The role of diet and exercise in Type 2 diabetes prevention
Pamela A. Dyson, BSc, SRD, Diabetes Specialist Dietitian.
Oxford Centre for Diabetes, Endocrinology and Metabolism, Churchill Hospital, OxfordThe number of people in the world diagnosed with diabetes is set to double from a current figure of about 150 million to 300 million by 2025, researchers have warned (King et al, 1998). These predictions are largely accounted for by an epidemic rise in Type 2 diabetes, and will have enormous resource implications for health services worldwide (Amos et al, 1997). In the UK alone 1.4 million people are diagnosed with diabetes and the charity Diabetes UK has highlighted a 'missing million' who remain undiagnosed.
Type 1 and Type 2 diabetes are now defined by the different metabolic processes of the disease and not, as formerly, by treatment. Type 1 diabetes, which accounts for approximately 10-20% of cases, was formerly known as either early-onset or juvenile diabetes or insulin-dependent diabetes (IDDM). Its onset is usually sudden and results in beta cell failure leading to total loss of insulin secretion. Treatment is by diet and insulin injections. This type of diabetes is more commonly seen in children and young adults, although it can occur at any age. There has been a suggestion that the early introduction of cow's milk into the diet may cause Type 1 diabetes in children (Gimeno and deSouza, 1997), although a more recent study does not support this hypothesis (Esfarjani et al, 2001).No real evidence has emerged as yet for the role of diet in preventing Type 1 diabetes, while two large studies are evaluating the role of insulin treatment and nicotinamide supplementation in high-risk individuals (Schatz, 2001).Type 2 diabetes was formerly known as maturity-onset diabetes or non-insulin dependent diabetes (NIDDM) and accounts for most cases (80-90%). Its onset is gradual and it is usually seen in middle-aged and older people; although, as obesity rises, Type 2 diabetes is more frequently being diagnosed in children and teenagers (ADA, 2000). Type 2 diabetes is caused by progressive loss of beta cell function and is commonly associated with insulin resistance. It is treated by diet alone or diet in combination with oral hypoglycaemic agents and/or insulin. Of the 1.4 million people with diabetes in the UK over one million have Type 2 diabetes. Evidence is growing to support the role of diet and lifestyle changes in the prevention of Type 2 diabetes.The importance of prevention
There is a strong argument for the economic benefits of diabetes prevention as the disease is costly to health services (Jonsson, 2002). Treatment of the complications accounts for the major costs. Diabetes accounts for 5-10% of total health-care resources in Europe, although it affects only 3% of the population.Diabetes is the fourth-leading cause of death in most developed countries and typically reduces life expectancy by 8-10 years. Cardiovascular disease is the major cause of death in people with Type 2 diabetes with a four- to five-fold increase in macrovascular disease. Diabetes also leads to long-term tissue damage and is a major cause of blindness, renal failure and amputation, all of which have an enormous impact on health and quality of life. Effective treatment can prevent some of these complications (UKPDS, 1998; DCCT, 1993) but cannot eliminate them entirely. Preventing diabetes would result in benefits for both the individual at risk and for society as a whole.The National Service Framework for Diabetes (DoH, 2002) stresses the importance of prevention of Type 2 diabetes by stating its first standard as: 'The NHS will develop, implement and monitor strategies to reduce the risk of developing Type 2 diabetes in the population as a whole and to reduce the inequalities in the risk of developing Type 2 diabetes.'What causes diabetes?
