Understanding BMI, Body Composition, and Body Fat

by Jeff Godin, Ph.D., CSCS, of Spartan Coaches


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.

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6 Responses

  1. avatar

    Thank you for this article, is show me that i don’t feel bad to my body and continued to trainee hard.

  2. avatar

    I use a bio-electric impedance scale that also measures hydration levels. Like Dr. Godin said, there is a high variance in the percentage calculated. I’ve also never read any statics on the reliability of, or the science behind the how it calculates hydration percentage. One thing that has come to my attention over the course of gathering these readings is thehigh level of variance based solely on whether or not your skin is wet (mainly the bottoms of your feet). I’ve taken reading before and after a shower that have differed as much as 12%. Again, I’m using all of these measurements as a baseline to determine progress, and paired with progress photos and general “how do I feel” to plan further training and diet, not as a be-all-end-all sign of fitness.

    With that said, I do still have a question. Rather than just taking 1 measurement, 1 day a week, I set a side a day of the week (usually Monday) and measure myself several times that day, noting the time of day and conditions present during the measurement. Upon waking, before 1st workout, after 1st workout, at lunch, before 2nd workout, after 2nd workout, and just before bed would be common times for me to gather a reading. After I’ve gathered a day’s readings, I’ll pull out the calculator and create an average. I’m certainly not a doctor, and all my knowledge of the body and equipment used to measure it is self taught, but it just seems like common sense that given the opportunity and ease of taking multiple readings throughout the day, why not? It seems to me like it would provide a slightly more accurate reading since you never will be able to control the little variables as much as the method requires.

    Does anyone with more knowledge on the bio-electric impedance scale or body fat measurement have anything to offer for or against the “average” method as opposed to the “1 time” method?

  3. avatar

    Interesting read!

    Having dropped 57lbs so far, my body fat has gone from 40% to 28.8% (as of yesterday). I have set the Spartan Sprint in London as a goal/target/challenge! Bring it on!

    Not quiet up to standard of some of the workouts (pull ups being one). But train 5-6 days per week and am going to lose another 40lbs or so by race day.

  4. avatar

    Great article!

  5. avatar

    Damn it, Jeff! I thought you were leaking that the legendary Doc’s Barbecue was providing the after-Beast meal, but NOOOOO, the whole article was all about body fat measurement.

    I’m so disappointed right now.

    Seriously though, thanks for the info. As an aspiring personal trainer, it’s great to get good intel from a pro.

  6. avatar

    An additional method of checking your body fat is to use the home scales that you can get which automatically determine it for you. There is a lot of controversy about if the scales are all that exact or not although ultimately, it may offer you an idea about the amount of excess weight you’re carrying around.
    ideal body fat percentage

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