Non-smoking adults with a Body Mass Index (BMI) from 20 to 24.9 have a much lower risk of premature death than overweight or obese individuals, as well as extremely thin people, researchers wrote in the NEJM (New England Journal of Medicine). If your BMI is between 25 and 29.9 you are overweight, you are obese if it is over 30, severely obese if over 35. The World Health Organization defines those with a BMI of between 18.5 and 24.9 as being within normal ranges. Many countries define the ideal as between 20 and 24.9.

The authors explained that prior research had been confusing and produced conflicting results; with some showing only a slight association between overweight and mortality, while others suggested the opposite. There had been no clear study examining different levels of obesity with health risks and mortality.

Scientists from the NCI (National Cancer Institute), as well as other centers of excellence from around the world set out to determine what the link might be between different BMI levels and risk of death.

Your BMI (body mass index) is your weight in relation to your height:

BMI metric units:

Your kilogram weight divided by the square of your height (meters).

For example. If you are 1.8 meters tall and weigh 80 kilograms

1.8 2 meters = 3.24

80 ÷ 3.24 = 24.69

BMI = 24.69

Your kilogram weight divided by the square of your height (meters). For example. If you are 1.8 meters tall and weigh 80 kilograms 1.8 meters = 3.24 80 ÷ 3.24 = 24.69 BMI = 24.69 BMI Imperial Units:

Your weight in lbs (pounds) multiplied by 703, divided by the square of your height in inches.

For example. If you are 6ft tall (72 inches), and weigh 190 pounds

722 = 5,184

190 x 703 ÷ 5,184 = 25.76

BMI = 25.76

It is well known that people with obesity have a higher risk of premature death from cancer, stroke and heart disease. According to the NIH (National Institutes of Health), approximately two-thirds of American adults are either obese or overweight. Of concern is the growing numbers of severely obese individuals – 17% of US adult women and 11% of men are severely obese.

The researchers gathered data from 19 long-term studies which monitored individuals from between 5 to 28 years, involving 1.5 million white adults.

The scientists found that:

Overweight females had a 13% higher risk of death during the study period compared to those whose BMI was between 22.5 and 24.9 (lifetime non-smokers in both groups). The risk was significantly greater for obese and severely obese women.

Adults with a BMI of 30 to 34.9 had a 44% higher risk of death than those with a 22.5 to 24.9 BMI

The risk of death during the study period was 88% higher for those with a BMI of 35 to 39.9

There was a 250% higher risk of death for those with a BMI of 40 to 49.9

There was a 31% higher risk of death for every 5 unit BMI increase

Lead study author, Amy Berrington de Gonzalez, D.Phil., said:

“By combining data on nearly 1.5 million participants from 19 studies we were able to evaluate a wide range of BMI levels and other characteristics that may influence the relationship between excess weight and risk of death. Smoking and pre-existing illness or disease are strongly associated with the risk of death and with obesity. A paramount aspect of the study was our ability to minimize the impact of these factors by excluding those participants from the analysis.”

The authors wrote that their calculations were made after taking into account various factors that can influence human health and mortality rates, such as levels of physical activity, present and past smoking status, alcohol intake and levels of education.

The link between risk of premature death and increasing >25 BMI rates exists within all age groups. The elevated risk of death is more prominent for those aged 50 or less.

Many experts say that using BMI can be misleading, because it does not take into account the individual’s body measurements, such a hip, chest and waist sizes. An Olympic 100-meter-sprint gold-medal winner with a BMI of 25.5 will have a completely different body shape compared to a couch potato with a 25.4 BMI – the couch potato will have a bigger belly, less muscle, and more fat in various parts of the body, as well as in his/her blood. The athlete will enjoy far better health. However, if you use a pure BMI criteria, the couch potato with a one-tenth lower BMI is healthier.

BMI does not take into account bone mass (bone density). An individual with osteoporosis may have a lower BMI than another person because of it, even though their waist size may be bigger than the other person’s.

Experts say BMI is only useful as a rough ballpark basic standard that may indicate population variations. There is a growing tendency to use waist-hip ratios to determine a person’s ideal weight and fitness – the ratio of the circumference of a person’s waist to that of their hips.

BMI underestimates how much fat an obese person is carrying, and it tends to overestimate fat content in muscular or lean individuals.

The authors concluded:

“In white adults, overweight and obesity (and possibly underweight) are associated with increased all-cause mortality. All-cause mortality is generally lowest with a BMI of 20.0 to 24.9.”

“Body-Mass Index and Mortality among 1.46 Million White Adults”

Amy Berrington de Gonzalez, D.Phil., Patricia Hartge, Sc.D., James R. Cerhan, Ph.D., Alan J. Flint, Dr.P.H., Lindsay Hannan, M.S.P.H., Robert J. MacInnis, Ph.D., Steven C. Moore, Ph.D., Geoffrey S. Tobias, B.S., Hoda Anton-Culver, Ph.D., Laura Beane Freeman, Ph.D., W. Lawrence Beeson, Dr.P.H., Sandra L. Clipp, M.P.H., Dallas R. English, Ph.D., Aaron R. Folsom, M.D., D. Michal Freedman, Ph.D., Graham Giles, Ph.D., Niclas Hakansson, Ph.D., Katherine D. Henderson, Ph.D., Judith Hoffman-Bolton, Jane A. Hoppin, Sc.D., Karen L. Koenig, Ph.D., I-Min Lee, Sc.D., Martha S. Linet, M.D., Yikyung Park, Sc.D., Gaia Pocobelli, M.S., Arthur Schatzkin, M.D., Howard D. Sesso, Sc.D., Elisabete Weiderpass, Ph.D., Bradley J. Willcox, M.D., Alicja Wolk, Dr.Med.Sci., Anne Zeleniuch-Jacquotte, M.D., Walter C. Willett, M.D., Dr.P.H., and Michael J. Thun, M.D.

N Engl J Med 2010; 363:2211-2219December 2, 2010

Written by Christian Nordqvist