Measuring Body Composition

I came up with the idea for this piece after a good laugh I had the other day. I recently decided to take up running again and thought it would be great to use my heart rate monitor periodically to track my fitness. The watch required a new battery which meant that all the previous settings were lost. When I entered my height and weight, to my surprise, the watch displayed, “BMI … 25.0 … Overweight.” Apparently well-defined muscles is overweight these days.  

BMI or Body Mass Index is calculated by taking your mass in kg and dividing it by your height in m squared. A BMI of 25.0 is on the edge of normal and overweight. The problem with BMI is that it assumes that everyone that’s heavy is heavy because they’re fat. Well-muscled people also happen to be heavy. BMI is best used to look at populations, but is inappropriate for individuals. If the population has an average BMI of 33, this is a cause for concern as the average individual is obese.

            Similarly, simply weighing yourself is a poor method for measuring changes in body composition. The scale may not have budged, but you could’ve lost several pounds of fat and replaced that with muscle thereby improving your body composition. Conversely, the scale may read a lower number but most of the weight lost came from muscle and water loss.

Dual energy x-ray absorptiometry (DEXA): This is the most accurate method of measuring body composition, but it’s not readily accessible for most.

Hydrostatic underwater weighing: This method is also very accurate, but again it’s not readily accessible for most.

Skinfold measurements: While less accurate than DEXA, it’s more accurate than the scale and it can be done at home. Instead of converting the numbers to a percent body fat it’s better to just look for improvements in each measurement over time as body fat can redistribute itself.

Bioelectrical impedance: The device for measuring this is easy to obtain, but it can be thrown off by fat distribution.  

Waistline measurements: This is a little less accurate than measuring skin folds, but it’s quicker and requires less skill.

Looking in the mirror: While this method is qualitative I still prefer it over the scale. If you look better your body composition has improved.

Clothing fit: If your clothes fit better you’ve lost body fat. If you’re trying to gain muscle your clothes will feel tighter.

If you’re looking to improve body composition focus on more subjective measures such as looking in the mirror and comparing clothing fit. On a weekly basis use skinfold or waistline measurements and only occasionally should you look at the scale.

Stay fit,

Jonathan Pratt

Pratt's Pro Fitness