How do those body fat scales work?

If you're referring to being weighted under water, it's the relationship between weight and buoyancy.

I bought a talking scale at Sears. Got on it this morning and it said "One person at a time, please."

They send a small electrical current through your body and measures the resistance. This is why you have to be barefoot in order for it to calculate.

Water, Bone, Fat & Muscle all have different levels of electrical resistance (how easily electicity will flow threw them). Your scale actually sends a series of (very small) electical pulses through your body. By determining the amount of time it takes this pulse to travel from one sensor to the other and through some mathematics it can determine each of the relative percentages.

Body fat scales use a technique called Bioelectrical Impedance Analysis... or BIA for short. Very simply...

A small and completely harmless electrical current is passed through your body. The electrical current passes more quickly through fat free tissue like muscle than it does through fat or bone tissue. So...

The amount of resistance to the electrical current relates to how much fat-free mass a person has and their body density.

Here's the first challenge...

Like all body fat tests, body fat scales don't actually measure your body fat percentage. They determine your body density. The examiner (or the scales) then uses a formula to calculate body fat percentage based on body density.

Here's the key...

These formulas just predict your body fat. Unfortunately there is no one formula that accurately predicts body fat for the whole population. Differences in age, gender, ethnicity, body size, and fitness level all have a significant affect on the results.

Most scales can account for some of the basic differences such as age and gender, but take the actual body fat percentage they give you with a pinch of salt.

What does this all mean for you?

Well, whether body fat scales measure your "true" body fat percentage or not doesn't matter! As long as they can accurately monitor changes in your body composition over time, that's all you need.

Your body position, the amount of water in your body, your food intake, skin temperature and recent physical activity can all adversely affect the results of body fat scales.

So...

To achieve accurate, consistent results, you must standardise the way you perform each test. That simply means making each test with your scales as similar as possible.

Are body fat scales any better than plain old weighing scales?

Absolutely!

One of the biggest mistakes people on a weight management program make is gauging their progress by weight alone. If you are working out, as well as controlling your diet, body fat is the best measure of your progress....much better than weight as muscle weighs more than fat. Always measure yourself for progress checks too!

Good luck!