Decoding the numbers: How accurate are body fat scales?

You've been doing all the right things to get in shape, like eating healthy, drinking plenty of water, and exercising. You jump on the scales with a glimmer of hope in your eye, expecting to see the fruits of your labour — but are dismayed to see that the number has, in fact, gone up.

While this can be a frustrating and disheartening experience, it's not always a true indication of reality. There are many factors that affect body composition, and your run-of-the-mill bathroom scales can't always tell you how much of your 'weight gain' is fat or muscle.

This is where body fat scales come in. Instead of focusing solely on your body weight, these high-tech scales provide you with a range of other health metrics, including body fat percentage — the percentage of your total mass that is made up of body fat, as compared to lean muscle, bone, tissue, and water. These measures can give you a much more nuanced perspective of your progress, especially if your goal is to tone up, not just shed kilos.

But, just how accurate are these at-home body fat scales, and how do they work to estimate your fat mass? Plus, how do they compare to other methods of measuring body composition? Here, we break down everything you need to know.

How do body fat scales work?

While standard scales simply measure physical load, body fat scales or 'smart scales' can tell you what percentage of your total weight is fat-free mass, and how much is actually fat.

You might be wondering — how exactly do these scales know how to differentiate body fat versus muscle, tissue, and bone mass?

Powered by 2 electrodes inside the scale, these gadgets use a technology called bioelectrical impedance analysis (BIA) to estimate how much fat you're holding. When you step on one, a light electrical current runs up and down your body.

This signal is 100% safe and painless, and you won't be able to feel it. However, behind the scenes, it's using an arm-to-leg or foot-to-foot analysis (depending on the type of scales) to estimate your body composition.

Body fat, tissue, and muscle all hold varying levels of water [1]. This creates different levels of resistance (otherwise known as impedance), which affects how easily the signal can pass through your body. For example, body fat has a lower water content than muscle, which means the current will move more slowly when it comes into contact with it. Therefore, the bioelectric impedance measure can give you an estimate of your fat-free mass, based on how long it takes the signal to pass through your body.

As well as storing your data inside the scales, many smart scales are Wi-Fi enabled. This means you can connect it with health and fitness apps, to easily keep track of your measurements over time.

Do scales measure body fat accurately?

Body fat scales are a safe and convenient way to measure body composition at home. But, when it comes to the question 'Are body fat scales accurate?' the answer is 'it depends.'

There are many different smart scales on the market, and some can underestimate or overestimate body fat percentage. However, the best scales using bioelectrical impedance are said to estimate body composition with around 3-4% accuracy [2].

One factor that can affect body fat scale accuracy is the number of measurement points on the scale. The more sensitive scales that collect measurements from 4 points of your body (such as your hands and feet) will always give a more accurate reading than those that only measure 2 points.

Hydration can also play a role as if you're dehydrated, you'll likely see your body fat percentage is overestimated (and vice versa). Other factors like water weight and swelling can impact the accuracy of your measurements.

No form of body fat measurement is without its limitations, and for this reason, it's better to treat the number as a rough estimate rather than an absolute.

As Melynda Dennis, a physician assistant specialising in weight management at Houston Methodist advises: "Don't worry over a particular reading on your scale at home," says Dennis. "Instead, use it as a way to track body composition trends over time. When the same scale is used under the same circumstances regularly, at-home body composition scales can be a great way to track your progress over time or uncover a significant shift in your body fat." [3]

What's the most accurate test to measure body fat at home?

Many personal trainers will tell you that the most accurate way to measure body fat at home is with the pinch test. This is when you use body fat callipers to pinch the skin folds and take measurements from various sites on the body.

These measurements are then plugged into a formula, to calculate your body fat percentage [4]. In the right hands, this can be an accurate way to calculate fat mass. However, it is highly prone to human error and can be quite uncomfortable and challenging to do yourself.

Some people also swear by the old-school method of using a measuring tape to take measurements from different sites of the body. However, this is mostly useful for collecting circumference measurements, rather than assessing body fat.

With this in mind, smart scales are the most convenient and accurate way to track your body fat percentage at home. While the number may not be exact, it can give you a fairly good indication of your starting point and whether you're losing fat, muscle or both.

Other ways to measure body fat

Throughout history, humans have found countless ways to measure body fat — but not all of them should be attempted at home.

Currently, the DXA scan (dual-energy X-ray absorptiometry, or DEXA) is considered the gold standard to measure body composition. Originally designed to assess bone density and diagnose osteoporosis, this scan can also measure body fat percentage with around 1-2% accuracy [5].

Usually carried out in a medical imaging lab, it involves lying still under a machine, while dual low-power X-ray beams scan your body from head to toe. The amount of X-rays that come through the tissue from each of the 2 sources is measured by a detector, which provides an estimate of tissue density, muscle mass, abdominal fat, and more.

While this test measures body fat with great precision, thus far nobody has found a safe and effective way to integrate X-ray absorptiometry technology into at-home smart scales. Plus, as excessive exposure to radiation carries health risks, this type of scan is not designed to be used regularly.

Another strategy is hydrostatic or underwater weighing, which is a type of air displacement plethysmography. This is based on Archimedes' principle that the amount of water an object displaces is equivalent to the volume of an object [6].

The logic here is that, because muscle sinks and fat floats, someone with more muscle will weigh more underwater. To facilitate underwater weighing, the subject is first weighed on dry land and then submerged in a tank. They are required to expel all air from their lungs, before being weighed underwater.

This method is surprisingly accurate, which is why it's often used for athletes in sports labs. However, as you may have guessed, it's not exactly convenient for the average person.

The best body measurement system is the one you can do consistently, which is what makes smart scales the ideal option for most people who are trying to lose weight.

Why measure body fat percentage?

Typical scales only capture your total body weight, which can be used to calculate your body mass index (BMI).

However, at the end of the day, your BMI is only 1 component of your health and well-being — and considering that muscle weighs more than fat, it's not always the best indicator of your body composition.

Methods that measure the percentage of body fat have been found to be a much better predictor of overall health, including cardiovascular risk factors [7]. Plus, if you're looking to achieve a stronger, fitter look, increasing lean muscle mass (and reducing body fat) is key.

Many smart scales also measure other key biometrics, including visceral fat, water percentage, bone density, muscle mass, and BMR (basal metabolic rate). These measures can help paint a clearer picture of how your body is responding to all your hard work, which is far more motivating than looking at the number on the scales alone.

Smart people need smart data to keep them on track. By joining Juniper's Weight Reset Programme, you not only get the medically-backed resources you need for sustainable weight loss but also a free pair of digital body fat scales with your first month's delivery. With the scales connecting directly to the members-only app, this gives you a birds-eye view of your progress, plus all the encouragement you need to keep going.

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