The high percentage of thrifty phenotypes is thought to relate to poor prenatal care, with associated drug, tobacco and alcohol use. In addition, preeclampsia, which comes from an inherently abnormal placenta, affects up to 10% of pregnancies in some populations. I agree, it’s certainly not calorie restriction, but these other factors certainly contribute. Cocaine use is particularly damaging to placental function, and is associated with total separation of the placenta, a condition known as abruption, which will kill the fetus if it occurs outside a hospital setting.
As far as your comment about other ethnic groups and their increase in obesity (and diabetes), it is unquestionably true.
