Physical activity for health, weight loss, and diabetes prevention and treatment: What should be recommended?
The health benefits of physical activity and exercise are well documented, but the reality is that most people are not active enough. Perhaps the greatest challenge for health professionals is persuading people to be more active, but to do that with confidence, it is vital that the evidence base supporting its benefits is fully understood. The definitions of some key terms are provided in Box 1 below.
|Box 1. Definitions|
Physical activity for health
There is extensive epidemiological and interventional evidence that people who are more physically active are fitter and less likely to develop chronic conditions such as diabetes than those who are less physically active. Current guidelines recommend 150 minutes of moderate-to-vigorous activity each week. However, less than 60% of adults in the UK report meeting this target, with women being less likely to do so than men and the proportion falling with increasing age (Fig. 1). People with diabetes are even less active . Sedentary time is also an independent risk factor for obesity and diabetes .
Figure 1. Adult physical activity levels by age.
Physical activity for weight loss
There is good evidence that increasing physical activity should be part of a multicomponent weight management program for people who are overweight or obese, but it is very difficult to lose weight only by increasing physical activity (without also reducing energy intake). This is clearly shown in the following example (which is deliberately extreme to help emphasize the point). A fit 30-year-old man running a marathon in 3 hours will consume about 250 g of pure lipid; thus, to lose 1 kg of fat per month means running a marathon every week (assuming nothing else changes, particularly food intake, which would only need to increase by 200 kcal day to negate the effect). Most people who are obese are unable to undertake anything like this amount of activity . Clinical trials where an increase in supervised exercise alone, at levels consistent with recommendations (without an attempt to change food intake), is used to support weight loss are consistent with this, showing about 2 kg weight loss over 12–18 months compared with 7 kg with combined diet and exercise programs . Once weight has been lost (by reducing energy intake), the amount of physical activity that is optimal to help weight maintenance is nearly double the recommendation for people of a healthy weight (240–300 minutes per week), due to the reduced rate of metabolism that occurs in response to weight loss. Thus, increased physical activity makes a very small contribution to weight loss, but can support weight maintenance.
Physical activity for diabetes prevention
All of the major diabetes prevention trials, such as the Finnish Diabetes Prevention Study  and the US Diabetes Prevention Program , included physical activity as part of the intervention, although the contribution of the physical activity component on the successful outcomes of these trials was uncertain. In the Da Qing diabetes prevention study , conducted in China, the effect of the exercise intervention alone was similar to diet alone. Both observational and controlled studies show that physical activity can help reduce waist circumference, increase insulin sensitivity, and reduce glucose levels in people with prediabetes, and show that it is the overall amount, rather than intensity of physical activity, that is most important. Thus, although physical activity is relatively ineffective for weight loss, it can be very effective as part of a lifestyle intervention aimed at preventing diabetes in those at high risk.
Physical activity in people with type 2 diabetes
Once type 2 diabetes has developed, people are less active and have lower physical fitness. This, together with an increase in sedentary time, contributes to the greater risk for cardiovascular disease in people with type 2 diabetes . The reasons for this are not fully understood, but include genetic factors, presence of other illnesses, and the metabolic effects of hyperglycemia. There are relatively few intervention studies looking at the effects of increasing physical activity and reducing sedentary time in people with type 2 diabetes, although this was a component of the Look AHEAD study, which focused on effects of weight loss . The recently reported 4-month data from the Italian Diabetes and Exercise Study (IDES_2) suggest that it is possible to increase both light and moderate to vigorous physical activity in people with diabetes and reduce sedentary time with an intensive behavioral intervention. Although the overall effect of the intervention was modest (an HbA1c reduction of 0.35%), with no significant change in cardiovascular risk factors, those individuals in the highest tertile of changes in light physical activity (an increase of about one hour per day), moderate-to-vigorous physical activity of 7.33 minutes per day and those who reduced sedentary time by over an hour per day achieved an HbA1c reduction of about 1% (9 mmol/mol), together with improvements in cardiovascular risk factors . The ongoing follow up of this study will inform us whether this is sustained in the long term.
Summary and conclusions
Physical activity has significant health benefits, but most people are not active enough. Those at risk for diabetes, and those with diabetes, have lower activity levels than the general population. Interventions to increase physical activity are beneficial to health and can contribute to diabetes prevention and diabetes treatment, although they are unlikely to result in significant weight loss.
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