Effective prevention strategies can operate only if the causes of diabetes are identifiable. However, these still remain unclear, although genetic factors play a part. The presence of antibodies in Type 1 diabetes indicates autoimmune disease and the seasonal variation in incidence suggests a viral component. Type 2 diabetes is associated with obesity and low levels of physical activity and the predicted global increase in diabetes can be explained by the increased incidence of Type 2 diabetes, associated with the recent epidemic rise of obesity throughout the world (Costacou and Mayer-Davis, 2003).The risk factors for Type 2 diabetes are shown in Box 1 and can be sub-divided into lifestyle or modifiable factors - obesity, waist measurement and physical activity - and into factors that cannot be modified - age, family history, ethnic origin and gestational diabetes. Most published prevention studies for Type 2 diabetes have addressed the modifiable lifestyle factors.Lifestyle and prevention of Type 2 diabetes
A number of epidemiological studies have shown that people who maintain a normal body weight and have physically active lifestyles are less likely to develop Type 2 diabetes (Hu et al, 2001; van Dam et al, 2002a). There is now clear evidence that obesity and low levels of physical activity are the most important modifiable risk factors for Type 2 diabetes. Over the past six years, studies in China, Finland and the USA (Pan et al, 1997; Tuomilehto et al 2001; Knowler et al, 2002) have confirmed that lifestyle changes that reduce body weight and increase physical activity can delay progression to diabetes in high-risk individuals. In all these studies, people at high risk were defined as having impaired glucose tolerance (IGT), a well-recognised factor in the development of Type 2 diabetes. Box 2 shows the World Health Organization criteria for diagnosis of IGT and Type 2 diabetes.Prevention trials - diet and exercise
The rise in incidence of Type 2 diabetes is significantly associated with rises in obesity and it would be logical to conclude that reduction in body weight would reduce incidence of diabetes.Of the three major trials that have examined the effects of lifestyle changes on incidence of diabetes, only one in China - the Da Qing study (Pan et al, 1997) - has addressed the issue of the separate effects of diet and exercise or a combination of the two. This study followed 577 people with IGT randomised by clinic into four groups: diet; exercise; diet plus exercise; and a control group.The subjects were followed for six years and showed a significant reduction of progression to diabetes in all the intervention groups compared with the control group, although there was no significant difference between each intervention group. This study suggested that the separate effects of diet and exercise are of no greater importance than a combination approach. As a result, the two most recent studies assessed the effects of diet and exercise as a combination strategy.The Diabetes Prevention Programme (DPP) (Knowler et al, 2002) was a three-year study in the USA that evaluated the effect of diet and exercise and metformin in preventing progression to diabetes over three years in 3234 people with IGT. Those randomly allocated to the intensive lifestyle arm reduced their risk of diabetes by 58%, compared with a risk reduction of 31% in those allocated metformin. The most recent study, the Finnish Diabetes Prevention Study (DPS) (Tuomilehto et al, 2001), showed a similar risk reduction of 58% over 3.2 years in 552 subjects with IGT randomly allocated to an intensive lifestyle programme compared with a control group. Although these studies both show a significant reduction in progression to diabetes among these high-risk individuals, no data examine the effects of lifestyle change over a period greater than three years.Practical application of prevention trials
These major trials demonstrate that changes to diet and physical activity levels can have an impact on progression to Type 2 diabetes in high-risk individuals. Both the DPP and DPS offer practical advice on the extent of lifestyle change necessary to result in the significant risk reduction seen in both these trials and this is summarised in Box 3.Dietary components
Diet may have a significant role in the development of diabetes. During rationing in both world wars it was observed that both prevalence and mortality from Type 2 diabetes decreased.Two large US epidemiological studies have attempted to identify the components of dietary intake that may help prevent the onset of Type 2 diabetes. The most compelling data come from these studies, investigating a cohort of 85 000 female nurses and 40 000 male health professionals who were followed up over 16 and 12 years respectively.During this time they completed regular dietary questionnaires that were independently validated and were used to calculate subsequent risk of diabetes.The following dietary components were hypothesised as protective of Type 2 diabetes: carbohydrate foods of low glycaemic index (Willett et al, 2002), whole-grain foods (Liu et al, 2000; Fung et al, 2002), high-fibre cereal intake (Montonen et al, 2003), high intake of nuts and peanut butter (Jiang et al, 2002) and regular alcohol consumption (Conigrave et al, 2001).An epidemiological study carried out in Ely, UK, has also reported that high consumption of vegetables protects against Type 2 diabetes (Williams et al, 1999).The components of the diet that may be causative for Type 2 diabetes in men include high total fat and saturated fat intake (although the significance of this disappears after correction for BMI), processed meat intake (van Dam et al, 2002b) and high intakes of trans-fatty acids in women (Salmeron et al, 2001). Interestingly, no association is found between sugar intake and incidence of Type 2 diabetes (Janket et al, 2003).The challenge for nurses is to provide advice which is received enthusiastically by the patient. In practice, many people diagnosed with diabetes will receive dietary advice from the primary health-care team. Nurses, dietitians, doctors and other health-care professionals may all be involved and it is vital that everyone provides consistent advice.The responsibility for providing educational material and providing training for other health professionals should rest with the diabetes specialist dietitian. Combining motivation and sensible consistent advice on diet and lifestyle, with supervision and support, is an approach that is likely to succeed.Conclusion
It has become clear that the epidemic rise in Type 2 diabetes is related to relatively recent lifestyle changes and that the key to prevention is to develop strategies that will reduce the burden on both the individual and society. Recent large preventative trials have shown that moderate changes in lifestyle can substantially reduce the risk of developing Type 2 diabetes. With this strong evidence and the publication of the National Service Framework for Diabetes (DoH, 2002), it is hoped that strategies to reduce obesity and increase physical activity can be developed and directed at those of high risk with subsequent positive results on the incidence of Type 2 diabetes.
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