DietDoc takes a look at the latest research.
In the past two weeks, we have been considering some of the challenges that people with type 1 or 2 diabetes face on a daily basis. I was, therefore, most interested to attend a lecture on “Nutrition in the Prevention and Treatment of Diabetes: Science and Mythology”, arranged by the Academy of Science of South Africa (ASSAf) a few weeks ago. The lecture was presented by Jim Mann, Professor in Human Nutrition and Medicine, University of Otago, Dunedin, in New Zealand. Prof Mann, is one of the world’s leading experts in nutrition and its effects on diseases of lifestyle, including diabetes. Prof Mann was visiting South Africa as a guest of North-West University, who have bestowed an Honorary Doctorate on him for his contributions to nutrition.
In his introduction, Prof Mann briefly outlined the history of diabetes and its treatment. This disease was known as far back as 400 BC and is mentioned in ancient Chinese texts. In 1889, a Dr Schmée diagnosed type 1 diabetes for the first time and prescribed a low carbohydrate diet for his patients, an approach that persisted for the next 100 years. From what my readers report to me, many doctors and healthcare professionals still only say, “Cut out all sugars and carbohydrates” when they counsel their diabetic patients.
In the 1970's, a researcher called Hugh Trowell, for the first time suggested that diabetics could eat carbohydrates provided they were rich in dietary fibre. Dr Trowell based his recommendations on the observation that populations eating traditional diets which were rich in plant fibre and relatively high in carbohydrate, had a low incidence of diabetes compared to populations eating western diets containing an excess of highly processed low-fibre foods.
Subsequently a number of scientific studies have produced results showing that diabetics can use carbohydrates in their diet, provided such carbohydrates have a high dietary fibre content and are of the correct type (see below).
Prof Mann also described the Finnish Diabetes Prevention Study (DPS), which was conducted with 522 middle-aged, overweight subjects with impaired glucose tolerance. The study achieved significant results that can be applied in everyday life (Lindström et al, 2003) . The goals of the DPS were as follows:
- to decrease the body weight of the participants by 5% or more
- to get participants to do moderate physical activity of 30 or more minutes a day
- to reduce the subjects’ intakes of dietary fat and saturated fat significantly
- to increase their dietary fibre intake to 15 g or more per 1000 kcal (a man consuming 2000 kcal per day, would thus be expected to eat 30 g of fibre a day)
The subjects in the DPS were divided into two groups - an experimental group that received intensive lifestyle interventions (individual dietary counselling from a dietician, circuit-training and exercise advice) and a control group who received standard medical care (Lindström et al, 2003).
Prof Mann emphasised the success of this Finnish study, where those subjects who had achieved at least 3 of the above mentioned goals, did not develop type 2 diabetes even 15 years after the initial intervention. The two most important interventions were identified as weight loss and increasing dietary fibre intake.
Perhaps the most encouraging result of the DPS was the finding that type 2 diabetes is the most preventable of all the lifestyle diseases!
What type of carbs should diabetics eat?
According to Prof Mann, it is important for diabetics to eat the right kind of carbohydrate, such as legumes (cooked or canned dry beans, peas, lentils, soya) and pulses, chickpeas, low-GI (glycaemic index) bread, and low-GI vegetables.
In other words, the old idea that all so-called "complex" carbohydrates benefit diabetics by keeping their blood sugar and insulin levels low and steady, such as high-fibre carbohydrates (wholewheat bread, high-bran breakfast cereals, etc), is no longer valid. Prof Mann pointed out that the idea that complex carbs are "good" and simple carbs are "bad", has been disproved with the introduction of the glycaemic index (GI). So if you are diabetic, it is important to select carbohydrates with a low-GI-value, such as the ones listed above.
Other aspects of the dietary treatment of patients with diabetes or insulin resistance, are also being investigated. It has been suggested that the type of fat used in the diet (saturated, polyunsaturated or monounsaturated), can also have an important effect on insulin reactions.Researchers working in Australia, have investigated the effect of substituting carbohydrates with either monounsaturated fat or protein (Luscombe-Marsh et al, 2005). In this study, 57 overweight or obese subjects with insulin resistance were either given a low-fat, high-protein diet or a high-fat (mainly monounsaturated fat), low-protein diet for 12 weeks, to study the effects of the 2 diets on weight loss, blood fats, appetite regulation and energy output after each test meal.
The results showed that there was no significant difference between the amounts of weight the two groups lost - the low-fat, high-protein subjects lost 9.7 kg, while the high-monounsaturated fat, low-protein subjects managed to lose an average of 10.2 kg in the 12-week study period. However, the low-fat, high-protein diet did suppress appetite to a greater extent than the high-fat, low-protein diet.
The researchers concluded that the weight loss and improvements in insulin resistance and other risk factors (e.g. risk of heart disease due to increased blood fat levels), were similar on both diet treatments and that neither diet affected bone turnover or kidney function negatively (Luscombe-Marsh et al, 2005).
It may thus be a good idea for diabetics to use monounsaturated fats in their diets to replace other sources of fat. Avocados, nuts and olives; olive, canola, grapeseed, peanut, sesame, safflower and avocado oils, as well as foods made with these oils, are rich sources of monounsaturated fats.
Different population GI responses to foods
Prof Mann also cautioned that different populations may exhibit different blood sugar and insulin reactions to foods. A study which compared the glycaemic response of people of European and Chinese ethnicity, found that on average the GI of parboiled rice in the Chinese subjects was 20% higher than in the Caucasian group: an average GI of 72 was obtained with the Chinese subjects, compared to an average GI-value of 57 with the European subjects, which would make parboiled rice a high-GI food for the Chinese subjects, while it can still be regarded as a low-GI food for the subjects of European descent.
This indicates that it is important to determine the GI of foods in different populations, particularly staple foods that are used as the basis of a specific population’s diet (rice in China and other Eastern countries, maize meal in large parts of Africa, including South Africa).
It is good to know that scientific researchers are hard at work trying to improve the lives of patients with insulin resistance, and types 1 and 2 diabetes. The most encouraging news is that type 2 diabetes can to a large extent be prevented by losing weight, increasing the intake of dietary fibre from legumes, pulses and low-GI foods, and doing some physical exercise as often as possible.
- (Dr Ingrid van Heerden, DietDoc, May 2011)
(Lindström J et al (2003). The Finnish Diabetes Prevention Study (DPS): Lifestyle intervention and 3-year results on diet and physical activity. Diabetes Care, Vol 26(12); Luscombe-Marsh ND et al (2005). Carbohydrate-restricted diets high in either monounsaturated fat or protein are equally effective at promoting fat loss and improving blood lipids. Am J Clin Nutr, Vol 18(4):762-772; Mann, J (2011). Nutrition in the Prevention and Treatment of Diabetes: Science and Mythology. Lecture presented on 19 May 2011, at ASSAf in Pretoria.)