Obesity and T2DM
Obesity is one of the strongest risk factors for type 2 diabetes. The greater the body mass index (BMI) (defined by (weight in kg)/(height in metres squared)) the more likely people will have type 2 diabetes in the future, and it is not just the obese (those with BMI ≥ 30 kg/m²) who are at risk, but those in the lower overweight range also (BMI: 25-30 kg/m²).
Body mass index and the incidence of diabetes
Figure 1: The incidence of diabetes, 9 years after the measures of body mass index. The French D.E.S.I.R. study. The data in Figure 1 below comes from a French population (the D.E.S.I.R. cohort). The people did not have type 2 diabetes at inclusion. After 9 years of follow-up, we recorded the frequency of new cases of diabetes, according to their BMI at inclusion. The incidence appears to increase exponentially with increasing BMI. People who had been newly treated with drugs for diabetes, and those with a fasting plasma glucose of more than 7.0 mmol/l were defined to have type 2 diabetes.
Most of the men and the women in this French population were within the BMI range of 20-30 kg/m², but there were some people who became diabetic even though their BMI was under 25 kg/m² and one would suspect their diabetes was due to factors other than obesity and insulin resistance. Although the incidences increased exponentially with BMI, only 1% of the men and women had a BMI over 30 kg/m², and 13 men and 24 women a BMI over 35 kg/m².
Comparing men and women
Note that the risk of diabetes was much greater in men than in women for the same level of BMI. In Caucasian populations it is common that diabetes is more frequent in men than in women. In parallel, for a given age, the BMI of men is lower at the diagnosis of diabetes than it is in women, in data from a Scottish diabetes register. 
Adiposity measured by the waist circumference
Figure 2: The average BMI at the diagnosis of diabetes, by age, for men and women with BMI > 25 kg/m². The best way of measuring obesity or adiposity is still disputed – is the traditionally used BMI the best measure? Carrying weight preferentially around the abdominal area rather than around the hips, or as lean muscle is associated with metabolic disturbances, and the waist circumference is one way of quantifying abdominal fat. The risk of becoming diabetic increases with increasing waist circumference (Figure 2). However, comparing men and women with the same waist circumference, they had similar risks of diabetes up until a waist circumference of 100 cm.
Figure 3: The incidence of diabetes, 9 years after the measures of waist circumference. The French D.E.S.I.R. study.
The higher incidences correspond to higher waist circumferences; there were 14% of the men and 3% of the women with a waist circumference 100 cm or more, and 1.9% of the men and 0.6% of the women had a waist circumference of 110 cm or more – so there were few people in these high incidence classes in this French population.
Figure 4: The incidence of type 2 diabetes in European women, according to BMI (normal < 25 kg/m², overweight 25-30 kg/m² and obese ≥ 30 kg/m²) and waist circumference (red line < 80 cm, blue line 80-87.9 cm, black line w ≥ 88cm.) The InterAct Study. Even for people with a normal BMI (< 25 kg/m²), those with a larger waist circumference have a greater incidence of diabetes (Figure 4).
Diabetes Risk Scores and adiposity
Figure 5: The FINDRISC score, to evaluate the risk of having diabetes or being at risk for diabetes in the future. Many diabetes risk scores have been developed to incite people in the general population with a high risk score, to go to be screened by a blood test for diabetes; all of these scores include a measure of adiposity, usually BMI. A commonly used score that evaluates the risk of either having diabetes (prevalent diabetes) or being at risk for getting diabetes in the future (incident diabetes), is the Finnish FINDRISC score, and this score includes both the body mass index and waist circumference (Figure 5) to evaluate the risk.
A life-course approach to the relation between adiposity and diabetes
Another important factor is the timing of being overweight or obesity. Adults born small for gestational age have been shown to be at increased risk of metabolic diseases, including diabetes, when they are adult. However, the life course approach, following individuals over their life-time, can identify trajectories associated with diabetes later in life. In a French population of close to 90,000 women, followed over 15 years, indeed those with a low birth weight (<2.5kg) had a 40% higher risk of diabetes than those with a birth weight between 2.5 and 4.0 kg. The women also noted their body silhouette at various ages over their life-time using the silhouettes shown in Figure 6 below.
Figure 6: Silhouettes to describe adiposity. Women who were thinner (silhouettes 1 & 2) at age 8, at menarche and at 20-25 years had a higher risk of diabetes over the follow-up, in comparison to silhouette 3, whereas those who were not thin (silhouettes ≥4) were either at a lower risk or their risk did not differ from women with silhouette 3. However, at age 35-40 years, these relations
Figure 7: The risk of type 2 diabetes in women from the French E3N study, according to their silhouettes at age 8 years, at menarche, at 20-25 years and at 35-40 years. were reversed – thinner women were not at risk, whereas those with a larger silhouette (silhouettes ≥4) were at a higher risk than silhouette 3. The relations shown did take account of BMI, as reported every two to three years by the women during the follow-up. A cross sectional study did show a high correlation between measured BMI and the self-reported silhouettes in this study.
Prevention of Diabetes
The main target in programs to prevent diabetes is the reduction of weight – by changing eating habits and increasing physical activity – changing the energy balance. In the Finnish Diabetes Prevention Study, weight change was pronounced in the people who were actively treated using a lifestyle approach, and this was reflected in the numbers of new cases of diabetes in these two groups. The active intervention was only over the first four years, but the differences between the intervention and the control group for the occurrence of new diabetes persisted over 15 years.
Figure 8a - weight change in the lifestyle intervention group (full line) and in the control group (dashed line) in the Finnish Diabetes Prevention Study over the 4 years of the study
Figure 8b - the probability of remaining free of diabetes 
^ Balkau B et al. (2008) Predicting diabetes: clinical, biological, and genetic approaches: data from the Epidemiological Study on the Insulin Resistance Syndrome (DESIR). Diabetes Care 31:2056-2061
^ Logue J et al. Scottish Diabetes Research Network Epidemiology Group (2011) Do men develop type 2 diabetes at lower body mass indices than women? Diabetologia 54:3003-3036.
^ InterAct Consortium et al. (2012) Long-term risk of incident type 2 diabetes and measures of overall and regional obesity: the EPIC-InterAct case-cohort study. PLoS Med 9:e1001230
^ Lindström J, Tuomilehto J (2003) The Diabetes Risk Score: a practical tool to predict type 2 diabetes risk. Diabetes Care 26:725–731
^ de Lauzon-Guillain B et al. (2010) Birth weight, body silhouette over the life course, and incident diabetes in 91,453 middle-aged women from the French Etude Epidemiologique de Femmes de la Mutuelle Generale de l'Education Nationale (E3N) Cohort. Diabetes Care 33:298-303.
^ Sörensen TIA et al. (1983) The accuracy of reports of weight: children’s recall of their parents’ weights 15 years earlier. Int J Obesity 7:115–122.
^ Lindström J et al. Finnish Diabetes Prevention Study (DPS) (2013) Improved lifestyle and decreased diabetes risk over 13 years: long-term follow-up of the randomised Finnish Diabetes Prevention Study (DPS). Diabetologia 56:284-293.