A unique study of 16 pairs of identical twins in which one twin is obese and the other lean has yielded some surprising results. In 8 of the pairs of twins, the obese twin was as 'metabolically healthy' as his or her lean co-twin, while in the other 8 pairs, the obese twin had a poorer blood fat profile, higher liver fat and increased insulin production and resistance, and higher blood pressure -- all hallmarks of unhealthy obesity.