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.Many more?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.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.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.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."