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Dietary prevention and treatment of type 2 diabetes

Alyson Hill
Chief Dietitian
Diabetes and Endocrinology Department
Royal Hospitals Trust

Diabetes mellitus is a chronic disease caused by deficiency of insulin or by resistance of the body to the insulin produced. This results in increased concentrations of glucose in the blood, which in turn damages the body's systems, in particular the blood vessels and nerves.(2)
Glucose is the primary energy source for all cells and is provided by digestion of carbohydrates from the diet. Insulin is required to move glucose from the bloodstream into the body's tissues, where it is converted to energy.
People with diabetes are classified as type 1 or type 2.(3)

Type 1 diabetes
Type 1 diabetes, formerly known as insulin-dependent diabetes, is characterised by partial or total failure of insulin production by the beta-cells of the pancreas. This type of diabetes usually appears before the age of 40. Treatment with insulin is required for survival.

Type 2 diabetes
Type 2 diabetes, formerly known as ­noninsulin-dependent diabetes, occurs when there is insufficient or ineffective insulin (insulin resistance). It is much more common than type 1 and accounts for approximately 85% of whites and 95% of nonwhites with diabetes.(1)
Type 2 diabetes commonly occurs in middle age and later life, although it is increasingly being reported in obese children and adolescents.(4) Diet and lifestyle modifications are sufficient treatment for many patients; however, about 40% require oral hypoglycaemic agents and about 30% require insulin therapy to improve glycaemic control.(2)
Prevention of diabetes
There is unequivocal evidence that type 2 diabetes can be prevented or at least delayed by moderate weight loss (5-7% of body weight) and regular physical ­activity (30 minutes daily).(5,6)
The risk of developing type 2 diabetes increases by up to 10 times in people with a BMI over 30kg/m(2).(1) The UK has one of the fastest-growing rates of obesity in Europe - over the last 10 years rates have increased from 14% to 22% of adults.(7) Those with abdominal obesity (apple-shaped) are at a greater risk of developing type 2 diabetes than those with gluteal obesity (pear-shaped).(8) There is some evidence that eating at least three portions of wholegrain, high-fibre foods each day is beneficial in the prevention of diabetes.(9)
Risk factors for type 2 diabetes include:

  • Increasing age.
  • Family history.
  • BMI >25, particularly abdominal obesity.
  • History of gestational diabetes.
  • Certain ethnic groups are more prone - Afro-Caribbean, Chinese and South Asian populations.

Diet in diabetes
Nutritional advice and information is essential for the prevention of diabetes in those at risk of type 2 diabetes and for the effective management of both type 1 and type 2 diabetes. Dietary guidelines for the management of diabetes promote a healthy diet,(10,11) which should be individually adapted for age, lifestyle, usual eating patterns, culture and diabetes medication.

Weight management
Weight loss and stabilisation are major priorities for those who are overweight or obese.(10) Weight loss is known to reduce insulin resistance, improve glycosylated haemoglobin, dyslipidaemia and blood pressure.(11)
Modification of existing eating habits to reduce energy-dense, high-fat foods and increasing activity are recommended to achieve weight loss. Modest amounts of weight loss and weight maintenance should be the aim of those unable to achieve a BMI under 25kg/m(2),(12) as a weight loss of 11% will reduce total mortality by 25% and cardiovascular and diabetes mortality by 28%.(13)
Achieving long-term weight loss is difficult, and it is known that standard weight reduction diets used in ­isolation are often unsuccessful.(10) Evidence indicates that structured intensive-lifestyle programmes that emphasise reduced energy and fat intake, regular physical activity and regular participant contact can produce a long-term weight loss of 5-7%.(11)
Regular physical activity is of benefit to everyone with diabetes, assisting in weight loss and improving insulin sensitivity, lipid profiles and blood pressure.(8)
High-protein, low-carbohydrate diets used for weight loss are not recommended for people with diabetes. Evidence indicates that subjects on these diets had regained more weight at one year than on conventional diets,(14) and that the long-term effects of these diets are detrimental to glucose control or insulin sensitivity. There is also concern regarding adverse renal consequences.(15)

People with diabetes have a higher risk of cardiovascular disease(16) and should therefore be encouraged to make adjustments to the type and amount of fat consumed. Reducing the total and saturated fat content of the diet has been shown to have beneficial effects on cardiovascular disease risk factors,(17) and has been shown to improve glycaemic control.(18) Therefore diets should be low in total fat, especially saturated and trans-unsaturated fats.(11)
Dietary fat intake can be reduced by eating lower-fat foods. Foods known to be high in saturated and trans-unsaturated fats (for example, cheese, butter, biscuits and chocolate) should be limited. Foods containing monounsaturated fat (for example, olive oil, rapeseed and canola oil) and polyunsaturated fat (for example, sunflower and corn oil) should be used as a substitute; however, it must be remembered that all fats contribute to total energy.
Fish oils are known to be cardioprotective,(19,20) ­and therefore two to three servings of oily fish such as salmon, herring or mackerel should be encouraged each week.(11) The use of pharmacological doses of fish oils are not recommended,(10,11) as they have been found to have deleterious effects in people with diabetes by increasing total and LDL cholesterol and increasing fasting plasma glucose and HbA(1c).(21)

