An evaluation of national data by UC Davis researchers has found that extra weight is not necessarily linked with a higher risk of death.

When compared to those with normal weight, people who were overweight or obese had no increased risk of death during a follow-up period of six years. People who were severely obese did have a higher risk, but only if they also had diabetes or hypertension.

The findings, which appear in the July-August issue of The Journal of American Board of Family Medicine, call into question previous studies -- using data collected when obesity was less common -- linking higher short-term mortality with any amount of extra weight.

"There is currently a widespread belief that any degree of overweight or obesity increases the risk of death, however our findings suggest this may not be the case," said Anthony Jerant, professor of family and community medicine and lead author of the study. "In the six-year timeframe of our evaluation, we found that only severe obesity was associated with an increased risk of death, due to co-occurring diabetes and hypertension."

Based on the study, Jerant recommends that doctors' conversations with patients who are overweight or obese, but not severely obese, focus on the known negative effects of these conditions on mental and physical functioning, rather than on an increased short-term risk of death.

By contrast, Jerant added that it is important for doctors to talk with severely obese patients who also have diabetes or hypertension about their increased short-term mortality risk and treatment, including weight loss.

"Our results do not mean that being overweight or obese is not a threat to individual or public health," said Jerant. "These conditions can have a significant impact on quality of life, and for this reason alone weight loss may be advisable."

In conducting the study, Jerant used nationwide data from 2000 to 2005 of nearly 51,000 adults aged 18 to 90 years who participated in the Medical Expenditure Panel Surveys on health-care utilization and costs. The surveys include information on health conditions such as diabetes and hypertension.

Body mass index (BMI), or weight adjusted for height, was calculated for each respondent. The study categorized people as underweight (BMI < 20), normal weight (BMI 20 to < 25), overweight (BMI 25 to < 30), obese (BMI 30 to 35) or severely obese (BMI > 35).

Mortality was assessed using the National Death Index. Of the 50,994 people included in the UC Davis analysis, just over 3 percent (1,683) died during the six years of follow-up.

The investigators found that severely obese people were 1.26 times more likely to die during follow-up than people in the normal weight group. However, if people with diabetes or hypertension were eliminated from the data, those who were overweight, obese or even severely obese had similar or even lower death rates than people of normal weight. Consistent with a number of prior studies, underweight people were nearly twice as likely to die than people with normal weight, regardless of whether diabetes or hypertension was present.

The prevalence of overweight and obesity has increased dramatically in recent decades. An estimated one-third of all U.S. adults over age 20 are obese and another one-third are overweight. In addition to diabetes and hypertension, health problems associated with these conditions include heart disease, osteoarthritis and sleep apnea.

The relationship between weight and mortality is a controversial topic in public health. Although studies based on data collected 30 years ago showed that mortality risk rose as weight increased, analyses of more recently collected data, including the current one, call this assumption into question.

"Our findings indicate that the risk of having an above-normal BMI may be lower than in the past," said Jerant. "While this study cannot explain the reasons, it is possible that as overweight and obesity have become more common, physicians have become more aware of associated health issues like high blood pressure, cholesterol and blood sugar, and are more aggressive about early detection and treatment of these conditions."

Jerant said that the six-year period of his investigation limits the ability to make assumptions about the link between unhealthy weight and the risk of death over a longer timeframe.

"We hope our findings will trigger studies that re-examine the relationship of being overweight or obese with long-term mortality," said Jerant.

The study co-author was Peter Franks, professor in the UC Davis Department of Family and Community Medicine. Franks and Jerant used public access data in conducting the study, which involved no external funding.