Understanding BFP and BMI

All you need to know about BMI

Does This Machine Make Me Look Fat?

BFP vs BMI: Remember when everybody was talking about BMI being the latest and greatest measure of a healthy body composition?  BMI is short for Body Mass Index, which is the percentage of weight in relation to height.  BMI is supposed to be an indicator of how much of your body composition comes from fat.  New research then came along showing that BMI was not such a good indicator as previously thought.

Scientists today have determined that what really matters is something called BFP, or Body Fat Percentage.  BFP is the amount of fat in your body compared to everything else.  To understand how to measure it, it is necessary to first understand some basics about body composition.

Body Composition Basics

Body mass is composed of four elements:  fat, water, bones, and protein (muscle). Thus, fat mass is total body mass less water, bones, and protein. The largest of these components by far is water.

So let’s talk about body water.  When we are born, water accounts for about three-quarters of our total body weight.  By adulthood that percentage has dropped to about half, with variances according to factors such as age, physical condition, muscle mass, environment, ethnicity, and total weight.

There are a lot of different ways to estimate BFP. I should note that none of them are as accurate as an autopsy, but since that requires the body in question to be dead we’ll just skip right on past that one and talk about the other methods.

Another thing we’ll skip the past is low-tech methods such as skin folds, waist measurements, and fitness calculators. All of these methods can be useful, but this article is about the more sophisticated, high-tech methods available today.

BFP estimation techniques are categorized based on how many components are used in the calculation. Two-compartment models are the most prevalent so we’ll talk about those first.

Two-Compartment Models

Two-compartment techniques for measuring BFP divide total body mass into fat mass (FM) and fat-free mass (FFM). The primary weakness of the two-compartment model is that it lumps all the nonfat components together.

As such, the results can be skewed by water weight. Remember, FFM includes total body water, which can vary based on the factors mentioned in the previous section, plus how hydrated you are when you take the test. Two compartment methods include DEXA scanning, Underwater Weighing, Bod Pod, and Body Fat Scales.  Some of these are more accurate than others. All are estimates.

Here’s how each of them works.

DEXA Scanning

DEXA stands for Duel-Energy X-ray Absorptiometry.  It is also known as a bone density scan because it was originally developed for the purpose of measuring bone density. DEXA scanning is widely considered to be the most accurate two-compartment method for measuring BFP.

Getting scanned is rather like getting an X-ray.  You lie down on a table face up while a large scanning arm slowly passes over your body. As the arm moves, a narrow beam of low-radiation x-rays is passed through your body.

Some of the x-rays are absorbed by tissue such as fat and bone. The rest pass straight through.  The machine detects this and uses the information to produce an image of the scanned area. This is used to identify a fat mass, lean mass, and bone mass. The procedure is generally performed by a radiographer, or x-ray technician, and takes five to ten minutes.

DEXA scanners are at almost all large university physiology departments and some healthcare facilities. Most places that have scanners will provide evaluations upon request.

Underwater Weighing

Underwater weighing is a method for calculating body density. It is based on the principle that fat-free mass is denser than fat mass. As such, fat-free mass has a tendency to sink in water, whereas fat mass will float.

The idea is to submerge the subject in water, and then simultaneously weigh them and measure the amount of water they displace. This is used to determine body volume. Using the principle just described, body density can be calculated. Body density is then used to estimate fat and fat-free mass based on algorithms.

This is what happens. The person being weighed puts on a swimsuit and sits on a sort of a swing suspended over a water talk. The person then expels as much air as possible from their lungs before being lowered into the tank while a machine performs its measurements.

This process usually has to be repeated several times to ensure a good measurement. The whole thing takes about twenty to thirty minutes. I’ve seen videos of the procedure on YouTube and, frankly, it does not look fun.

Facilities offering underwater testing are not that hard to find in the larger cities, but you might have to hunt around if you live in a small town or rural area. There are mobile facilities that perform this procedure as well.

Bod Pod

Like Underwater Weighing, the Bod Pod is designed to measure body density, only it does so through air displacement rather than water displacement. The technology behind the Bod Pod is known as an Air Displacement Plethysmograph.

This technology measures body volume based on the physical relationship between pressure and volume.  Body density is then calculated as total mass divided by volume.  From body density, the relative proportions of fat mass and fat-free mass can be estimated.

The machine itself is quite cool looking.  It is an egg-shaped contraption that the subject is sealed into while all the measuring goes on. Bod Pods are produced by COSMED. They are usually located in fitness centers or universities.

Body Fat Scales

Body fat scales utilize Bioelectrical Impedance Analysis (BIA) to measure fat mass in relation to fat-free mass.  Body fat scales are becoming all the rage with fitness enthusiasts because they provide a quick and easy method for estimating BFP.

They work by running a light electrical current through your body.  Fat-free mass has less resistance to electrical current because it consists mostly of water. Thus, by determining the resistance of the current running through your body, it is possible to calculate how much fat mass and fat-free mass you have.

In theory.

In reality, not so much. That’s not to say body fat scales don’t work at all, there are just lots of things that can make it less accurate. For one, electrical current will follow the path of least resistance through the body, which means that it does not hit subcutaneous fat if you have it.

Additionally, most  BIA devices miss entire portions of the body. Still, the technology is relatively new and improvements may minimize these issues. For now, however, body fat scales are considered to be among the least accurate methods out there for measuring BFP.

Four-Compartment Model

And now we get to the highest echelon, the be-all, and end-all, the ultimate gold standard of BFP measurement—the four-compartment model.

The four-compartment model divides the body into four parts: mineral, water, fat, and protein.  Measuring them requires utilizing a combination of the techniques already discussed.

This model is obviously more accurate than the two-compartment model which only looks at fat and fat-free mass, which can be greatly skewed by variances in total body water.

Here are the steps.

First, measure body density. This is done by using either the Bod Pod or underwater weighing, both of which are considered to be very accurate for that purpose.

Second, measure total body water. This involves using a technique called deuterium dilution. Also very accurate.

Third, measure bone mineral content. This is done with DEXA scanning. Recall, DEXA scanners were originally built to measure bone density.

At this point, we have two of the four components: water and mineral.

The final step is to use these two measurements plus bone density to estimate the remaining two. This is done using an algorithm developed by the scientists performing the analysis.

Yes, I said scientists. Today, this kind of analysis occurs only in laboratory settings for purposes of research.  It is the most accurate measurement method available (outside of autopsy) but it is important to remember it is still an estimate.

What is a good BFP?

A person’s target BFP will depend on their gender and overall fitness goals. Following is a handy chart to use as a guideline.

Fitness Level Male Female
Obese 25% or higher 31% or higher
Average 18-25% 25-31%
Fit 14-17% 21-24%
Athletic 6-13% 14-20%