Understanding BMI, Body Composition, and Body Fat
The Difference between BMI and Body Composition
It is no secret that the incidence of obesity is on the rise and that the approximately 34% of Americans are obese. These statistics are based off of data collected using the Body Mass Index or BMI.
BMI is calculated by dividing a person’s body weight in kilograms by the square of their height in meters.
A score between 18.5 and 24.9 is considered normal, 25-29.9 is overweight, 30-34.9 is obese, and greater than 40 is extreme obesity. The advantage of the BMI is that it can be easily administered, can be administered to large groups, and it does correlate with other chronic disease risk factors. The negative, for some, especially people that are Spartan fit, it can classify people that are of normal weight as overweight or even obese.
BMI doesn’t take into account body composition. Body composition is the relative amounts fat mass versus fat free mass. For example, we could have two people of identical height and weight, one is sedentary the other is a highly trained athlete. They would have an identical BMI, yet if you compared them side by side they would have widely different physiques. For this reason, in athletes I coach we don’t use BMI, I measure their body composition and actually calculate percentage of body fat. But don’t misunderstand, BMI is a valuable tool and will correctly classify 85% of the population, but for people that train regularly (5+days per week) it may not be the best tool in the chest.
Measuring percent body fat isn’t a perfect science either. The margin of error is dependent on the method used. For the two most common methods used, skinfolds and bioelectrical impendence, the standard error is 3-5%.
For example, if an athlete’s measured body composition is 10%, their actual body composition is between 7-13%. It takes experience and practice under a trainer technician’s supervision to be proficient in skinfold measurement, so the error may actually be larger in many cases. Bioelectrical impedance varies widely depending on time or day, hydration, previous exercise, and food intake. Are they useless? Yes and no.
When people use the score as an end themselves, yes they are useless. Yay I am 5% body fat!!! Pointless. But as a tool to track changes and progress it is excellent. In order for the measurements to be reliable, they should be made by the same person and under the same conditions. Even though the process is standardized, if one person deviates slightly from the protocol, or uses a different piece of equipment, or does it under different conditions the score may differ. I monitor my % fat with a bioelectric impedance scale. I measure it every Friday morning after voiding, before exercise, and before eating. The numbers are reliable and will track changes. I use it to make sure it is going in the right direction or staying the same.
What is ideal?
But what is ideal? For most of us to have a body fat % lower than it is now would be better, but what is that target number? Somewhere between 10-20% and 20-30% is considered healthy for men and women respectively. But what about those who want to race competitively in a Spartan Race? Like many endurance sports, having extra body mass is a hindrance to performance. An athlete has to carry that extra mass up hills, over obstacles, or crawl with it under barbed wire. Body composition recommendations for athletes have been determined by taking the average body weights and fat percentages from large groups of athletes in various sports. These body fat percentages ranged from 5-15% in males and 10-20% for female athletes. But that is not to suggest that they represent an ideal for a particular person, and actually for some having a body fat percentage too low may actually be a hindrance. Very low body fat is associated with increased risk of infections, maladaptation to training or overtraining, amenorrhea and poor reproductive health in women, and chronic fatigue.
A logical strategy would be to compare present body composition to standards for good health. The first goal should be to make sure it is within that range. Next, if the goal is to improve performance try and reduce body fat percentage to the mid to upper range cited for athletes. Notice any significant improvements in performance? Are you recovering well, do you have energy, have you been avoiding infections? If yes, this new body composition is appropriate for you. You could then set a new goal and strive for a percentage slightly lower and monitor those same feelings. You may also just decide to maintain your current level. Essential fat for men and women is 3 and 12% respectively. Striving for body fat percentages below essential fat will be detrimental for just about anyone. There is no “ideal” body fat percentage for everyone. Get healthy first, then if you desire, see what you can accomplish that is reasonable for you.