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The Role of Carbohydrate in the Management of Diabetes JANETTE C. BRAND-MILLER AND SUSANNA H. A. HOLT University of Sydney, Sydney, Australia
INTRODUCTION
Diet is said to be the ‘cornerstone’ of management of diabetes, yet the recommended dietary guidelines remain controversial and relatively few patients succeed in being well controlled on diet alone (1).
This may imply that dietary treatment is not sufficient in itself or the dietary changes are too difficult to comply with or even that the wrong type of diet is being recommended.
Many experts argue against the current dietary recommenda- tions for diabetes, with both the quantity and quality of carbohydrate being at the centre of the controversy.
This chapter is designed to critically address the issues of how much and what type of carbohydrate should be recommended for people with diabetes.
It takes an evidence-based approach, giving greater weight to the results obtained from randomised controlled intervention studies.
Important questions addressed in this chapter include: .
What is the scientific basis for recommending high-carbohydrate diets? .
What are the potential adverse effects of high-carbohydrate diets? .
What is the scientific basis for recommending diets high in monounsatu- rated fat (MUFA)? .
What is the scientific basis for recommending low glycaemic index diets? .
What is the optimal diet for improving insulin sensitivity? .
What is the optimal diet for weight loss? .
What is the evidence for a restricted versus liberal intake of sucrose?
Nutritional Management of Diabetes Mellitus. Edited by G. Frost, A. Dornhorst and R. Moses & 2003 John Wiley & Sons, Ltd. ISBN 0 471 49751 7
170 NUTRITIONAL MANAGEMENT OF DIABETES MELLITUS
THE OBJECTIVES OF THE DIETARY MANAGEMENT OF DIABETES
The goals of dietary management are clear (beyond dispute) and they apply equally to both Type 1 and Type 2 diabetes.
They should achieve: .
Near normal blood glucose with minimal risk of hypoglycaemia; .
Reduced risk of microvascular and macrovascular complications (as assessed by a variety of direct and surrogate measures, including blood lipids, clotting factors, blood pressure); .
Weight loss in overweight patients; .
Normal growth and development in children; .
Healthy outcomes for mother and child in diabetic pregnancy.
Dietitians have the enormous challenge of not only achieving all of the above, but tailoring each person’s diet to suit their individual taste preferences and lifestyle.
GOOD GLYCAEMIC CONTROL IMPROVES PROGNOSIS
Good glycaemic control as indicated by near-normal HbA1c levels has been shown to reduce the risk of developing microvascular complications in both Type 1 and Type 2 diabetes (2,3).
In addition, there is increasing evidence that it also reduces the development and progression of macrovascular disease (1,4,5).
People with diabetes are two to four times more likely to die of coronary heart disease than people without diabetes, even when total cholesterol level and blood pressure are the same.
Thus, preventing the excess cardiovascular morbidity and mortality associated with diabetes is arguably the most pressing treatment goal.
However, this does not mean that normalising blood lipid or clotting factors is more important than normalising blood glucose.
Indeed, high blood glucose levels are now recognised to contribute directly to the pathogenesis of macrovascular disease in both diabetic and non-diabetic subjects (6,7).
Until recently, HbA1c levels were thought to be mainly influenced by fasting and pre-prandial blood glucose levels.
However, human beings spend much of their time in the post-prandial state and therapies which specifically reduce post-prandial glycaemia may be superior for improving overall glycaemic control and reducing the risk of complications (8,9).
The degree of post- prandial glycaemia appears to adversely alter vascular function and directly contribute to thickening of the intima wall (10).
For this reason, both the amount and type of carbohydrate are probably more important than presently recognised.
THE ROLE OF CARBOHYDRATE 171
HISTORICAL PERSPECTIVES ON CARBOHYDRATE
Before the discovery of insulin in 1922, diets prescribed for diabetes were very low in carbohydrate, around 5% of energy, and very high in fat, around 75% [reviewed by Truswell (11)].
Even after the advent of insulin, doctors were cautious and very low carbohydrate diets continued.
By 1930, diet prescriptions of carbohydrate had risen to 15% of energy.
In the 1940s and 1950s, carbohydrate allowances had come up to 25, 30% of energy and carbohydrate exchange lists came into use.
There was little questioning of the principle that carbohydrates were bad for people with diabetes and focus was on the insulin treatment.
By the 1970s pharmaceutical treatments had expanded with the introduction of oral hypoglycaemic drugs and the average carbohydrate intake rose to about 40% energy.
Prohibition of sucrose was now the main message.
With extreme caution, several experimental studies compared higher carbohydrate diets (450% energy) with the traditional diabetes diet and found improved glucose tolerance or insulin sensitivity (12, 14).
