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From the Clinical Research Centers |
Department of Medicine, Stanford University School of Medicine, Stanford, California 94305
Address correspondence and requests for reprints to: G. M. Reaven, M.D., Shaman Pharmaceuticals, Inc., 213 East Grand Avenue, South San Francisco, California 94080.
| Abstract |
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| Introduction |
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Although the existence of the correlations noted above has received general acceptance, a diametrically opposed formulation of the causal nature of the relationships has emerged. Specifically, the results of studies, both in vitro (4, 5) and in vivo (6, 7, 8, 9) have demonstrated that insulin acutely inhibits hepatic VLDL-TG secretion. Based on these findings, it has been suggested that the physiological effect of insulin is to inhibit, not enhance, hepatic VLDL-TG secretion. As a consequence, it is argued that hypertriglyceridemia occurs in association with insulin resistance due to the loss of insulins ability to inhibit VLDL-TG secretion in resistant individuals.
The present study was initiated to evaluate these two disparate views of the role of hyperinsulinemia in regulation of plasma TG concentrations. To accomplish this goal, we took advantage of the ability of carbohydrate (CHO)-enriched diets to increase both plasma TG and insulin concentrations (10, 11). Specifically, we compared fasting and day-long plasma glucose, insulin, free fatty acid (FFA), and TG concentrations in 12 healthy volunteers, consuming two different dietsone relatively high and the other relatively low in CHO. By determining the relevant hormone and substrate concentrations on each diet, and relating them to the dietary-induced changes in plasma TG concentration, we endeavored to gain insight into which of the two dichotomous versions of the causal relationship between hyperinsulinemia and hypertriglyceridemia was most consistent with the experimental results.
| Subjects and Methods |
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Subjects were randomized to one of two, 14-day, eucaloric diet phases, varying in composition of CHO and fat. The macronutrient composition of one diet was 40% CHO, 15% protein, and 45% fat; the alternate diet composition was 60% CHO, 15% protein, and 25% fat. The CHO in both diets consisted of starches and sugars, mainly from fruits and vegetables. The glycemic index and the ratio of complex to simple carbohydrates were similar on the two diets. Similarly, saturated fat and the ratio of polyunsaturated fat to monoundaturated fat was 1.0 in both diets. Dietary fiber intake was also quite similar, 13.5 g/1000 kcals in the high CHO diet and 10.5 g/1000 kcals in the low CHO diet. Finally, the Harris-Bendict equation (13) was used to estimate each volunteers basal energy expenditure, and an activity factor was added to estimate total caloric requirement (basal energy expenditure x 1.31.5).
The experimental diets consisted of three rotating menus prepared in the Stanford GCRC Research Kitchen. Subjects were scheduled to visit the GCRC three times per week during the diet intervention phases. At each visit subjects met with the dietitian to discuss compliance, verify body weight maintenance, eat the noon meal, and pick up their research meals. On the 15th day of each dietary phase, subjects were admitted to the GCRC for metabolic testing. Subjects were studied for an 8-h period, during which test meals, with the same proportion of CHO, protein, and fat as the study diet, were given at 0800 and 1200 h, with breakfast comprising 20% and lunch 40% of the estimated daily caloric requirement. Blood was drawn fasting, and then hourly, beginning 1 h after the first study meal, for measurement of plasma, glucose (14), insulin (15), FFA (16), and TG (17) concentrations.
Subjects then entered a 2-week washout phase, followed by randomization to the other diet. After 2 weeks on the second diet, subjects were studied in the GCRC as described above. The subjects weighed 74.6 ± 3.9 kg and 74.2 ± 3.8 kg at the beginning of the 60% and 40% CHO diets, respectively, and weighed 0.0 ± 0.3 kg less at the end of both dietary periods.
