In 2005, a study from the Centers for Disease Control and Prevention (CDC) caused a stir when researchers concluded that overweight individuals, who have a body mass index (BMI) of 25 – 25.9, have significantly lower mortality rates compared to underweight (BMI < 18.5) and obese individuals (BMI > 30). The same researchers extended their work using additional data with longer follow-up, to examine the association of mortality with weight. The new study was published in JAMA in early November.

The researchers looked at 2004 mortality data for 2.3 million U.S. adults age 25 years and older with cause of death grouped into three categories: cardiovascular disease (CVD), cancer, and all other causes. Data was also divided into three age groups: 25-59 years, 60-69 years, and 70 years and older.

Results showed that for CVD, obesity but not overweight was significantly associated with excess mortality. Neither underweight nor overweight was associated with significantly increased or reduced mortality from CVD. Cancer had no significant association of excess mortality with any BMI category. Overall, obesity was associated with a significant increased number of deaths from cancers considered obesity related; overweight was not associated with excess deaths from cancer. For all other causes of mortality (non-cancer, non-CVD), underweight was associated with a significantly positive number of excess deaths, overweight was associated with a significantly negative number of excess deaths, and obesity was not associated with any significant positive or negative excess mortality. Upon further investigation, researchers found that diabetes and kidney disease combined were associated with significantly increased mortality in overweight and obesity.

The researchers conclude that evidence from other studies suggests that moderately overweight may improve survival in certain circumstances including recovery from infections or medical procedures, and findings may be due to nutritional reserves or higher lean body mass associated with overweight. Lead researcher of both studies, Katherine Flegal, PhD, cautions that "the take-home message is that the relationship between fat and mortality is more complicated than we tend to think." Health professionals who criticized the study emphasized that mortality data does not take into account quality of life benefits gained from achieving or maintaining a normal weight.

The abstract for the JAMA article, with a link to the full article for JAMA subscribers, can be accessed at: http://jama.ama-assn.org/cgi/content/abstract/298/17/2028