June 18, 2012
How Flawed And Outdated Is The Body Mass Index (BMI) Measurement?
The body mass index (BMI) is a poor measurement and a controversially inaccurate indicator to assess health. Despite this reality, almost every doctor is still trained in this useless analytical tool which is meant to provide a heuristic proxy for human body fat based on an individual's weight and height. The problem is, BMI doesn't actually measure percentage body fat or lean muscle tissue and makes absolutely no distinction between either of them.
While the formula previously called the Quetelet Index (invented by Adolphe Quetelet) for BMI dates to the 19th century, the new term "body mass index" for the ratio and its popularity date to a paper published in the July edition of 1972 in the Journal of Chronic Diseases by Ancel Keys, which ironically found the BMI to be the best proxy for body fat percentage among ratios of weight and height. It was designed as a simple numeric measure of a person's "thickness" or "thinness".
When it comes to assessing health status, most of the inaccuracy related to the BMI measurement comes from its reliance on population studies without assessing individual diagnosis. Keys himself admitted this shortfall.
Consequently, it makes absolutely no distinction between body weight from muscle and body weight from fat which labels a broad segment of the athletic and similar healthy populations as overweight and obese. An ideal BMI (which the measurement considers to be healthy) is between 20.5 and 21.5. Between 30 and 35, a person is considered to be moderately obese. Yet from the graphic above, we can see that a heavily muscled person is also classified as obese. High BMIs (35 and over) are linked to increased risk of cardiovascular events but not to increased mortality overall. BMI is extremely limited in its ability to predict deaths caused by heart attacks or strokes.
Researchers from the Mayo Clinic College of Medicine, Rochester, Minnesota, found that patients with a low BMI had a higher risk of death from heart disease than those with normal BMI. At the same time overweight patients had better survival rates and fewer heart problems than those with a normal BMI.
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This apparently perverse result, drawn from data from 40 studies covering 250,000 people with heart disease, did not suggest that obesity was not a health threat but rather that the 100-year-old BMI test was too blunt an instrument to be trusted.
"Rather than proving that obesity is harmless, our data suggests that alternative methods might be needed to better characterize individuals who truly have excess body fat compared with those in whom BMI is raised because of preserved muscle mass," said lead researcher Francisco Lopez-Jiminez.
"This unfortunately has become a broad indicator of general health and it's a standard diagnostic tool of heart disease risk, but it gives us virtually no real data on causative or even specific correlating factors for disease," said cardiologist Dr. Jean Lahane.
Maria Grazia Franzosi from the Instituto Mario Negri in Milan, writing in the same issue of the Lancet, noted that a 52-country study comparing four different tests -- BMI, waist-to-hip ratio, waist measure and hip measure -- found that waist-to-hip was the best predictor of heart attack risk.
"BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular risk," she said.
"Uncertainty about the best index of obesity should not translate into uncertainty about the need for prevention policy against excess bodyweight," she cautioned.
A study published on April 2, 2012 in PLoS One, researchers in New York City used a modified test and a revised standard of fatness to determine whether or not their 1,400 participants were at a healthy weight.
The standard they applied is the fat-to-lean muscle mass ratio, and the test they used to make this determination is dual-energy X-ray absorptiometry (DEXA), a scan that is widely used to measure bone density but can also reveal percentages of body fat and muscle mass.
Based on their findings, the investigators noted that if this more accurate method were substituted for the BMI, the almost four in 10 adults now identified as "overweight" would be reclassified as obese, with all the health risks that implies. They contended that the BMI number that now signals obesity - 30 - should be lowered to 24 for women and 28 for men. This would mean that a 5-foot 6-inch woman weighing 150 pounds would be considered obese, as would a 5'11" man weighing 200 pounds.
Dissatisfaction with the BMI is widespread, and various researchers are emphasizing that waist-to-hip ratio and even waist-to-height ratios are far more accurate. The research team that conducted the latest study also tested their participants' levels of leptin, a hormone secreted by body fat, and reported that the results seem to mirror DEXA findings. They suggested that testing leptin levels may be an easy and inexpensive way to distinguish between normal weight, overweight and obesity.
Here are some health calculators to help you assess your health status:
- Body Composition (body caliper required)
- Body Fat Percentage (estimate)
- Waist-to-Height Ratio (WHtR)
- Waist-Hip Ratio
Tammy McKenzie is a certified personal trainer and fitness specialist with a speciality in women's fitness.