How accurate are body fat scales? The short answer is that they’re not accurate at all.
In fact, you can’t rely on any of the body fat tests out there to track fat loss and muscle growth over time.
Using the results to guide your training and diet decisions may well send you off in completely the wrong direction.
I’ll explain why in a moment. First, I want to take a step back and explain why I don’t trust any of the body fat tests out there.
What Is the Most Accurate Way to Measure Body Fat?
No body fat test, be it body fat scales, DEXA scans (dual-energy x-ray absorptiometry), air displacement plethysmography (the Bod Pod), underwater weighing or skinfold calipers, can accurately measure how much fat and muscle mass you have.
The only way to measure your body fat is to have it stripped out, placed on a scale, and weighed. Although this method, known as carcass analysis, is highly accurate, you have to be dead in order for it to happen.
In fact, a body fat test is nothing of the sort. It should be renamed body fat speculation, because that’s what it is. It’s not a test, it’s conjecture.
Put differently, it’s less of a measurement than it is a (very) rough estimate. A guess about what your body fat percentage really is.
While this guess often comes dressed up in complex equations and fancy charts, it’s still a guess, one that’s often a lot less accurate than many believe.
How Do Body Fat Scales Work?
Body fat scales are one of the most popular ways to measure how much fat and muscle mass you have, mainly because they’re quick and easy to use. They use a method known as bioelectrical impedance, or BIA for short, to estimate your body composition.
You take your socks off and step on the scales, which then send out a weak electrical current. This electric current passes up one leg and down the other. The body fat scales then measure the degree of resistance (or impedance) to the flow of the current .
Different tissues provide varying levels of resistance, with fat-free mass providing less resistance than fat due to its higher water content.
The idea is that by determining the level of resistance to the current, the scale will know how much lean tissue and fat mass you have, which it then uses to estimate your body fat percentage.
Some so-called smart scales also sync with fitness apps on your smartphone via bluetooth or wi-fi, transferring data to Apple Health, Fitbit, or Google Fit.
You’ll then be presented numerous graphs, charts and diagrams, showing you what your body fat percentage is, how much visceral fat, bone mass and muscle mass you have, what your basal metabolic rate (BMR) is, and various other metrics.
But while it might all look very scientific and official, much of this data is completely useless.
How Accurate Are Body Fat Scales?
Most research shows that body fat scales aren’t accurate at tracking individual changes in body composition over time.
In fact, you could lose fat and gain muscle mass over a period of several months, but body fat scales might say that your body fat percentage has gone up.
The first and perhaps most obvious problem with body fat scales is that they miss out large segments of your body.
Stand on a set of foot-to-foot body fat scales, for example, and the current will simply go up one leg and down the other .
So the device is really only gathering data from your legs, and making assumptions about what’s going on in the rest of your body.
Changes in hydration status will also have a big impact on the results. In fact, bioelectrical impedance analysis seems to interpret a change in body water as a change in fat mass .
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Levels of total body water can also be affected by the type of training you do. In a group of men using strength training to lose weight, changes in body fat measured by underwater weighing and bioelectrical impedance analysis agreed reasonably well .
But in those who lost weight using cardiovascular exercise, bioelectrical impedance analysis underestimated fat loss and overestimated the loss of fat-free mass.
This discrepancy appears to have been caused by a change in total body water resulting from an increase in plasma volume, which is one of the adaptations to cardiovascular training.
One of the arguments in favor of body fat testing is that even if a given test isn’t accurate, at least it’s consistent.
In other words, it doesn’t matter if a body fat test is out by a few percentage points here or there. As long as it’s consistently inaccurate, you can use it to track your progress over time.
The problem with this idea is that a change in weight causes a shift in the density of various tissues. Different types of training also have different effects on the density and composition of fat-free mass .
But bio-electrical impedance analysis wrongly assumes that the density of various tissues is the same from person to person, and remains constant over time as you lose weight.
In fact, foot-to-foot bio-electrical impedance analysis has been shown to overestimate body fat percentage in lean subjects, while underestimating it in obese subjects .
That is, the more fat you have, the more likely it is that body fat scales will underestimate your body fat percentage.
On the flip side, the leaner you are, the more likely it is that you’ll end up with an estimate of your body fat percentage that’s higher than it really is.
For example, let’s say that your true body fat percentage is 35%. You step on a set of body fat scales, and they underestimate your body fat percentage by 5%, putting you at 30%.
You then train hard and diet until your true body fat percentage hits 15%.
At this point, the body fat scales are more likely to overestimate your body fat percentage, and might put you at 20%.
If the body fat scales were consistently inaccurate, and always over or underestimated your body fat percentage by the same fixed amount, they’d do a decent job of tracking your progress over time.
But that isn’t the case. Not only are they inaccurate, they’re inconsistently inaccurate.
Consistency vs Accuracy
I often see forum posts from people who own a body fat scale and swear blind that it’s accurate, mainly because their body fat percentage and weight both trend down over time.
The body fat scales might not be accurate, they say, but they’re useful for showing a trend.
Given that total weight is one component of the prediction equation used by body fat scales to estimate body fat percentage, it would be very surprising if there wasn’t a downwards trend.
That is, when body fat scales estimate your body fat percentage, the resistance to the flow of the current is only one component of the equation.
