Aims of nutritional therapy
The aim of nutritional therapy for people with diabetes mellitus is to reduce the impact of the condition in terms of both its acute and chronic complications. These may differ according to the type of diabetes. The primary aim for individuals with type 1 diabetes (T1DM) is to match insulin doses with dietary carbohydrates (or vice viscera). So, nutritional interventions focus on identifying and estimating carbohydrate contents of food. The goal of treatment in T1DM is to optimise glycaemic control and minimise the risk and effects of hypoglycaemic episodes. Whereas, in type 2 diabetes (T2DM), with its increased risk of cardiovascular disease and with around 90% of cases being associated with excess bodyweight, have these as its aims for nutritional therapy. The goals of treatment both in terms of reducing the development of T2DM in high-risk individuals and its management can be aided by a 5-10% reduction in body weight. Cardiovascular risk is managed by reducing intake of fats (especially saturated fats, mainly from animal products) along with increased intakes of fruit and vegetables in the form of a Mediterranean type diet.
The history of the nutritional management of diabetes initially focused upon carbohydrate control, mainly as a treatment for T1DM. This was based on the assumption that blood sugar (glucose) are derived from mainly dietary sugars and to possibly a lesser extent other carbohydrates (starches). This resulted in the development of exchange-based systems and the low sugar diet. This has been the mainstay of the nutritional management of diabetes. The work of Jenkins et al. (1981), and later Foster-Powell et al. (2002), has challenged this perspective, demonstrating that the postprandial rise in blood glucose can be more rapid from dietary starches than sugars. Thus, leading to a more relaxed dietary approach to the management of diabetes. The recent changes in nutritional management approaches have largely came from a move towards evidence-based practice and the introduction of patient education over the last decade.
This has culminated in the Diabetes UK Nutritional guidelines of 2011 (Dyson et al., 2011). This report clearly states that education is a key element, as people with diabetes need to be empowered, so that they are fully enabled to implement changes to their lifestyle, including diet. Healthcare professions can easily overlook this aspect, but these are critical, as these lifestyle changes need to be maintained for life. Unfortunately, for many although initially effective, these lifestyle changes can be difficult to maintain for more than a few months. It is felt that this perhaps represents the deficiency in the quality of the delivery of these messages. It is also reflective that dietary changes, although they can significantly improve health status and the risks associated with diabetes, they can reduce quality of life, and therefore are not maintained. This has been identified that one of the biggest negative impacts upon quality of life is the reduced control or ability to make free food choices.
As the majority of people with diabetes have T2DM, the remainder of this review will focus on this form of the condition. Approximately 85% of people with diabetes have T2DM, and 80-90% of this is associated with overweight and obesity. This leads to the logical conclusion that T2DM can be best managed and even prevented by weight management. The key strategies for weight management are reduced energy (calorie) intake and/ or increased energy expenditure (physical activity). Consensus opinion, historically has suggested that this is best achieved by following a low fat diet. This has been somewhat challenged by more recent evidence, that now suggests any safe and clinically appropriate approach to weight loss can be considered. This includes, bariatric surgery, weight loss drugs (although currently only orlistat is licenced in the Europe) and other nutritionally adequate approaches (possibly including low carbohydrate diets, very low energy diets and meal replacements) as well as healthy eating based on the Eat Well Plate (or other national healthy eating guidelines).
Fig 1. Eat Well Plate, Reproduced with acknowledgment of Dept. of Health
Healthy eating, is still the most used basis for dietary advice, however, it is often most successful in achieving weight loss when combined with an energy restriction. The simplest way of achieving this reduction is by the reduction of food portion sizes. An example of a simple guide to portion size estimation is the Zimbabwe Hand Jive of the Canadian Diabetic Association. This has the advantage that it can easily be communicated to individuals even with the lowest levels of literacy.
Fig 2: Zimbabwe Hand Jive, which can be used to help practically show individuals ways to construct meals to help plan meals. Canadian Diabetes Association (2005).
In addition to weight management, cardiovascular risk management is the other key goal of nutritional management. The evidence basis for this is partially limited to the dietary approaches used within the published clinical trials or the factors tested in prospective cohort studies. This is very different to the type of data and assumptions derived from pharmaceutical trials, where only the drug is the factor, which changes in the study. In nutritional studies, it is not possible to change one nutrient in isolation, equally issues including control and blinding are more difficult in food-based studies.
Data derived from the diabetes prevention studies; in high risk groups with impaired glucose tolerance suggest that an energy restriction and weight loss reduce cardiovascular risk. Additionally a Mediterranean diet, which is low in animal (saturated) fats with moderate amounts of monounsaturated (fatty acids) fats along with larger quantities of fruits and vegetables is known to reduce cardiovascular risk. Evidence from studies such as DASH (Dietary Approaches to Stop Hypertension) and Portfolio (an approach which includes nuts, soya, soluble fibre and fruit and vegetables) diets can significantly reduce blood pressure and cholesterol significantly to the same degree as a number of pharmaceutical agents. However this can prove to be a challenge to stick to these types of dietary approach, due to palatability and difficulty to follow. Additionally, sterol esters as found in a number of cholesterol lowering margarines and yoghurt drinks have been shown to significantly reduce cholesterol by about 14%. This needs to be considered in view of the randomised controlled trials being sponsored by the manufacturers, other dietary approaches aimed at reducing cholesterol levels in diabetes have not been investigated in the same way.
The role of low carbohydrate diets in T2DM is controversial, although physiologically there is some logic to this approach, there has been some debate whether this approach may have other effects, including increased risk of a number of cancers. It must be noted that increases in cardiovascular risk has not been observed, in many of the studies a reduction in markers of risk have been reported.
In summary, T1DM and T2DM (or at the very least insulin and non-insulin managed patients with diabetes) should be managed with different approaches. With T2DM, the focus needs to be placed on weight management and reducing cardiovascular risk, whereas for T1DM, carbohydrate counting and potentially dose adjustment of insulin is the nutritional approach with the most evidence for its efficacy.
^ Jenkins, D.J. et al. (1981) Glycemic index of foods: a physiological basis for carbohydrate exchange. Am. J. Clin. Nutr. 34(3), pp. 362-6.
^ Foster-Powell, K. et al. (2002) International table of glycemic index and glycemic load values: 2002. Am. J. Clin. Nutr. 76(1), 5-56.
^ Dyson, P.A. et al. (2011) Diabetes UK evidence-based nutrition guidelines for the prevention and management of diabetes. Diabetic Medicine, 28, 1282–1288.
^ Wadden, T. A. et al., (2009) One-year weight losses in the Look AHEAD study: factors associated with success. Obesity, 17(4), pp.713-22.
^ Department of Health (2011) Eat Well Plate. Accessed online [14th Dec 2011] http://www.nhs.uk/Livewell/Goodfood/Documents/Eatwellplate.pdf
^ Canadian Diabetes Association (2005) Zimbabwe Hand Jive – Plan your portions. Accessed online. [14th December 2011]. http://www.diabetes.ca/Files/plan%20your%20portions.pdf