BMI is a baseline measure, but also a helpful one

Lots of people put tons of emphasis on BMI. It's a good baseline test for figuring out what kind of shape your body is in. The US Department of Health and Human Services National Institutes for Health (NIH) have composed guidelines for healthy BMI and are what physicians use in the initial assessment of whether or not a person is underweight, healthy weight, overweight or obese. BMI is somewhat limited in that it does not take into account a person's body composition, but the majority of the population (not elite athletes) this isn't too big of a problem. When you consider athletes BMI hits a little bit of a wall because athletes tend to have less body fat which means more of their weight is from muscle. This does not mean that BMI is invaluable, it simply needs to be used carefully. For this reason I have often given BMI less credence than perhaps it deserves.

The NIH recently released information regarding a study published in the New England Journal of Medicine on BMI and it's predictive power concerning longevity. It turns out that your BMI is a little bit bigger deal than I have thought in the past. The study pooled 19 other investigations that examined BMI and the probability of mortality (dying). This study actually allowed them to look at data concerning 1.5 million people which is 0.5% of the total population in the US (not bad considering the number of people in the US). What it found was that overweight people had significantly increased mortality rates and that obese people were even more at risk. This was also examined with other possible concurrent risk factors for death and found that an increased BMI by itself increases the likelihood of death within 10 years compared to an individual with a health BMI (defined as between 20 and 25).

Interestingly, a BMI that is too low is also bad for you (there is such a thing as being too skinny). But before you go pounding down twinkies to try and protect yourself from a low BMI, consider this. The increased risk to your health begins around a BMI of 18 which would be a 5' 6" tall person who weighed less than 118 lbs or a 6' tall person under 140 lbs.

If you're curious you can go to the NIH's BMI Caculator and see for yourself what your BMI is. If that's not enough for you, the next step to finding out more about what kind of shape you're in would be to find out your body composition or % body fat, but that's a conversation for another time.