Cause-Specific Excess Deaths Associated With Underweight, Overweight, and Obesity
http://jama.ama-assn.org/cgi/content/short/298/17/2028 (Abstract. My e-mail is on my profile if you want me to send the full article.)
Three is a New York times article here: (mentions weights) http://www.nytimes.com/2007/11/07/health/07fat.html
Basically, they investigate the claims about obesity or being underweight increases mortality risk.
One of the main criticisms here (in this LJ community) is that the studies did not account for people "wasting away" due to cancer. Most of the deaths for underweight individuals actually came from non cancer and non cardiovascular issues, and this is the highest risk factor for those individuals. They also had an increased death rate from some cancers.
Obesity was correlated with higher deaths from "obesity related cancers," but was actually protective for other cancers. Overall, cancer balanced out among the weight classes. Obesity had a much higher death rate from cardiovascular issues, and diabetes was only correlated when it was combined with kidney disease.
From the conclusions section: (Separated by me for easier reading_
We found that underweight was associated with increased mortality, primarily from non-cancer, non-CVD causes.
Overweight was associated with significantly increased mortality from diabetes and kidney disease combined; was associated with significantly decreased mortality from other non-cancer, non-CVD causes; and was not associated with mortality from cancer or cardiovascular disease. The net result was that overweight was associated with significantly decreased all cause mortality overall.
Obesity was associated with significantly increased mortality from CVD, from some cancers, and from diabetes and kidney disease combined but showed little or no association with other cancers and with other causes of death. Obesity also was associated with increased mortality
overall, primarily due to its association with CVD mortality.
Thus, our data indicate that the association of BMI with mortality varies considerably by cause of death.