In the late 1970s, there was a revolution in thinking about diabetic diets and a spurt of experimental studies indicated that high-carbohydrate diets were no worse, if not better, for people with diabetes because they lowered blood cholesterol levels (see below).
By then, low-fat, high-carbohydrate diets were being recommended for the prevention and treatment of cardiovascular disease in the general population.
Since 1980, dietary recommendations for people with diabetes have unanimously emphasised reducing saturated fat intake.
However, if saturated fat intake is reduced, the energy has to be replaced by some other nutrient.
Because there are concerns about potential adverse effects of high-protein diets on renal and bone health, the choice is either more carbohydrate or more unsaturated fat.
And here lies the controversy.
Since carbohydrate is the main glycaemic element in the diet (being the main precursor of blood glucose), an increase in dietary carbohydrate might be expected to result in greater post- prandial glycaemia and compromise diabetes control.
An increase in fat, on the other hand, might promote weight gain and decrease insulin sensitivity.
CURRENT RECOMMENDATIONS FOR CARBOHYDRATE INTAKE
For the past 20 years, most diabetes associations around the world have recommended high-carbohydrate diets that are low in fat and high in fibre for people with diabetes (15,16). The British Diabetic Association’s recommenda- tions state that carbohydrate should provide 50, 55% of the total energy content of the diet while fat should contribute 30, 35% of energy intake, of which 510% should be saturated fat, 510% polyunsaturated fat (PUFA) and
172 NUTRITIONAL MANAGEMENT OF DIABETES MELLITUS
10, 15% monounsaturated fat (MUFA) (17).
However, there is concern in some quarters that 50, 55% of the total energy intake as carbohydrate may have adverse effects on blood triglyceride (TG), HDL-cholesterol and glucose levels compared with high-fat diets (435% total energy) enriched with MUFA (18,19).
During the 1990s, this issue has been the focus of much research.
On the basis of the resulting evidence, the American Diabetes Association’s guidelines now recommend that 60, 70% of energy be divided between carbohydrate and monounsaturated fat, depending on patient preference and the appropriate nutritional goals for their medical status (20).
WHAT IS THE SCIENTIFIC BASIS FOR RECOMMENDING HIGH CARBOHYDRATE INTAKE?
There is no doubt that the goal of increasing carbohydrate intake was actually to reduce fat consumption, especially saturated fat.
People with diabetes were no longer dying of diabetic ketoacidosis but coronary heart disease.
In fact, some experts suspected that the prescribed high-fat (and high saturated fat) diabetic diets might actually be partly responsible for the heightened risk of cardiovascular disease among people with diabetes.
Several well-designed intervention studies in diabetic subjects were undertaken and showed that high- carbohydrate diets (55, 70% energy) could result in lower blood cholesterol and TG levels with no deterioration in glycaemic control compared to traditional ‘diabetic’ diets containing less carbohydrate and more saturated fat (12, 14, 21, 25).
Indeed, much to their surprise, HbA1c, glucose tolerance and fasting glucose were often improved following treatment with a high-carbohydrate diet.
This implied that insulin sensitivity was improved on a higher carbohydrate intake as had been earlier demonstrated in non-diabetic subjects (26).
Thus, in the 1980s, diabetes associations in the United States, Canada, Australia and Britain independently agreed that there was sufficient evidence to advocate an increase in the carbohydrate content of the diabetic diet.
It is important to note that these early studies used high-carbohydrate diets that were heavily based on wholegrain cereals, vegetables and legumes that concomitantly contained very large amounts of fibre (upwards of 75 g per day).
This is more than three times that normally consumed and presented a very real challenge for the average person with diabetes.
In addition, unless the fibre was of the soluble, viscous or leguminous type, then post-prandial blood glucose peaked at higher levels on the high-carbohydrate diet (27).
Not surprisingly, some degree of weight loss was seen after subjects completed the high-fibre, high-carbohydrate dietary treatments, but not after the low- carbohydrate, high-fat diets.
We now know that energy restriction per se, even before significant weight loss is evident, improves all aspects of diabetes control (28).
A deficit of calories, rather than a high-carbohydrate intake, may
THE ROLE OF CARBOHYDRATE 173
well have explained much of the improved profile.
Even without weight loss, consistent benefits of high-carbohydrate diets have been reported only when the diets incorporate relatively unrefined, high-fibre foods (legumes, whole- grains, cruciferous vegetables, fruit) , resulting in meals that are somewhat different from those eaten by the general population.
Because very high fibre intakes are perceived as being unpalatable and hard to achieve, many health professionals took the view that simply increasing total carbohydrate intake was the main priority because this achieved the objective of lowering saturated fat.
The American Diabetes Association, for example, presently recommends a moderate intake of dietary fibre of 20, 35 g per day to help lower LDL-cholesterol and does not consider that dietary fibre offers significant benefits for glycaemic control (20).