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About 80% of people with type 2 diabetes are obese, and the risk of developing diabetes increases progressively as the BMI (weight (kg)/height (m)²) increases (Figure 7.1). A BMI >35 kg/m² increases the risk of type 2 diabetes developing over a 10‐year period by 80‐fold, as compared to those with a BMI <22 kg/m². Small increments in body weight translate into large increases in type 2 diabetes risk (Figure 7.10).

The latest data from the US NHANES survey confirms a 6–10‐fold higher lifetime risk of type 2 diabetes for 18‐year‐olds with a BMI >35 kg/m² compared to those with BMI <18.5 kg/m², with an associated 6–7 year reduction in overall life expectancy.

Obesity is still widely defined as a BMI >30 kg/m² although BMI is not an accurate reflection of fat mass or its distribution, particularly in Asian people. A simple waist circumference may be better. The pattern of obesity is also important in that central fat deposition is associated with greater insulin resistance and confers a much higher risk for developing diabetes compared to gluteofemoral deposition (Figure 7.11). In clinical practice, ‘central’ obesity can be assessed by measuring the ratio of waist:hip circumference, but it is unclear whether this has any advantage over a simple measurement of waist circumference alone.

Fat deposition at other sites, particularly in skeletal muscle, liver and in the pancreatic islets, may contribute to metabolic defects and insulin resistance. This ‘ectopic’ fat deposition leads to lipotoxicity, which in turn causes insulin resistance and (in islets) impairs insulin secretion.