In July 1998, the National Institutes of Health changed what it means to be overweight, defining it as a BMI (body mass index) of 25 or greater for adults. The cut-off had been 28 for men and 27 for women, so suddenly about 29 million Americans who had been considered normal became overweight even though they hadn’t gained an ounce.
The change, based on a review of hundreds of studies that matched BMI levels with health risks in large groups of people, brought the country in line with definitions used by the World Health Organization and other health agencies. But it also prompted many to question the real meaning of BMI and to note its potential drawbacks: labelling some healthy people as overweight or obese who are not overly fat, and failing to distinguish between dangerous and innocuous distributions of body fat.
More recent studies have indicated that many people with BMI levels at the low end of normal are less healthy than those now considered overweight. And some people who are overly fat according to their BMI are just as healthy as those considered to be of normal weight, as discussed in a new book, ‘The Obesity Paradox,’ by Dr Carl J Lavie, a cardiologist in New Orleans, and Kristin Loberg.
According to current criteria, those with a BMI below 18.5 are underweight; those between 18.5 and 24.9 are normal; those between 25 to 29.9 are overweight; and those 30 and higher are obese. The obese are further divided into three grades: Grade 1, in which BMI is 30 to 34.9; Grade 2, 35 to 39.9; Grade 3, 40 and higher.
Before you contemplate a crash diet because your BMI classifies you as overweight, consider what the index really represents and what is now known about its relationship to health and longevity.
The index was devised in the 1830s from measurements in men by a Belgian statistician interested in human growth. More than a century later, it was adopted by insurers and some researchers studying the distribution of obesity in the general population. Though never meant to be an individual assessment, only a way to talk about weight in large populations, BMI gradually was adopted as an easy and inexpensive way for doctors to assess weight in their patients.
For one thing, body weight is made up of muscle, bone and water, as well as body fat. BMI alone is at best an imprecise measure