What's Wrong With BMI

In studies dealing with body weight in relation to mortality scientists often use BMI (body mass index) to determine whether or not a particular individual is overweight.  In recent studies a high BMI has been a good predictor of mortality in young people but not in seniors.  Some scientists have been puzzled by these results.  I think the problem is with BMI as a tool; it is not a very accurate indicator of whether an individual is fat.  To determine whether a person is obese you need to measure their body fat.

BMI is calculated by relating weight to height.  You multiply your weight in pounds by 4.88, then divide it by your height in inches squared.  For example I weigh 143 pounds (x 4.88=697.84) and I’m 5’5.5” tall (squared = 29.81) so my BMI is 23.4, at the high end of the normal range   (For an online BMI calculator go to http://www.nhlbisupport.com/bmi/)  In people who are muscular (like me) BMI tends to be high even though body fat may be quite low.  At the other end of the spectrum are very thin people with very little muscle mass; their BMI’s will be normal even though their body fat might be high. 

Getting back to those confusing studies, let’s imagine a group of people in their 30’s whose BMI’s are high, some because they are too fat, others because they are more muscular than average.  So BMI is lumping together people who are in poorer than average physical condition and people who are in better than average shape. 

Fast forward 40 years and measure the BMI’s of the same group.  In the Too Fat sub-group some have died, while others have moved into a lower BMI category because of illness, malnutrition, or other weight loss.  The More Muscular Than Average sub-group has tended to change less.  As compared with the TF’s, fewer have died and more have exercised regularly because it is easier for them to do so.  Because of the exercise they have less illness and less age-related weight loss.  Even MMA’s who never exercise (like my late mother-in-law) tend to live longer because their hearts are likely to be strong like the rest of their muscles.  So this later version of the high BMI group is going to have more people in better than average shape than the earlier version.

The good news is that it is possible to build muscle mass at any age, given reasonable health and nutrition that is sufficient to support muscle development.  I am a case in point.  At 30 I couldn’t do a single pushup; now, at 65, I can do at least 40.  Protein is an important part of this picture and many older people do not get enough in their diets or their bodies don’t adequately process the protein they do ingest.  Fortunately, protein supplements and proteolytic enzyme supplements (to help digest protein) are widely available in stores and on the internet.