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Effect ofVariations in Amount and Kind of Dietary Fat and Carbohydrate in the Dietary Management ofType 2 Diabetes GERALD M. REAVEN Stanford University, Stanford, CA, USA
INTRODUCTION
Treatment of patients with Type 2 diabetes must focus on the prevention of the long-term vascular complications of this syndrome.
The results (1) of the United Kingdom Prospective Diabetes Study (UKPDS) have clearly shown that improved glycaemic control will decrease the development of micro- vascular disease in patients with Type 2 diabetes.
Therefore, consideration of the role of energetic macronutients of dietary fat and carbohydrate (CHO) in the management of patients with Type 2 diabetes must evaluate the impact of any recommendations in light of their effect on glycaemic control.
The results of the UKPDS were less encouraging concerning the ability of improved glycaemic control to reduce macrovascular disease (1).
Indeed, neither lowering plasma glucose concentration (1), nor blood pressure (2), decreased myocardial infarction to the same degree as reported for the microangiopathic endpoints.
The reason for this disparity between micro- vascular and macrovascular disease incidence in the UKPDS is not fully understood, but is at least partly due to the importance of abnormal lipoprotein metabolism in the genesis of coronary heart disease (CHD) in
Nutritional Management of Diabetes Mellitus. Edited by G. Frost, A. Dornhorst and R. Moses & 2003 John Wiley & Sons, Ltd. ISBN 0 471 49751 7
190 NUTRITIONAL MANAGEMENT OF DIABETES MELLITUS
patients with Type 2 diabetes.
Consequently, recommendations concerning the CHO content of diabetic diets must take into account how they might affect dyslipidaemia in patients with Type 2 diabetes.
Although Type 2 diabetes is defined by hyperglycaemia, an increase in ambient glucose concentration is not the only metabolic abnormality in patients with Type 2 diabetes.
In addition to diabetic dyslipidaemia, there is a cluster of abnormalities related to insulin resistance and circulating plasma insulin concentrations in patients with Type 2 diabetes (3, 5).
Since these changes may contribute to the increased prevalence of CHD, they must also be taken into account when dietary guidelines are proposed.
Finally, it is important to distinguish between the fat and CHO content of weight loss versus weight maintenance diets.
The metabolic impact of variations in macronutrient content will vary enormously in these two situations, and this important difference cannot be ignored.
In this chapter an attempt will be made to discuss how variations in relative amount and kind of CHO could affect both microvascular and mascrovascular outcome in patients with Type 2 diabetes, taking into account the considerations discussed above, and results of clinical studies of patients with Type 2 diabetes.
CALORIE-RESTRICTED DIETS
There is little doubt that weight loss in response to calorie-restricted diets will improve both glycaemic control and dyslipidaemia in patients with Type 2 diabetes (6, 11).
However, there are two questions concerning this issue that deserve some attention.
In the first place, do variations in the relative proportion of macronutrients have any effect on the ability of patients with Type 2 diabetes to lose weight?
Three relevant papers bearing on this issue have been published in the last few years, comparing weight loss in response to calorie-restricted diets, varying only in terms of relative proportions of CHO and fat.
The results showed that weight loss was identical when dietary CHO varied from 10% to 70% of daily calories, with proportionate changes in fat content (9, 11).
Since the longest of these studies only lasted for 12 weeks (8), it could be argued that differences in rate of weight loss might have emerged if the patients had been followed for longer.
On the other hand, there is no evidence that relatively large variations in the relative amounts of dietary CHO and fat present in energy-restricted diets have a discernible effect on the ability of patients with Type 2 diabetes to lose weight.
Variations in relative amounts of dietary CHO and fat did not prevent the improvement in glycaemic control associated with weight loss, but in two of the studies (9,10) the fall in plasma glucose concentration was significantly greater on a higher monounsaturated fat (MUF), lower CHO diet.
Plasma triglyceride
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(TG) and high-density lipoprotein (HDL) cholesterol concentrations fell with weight loss, irrespective of macronutrient content, but the decrement in TG concentration was greater, and the fall in HDL cholesterol attenuated, in response to calorie-restricted diets relatively high in MUFA and low in CHO.
