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Body Fat Measurements

Posted Feb 24 2011 8:13am

Exercise Update! The thighs are beginning to feel a little bit more flexible today.  They’re still achy, but I’m going to try and attempt to squeeze in some lunges later.  As for upper body, the experts (you) have spoken- push-ups it is! I’m going to look into the 100-pushup-challenge as Melissa suggested… but I didn’t have time to do so yesterday, so I did 15 girly push-ups and called it a day ;)

There were some other fabulous suggestions, including yoga, which I agree, those downward dogs and chaturangas totally work my arms!

Moving on…

I spend a lot of my time with clients explaining body composition and the importance of tracking your body fat rather than just what the scale says.  Many people are confused when gyms measure their body fat and it is different than the measurement we come up with while using calipers or the Body Gem, which I use to measure a person’s resting metabolic rate. I explain to them that all these methods of measuring body fats aren’t 100% accurate and all contain some margin of error, usually about 2%.  However, some are more accurate than others…

Methods for Testing Body Fat


DEXA (dual energy X-ray absorptiometry)

This test is used to measure bone density, but it also measures body fat percentage as well as where most of your fat is (as if you didn’t know). The facts about DEXA:

  • DEXA uses a whole body scanner and two different low-dose x-rays to read bone mass and soft tissue mass.
  • It takes about 10-20 minutes to do a body scan
  • It provides a high degree of precision with a 2-3 % margin of error.
  • This is considered a gold standard for measuring body fat and bone density
  • It’s painless

Hydrostatic Weighing

This is just a fancy way of saying underwater weighing. Like DEXA, this test is one of the more accurate ones. How it works:

  • You sit on a scale inside a tank of water and blow out as much air as you can
  • You are dunked underwater, where you blow out even more air
  • Since fat is lighter than water, the more fat you have, the more you’ll float. The scale measures underwater weight to figure out body density.
  • The margin of error is around 2-3%, but the accuracy depends on the amount of air you expel. You have to blow it ALL out or it won’t be as accurate.
  • This is a difficult way to measure body fat since it can be uncomfortable and even scary to be dunked underwater with no air in the old lungs.

Calipers (aka, the Pinch Test)

This method uses calipers to measure skinfold thickness at several areas of your body. How it works:

  • An expert pinches your skin at different areas and measures them with calipers.
  • The results are plugged into a formula to determine your body fat.
  • The result is based on the idea that thickness of fat under the skin reflects total body fat.
  • It CAN be as accurate as the methods mentioned about, but usually isn’t. It depends on the skill of the tester at separating your fat from your muscle and finding the right spots to pinch.
  • The results can also be skewed if you’re older (since fat moves inward with age) or if you are nonwhite, since formulas are based on white subjects.
  • This is one of the more accessible ways to check body fat and is generally painless.

Bioelectrical Impedance

This is one of the quickest methods of testing body fat using a BIA scale.

  • Either using a handheld scale or standing on a BIA scale, a signal passes either from hand to hand or foot to foot.
  • The faster the signal travels, the more muscle you have.
  • The results are based on the fact that water conducts electricity. Fat contains almost no water while muscle is about 70% water.
  • This method CAN be accurate (4% margin of error) but the results are affected by hydration, food intake and skin temperature. If you’re dehydrated, you’re body fat percentage will read higher than it is.

Body Gem Metabolic Machine

Along with it’s other many benefits, the Body Gem measures your body fat percentage.

  • RMR is highly related to muscle tissue, in theory, one should be able to determine the amount of fat-free weight from an RMR measurement.
  • Results indicate the measurement is within +/- 2% of DEXA measurements (the gold standard). 

Phew… that’s a lot of info… if you’re still reading, I’m impressed!

Have you ever had your body fat measured?

With more than 1 method?

Did the measurements differ?

Thoughts of body fat measuring?

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