A worldwide "gold standard" formula used to distinguish between healthy and unhealthy weight may be misclassifying dangerously fat people as the perfect body shape.

A U.S. study has found that the body-mass index BMI, the 200-year-old formula widely used by medical experts, health insurers and the fitness industry, may be categorizing almost half of women and just over 20 percent of men as healthy when their body-fat composition suggests they are obese.

The BMI, or Quetelet index, is a proxy for human body fat based on an individual's weight and height. BMI does not actually measure the percentage of body fat.

The PLoS One study, of 9,000 people, used a patient’s ratio of fat-to-lean muscle mass for detecting obesity and suggests that it may be a bellwether of an individual’s risk for health problems.

The study found that many women, particularly those over 50, whose BMIs suggest they are the picture of health are, in fact, dangerously fat.

Click here to read more on this story from the Herald Sun.