Do you know your body mass index? Recent studies are beginning to cast doubt on its importance.
The Body Mass Index (BMI) is used extensively to assess the nation’s obesity rates and often the nation’s health. Simply it’s a measure of weight for height. Ranges of BMI are used to define weights for height that correspond to underweight, normal weight, and obesity in adults. It has the advantage of being easy to calculate and provides a single measure for men or women.
However the BMI is being debated lately in its ability to predict people’s health status. A number of “experts” caution that the tool is fairly imprecise and often wrong when applied to health risks
Additionally, the Centers for Disease Control (CDC) uses the BMI to project the nation’s weight problem with the most recent statistics warning that 1/3 of Americans are overweight and an additional 1/3 are classified as obese. These statistics may not accurately reflect the number of obese or overweight in the population due to gender, age and ethnic differences.
The health problems associated with obesity keep getting longer and longer from diabetes type 2, heart disease, dementia, infertility, certain cancers, to sleep apnea. Obese people are victimized and harassed; children are bullied, and bias against “fatness” abounds in our culture.
The BMI has a long history. It was invented by a Belgium mathematician, Adolphe Quetlet, between 1830 and 1850 to fit existing data on 19th century Europeans. Now it’s used by doctors and weight loss programs to counsel patients on weight loss and to warn them of health risks associated with being overweight or obese. It was never intended to be assess a person’s health risks. It is not accurate for children, body builders, African Americans, Asians, or for anyone who is very muscular. When I taught this calculation to college students, most of the athletic males in the class looked very disturbed to find they were classified as obese. I had to quickly reassure them that they were not obese and their weight was more than likely due to muscle weight, not fat weight.
The proponents of the BMI were shocked when a series of studies came out showing that patients diagnosed with heart disease fared better and survived longer when overweight or obese than those heart patients of normal, “healthy” weight. The “obesity paradox” was quite shock since obesity is often equated with an early death.
Since the BMI is merely a screening tool and most screening tools give inaccurate results a lot of the time, better methods are needed to assess weight and its association with health status. We can measure body fat percentage, but most of the best techniques are expensive and not practical in the doctor’s office.
One of the easiest measures is waist circumference, which has been a fairly good predictor of type 2 diabetes risks. If the waist measure is greater than 40 inches for men and 35 inches for women, the risk of heart disease and type 2 diabetes goes up.
A new way to measure body fat that relies only on measurements of your height and your hip circumference is an improvement on the BMI and is called the body adiposity index (BAI). The BAI also yields the percentage of fat itself, rather than just a correlate (or index) of it, which is what the BMI does. This calculation should be easy for nutritionists and physicians to perform for patients with a simple calculator.
Another simple measure is the use of Bioelectrical Impedence equipment, some of which is fairly inexpensive and a somewhat reliable measure of body composition. It’s best use is to measure change over time rather than predict actual percentages of body fat or lean body mass.
“Fit but fat” should be more emphasized by health providers since losing weight and maintaining weight is challenging for many people. Health advice should include more advice on eating healthy foods, and encouraging exercise rather than concentrating only on how much weight a patient should lose. Weight loss is important, but it may be better to shift the importance of physical activity, building muscles, and stamina to assess health risks. So many weight loss programs overemphasize pounds lost and ignore the more holistic approach of “health at any size”.
An easy way to calculate your BMI is:
Divide your weight in pounds by your height in inches (squared) and multiply by 703.
Underweight = below 18.5
Normal weight = 18.5-24.9
Overweight = 25.0 – 29.9
Obese = 30.0 and over
- Obesity expert: A better fat measure than BMI (newscientist.com)