Sometimes, health tracking accuracy is overrated

This is Optimizer, a weekly newsletter sent from Verge senior reviewer Victoria Song that dissects and discusses the latest gizmos and potions that swear they’re going to change your life. Opt in for Optimizer here.

About three years ago, a doctor told me I had to lose abdominal fat. She didn’t care about my lower belly fat. That, she said while pinching me, was subcutaneous fat. What she wanted was for me to target about five pounds of fat loss above my belly button because that’s where the visceral fat lived. Not weight loss; my BMI and weight, she said, were fine. I needed fat loss because I had borderline high cholesterol and one mildly elevated liver enzyme. Given my diagnosis of polycystic ovary syndrome (now known as polyendocrine metabolic ovarian syndrome), she was convinced that Type 2 diabetes lurked just around the corner for me unless I immediately overhauled my body composition.

She suggested I invest in a smart scale — as well as commit to a vegan diet. I ignored the latter (and, a few months later, switched doctors after she recommended a dubious dieting podcast). But I did start dabbling a bit more seriously with smart scales, embarking on a multi-year journey of frustration.

Smart scales use a method called bioelectric impedance analysis (BIA) to give you a breakdown of your body fat, lean mass, and bone mass. (Some high-tech smart scales include other metrics, but it depends on the device.) The gist is that a weak electrical current is sent through your body. Fat, muscle, and other types of tissue all have different levels of conductivity, so depending on the resistance measured, an algorithm then estimates how much of each you have. They’re more accessible for consumers than other clinical methods, but they can be notoriously inaccurate. It’s very easy to have two BIA scales and get wildly different results.

Case in point, this morning I stepped on three separate smart scales: two from Withings (one only measures through your feet; the other has more advanced segmented body analysis), and a third from a company called Twin Health. When I go for my monthly doctor follow-ups, I get measured by an InBody scale, which is a fancier version of the smart scales people have at home. Two weeks ago, I went to a sports clinic to get my first Dual-Energy X-ray Absorptiometry (DEXA) scan, which shoots low-power X-rays through your body to provide a breakdown of your body composition, including bone density. It’s often considered the “gold standard” in clinical settings. None of these devices has ever given me the same body fat percentage.

In fact, here are my most recent readings for each scale:

Granted, I couldn’t take all of these measurements on the same day, so some variance is expected. However, they were all taken within two weeks of each other, after fasting, and first thing in the morning. All of these devices claim a degree of accuracy. Withings claims its BodyFit scale’s tech — which involves a retractable handle and a total of eight electrodes — underwent an internal clinical study of 80 participants and found a 99 percent correlation with DEXA scans. Let’s say the DEXA scan is the most accurate. I’m looking at a variance of 5 to 12 percentage points between it and the other devices. My highest and lowest readings differ by 17 percentage points. That is a significant difference.

And that’s just one metric. My metrics on lean mass (everything but fat) also wildly differ. DEXA, the Withings BodyFit, and InBody break down your body composition based on body parts, like torso, legs, and arms. Those readings also differ by 5 to 8 percentage points. Some also estimate bone density and visceral fat. Would you believe my measurements have some significant variance there, too?

For example, the DEXA scan says I have extremely excellent bone density and that my skeleton makes up about 4.1 percent of my total body weight. Withings says 3.7 percent. On a scale of 1 to 20, the two Withings scales estimate my visceral fat at 1.9 and 2.1, respectively. (1 to 12 is fine, 13 to 20 is high risk.) InBody estimates at 14. My DEXA scan says I have 1.24 pounds of visceral fat and that, ideally, I’d get that number to one pound or less.

Am I good? Frankly, that depends on who you ask. I’m sure the most hardcore fitness influencers would say that I’m obese and in dire need of locking in and shaping up. DEXA and InBody have both given me a C- score for my body composition. Ten years ago, I probably would’ve had immense health anxiety and spiraled trying to find which of these devices was “most accurate” and what numbers were “most true.”

The real truth is, it doesn’t matter all that much.

I don’t have long-term DEXA results. Those scans are expensive (ranging from $75 to $300 a pop), and generally, it’s recommended you only take those every three to six months. But the scan was something I did to prove a point: the accuracy of these devices doesn’t matter as much as the consistency. DEXA scans are highly accurate, with a clinical margin of error of about 1 to 2 percent — but it also depends on the facility you go to, machine calibration, and your hydration levels. The first DEXA scan takes stock of where you’re at. The subsequent scans are where you start to see useful data about whether you’re progressing. That’s true regardless of which body composition method you choose — be it a pricey clinical measurement or the cheapest-possible smart scale you copped during a Prime Day sale. So long as each method is “inaccurate” by roughly the same margin of error each time, the most useful data they provide is your long-term trends.

Again, none of these devices ever recorded the same numbers for me. Even something as simple as my weight could vary by one- to three-tenths of a pound, all other factors controlled for. But what they all did do was tell the same overall story with my trends. Body fat and visceral fat percentage uniformly went down by a significant amount. Lean mass percentage either stayed flat or increased slightly. Of the methods that measured skeletal muscle, that slightly decreased. My bone density has remained the same. Overall, these are all signs my treatment plan is working and point to the same overall recommendations: prioritize rebuilding lost muscle, continue monitoring side effects, review my blood tests every quarter, and keep on keeping on until my doctor is happy with where I’m at.

This concept also applies to other wearables, like smartwatches and more basic fitness trackers. The daily numbers don’t mean nearly as much as your weekly and monthly trends. If you want to move more, it doesn’t matter if an Apple Watch says you took 9,000 steps and the Whoop says you took 8,000. Fretting over which one is “more right” misses the point entirely. What matters is if whatever tracker you choose measures the same mile-long walk as roughly the same number of steps (or GPS distance) each time.

There are nuances to this. I started taking body composition analysis more seriously three years ago because a doctor scared the bejeezus out of me. Since then, I’ve experimented with about eight different scales and body composition tech of varying degrees of smartness. Until my DEXA scan, I never had a reliable sense of what my body fat percentage was. I still kind of don’t. Here’s what I do know: My body composition didn’t respond to traditional lifestyle interventions. Turns out, I have insulin resistance. That makes it easier to put on fat, which in turn increases insulin resistance. I’d never have learned that from any BIA gadget! Fixating on any individual metric — be it from a wearable, scale, or a blood test — was never going to be the key to unlocking my metabolic dysfunction. I was never going to self-optimize my way out of unlucky genetics.

Health and fitness-wise, I’ve felt better. I’ve also felt much worse. All my numbers right now paint a very mixed picture. I could get lost in the weeds if I’m not careful. But the overall trend? I’m on a better track than I was, and that’s what’s most important.