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 of how fat a person may be. Another problem: the distribution of excess body fat makes a big difference to health. Those with lots of abdominal fat, which is metabolically active, are prone to developing insulin resistance, elevated blood lipids, high blood pressure, diabetes, premature cardiovascular disease, and an increased risk of erectile dysfunction and Alzheimers disease.
But fat carried in the hips, buttocks or thighs is relatively inert; while it may be cosmetically undesirable, it is not linked to chronic disease or early death.
Furthermore, a persons age, gender and ethnicity influence the relationship between BMI, body fat and health risk. Among children, a high BMI is a good indicator of excess fat and a propensity to remain overly fat into adulthood. But for an elderly person or someone with a chronic disease, a BMI in the range of overweight or obesity may even be protective. Sometimes after a heart attack or major surgery, for example extra body fat can provide energy that helps the patient to survive. An added layer of fat can also protect against traumatic injuries in an accident.
On average, women have a higher percentage of body fat in relation to total weight than do men, but this does not necessarily raise their health risks. And African-Americans, who tend have heavier bones and weigh more than Caucasians, face a lower risk to health even with a BMI in the overweight range.
While experts continue to debate whether a person can be fit and fat, Keri Gans, a dietitian in New York and former spokeswoman for the Academy of Nutrition and Dietetics, points out that physical activity and a healthy diet tend to offset the risks of being overweight.
People with a low-normal or below-normal BMI (less than 18.5) face different health risks. They may lack sufficient reserves to survive a serious health problem, and they are prone to osteoporosis, infertility and serious infections resulting from a weakened immune system.
- Jane E Brody