Title: Diabetes - Dietary treatment

Key words: Glycaemic control, basal metabolic rate, physical activity level, dietary fat, protein, carbohydrate, polyunsaturated and mono-unsaturated fats, compliance

Date: April 1999

Category: 13. Specific Conditions

Type: Article

Author: Dr van Rhijn

Diabetes Mellitus

A multi-disciplinary team approach is required for proper management of diabetes mellitus, including education¹, self-management skills training and advice regarding exercise and diet. Dietary treatment is notoriously difficult, due to major differences in insulin resistance, body size and insulin secretion. Good glycaemic control is unlikely to be achieved when diet is neglected. This paper will focus on the nutritional aspects of the treatment of diabetes mellitus.

The therapeutic goals for diabetes are to achieve near-normal blood glucose levels, carbohydrate, lipid and protein metabolism and to avoid complications, such as hypoglycaemia, hyperglycaemia, ketoacidosis and long term complications ^{2, 3} (microvascular and macrovascular).

The diet should provide appropriate energy, protein and other nutrients to meet established standards for normal growth and development. The current guidelines are not to have a single dietary regime but to tailor the diet according to the individual circumstances^{4, 5} depending on specific abnormalities and treatment goals.

Patient education and empowerment, ongoing nutritional self-management^{6, 7}, appropriate self-monitoring of blood glucose, individualised and prudent glycaemic goals and ongoing professional support are fundamental⁸. Identification of obstacles to dietary adherence, such as: negative emotions, resisting temptation, eating out, feeling deprived, time pressure, tempted to relapse, planning, competing priorities, social events, family support, food refusal, and friends' support⁹, should be identified and assessed.

With regard to dietary advice, the type of diabetes is relatively less important than any substantial difference in the timing (rather than the content) of the meals. Post-prandial hyperglycaemia, which has been shown to vary widely, is a major concern in the management of diabetes.

To reduce daily glucose fluctuations, the current trend is to advise smaller, more frequent meals through the day, rather than the standard 3 large meals. Increasing meal frequency reduces the plasma concentrations of total and low density lipoprotein cholesterol and phospholipids, with a reduced demand for insulin in Type II diabetes¹⁰. The British Diabetic Association recommends the use of more than one diet composition¹¹.

Estimating the average daily energy needs is crucial. This is based on estimating the basal metabolic rate (BMR), relating to gender and body weight. This is calculated from tables¹² depending on the age of the individual and expressed in MJ/day or kJ/hour, and multiplied by their physical activity level (PAL). Although this is only a preliminary guide, it is more accurate than estimations based on 24-hour recall. It requires motivation and discipline to restrict calorie intake and to spread meals out throughout the day, ensuring they consist of foods that are slowly digested and absorbed.

Carbohydrate (CHD) content of the diet should provide 40 - 50% of the total calories. Higher amounts fail to achieve glycaemic control and promote hypertriglyceridaemia, dyslipidaemia and insulin resistance syndrome. CHDs are absorbed at very different speeds and so can cause enormous fluctuations in blood glucose in response to a meal. This is expressed as glycaemic index ¹³. The smaller the response the more favourable the food is considered to be. For example, cereals, pulses, vegetables and fruit contain pectins and hemicelluloses in the non-starch polysaccharides (NPS) fractions. These restrict hydrolysis of their starches, thus retarding the diffusion and absorption of glucose ¹⁴. Glucose is absorbed more rapidly than galactose, sucrose or fructose and is important to consider this when organising meals.

Disposal of carbohydrate is under immediate metabolic control, provided insulin is available. Numerous studies have shown that sucrose has no adverse effects on glycaemia¹⁵, and that fructose has no adverse effects on serum total cholesterol or triacylglycerol. Studies have shown that a high carbohydrate diet, avoiding excess of quickly absorbed forms (low glycaemic index)¹⁶, achieved at least as good glycaemic control in diabetics as did the traditional "low carbohydrate" regime¹⁷.

Recommended intake for protein is set at 10 - 20% of total energy requirements, sufficient enough to ensure normal growth, development and maintenance of body functions. The intake should be restricted to 0.8 g/kg body weight in renal failure, and increased in individuals during pregnancy, childhood or with very high physical activity.

The total fats should not contribute more than 30 - 40% of the total calorie intake, with no more than 10% from saturated fat¹⁸, and a ratio of polyunsaturated to monounsaturated fatty acids (PUFA:MUFA) of around 2:3. Saturated dietary fat is absorbed by a different mechanism. It is transported from the intestine via the lymph rather than the portal stream and is readily deposited in adipose tissue by a pathway involving the non-insulin sensitive enzyme, lipoprotein lipase, thus bypassing the fine insulin control mechanism. This readily leads to dyslipidaemia and promotion of cardiovascular disease (atherosclerosis and thrombosis) to which diabetics are so prone. PUFA's and especially MUFA's are beneficial and reduce plasma triglycerides¹⁹, increase high density lipoprotein (HDL) cholesterol and improve glycaemic control²⁰. They may, however, aggravate hyperglycaemia and increase low-density lipoproteins cholesterol (LDL-C) if taken in excess (4-6 g/day). This is a major risk factor in coronary heart disease (CHD), whereas high-density lipoprotein cholesterol (HDL-C) is protective.

MUFA's, however, enhance insulin sensitivity and also reduce the susceptibility of LDL to lipid peroxidation ²¹, reduce LDL uptake by macrophages and may have beneficial effects on platelet and factor VII activity. It is advised that dietary cholesterol should not exceed 300 mg/day. Frequent monitoring of glycaemic control and plasma lipids is necessary to evaluate and adjust the treatment plan. Partial replacement of complex digestible carbohydrates with monounsaturated fatty acids (avocado as one of its main sources) in the diet of patients with non-insulin-dependent diabetes mellitus improves the lipid profile favorably, maintains an adequate glycemic control, and offers a good management alternative²².

Vegetables & fruit provide NSP and antioxidants. Only soluble NSPs are effective in improving glycaemic control²³. An intake of 20g/day of soluble fibre (oat bran/gums/legumes) will produce about 10% improvement in plasma glucose²⁴, glycosylated haemoglobin, LDL cholesterol and triacylglycerol²⁵. This is due to delayed absorption of glucose, preventing hypoglycaemia.

Obesity is a major cause of reduced insulin sensitivity and should be addressed if proper glucose control is to be achieved. Physical activity and weight loss have been shown to lower plasma glucose, fasting and post prandial insulin concentrations, triglycerides, very low-density lipoproteins (VLDL) and improve insulin sensitivity in a high proportion of individuals with Type II diabetes. More aggressive weight reduction programmes have been proposed²⁶ if conventional measures fail.

Alcohol (ethanol) is a potent inhibitor of gluconeogenesis and may be responsible for hypoglycaemia after 2-3 hours. Moderate intakes with food, however, will not cause problems with glycaemic²⁷ control nor lipid metabolism and may even reduce cardiovascular mortality and morbidity. High-intensity sweeteners (aspartane/saccharin) are currently preferred to natural sweeteners as they do not provide energy nor increase blood glucose concentrations.

Diet is the cornerstone of diabetes management but failure to follow dietary advice is common. Only about 66% of diabetics manage to eat within 30% of the intended consumption. Compliance in adhering to measures known to improve diabetic control i.e. energy restriction, weight reduction and physical activity is better if tailored to the patient and reinforced by self-monitoring. Sodium retention and appropriate adjustment in the diet to treat hyperlipidaemia are essential to reduce the risk of long-term complications of diabetes mellitus.