There were no statistical differences in any metabolic measurement as a function of either gender or the order in which the two diets were consumed. Thus, the data were combined for analysis of the statistical significance of differences in the metabolic responses to the two diets. Data are expressed as the mean ± SEM. Metabolic responses during the 8-h study period following each of the two dietary phases were compared by two-way ANOVA, in which the dependent variables (metabolic responses) were analyzed with respect to diet (high vs. low CHO) and time (hour during 8-h test period). In addition, the total integrated responses during the 8-h period of observation were compared by Students t test, using log-transformed data because the metabolic variables were not normally distributed.
| Results |
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| Discussion |
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These data strongly support the view that although exogenous hyperinsulinemia may acutely inhibit hepatic TG secretion (4, 5, 6, 7, 8, 9), these observations have little, or no, relevance to the metabolic impact of endogenous hyperinsulinemia resulting from the pancreatic ß-cell response to the 60% CHO diet. To put it most simply, there is no reason to believe that the metabolic effects of an acute increase in exogenous insulin will be similar to the chronic effects of a compensatory elevation of endogenous insulin secretion.
Perhaps the most persuasive evidence of the profound differences on TG
metabolism of acute vs. chronic hyperinsulinemia can be
found in the recent publication by Aarsland et al. (22). In
this study, hyperinsulinemia was induced for 4 days by feeding
volunteers hypercaloric, high CHO diets. Plasma insulin concentrations
increased to
60 µU/mL after 1 day and remained elevated for the
remainder of the study. Both VLDL-TG secretion and plasma TG
concentration did not change statistically after 1 day of the dietary
intervention. However, by day 4, both VLDL-TG secretion and plasma TG
concentration had increased
5-fold. On the basis of their
observations, the authors concluded that the increases in VLDL-TG
secretion and plasma TG concentration seen in association with chronic,
endogenous hyperinsulinemia were due to an increase in the use of FFA
by the liver for TG synthesis.
In this study, we have elevated the endogenous plasma insulin concentration by increasing the CHO intake of a group of healthy volunteers. The use of this protocol permitted us to provide experimental evidence that the increase in plasma TG concentration produced by this dietary manipulation was inconsistent with the view that the primary effect of insulin is to inhibit hepatic TG secretion. However, this should not be construed to mean that high CHO diets inevitably lead to hyperinsulinemia and hypertriglyceridemia. For example, if weight loss in overweight individuals occurs, with ad libitum high CHO diets, hypertriglyceridemia does not develop (23). Furthermore, the choice of the type of CHO consumed can modulate the associated increase in plasma TG concentration, as does a concomitant increase in physical activity (24, 25). Furthermore, the conclusion that CHO-induced hypertriglyceridemia does not result from the failure of insulin to suppress hepatic VLDL-TG secretion (i.e. hepatic insulin resistance) is independent of the conflicting results as to whether or not CHO-induced hypertriglyceridemia is due to an increase in hepatic VLDL-TG secretion (2, 3, 22) or a decrease in hepatic VLDL-TG removal from plasma (26). This latter controversy results from differences in the isotopic method used in the various studies, and our results provide no useful information as to which of these disparate views are most correct. On the other hand, our results do seem to make highly unlikely the view that hypertriglyceridemia is due to a failure of insulin to inhibit hepatic VLDL-TG secretion in insulin-resistant individuals, and that was the goal of our study. We did not plan the experimental protocol to evaluate the potential clinical use of diets varying in fat and CHO content.
In conclusion, observations (4, 5, 6, 7, 8, 9) that the addition of exogenous insulin to liver cells in culture, or the injection of exogenous insulin into human beings, acutely inhibit VLDL-TG secretion does not mean that the physiological role of circulating insulin is to inhibit hepatic VLDL-TG secretion or that the causal relationships between hyperinsulinemia and hypertriglyceridemia are due to a failure of insulin to inhibit hepatic VLDL-TG secretion.
| Footnotes |
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Received December 30, 1999.
Revised May 25, 2000.
Accepted June 15, 2000.
| References |
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