We often scans folks for body composition and at some point body mass index (BMI) comes up. First, we have to talk about what BMI is, this is done by working with a simple formula of the weight in kilograms divided by your height in meters squared.
This is a good tool if you are working with folks that don’t work out, but if you are working with resistance trained individuals they can have a higher weight for their height but this is often due to lean mass. I’m a good example of this, since I weigh about 210lbs. at a height of 5’9.5”. This gives me a BMI of over 30 which is considered obese, but my body composition by Dexa scan is just about 15% which puts me in the normal category of body fat levels. Mind you most guys that weigh over two hundred pounds at my height are likely that size because they carry excess body fat, but you need to understand the subtlety there.
Research in the area of BMI to longevity seems to show what is known as a “j” shaped curve, where people with higher BMIs (over thirty) and lower (under 18.5) have higher all-cause mortality risk, but the lowest risk seems to be at having a BMI of 25 which is the cusp of normal to overweight. This is due likely to a number of factors; people that are super big have higher risks for things like diabetes, heart disease, high blood pressure, etc. Then small folks have very low reserves of everything so when you lose weight due to illness like the flu it can quickly become a life and death situation. Also, this is data from massive amounts of humans and what we are really looking at is individual response.
This is not fully rationalizing being a massive human being with muscle since your heart still has to send blood to that entire larger frame which puts more stress upon it. Very large human beings don’t live as long as normal sized folks and there is no way around that.
This is what your weight is composed of. Most of the time it is referring to the two compartment model that divides the body in to lean mass (bone, muscle, most organs, ligaments, tendons, etc.) and fat mass (subcutaneous, brain, bone marrow (mind you the brain does not weigh 20 pounds, same with the bone marrow). So when you have a higher percentage of body fat it is typically due to increased subcutaneous fat which is the fat right under your skin, and visceral fat which is wrapped around your organs. Higher amounts of body fat are related to metabolic disease and other negative health outcomes and super low amounts of body fat are related to issues with fertility and other negative health outcomes. If your goal is to live as long as possible you want to have a body fat percentage between 6-20% if you are male and 15-32% if you are female. Being above that is not a death sentence but being above 25% for men and 40% for women we are talking about being obese and higher risks (I can go more in to optimal body fat in a later post if you are interested).
You can have a normal healthy BMI but have an obese amount of body fat percentage. So being a healthy weight is not always a good indicator of overall health. If you truly care about your health, learn about what your body fat percentage is and track it over time. You don’t have to be “shredded” or have super low body fat, but making sure you keep your level at a healthy or nearly healthy range can help you out. The average American tends to increase their body fat percentage each year of their life so do what you can to maintain yours (and decrease it if need be).
Finally, when your body weight changes from training or a diet if you take your body composition at the same time that you do that you can figure out what type of tissue you gained or lost. This is important, because sometimes you might gain muscle and lose fat, but on the scale not lose a single ounce. You can also do the body by Haagen-Dazs and gain fat while losing muscle, or go hard core training montage and lose weight while still gaining muscle.
BMI works well if you are working with sedentary folks, that being said you still need information on body composition to make some stronger assessments of health and wellness. Do your best to track your body comp so you can see the effectiveness of your diet or training. Thanks as always for reading and if you have any questions please drop in a comment.