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How Galaxy Watch 8 Measures Antioxidant Levels, Explained

Press a thumb against the back of a Galaxy Watch 8 for five seconds, and the watch hands back a number between 0 and 100. Samsung calls it the Antioxidant Index, and knowing how Galaxy Watch 8 measures antioxidant levels changes how you should read that score (Engadget reported this week).

In practice, the watch isn't measuring every antioxidant in the body. It's estimating one class of pigment called carotenoids, compounds found in yellow, orange, and green produce that build up in skin as a byproduct of eating fruits and vegetables (Engadget). Samsung markets this as the first smartwatch feature to measure the body's antioxidant status, framed against the World Health Organization's daily fruit-and-vegetable targets (Samsung Newsroom).

That distinction matters. A five-second scan that spits out an instant score invites people to treat it like a blood-pressure reading: useful today, forgotten tomorrow. The biology underneath doesn't move that fast. What follows covers what the sensor actually detects, how the scan works, why the number barely shifts day to day, and where Samsung's own evidence for accuracy stops and its marketing claims continue.

What does the Galaxy Watch 8 antioxidant score measure?

Samsung's explanation comes from Professor Hyojee Joung of Seoul National University, who described carotenoids as the red, yellow, and green pigments found in produce. Because the body can't produce carotenoids on its own, skin levels can reflect what someone has been eating (Samsung Newsroom). That's what makes carotenoids a usable proxy: track them, and you're indirectly tracking fruit and vegetable habits.

Think of the Index less like a blood panel and more like a thermometer built for one narrow signal. A thermometer doesn't diagnose what's wrong with a patient, just that something has shifted from baseline. The Antioxidant Index works the same way. It isn't measuring every antioxidant compound in the body, just this one measurable stand-in, and using it to infer a habit (Engadget).

Samsung translates the raw carotenoid reading into a 0-100 score, then sorts it against the WHO's benchmark of 400 grams of fruit and vegetables a day: under 50% of that target counts as "very low," 50 to 99% as "low," and 100% or more as "adequate" (Samsung Newsroom). At last year's Galaxy Unpacked, Samsung also described a color-coded layer on top of the number, where a sky-blue result signals sufficient intake of carotenoid-rich foods like leafy greens and carrots (Samsung Newsroom).

Understanding this framing upfront heads off the most common misread of the feature: treating the Antioxidant Index as a thorough health verdict rather than a diet-tracking proxy.

How Galaxy Watch 8 measures antioxidant levels with a thumb scan

The mechanism behind this Galaxy Watch 8 carotenoid scan is called multi-wavelength absorption spectroscopy. The BioActive Sensor on the back of the watch shines yellow, blue, purple, and green LED light onto skin and reads what bounces back using photodiodes (Engadget). Blue light matters most here, since carotenoids absorb it more strongly than other wavelengths, which makes the reflection pattern a usable fingerprint for carotenoid concentration.

Here's how to measure antioxidants on Galaxy Watch 8: remove the watch, open Samsung Health, select Antioxidant Index, tap Measure, then press a thumb against the rear sensor for about five seconds (Engadget). That last step, using a thumb instead of reading straight through the wrist, isn't an arbitrary design choice.

Samsung engineer Park explained that skin pigmentation and blood flow both distort optical readings. The team picked finger skin specifically to cut down on melanin interference, and had users press down to briefly restrict blood flow, reducing hemoglobin's effect on the signal (Samsung Newsroom).

That's the most interesting engineering detail in the whole feature. The mildly odd ritual of removing a watch to measure something the watch is wearing isn't a UX afterthought, it's a workaround for known optical noise. Samsung says its engineers began researching sensors for this feature back in 2018, and Park has described how sensitive the setup turned out to be during development, noting that even a slight change in how LEDs and photodiodes were arranged could throw off results (Samsung Newsroom).

Before this kind of Samsung BioActive Sensor carotenoid measurement existed on a wearable, checking antioxidant status required a blood draw and lab processing (Samsung Newsroom). Compressing that into a five-second thumb press is the core achievement here, even if the number it produces moves on a much slower clock than the scan itself suggests.