For example, here’s one prediction equation used to estimate fat-free mass in obese subjects, using underwater weighing as a reference method .
FFM = 9.3794 + (0.0009 x height(2)) – (0.015 x resistance) + (0.3 x weight) – (0.07 x age)
What this means is that any change in weight is also going to affect your body fat percentage, independently of changes in resistance to the flow of the current.
Step on a set of body fat scales while holding a 10-pound dumbbell. Do the same thing again, but without the dumbbell. Chances are the device will show your body fat percentage has gone down.
Assuming that your weight is going down, estimates of body fat percentage derived from BIA are also likely to trend down over time.
But that would hold true even if the scales didn’t collect any impedance data, and used a simple formula based on BMI, sex and age to predict body fat.
Some also point to the fact that when they take back-to-back readings (i.e. they get on the scales, get a reading, get off the scales, then step back on two minutes later) the scales show similar results.
That is, when they take back-to-back readings (i.e. they get on the scales, get a reading, get off the scales, then step back on two minutes later) the scales show similar results.
It’s giving me much the same number from one reading to the next, they think to themselves, so it must be right.
Problem is, they’re confusing consistency with accuracy. Multiple back-to-back readings will determine test-retest reliability — how close the numbers are when repeated measurements are made.
However, test-retest reliability isn’t the same thing as accuracy.
To determine accuracy, you need to compare the readings you get from the body fat scales to some kind of reference method.
As I mentioned earlier, the most accurate way, and indeed the only way, to measure your body fat (as opposed to estimating it) is carcass analysis (i.e. stripping the fat from a dead body and weighing it).
When it comes to estimating body fat, the current state of the art is something called the 4-compartment model, or 4C model for short.
The 4C model is an expensive method of measuring body composition that divides the body into four components (mineral, water, fat, and protein) and measures each one independently.
Total body water is measured using a method known as deuterium dilution, body density is measured with underwater weighing (or air-displacement plethysmography), while bone density is measured with a DEXA scan.
Short of killing someone, stripping off their fat and weighing it, the 4C model is as good as you’re going to get. It’s currently the benchmark test for body composition and the gold standard against which other body composition tests are measured.
Do Scales That Measure Body Fat Work?
When researchers have compared body fat scales with the 4C model, the results have been less than impressive.
In one study, researchers from Maastricht University looked at changes in body composition in a group of male bodybuilders .
They compared several body fat tests — including DEXA scans, underwater weighing and bioelectrical impedance analysis, the technology used in body fat scales — with the 4C model.
Bioelectrical impedance was the least accurate of all the methods. In fact, the error got as high as 8%.
So what does that mean exactly?
Let’s say you step on a set of body fat scales, and your body fat percentage comes out at 20%. You eat right and train hard for a couple of months, and get down to 15% body fat.
But when you go back for another test, the scales might say that your body fat percentage is still 20%.
Same thing holds true for muscle growth. You could gain 5-6 pounds (around 2-3 kg) of muscle over a period of several months. But the scales might show that you hadn’t gained any muscle at all.
You’d come away with the impression that whatever you’d been doing to generate those results didn’t work, when actually it did.
And you run the risk of ditching a training and nutrition program that’s working and replacing it with one that’s less effective.
Researchers from Texas also tested a number of bioelectrical impedance analysis units, and found an equally large error when compared to the 4-compartment model .
Of the three bioelectrical impedance analysis devices they tested, the Omron HBF-306 (single-frequency hand-to-hand electrodes) had the largest error for estimating body fat percentage, clocking in at 7.9%.
That is, if the Omron puts you at 15% body fat, it could really be anywhere between 22.9% and 7.1%.
Both the single-frequency (Tanita TBF-300A, which has foot-to-foot electrodes) and multi-frequency (Seca mBCA 514/515, which has electrodes for both the hands and feet) units didn’t fare much better, with an error of 7.6%.
Assuming they give you a body fat reading of 15%, your true body fat could be as low as 7.4% or as high as 22.6%.
BMI vs Body Fat Scales
It is possible to estimate body fat percentage using just your body mass index, or BMI for short.
The prediction equation is based on BMI, age and gender. Here’s what it looks like:
Body fat percentage = (1.2 x BMI) + (0.23 x age: years) – (10.8 x sex: male = 1, female = 0) – 5.4
Despite the fact that BMI is simply a function of your height and total body weight (which you can get from regular bathroom scales), in some cases this equation has done a surprisingly good job at predicting changes in body composition.
In one study, with the exception of one individual, BMI did just as well as BIA for estimating changes in body fat percentage .
In another, the BMI body fat equation actually had a smaller error than BIA for detecting changes in body fat percentage over time .
A body fat test is meant to let you know when you’ve achieved a specific goal (such as reaching a certain body fat percentage), to let you know if what you’re doing is or isn’t working, as well as help you assess your risk for various health conditions, such as diabetes or heart disease.
Body fat scales fail on all three counts, simply because the margin of error is so large.
I’ve worked with people who have clearly lost fat and gained strength over a period of several months. They were stronger. Their clothes fitted better. They looked better in the mirror.
But the body fat percentage scales said that their fat percentage had actually gone up.
This left them feeling like all their hard work had been for nothing. Such was their faith in technology that they were more willing to believe a machine than what their eyes were showing them.
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