The obesity problem in the US may be much worse than previously thought, according to researchers.
They said using the Body Mass Index or BMI to determine obesity was underestimating the issue.
Their study, published in the journal PLoS One, said up to 39% of people who were not currently classified as obese actually were.
The authors said “we may be much further behind than we thought” in tackling obesity.
BMI is a simple calculation which combines a person’s height and weight to give a score which can be used to diagnose obesity. Somebody with a BMI of 30 or more is classed as obese.
The US Centers for Disease Control says at least one in three Americans are obese.
Other ways of diagnosing obesity include looking at how much of the body is made up of fat. A fat percentage of 25% or more for men or 30% or more for women is the threshold for obesity.
Prevalence of Obesity in the United States, 2009–2010
Data from the National Health and Nutrition Examination Survey, 2009–2010
• More than one-third of adults and almost 17% of youth were obese in 2009–2010.
• There was no change in the prevalence of obesity among adults or children from 2007–2008 to 2009–2010.
• Obesity prevalence did not differ between men and women.
• Adults aged 60 and over were more likely to be obese than younger adults.
One of the researchers Dr Eric Braverman said: “The Body Mass Index is an insensitive measure of obesity, prone to under-diagnosis, while direct fat measurements are superior because they show distribution of body fat.”
The team at the New York University School of Medicine and the Weill Cornell Medical College, New York, looked at records from 1,393 people who had both their BMI and body fat scores measured.
Their data showed that most of the time the two measures came to the same conclusion. However, they said 539 people in the study - or 39% - were not labelled obese according to BMI, but their fat percentage suggested they were.
Between 1999–2000 and 2009–2010, the prevalence of obesity increased among men but not among women.
In 1999–2000, 27.5% of men were obese, and by 2009–2010 the prevalence had increased to 35.5%. Among women, 33.4% were obese in 1999–2000 with no significant change in 2009–2010 (35.8%). In 1999–2000, the prevalence of obesity was higher in women than in men.
Between 1999–2000 and 2009–2010, the difference in the prevalence of obesity between men and women decreased so that in 2009–2010, the prevalence of obesity in men was virtually equal to that in women. There was no significant change in the prevalence of obesity from 2007–2008 to 2009–2010 overall or among men or women.
They said the disparity was greatest in women and became worse when looking at older groups of women.
“Greater loss of muscle mass in women with age exacerbates the misclassification of BMI,” they said.
They propose changing the thresholds for obesity: “A more appropriate cut-point for obesity with BMI is 24 for females and 28 for males.”
A BMI of 24 is currently classed as a “normal” weight.
“By our cut-offs, 64.1% or about 99.8 million American women are obese,” they said.
It is not the first time BMI has been questioned. A study by the University of Leicester said BMIs needed to be adjusted according to ethnicity.
The most recent national data on obesity prevalence among U.S. adults, adolescents, and children show that more than one-third of adults and almost 17% of children and adolescents were obese in 2009–2010. Differences in prevalence between men and women diminished between 1999–2000 and 2009–2010, with the prevalence of obesity among men reaching the same level as that among women.
Age differences in obesity prevalence varied between men and women. The prevalence of obesity was higher among older women compared with younger women, but there was no difference by age in obesity prevalence among men. Among children and adolescents, the prevalence of obesity was higher among adolescents than among preschool-aged children.
There has been no change in obesity prevalence in recent years; however, over the last decade there has been a significant increase in obesity prevalence among men and boys but not among women and girls overall. The Healthy People 2010 goals of 15% obesity among adults and 5% obesity among children were not met.
Last year in the BBC’s Scrubbing Up column, nutrition expert Dr Margaret Ashwell advocated using waist-to-height ratio to determine obesity.
She said: “It is a real worry that using BMI alone for screening could miss people who are at risk from central obesity and might also be alarming those whose risk is not as great as it appears from their BMI.”