Factors modifying the relationship between obesity and diabetes

A large number of factors influence the relationship between obesity and diabetes and many of them are closely inter-related. That obesity on its own is not sufficient to cause diabetes is apparent from the observation that 20 per cent of patients with type 2 diabetes are not obese and even in the highest risk group with high BMI and high waist–hip ratio over 80 per cent will escape type 2 diabetes (Colditz et al., 1995).

Other factors include body fat distribution, duration of obesity, weight gain, age, physical activity, diet, the in utero environment, childhood stunting and genetic factors. Methodological issues are also important in examining the relationship between obesity and diabetes. Some of the observed increase in diabetes prevalence attributed to obesity could be related to more awareness and detection of type 2 diabetes, rather than a true increase in numbers (previous Diabetes UK estimates are that 50 per cent of patients do not know they have type 2 diabetes).

The change in the diagnostic criteria for diabetes introduced by the American Diabetes Association in 1997, with less use of the oral glucose tolerance test and more emphasis on the fasting glucose, appear to underestimate the prevalence of diabetes in obese subjects who may have a relatively normal fasting blood glucose in the presence of a high post-load glucose (Melchionda et al., 2002; Richard et al., 2002).

Abdominal obesity may be an even better predictor of the development of type 2 diabetes than BMI (Larsson et al., 1984; Ohlson et al., 1985). The predictive value of high waist–hip ratios and high waist circumferences in mediating the risk of diabetes appears to be of most importance in those in the highest quintile of these measures (Chan et al., 1994) and perhaps in leaner subjects.

For a given BMI many Asian populations have a much higher risk of type 2 diabetes, even at a BMI well within the normal range (Mather and Keen, 1985).

WHO is currently studying the use of a more limited range of normal BMI (18.5–22.9 kg m^-2) in these groups together with use of waist circumference (James et al., 2001). The duration of exposure to obesity is an important modulator of the risk of diabetes. In Pima Indians, subjects whose BMI was greater than 30 kg m^-2 for more than 10 years had over twice the risk of type 2 diabetes compared with those who had been obese for less than 5 years (Everhart et al., 1992). The epidemic of childhood obesity allied with the influence of obesity duration suggests both increasing frequency of diabetes and its earlier onset.

Weight gain during adult life acts in addition to BMI per se to modify the risk of diabetes. In the Health Professionals' Follow-up Study men who gained more than 13.5 kg over the five years of the study had a 4.5-fold increased risk of diabetes in comparison with those men who remained within 4.5 kg of their weight at entry to the study (Chan et al., 1994). Similar findings apply to women as described in the Nurses' Health Study where the relative risk of diabetes was 2.7 in those who gained 8–10.9 kg compared with those who were weight stable over a 14-year period (Colditz et al., 1995).