Why the score changes slowly, and what else moves it

Despite the instant readout, Samsung says carotenoids typically take two to three weeks of sustained dietary change before they build up to detectable levels in skin (Samsung Newsroom). Eating a salad doesn't move the needle by dinner.

Samsung-cited research puts that window somewhere between one and three weeks, not a single fixed number. In a separate interview, Professor Joung described a Seoul National University study that fed 80 participants meals with varying carotenoid content over six weeks and saw skin levels shift within one to two weeks (Samsung Newsroom). Neither figure contradicts the other, but together they point to a rough range worth keeping in mind if the app doesn't respond as fast as expected.

Diet isn't the only variable, either. Samsung says UV exposure, alcohol, stress, and unusually intense exercise can all temporarily lower the score on top of whatever someone has been eating (Engadget). The company attributes this to free radicals generated by these stressors, which can deplete carotenoid levels and show up as a dip in the Index (Samsung Newsroom).

Real-world testing backs up the slow-moving picture. Engadget's Amy Skorheim deliberately overhauled her diet for a single day and watched her score drop by just two points; across later testing, her readings ranged from 56 to 60 (Engadget). Engadget's separate Galaxy Watch 8 review describes the number as stubborn, which lines up with what Samsung says about the underlying biology. Treat the score as a slow-moving trend line, not a report card on yesterday's lunch, and a single low reading after a rough week isn't much to go on.

How accurate is the Samsung Watch antioxidant measurement?

Samsung says it validated the sensor against Raman spectroscopy, a lab-grade technique for measuring skin carotenoids, and cross-checked readings against blood carotenoid tests and photographed food logs collected from several hundred clinical participants over the course of a week (Samsung Newsroom). Park has framed the miniaturized reflection-based sensor as a practical stand-in for Raman equipment, which is accurate but too bulky for everyday use outside a lab (Samsung Newsroom).

There's outside research that supports the general approach, with a caveat. A 2023 randomized controlled trial involving Samsung researchers found a strong correlation between reflection spectroscopy and Raman spectroscopy when measuring skin carotenoids (Engadget). That's useful evidence the underlying measurement method holds up, but it isn't an independent validation of the Galaxy Watch 8 specifically, since Samsung's own researchers took part in the trial.

Put together, the picture is this: the measurement method itself has supportive outside evidence, and Samsung reports its own internal validation against lab-grade methods. What's missing is a public, device-specific figure. Samsung hasn't published a consumer-facing accuracy percentage or margin of error for the Watch 8's implementation, and the company is explicit that the feature isn't meant to diagnose or treat any medical condition (Engadget).

There's also an unexplained gap in device support. The feature appears on the Watch 8, Watch 8 Classic, Galaxy Watch Ultra, Watch 9, and Ultra2, but Samsung's current comparison chart leaves it off the Galaxy Watch 7, and the company hasn't said why (Engadget).

Samsung has also tied the feature to Google Gemini, saying this marks the first time the assistant has been available on a smartwatch (Samsung Newsroom). Park has described asking Gemini to suggest nearby restaurants with antioxidant-rich food and getting workable options back, an example of where Samsung wants this data to go next: less a standalone number, more a starting point for everyday decisions (Samsung Newsroom).

What this means if you're actually using it

The Antioxidant Index measures one narrow, real signal: skin carotenoids as a stand-in for fruit and vegetable intake, not a full antioxidant panel.

Samsung's own guidance reflects that narrowness. The company recommends measuring once or twice a day at a consistent time and judging results over at least two weeks of steady habits, rather than reacting to any single reading (Samsung Newsroom). That's a different way of reading a number than checking heart rate, which is instant by design.

Samsung frames the bigger goal as chronic disease prevention, pointing to rising lifestyle-related illness among younger people as the reason a nudge toward better eating habits has value even without lab-grade precision (Samsung Newsroom).

That's a fair framing, as long as it's understood for what it is: a nudge toward eating more produce, not a substitute for a blood test or a doctor's advice. The engineering behind the sensor is genuinely clever. What's still missing is the kind of independently published, device-specific accuracy data that would let someone treat the score as more than a rough, slow-moving hint about their own habits.