Can you judge your health by a ratio of your weight to your height? There are calculators on many Internet sites to calculate your personal “body mass index,” BMI for short. You get a number that is supposed to indicate if you are too fat or not. It is a simple system, but its application to individuals as a measure of health is being questioned.

In 1970, a book titled “The Health of Americans,” edited by Boisfeuillet Jones, defined health as the complete physical, mental and social well-being of an individual. The publication suggested that it is an implicit goal of society to enhance each person’s opportunity for a healthy, reasonably happy and productive life. At that time, the weight of the nation was not a significant problem. Nor was BMI a common measure of a healthy weight. Now, however, BMI has come into common use as a measure of health.

Question: How well does BMI work?

Answer: Not that well. The BMI concept is most appropriately used in studies of large groups of people to classify them into various categories for research comparisons. The common practice is to classify adults with a BMI between 25 and 30 as overweight and those at 30 and above as obese. However, when BMI is used solely to classify an individual as overweight or obese, misinterpretation of a healthy weight for that individual can occur. For example, a BMI greater than 25 could reflect excessive amounts of body fat or significant muscle mass.

Overweight people who are physically fit and don’t display any of the commonly associated health problems may not benefit from extreme medical treatments to reduce their weight. Due to their genetic predisposition, being fit and fat may be reasonable and even preferable.

Q: Can BMI be used for children and adolescents?

A: Although BMI is being used to classify children and teens as overweight or obese, the “BMI report cards” that put parents on notice that their child is too fat have many potential complications.

Nobu Morimoto, a recent graduate of the University of Hawaii at Manoa Food Science and Human Nutrition program who was recently accepted into Tokyo Medical and Dental University, published a critical review on this topic in the Journal of Young Investigators. Morimoto found that using BMI with children and teens can often lead to incorrect conclusions about their body weight and health.

He found that when measurements of percent body fat were used in a study of nearly 1,000 children, more than 1 out of 4 children classified as obese by BMI actually had normal amounts of body fat. For children classified as overweight by BMI, more than 2 out of 3 had normal body fat. When children with normal amounts of body fat are told they are too fat, the physical and psychological ramifications can be severe and have lifelong impact.

Due to the limitations inherent in BMI, Dr. Arya Sharma, at the University of Alberta, proposed a ranking system using BMI in association with other aspects of health. Using this approach, called the Edmonton Obesity Staging System, the level of concern rises for a high BMI when it coexists with risk factors such as high blood pressure, elevated blood glucose, limitations in physical activity, psychological problems and other cardiovascular risk factors. This system helps to make more appropriate treatment decisions for overweight and obese individuals.