body fat %

so basically you need an instrument to do it? because i saw a scale in the store ( just a normal bathroom scale) and it claimed to be able to measure body fat %. it seemed like a gimmick to me though.

It uses BIA...


Changes in body composition during weight loss in obese subjects in the NUGENOB study: comparison of bioelectrical impedance vs. dual-energy X-ray absorptiometry.

Verdich C et al. Diabetes Metab. 2011 Jun;37(3):222-9.

"RESULTS:

At baseline, BIA significantly overestimated FFM and underestimated FM (by 1-3 kg on average) compared with DXA, and the limits of agreement were wide (mean ± 7-8.5 kg). "


A comparison of dual energy X-ray absorptiometry and bioelectrical impedance analysis to measure total and segmental body composition in healthy young adults.

Lehy S et al. Eur J Appl Physiol. 2011 May 26

"BIA was found to underestimate the percentage total body fat in men and women (p < 0.001). This underestimate increased in men with >24.6% body fat and women with >32% body fat (p < 0.001). Fat tissue mass in the trunk segment was overestimated by 2.1 kg (p < 0.001) in men and underestimated by 0.4 kg (p < 0.001) in women. BIA was also found to underestimate the fat free mass in the appendages by 1.0 kg (p < 0.001) in men and 0.9 kg (p < 0.001) in women. Compared to dual energy X-ray absorptiometry, bioelectrical impedance analysis underestimates the total body fat mass and overestimates fat free mass in healthy young adults."

and James Kreiger's excellent write up on bodyfat testing methods

I’ve heard people make the argument that, while BIA may not be that accurate, it should work fine when tracking change over time. The theory, they say, is that the error should be the same each time you use it.

The problem is that this isn’t true. As I mentioned in the article on hydrostatic weighing, the density and hydration of fat-free mass can change with weight loss. If this can affect the accuracy of hydrostatic weighing for measuring change over time, then you can be sure that the effect on BIA outcomes is going to be significantly larger.

Researchers have looked at the accuracy of BIA for tracking body fat change over time. In one study, the disagreement between BIA and the 4-compartment model ranged from -3.6% to 4.8% for measuring change. This means you could lose 3.6% body fat, but BIA would show no change. Or, BIA could tell you that you lost 8.8% body fat when you really only lost 4%. In fact, in this study, plain ol’ bod mass index (BMI) did just as well as BIA for predicting change in body fat, except for in one person.

BIA can be problematic because it’s a prediction based off of a prediction, so the error gets compounded. When you look at group averages for BIA measurements, there tends to be bias, with BIA often underpredicting how much fat you have. As with other techniques, the individual error rates can get high, with some research showing error rates of around 8-9%. In fact, BIA doesn’t do much better than BMI at predicting body fat in some cases. When it comes to measuring change over time, BIA can often underpredict the amount of fat loss, and the estimated change can be off by up to 8%.

got it. thanks! so it seems to me that a lot of people on the message boards measure their %body fat. in general would they use BIA? or some of the other mentioned methods? whats the easiest and most accurate way to do it? and does body fat % really mean that much? or is it akin to BMI?

My guess is most people get their bf% one of three ways; BIA, calipers or an online calculator that uses circumference measurements. The easiest and most accurate way would be to search for a DEXA scan (most places that cater to women have them) but it can be pricey, anywhere from $75-200. BF% is important, but I don't think you need a spot on number for most applications. BMI is not very accurate save for non athletic populations