In the wake of recent calls to take very fat children away from their parents, a reminder is in order:
Dr. Stunkard ended up with 540 adults whose average age was 40. They had been adopted when they were very young — 55 percent had been adopted in the first month of life and 90 percent were adopted in the first year of life. His conclusions, published in The New England Journal of Medicine in 1986, were unequivocal. The adoptees were as fat as their biological parents, and how fat they were had no relation to how fat their adoptive parents were.
(from the New York Times article Genes Take Charge, and Diets Fall by the Wayside)
It seems unlikely that if the BMIs of children adopted in their first year were not affected by their adoptive parents, that children who already have extremely high BMIs would become thin when placed with foster parents.