Low-density lipoprotein (LDL) cholesterol concentration decreased when either MUFA or CHO replaced saturated fat (SF) in the diet, but the improvement in LDL cholesterol concentration did not take place if dietary intake of SF was not decreased.
Finally, improvement in all of these variables in response to a diet relatively high in MUFA and lower in CHO persisted several weeks after a period of weight maintenance with the test diets.
In summary, weight loss in overweight patients with Type 2 diabetes is of substantial clinical benefit, and is almost certainly the most powerful lifestyle modification to improve clinical outcome in this population.
Although variations in relative proportion of dietary fat and CHO in energy-restricted diets do not seem to affect the amount of weight loss, the metabolic benefit associated with weight loss was somewhat greater when the diet was relatively higher in MUFA and lower in CHO.
METABOLIC EFFECTS OF VARIATIONS IN THE RELATIVE AMOUNTS OF DIETARY CARBOHYDRATE AND FAT CONTENT IN ISOCALORIC DIETS
As emphasised in the introduction, dietary recommendations for patients with Type 2 diabetes must take into account the impact of variations in macronutrient content on both microvascular and macrovascular disease. In this section attention will be focused on the effect of changes in the relative amount of CHO and fat in weight maintenance diets, evaluating the impact of such variations on the metabolic abnormalities characteristic of patients with Type 2 diabetes.
INSULIN RESISTANCE
The ability of insulin to stimulate muscle glucose disposal is decreased in the vast majority of patients with Type 2 diabetes (12,13). Although weight loss will enhance insulin-mediated glucose disposal in patients with Type 2 diabetes (6), there appears to be no evidence in patients with Type 2 diabetes that the frequently recommended relatively low fat, high CHO diets have any beneficial effect on insulin-mediated glucose disposal (14, 16). Indeed, there is evidence from one study (16) that insulin resistance is accentuated in response to low fat, high CHO diets as compared to diets higher in MUFA and lower in CHO.
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PLASMA GLUCOSE AND INSULIN CONCENTRATIONS Several studies have been published describing the effect of reciprocal increases in CHO and decreases in fat intake on plasma glucose and insulin concentration in patients with Type 2 diabetes (15,17, 20).
Furthermore, the results have been remarkably similar, given the differences in the experimental protocols, and quite consistent with what would have been predicted in view of the pathophysiology of this syndrome.
If CHO intake is increased in patients with Type 2 diabetes, plasma glucose concentrations will tend to rise, stimulating the pancreas to secrete more insulin.
If patients with Type 2 diabetes retain significant B-cell reserve, more insulin will be secreted in this situation, attenuating any rise in plasma glucose concentrations at the expense of higher plasma insulin concentrations.
Conversely, the less able the patient is to secrete additional amounts of insulin in response to an increase in CHO intake, the greater will be the rise in plasma glucose concentration, with minimal increases in ambient insulin concentration.
Obviously, these are two extreme examples of an almost infinite series of possible combinations of the changes in plasma glucose and insulin concentrations that will result from increasing the relative proportion of CHO in the diet.
In fact, most published data show that both plasma glucose and insulin concentrations increase in response to diets relatively low in fat and high in CHO.
Perhaps the best example of this general conclusion is the publication of Parillo and colleagues (21) showing that postprandial plasma glucose concentrations did not increase significantly when diet-treated patients with Type 2 diabetes consumed relatively more CHO, presumably due to the fact that the low fat, high CHO diets were associated with higher postprandial insulin concentrations.
The situation was reversed in sulphonylurea-treated patients, with higher post- prandial glucose and unchanged insulin concentrations, in response to increases in dietary CHO intake.
Based upon the above, there seems to be substantial evidence that postprandial glucose and/or insulin concentrations will increase when dietary fat content is decreased and CHO intake increased.
The best one can hope for is that low fat, high CHO diets may not lead to decreased glycaemic control.
However, even this can only be accomplished at the expense of increases in plasma insulin concentrations.
The role of endogenous hyperinsulinaemia as a risk factor for CHD in patients with Type 2 diabetes is still unclear, but it may not be prudent to ignore the possibility that this, or abnormalities associated with it, may contribute to the accelerated atherogenesis that characterises these patients (22,23).
DYSLIPIDAEMIA Diabetic dyslipidaemia is characterised by high plasma TG and low HDL cholesterol concentrations (22). Although less commonly measured, LDL