3/03/2009

Body Fat: Let's Not Get Carried Away with the Measurements!

Body Builders and certain competitive athletes may need to obsess about their % body fat, but most of us just need a simple reliable measurement of progress. There are more than a dozen different techniques for estimating body fat, including infrared and x-ray techniques, and one that involves drinking a radioactive isotope . Keep in mind that any approach gives you an estimate based on various measurements and mathematical formulas, you can't directly measure the fat like you can with weight and height. You would have to go through the inconvenience and mess of removing all the fat from the body, weighing it and replacing it to do that. It is best to look at the body fat measurement over time to see changes, and to have it performed by the same competent person using the same equipment. All of the methods have strengths and weaknesses, and research has been conducted to validate them. Here is a recap of the measurement methods:

Hydrostatic Testing - Immersion in a tank of water and measuring displacement is accurate within about 2% for most people. Adjustments by race are made, since African Americans tend to have denser bones while Asians have lighter bones. Air Displacement Plethysmography (The Bod Pod) is a similar but uses air displacement instead of water.
Skin Fold/Caliper Testing - Results are accurate within +/-3.5% for 90% of the population. Results for the other 10% can be off by much more. Variation by tester can be a big factor, so have the same person take the measurements and look for a trend in the results. This technique tends to overestimate body fat for lean individuals and underestimate for obese. Caliper testing needs to be done slowly, carefully and precisely, usually measurements are taken at least three times and averaged for a result.
Bioelectric Impedance - Results are accurate within +/-3% for 82% of the population, and can be off by up to 20% or more for the remaining 18%. Results are inaccurate for small changes and can be affected by hydration, skin temperature, exercise within 12 hours prior, alcohol within 48 hours, food within 4 hours, bladder and bowel content. A body scale once estimated me at 38%, almost 3x the measurement obtained from calipers around the same time.
Circumference Testing - using a few simple measurements, results are accurate within +/-5% for 86% of the population. (There is an online tester available at bodyfat.caryraffle.com, it measured me at almost the same as the calipers and is very easy to use so that you can track your results.)
Ask me - I can usually estimate someone's body fat within a couple of percent, not much less accurate than some of these techniques and a whole lot easier. (Be forewarned, you may have to show me your bare midriff!). If you get a body fat estimate that doesn't make sense - maybe from one of those diet supplement people who set up tables on the streets or at the mall - feel free to contact me for a second opinion.