You may well be aware that body mass index, or BMI, is an imperfect measure of what proportion of your body is made up of fat tissue. On the one hand, very muscular athletes may weigh enough that their BMI puts them into the “obese” range, while a slender person who has a “normal” BMI may in fact be carrying an unhealthy amount of visceral fat (that is, fat around the organs in your abdomen).
What the heck? I know…I know. As I’ve been learning the last several months in school, the methods we use to measure the overall health of a population (whether by geography, gender, ethnicity) are often not the best methods to measure the health of an individual. BMI is a perfect example.
If you have a BMI of, say, 25, that does not mean you have 25 percent body fat. BMI is a way of predicting risk of diseases that tend to be associated with being overweight or obese. When you are looking a huge group of people (a population), BMI is correlated with body fat. That is, if you took a million people and calculated their BMI and measured their body fat, and plotted those on a graph, you would see that as BMI increases, body fat increases, too. You would of course see some outliers (i.e., your muscular athletes and your “skinny fat” individuals), but by and large, BMI and body fat follow the same path.
You know how sometimes you sort of wonder about something, but you never get around to seeking out an answer? Well, I’d idly wondered for a long time why men and women share the same BMI chart. Today, as I was reading a chapter from my hefty Nutrition in the Prevention and Treatment of Disease textbook, I got my answer. 
If a man and a woman have the same BMI, the woman will have a higher percentage of body fat than the man (again, assuming they are average people and not outliers). But women tend to “carry fat” better than men. The typical female fat distribution pattern (hips and thighs) doesn’t carry the same health risks as the typical male fat distribution pattern (abdomen). Since BMI is primarily used to assess risk of weight-related disease, not to assess percentage of body fat. An average woman can carry more fat than an average man of her same BMI, yet she will have the same average risk of disease.
Yes, I am saying “average” a lot. That’s because when you look at an individual person, there are many factors that contribute to disease risk besides body weight. But I thought this was interesting and worth sharing. So what does this mean for you? 

  • If you are a muscular athlete, you probably already know that your BMI doesn’t matter. 
  • If you have a “normal” BMI but tend to carry a bit of weight around your middle, you may not be as healthy as your BMI might lead you to believe. That goes double if you tend to be physically inactive.
  • If you are Asian, don’t trust the BMI charts. Research has shown that many Asian people who have “normal” BMIs have unhealthy levels of fat around their organs. Many experts have suggested that there needs to be a different BMI chart for people of Asian descent.
  • If you are an older adult who is not terribly active, you have probably lost muscle mass. This means that having a “normal” BMI may hide the fact that you have a unhealthy body fat percentage.
  • If you are a child, adolescent, or teenager, BMI charts are not meant for you. The Centers for Disease Control and Prevention (CDC) has a set of growth charts that are more appropriate at these ages.
  • If you don’t fall into the above groups and you are moderately active, your BMI is a reasonable guideline of whether you are at a healthy weight, even though it doesn’t correspond to actual body fat percentage.

If you really want to know your body fat percentage, you should seek out a reliable assessment method, such as a DEXA scan, hydrostatic weighing, or multiple-location caliper measurements by someone who really knows what they are doing (it takes a lot of experience and practice on different body types to get good with the calipers, but someone who is skilled can produce results just about as accurate as the other,  more costly methods).
As I said, body weight is only one potential measure of health and risk of “lifestyle-related” diseases like type 2 diabetes and heart disease. Eating a healthful diet, getting regular exercise, reducing or managing stress and getting adequate sleep are also very important. If you know you have a family history of these or other diseases, talk to your doctor about what preventive screening exams you need (including when and how often).