After discussing the two rat experiment and applying it to Kitava, it must be pretty obvious that getting fat in the good times improves your chances of surviving the lean times.
Being fat is good for you.
Anyone going in to the next poor harvest with a BMI of 18.5 is significantly less likely to come out alive for the next good harvest than someone of BMI 29.9. Thinking Kitava here, same applies to hunter/gatherers. But is it possible to have too much fat?
Well a BMI of 18.5-25 is considered "ideal". Looking at this paper we find that:
"Overweight was not associated with excess mortality (-86,094 deaths; 95% CI, -161,223 to -10,966)"
That is BMI 25-30 has lowest all cause mortality. Lower mortality than our ideal target range. Perhaps it's time to move the goal posts. Maybe BMI <25 should be a cause for concern. I'm in trouble on that one!
My only consolation of running my own BMI at 20.5 is that being slim on a high fat diet is almost certainly not as bad for you as when you are on a carbohydrate based diet. It may be good for you. No one will ever find this out looking at USA health statistics. On their Food Pyramid everyone suffers.
PS I tried to understand this paper in depth and it is strictly for the statisticians. I'll have to take Katherine Flegal's word for the results being true.