Citations please. If that was true, I've learned a new fact that conflicts with most of what I know about obesity having an effect on, for example, heart disease.

The first link when googling for "lifespan by bmi" is:

http://wehrintheworld.blogspot.com/2009/05/bmi-and-life-expe...

Which cites two metareviews. The first is a collaboration between the UK Medical Research Council, several British organizations including one out of Oxford, and the US National Institute on Aging (which is under the NIH umbrella). It was published in _The Lancet_ (Impact factor: 33). I will quote the quoted part from said blogpost:

> BMI is in itself a strong predictor of overall mortality both above and below the apparent optimum of about 22·5–25 kg/m2. The progressive excess mortality above this range is due mainly to vascular disease and is probably largely causal. At 30–35 kg/m2, median survival is reduced by 2–4 years; at 40–45 kg/m2, it is reduced by 8–10 years (which is comparable with the effects of smoking). The definite excess mortality below 22·5 kg/m2 is due mainly to smoking-related diseases, and is not fully explained.

While I found a few studies that didn't find a correlation (including one in AMA), I can't actually find an academic source for the claim that people with a BMI of 25-30 have the longest lifespan and see lots of claims that the optimal BMI is in the low 20s. Do you have any citations handy?

If you run a search for '"slightly overweight" live longer', you'll turn up links to a number of different studies in a number of different jurisdictions that have shown this. For instance:

http://lifehacker.com/5303009/being-slightly-overweight-coul...

Is this not due to the fact that those with cancer and chronic illnesses tend to skew the death rate for the skinniest?

To this poster's benefit, I also vaguely remember an article about this, but it was for a specific disease (possibly heart related?). It was only a tiny benefit for the people who are in the first group of overweightness.

I'd be interested in the link to that study, if you have it.