Many cross sectional and prospective studies have confirmed the association between obesity and type 2 diabetes. Most people with type 2 diabetes are overweight or obese: more than 85% of people with type 2 diabetes in southeast Scotland in 2005 had a body mass index (weight in kilograms divided by height in metres squared) of over 25. Recent evidence indicates that high waist circumference may be an even better indicator than body mass index (BMI) of increased risk of type 2 diabetes.
The risk of developing diabetes over a 14 year follow-up period (among nurses aged 30-55 years at baseline) in the nurses' health study was 49 times higher among women whose baseline BMI was >35 than among women whose baseline BMI was <22. Even a a slightly raised BMI (22.0-22.9) at follow-up was associated with an age adjusted relative risk of diabetes that was three times higher than that in women with a BMI of <22.0 at follow-up.
Similar findings have been reported for men from a United States cohort of 51 529 male health professionals aged 40-75 in 1986 who were followed until 1992. Those with a BMI of ≥35 had a relative risk of developing diabetes of 42 (95% confidence interval 22.0-80.6) compared with men with a BMI of <23.0 at age 21, after adjustment for age, smoking, and family history of diabetes. Moreover, earlier onset of type 2 diabetes is associated with a higher BMI, and increasing prevalence of overweight and obesity is the most important factor in the increasing number of younger people diagnosed with type 2 diabetes.
These data have been derived from mainly white populations, and ethnicity modifies the relation between BMI and risk of diabetes. In an Indian population the increasing risk of diabetes associated with increasing BMI starts at even lower BMI levels (15 to 20) than in most other ethnic groups (in whom increasing prevalence of diabetes is only observed at a BMI of >25). This difference is only partly explained by patterns of fat distribution in different ethnic groups; south Asian populations are more likely to have a greater total percentage of body fat mass and larger amount of abdominal fat (reflected by high waist circumference) than other ethnic groups at a given level of BMI. High waist circumference increases the risk of glucose intolerance and diabetes, independent of the risk reflected by high BMI.
Studies in China, the US, and Finland have shown that diabetes can be prevented or delayed in people at high risk of diabetes through a combination of change in diet and lifestyle and modest weight loss. In the Swedish obesity study 69% of people with diabetes at baseline who lost weight after gastric bypass surgery did not have diabetes two years after follow-up. The challenges of maintaining weight loss and improvements in health among people with type 2 diabetes are summarised in Cochrane reviews of non-pharmacological and pharmacological interventions (see earlier article in this series).