A new study highlights why body mass index (BMI) is a flawed measure of one's health.
BMI, which is a simple calculation based on a person's weight and height, does not in fact assess a person's body fat. Because it is based solely on weight, BMI risks classifying those who are heavy due to muscle, rather than body fat, as overweight or obese.
To get a sense of the association between BMI and mortality, researchers did tests on 50,000 middle-aged and older women and 5,000 middle-aged and older men in Manitoba, Canada, who they monitored for seven years. In addition to recording the participants' height and weight, the researchers, through x-rays, recorded their precise level of body fat.
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The study found that those with the lowest BMIs were most likely to die. Women with BMIs below 22.5 were 44 percent more likely to die, and men with BMIs below 23.8 were 45 percent more likely to die. However, people with such low BMIs represent a very small percentage of the total population
The other major predictor for death was high body fat. Men whose body fat was above 36 percent were 59 percent more likely to die. For women, the risk was dramatically lower: Those with the highest levels of body fat were 19 percent more likely to die.
Those who were least likely to die were those who would traditionally be classified as overweight, based on BMI, but who had a moderate body fat level. That finding is consistent with previous research that has highlighted what is commonly referred to as the "obesity paradox," in which older people who are considered overweight have lower mortality rates than their trimmer counterparts.
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