There's no explanation yet, but pooled analysis of results from 5 longitudinal studies show that people who are lean at the onset of type 2 diabetes have a greater mortality risk than those who are obese.
RESULT: Association of weight status with mortality in adults with incident diabetes
JAMA (PubMed) | Aug 8, 2012 (Free abstract. Full text $30)
This new information from a longitudinal study of lifestyle interventions and diabetes is one of several to reveal that a genetic susceptibility may render some lean and normal-weight people at particular risk of (and from) type 2 diabetes. Together, these studies appear to urge both doctors and their patients not to take false hope in the idea that maintaining normal weight can avoid the risks of diabetes.
RESULT: Association of Type 2 Diabetes Susceptibility Loci With One-Year Weight Loss in the Look AHEAD Clinical Trial
Obesity | Jul 26, 2012 (Free abstract. Full text$32)