Surprisingly, being overweight -- defined as having a BMI between 25 and 30 -- was actually linked to fewer excess deaths than being of normal weight.

Researchers found that there were approximately 111,900 more deaths per year for obese people, defined as having a body mass index (BMI) of 30 or higher, than for people of normal weight.

Notably, they also found that those who were underweight were not out of the woods: There were nearly 33,700 excess deaths for people with BMIs below 18.5. BMI is a ratio based upon a person's weight and height.

The new findings, published in the April 20 issue of the Journal of the American Medical Association, update earlier estimates of excess deaths from obesity by the U.S. Centers for Disease Control and Prevention (CDC) and highlight the link between mortality and underweight.

"This shows the value of continuing research," said lead author Katherine Flegal, a researcher with the CDC National Center for Health Statistics. Flegal, who is a visiting scholar at the University of California, Berkeley's Center for Weight and Health, conducted the study while on campus.

"There is always room for new analyses as better methods and data become available," said Flegal.

Too Few Nutritional Reserves

This new study adjusts for potentially confounding factors, such as smoking, age and gender, when calculating excess mortality related to weight. Moreover, the study uses data from more recent death reports and nationally representative surveys where height and weight are measured rather than self-reported.

The study found that excess deaths associated with being underweight were greater for those 70 and older.

"Low weight by itself is associated with greater mortality risk on its own," said Flegal. "It may be that people who are underweight do not have a lot of nutritional reserves to call upon when illness does occur. Frailty is associated with poorer health and higher mortality, particularly in the elderly."

Overweight Linked to Fewer Deaths

Surprisingly, being overweight -- defined as having a BMI between 25 and 30 -- was actually linked to fewer excess deaths than being of normal weight.

"Our numbers suggest that the prevalence of weight-related mortality is not as great as previously thought," said Flegal. "However, we didn't look at issues such as quality of life in this study, so these results should not be overinterpreted to mean that we can all rest easy. There has been plenty of other research to indicate that excess weight is associated with health problems such as diabetes, heart disease and stroke."

Flegal explained that the impact of obesity on death rates may have decreased over time because of improvements in public health and advances in medical care. The study's authors noted in their published findings that mortality rates from ischemic heart disease have decreased, and life expectancy has increased overall in the United States.

Co-authors of the study are Barry Graubard and Dr. Mitchell Gail, both statisticians at the National Cancer Institute, and David Williamson, an epidemiologist at the CDC's Division of Diabetes Translation.

This article was adapted from a press release written by Sarah Yang.