A relatively high proportion of dietary energy should be derived from carbohydrate (45-60%).(10) The total amount of carbohydrate in meals and snacks is more important in determining blood glucose levels than the source (starch or sugar) or type of carbohydrate (low or high glycaemic index [GI]).(11)
The type of carbohydrate (low or high GI) can influence postprandial blood glucose levels.(11) Carbohydrate-containing foods can be categorised based on glycaemic response using the glycaemic index.

Glycaemic index
Glycaemic index (GI) is a measure of the change in blood glucose following ingestion of different carbohydrate-containing foods. It is calculated by comparing the glycaemic effect of a food containing 50g of carbohydrate with 50g of a reference food (usually glucose). The GI provides a ranking of individual foods, which are given a GI number according to their effect on blood glucose levels. Foods with a low GI are digested and absorbed more slowly than foods with a high GI. It has been proposed that low-GI foods may promote satiety and reduce hunger.
Many factors affect the GI of a food, including its physical form, the presence of fat or protein, cooking method and food processing. This results in some foods that are not necessarily unhealthy, such as bread and potatoes, having high GI values, and many high-fat foods having low GI values, making the glycaemic index ­difficult to interpret practically.
GI does not take into account the amount of carbohydrate in a food portion, which is where glycaemic load (GL) can be useful. The GL of a particular food is the product of the GI of the food and the amount of carbohydrate in a serving (GI ¥ grams of carbohydrate). Evidence indicates that GL can predict the blood glucose response to different foods across a range of portion sizes.(22)
Although the use of low-GI foods may reduce postprandial glycaemia, there is not enough evidence of long-term benefit to recommend general use of low-GI diets in patients with type 1 or type 2 diabetes.(11) GI should be used to assist with healthy food choices, rather than basing diets solely on low GI.(23) Therefore dietary advice on the total amount of carbohydrate consumed at meals and snacks is thought to be more important than the GI of individual foods.(23)

Traditionally, sucrose (table sugar) has been restricted in the context of a healthy diet for people with diabetes. However, it is now known that dietary sucrose does not increase glycaemia more than isocaloric amounts of starch.(10)
Modest amounts of sucrose can therefore be included in a healthy diabetic diet as long as it is consumed within the total daily energy allowance. Current guidelines suggest that sucrose intake should not exceed 10% of the total energy.(12) For the average adult consuming 2,000kcal/day, this would translate into 50g of sucrose.
Some foods containing large amounts of sugar are also high in fat and therefore energy dense, for example chocolate, cakes and puddings. Therefore overweight and obese people may wish to reduce their consumption of such foods to reduce overall energy intake.

Diabetic products
Foods marketed specifically for people with diabetes are not recommended.(10) These foods are sweetened with sorbitol and fructose instead of sugar and are often high in fat and calories, may be expensive and may cause osmotic diarrhoea if eaten in large quantities. As total avoidance of sugar is now considered unnecessary, ordinary cakes and biscuits can be consumed as part of a healthy balanced diet without being detrimental to glycaemic control.

People with diabetes should always discuss with their doctor whether or not they should consume alcohol. For those who consume alcohol, the recommended safe limit is two units for women and three units for men each day, with at least one to two alcohol-free days each week.(10)
Alcohol suppresses hepatic gluconeogenesis and can lead to hypoglycaemia.(24) Therefore those at risk should consume alcohol with or after carbohydrate-containing foods and never replace food with alcohol.
There is no evidence to suggest that people with diabetes should drink low-carbohydrate beers, lagers and cider in preference to regular varieties. The former usually have a higher alcohol content that may increase the risk of hypoglycaemia.(10) All alcohol contains calories and should therefore be restricted by those who are trying to lose weight.

Reducing sodium (salt) intake may help lower blood pressure in people with diabetes,(25) which reduces the risk of long-term complications.(26) Salt intake should therefore be reduced by avoiding adding salt to foods unnecessarily and limiting salty foods and snacks.
Regular dietary education to facilitate those with diabetes to make appropriate food choices and lifestyle changes is necessary to achieve good glycaemic control and optimise long-term health. It is essential that all members of the multidisciplinary team provide consistent advice, which is individualised, considering lifestyle, age, culture, usual eating habits and medication, to encourage compliance.


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