Actually, the lower death rates with moderate rather than lower BMI, was an early claim and was later shown to be the result of people being thinner as the result of previously undiagnosed illnesses. I don't remember the source, as I have read several books on the subject, I sort of think it was from Fumento's "The Fat of the Land", but it could have been several others (none of which supported the superiority of moderate over lower BMI, until you get down to starvation levels, ie BMI of less than 18).