose that adolescent and young women consume one gram of calcium daily, and that this intake be increased to 1500 mg at the time of menopause.     

The effects of chronic uremia and dexamethasone on triglyceride kinetics in the rat.

The effects of chronic uremia and dexamethasone administration on triglyceride (TG) kinetics were studied in the rat. Uremia was produced by a two-stage, $\text{5}\text{6}$ nephrectomy, and resulted in a fourfold rise in BUN levels. The elevation in BUN level led to a rise in mean ( ± SE) TG levels from $\text{46 ± 4 to 62 ± 6 mg}\text{100 ml}$. The increase in TG levels was associated with a marked prolongation in very low density lipoprotein (VLDL) removal rate from plasma, without any change in triglyceride secretion rate (TG-SR). The combined effects of uremia and glucocorticoid treatment were evaluated by treating chronically uremic rats with either low (0.05 mg/kg body weight) or high (0.25 mg/kg body weight) doses of dexamethasone. The low dose of dexamethasone led to a further rise in mean TG levels of chronically uremic rats ($\text{107 ± 14 mg}\text{100 ml}$), and the hypertriglyceridemia was further accentuated when chronically uremic rats were treated with high-dose dexamethasone ($\text{179 ± 15 mg}\text{100 ml}$). The elevation of TG levels in uremic rats receiving the low dose of dexamethasone was associated with a prolongation of VLDL removal which was not statistically significant. Treatment with the higher dose of dexamethasone resulted in an increase in TG-SR and a further prolongation of VLDL removal, both of which were statistically significant. Thus, hypertriglyceridemia in the chronically uremic rat is due to a defect in VLDL removal. Dexamethasone treatment is capable of accentuating this removal defect, as well as increasing TG-SR. The combined effect of both is to lead to a marked degree of hypertriglyceridemia.     

Prevention of age-related hypertriglyceridemia by caloric restriction and exercise training in the rat

Plasma triglyceride concentrations were determined in three experimental groups of rats as they grew from $\text{1}\text{1}\text{2}\text{–12}\text{mo}$ of age. One group was kept sedentary and allowed food ad lib (control), the second group was allowed to eat and exercise in a running wheel ad lib, and the third calorically-restricted in order to maintain weight equal to that of the exercising rats. The exercise-trained and calorically-restricted rats gained less weight than did the control rats, and the age-related rise in plasma triglyceride concentration in control rats was totally abolished in the other two groups. In addition, exercise training and caloric restriction inhibited the increase in plasma insulin concentrations noted to occur with age in the control rats. These data indicate that there are two effective ways to prevent the development of hypertriglyceridemia that occurs with age in otherwise normal rats.     

Disparate effects of prazosin and propranolol on lipid metabolism in a rat model

The effects of 14 days of daily intraperitoneal injections of prazosin hydrochloride (3.0 or 0.3 mg/kg) or propranolol (5 mg/kg) on various aspects of the lipid metabolism of normal rats was studied. The results indicate that plasma triglyceride and cholesterol concentrations were lower and the ratio of plasma HDL-cholesterol to total cholesterol concentration higher in prazosin-treated rats compared with propranolol-treated rats. The decline in plasma triglyceride levels in prazosin-treated rats was associated with a commensurate reduction in triglyceride secretion rates. Plasma free fatty acid levels were also lower in prazosin-treated rats, and this change may have contributed to the fall in triglyceride secretion rates. These data demonstrate that the disparate effects of alpha and beta receptor antagonists, previously shown to occur in hypertensive humans, can be duplicated in normal rats.     

Development of fasting hyperglycemia in uremic rats

Rats which had been fasted for the previous 24 hr were subjected to either sham surgery, bilateral nephrectomy, or bilateral ureterotomy. The fast was continued for another 24 hr before the animals were decapitated and blood was obtained for determination of serum glucose, insulin, and urea nitrogen levels. A moderate but statistically significant (p < 0.02) fall in serum glucose levels occurred in rats made uremic by bilateral nephrectomy. In contrast, rats made equally uremic by bilateral ureterotomy developed a significant (p < 0.001) elevation of both serum glucose and insulin levels. The combination of hyperglycemia and hyperinsulinemia suggested that insulin resistance had developed in these rats, and this was confirmed by demonstrating that the hypoglycemic effect of exogenously administered insulin was attenuated in rats following bilateral ureterotomy as compared to sham-operated rats. Unilateral ureterotomy did not lead to the same metabolic response, and the difference in serum glucose levels between sham-operated and bilaterally ureterotomized rats disappeared when a 5% glucose solution was substituted for tap water as the rat's drinking water. It is suggested that the coexistence of fasting and metabolic acidosis led to increased renal gluconeogenesis in rats subjected to bilateral ureterotomy, and the combination of increased renal glucose production and insulin resistance was responsible for the development of fasting hyperglycemia.