Ask Dr. Kasongo: BMI vs. Weight - Which is More Accurate?

Q: I would like to know how relevant the BMI is when it comes to muscle mass.  Isn't it true that muscle weighs more than fat?  For instance, when I was in high school, my weight was 140 pounds during a weigh in.  The girl in line behind me, who was visibly heavier than me by all accounts (I definitely looked slimmer) weighed in at 117 pounds.  Today, I don't care as much about the numbers on the scale but at the time I was a bit traumatized! People routinely under-guess my weight by at least twenty pounds.  I am definitely not making excuses, but I wonder where we draw the line between being truly "overweight" versus not measuring up to what may be an outdated standard.

Dr. Kasongo: We tend to be very obsessed by our weight and scales and while this may help you keep track of wieght gain or loss, it isn't telling you how healthy you are. Because the truth is that some people think they are overweight, when the amount of fat they have on their bodies is just fine, while others may carry hidden fat that the scale won't tell them about. Our bodies are made up of a lot of different kinds of tissues (plus a lot of water). There is muscle, fat, bone, and specialized tissue such as is in our various organs. The body fat percentage is just that –- the percentage of our weight which is made up of fat. Body fat percentage is similar to terms such as body fat ratio and body composition. It is important not to confuse BMI with body fat percentage. I get really irritated when official websites confuse the two. BMI (body mass index) is a formula based on height and weight. It was developed because in the general population, it is correlated with body fat. Women with a BMI of 25 to 29.9 are considered overweight, while women with a BMI of 30 or more are considered obese. All adults (aged 18 years or older) who have a BMI of 25 or more are considered at risk for early death and disability from being overweight or obese. These health risks increase as the BMI rises.  And yes, there are danger zones for fat storage  For most women, carrying extra weight around their waists or middle (with a waist larger than 35 inches) raises health risks more than carrying extra weight around their hips or thighs. These health risks include heart disease, diabetes, and cancer.

However, there are groups for whom BMI is not as accurate –- short women and muscular people, to name two. BMI also varies according to some ethnic groups (read Black). You weighed more than the girl in high school that you looked thinner than because muscle does weigh more fat. And people under guess your weight when you are muscular because muscle tends to make people look leaner. So for people who are interested in changing their body composition and not just their weight, knowing body fat percentage better than BMI. For example, if you are exercising to build muscle, which is preferable, knowing your body fat percentage is a good idea. Also, when losing weight, you want to preserve as much lean body mass as possible. Multiple websites have calculators that can help you determine your body fat percentage like  www.lowcarbdiets.about.com.  I hope this helps a bit to clear up confusion.

Mana Kasongo, MD is a board-certified emergency physician as well as a nationally published writer with expertise in emergency health care issues and women's health.  Email her at anovelistamedia@gmail.com with your women's health questions.  All correspondence will be kept strictly confidential and your name will not be used if your question is selected.  The Ask Dr. Kasongo column is for entertainment purposes only and the recommendations are merely suggestions.   If you have a true concern you should consult your personal physician. 

Comments

[BreathofLight]

Great information in this post! And I like this blog. My only issue is that you stated that muscle weighs more than fat. Which is not true. Muscle is more "dense" than fat. Which means you can fit more muscle in a space than fat. Which means someone with more muscle density will "look" smaller than someone who weighs the same but has a highter percentage of fat. A pound of muscle weighs the exact same as a pound of fat.