A $100 keto breath-sniffing device can help dieters know if they've been eating too many carbohydrates

• A new $99 breath test called Keyto can tell low-carb eaters whether they're in the fat-burning state of ketosis.
• The test isn't perfect: It won't necessarily register carb-heavy foods that were just eaten, and no single reading should be considered fully trustworthy.
• Instead, Keyto's founders say their breathalyzer is best used to check in on diet progress on a daily basis.
• INSIDER data editor Walt Hickey tried out the breathalyzer, and said that while it looked like he was using a vape pen, the breath test provided an easy way to monitor his keto dieting.
• Visit Business Insider's homepage for more stories.

Testing your breath can do more than reveal whether you need to brush your teeth - it can now yield clues about how much fat your body is using for fuel.

The pungent smell of acetone permeating from a keto dieter's gullet can be a fairly decent indicator of whether that person's low-carb eating strategy is on track. A new breathalyzer tool released yesterday aims to help people measure just that.

The goal of the popular keto diet is to move the body into a state of ketosis, in which the body burns fat for fuel instead of relying on carbs. Our systems naturally enter this mode when starving, but the keto diet hacks our systems to force ketosis by zeroing out the intake of sugar and carbohydrates and upping fat consumption.

But it can be difficult for keto dieters to determine how well their bodies are staying in ketosis without help from a urine test or finger prick for blood.

Now a startup called Keyto, founded by two Silicon Valley doctors, is offering a simple breathe-in-breathe-out test that can give people a sense of their metabolic state. Keyto's $99 breath-measuring stick is essentially a small handheld tube the size of a Sharpie that measures acetone on the breath in the same way alcohol breathalyzers measure ethanol.

Keto dieters exhale acetone

The state of ketosis can be triggered when a reliable 70-80% of a person's daily calories come from fat, and no more than 5% come from carbs. It can take several weeks to get into this mode, but once a person does, they may develop what some call "keto breath" (which might smell pleasantly like a baby's breath or may be mildly off-putting, depending on who you ask).

"I've had people come up to me and say 'your breath smells like a baby!'" Keyto founder Ethan Weiss told Business Insider. "I can't smell it, I'm not sensitive enough. But there are people out there who have really, really sensitive noses."

The scent is a byproduct of the body's work converting fat into fuel, since the process excretes three ketones, one of which is acetone. Ketones are the acids that are produced naturally by bodies as they break down fat for energy, which is a big shift from our metabolic default: fueling up on glucose from carbs.

The founders of Keyto - Weiss, a cardiologist at UCSF in San Francisco, and Ray Wu, a doctor who has developed other diet apps - re-jiggered an alcohol breathalyzer test, which traditionally measures ethanol, to monitor acetone levels instead. That makes it a fairly reliable ketosis meter. Weiss originally started developing the device to help more of his patients manage diabetes, obesity, and other metabolic issues without drugs.

"Keto is undeniably the best way to deal with Type-2 diabetes and insulin resistance," Weiss said.

But because the tool can give keto dieters instant feedback, it has garnered broad interest: More than 9,900 people backed Keyto's Indiegogo campaign, and the company says it has shipped 14,000 breathalyzers in its pre-launch phase.

The device is a battery-powered, handheld breath-measuring stick that pairs with a smartphone app. To perform a keto breath test, Keyto users open the app, then take a long, slow breath in and exhale into the tube for about 10 seconds. After that, the app reveals a "Keyto Level" ranging from 1 (not in ketosis) to 8 (deep ketosis), based on how much acetone vapor is detected on the breath.

The app also tracks a dieter's progress over time, linking their ketone level numbers to their weight. For an extra annual fee of $96, the app can also suggest meals that might help dieters get a better breath score; it rates foods based on how keto-friendly they are.

Trying out Keyto at the Business Insider office

Walt Hickey, the data editor at INSIDER (Business Insider's sister publication), is a longtime keto-diet fan, so he tried out the Keyto test for several weeks this spring.

He said the tool didn't always pick up a carb-heavy food like a donut if he tried to get a reading right after eating it. But for the most part, Keyto seemed to register how well he stuck to his low-carb plan: Hickey's readings plummeted when he indulged in some cake at a friend's wedding, yet remained relatively stable when he cooked for himself at home.

The device has to be connected to the internet to work, so Hickey said he couldn't get instant breathalyzer measurements on the subway during his morning commute. But he liked getting quick-fire feedback about how he was doing every day.

"I like certainty that I'm sticking to dietary choices, but with keto it's always a bit annoying," Hickey said. "We live in a society - I'm not going to bring a urinalysis test strip to the office. In that regard the Keyto was great, even if it by definition only approximated ketosis."

Plus, he added, Keyto looks close enough to a vape pen that he didn't stand out too much when using it.

"Aesthetically you look like someone taking a large hit from a vape pen but doing a particularly bad job of it. Which isn't the best look, but is decidedly not the worst," he said.

Keyto is not the only keto breathalyzer on the market. There is also the $219 Ketonix breath test and the Levl device, which costs $99 upfront then $99 a month for at-home testing. But the Keyto founders are hoping their cheaper, smaller device that pairs with a smartphone app will woo consumers.

Keto diets aren't for everyone

Keto diets are soaring in popularity, and mounting evidence suggests the eating plan can be a health boon for some people. Going keto may help reduce Type-2 diabetes risk, and the diet is well known as an effective approach to dealing with tough-to-treat epileptic seizures in children.

Many keto-lovers say that the high-fat plan both helps them lose weight and also lifts some mental haze.

Wu and Weiss said - much to their own surprise - they have also both become keto adherents. They have taken to using the breathalyzer daily on themselves, while sometimes stoking "friendly competitions" among Keyto users to see who can get the best ketone score.

"It's really a great detector of hidden sugar, whether that's in salad dressing or sauces, barbecue sauce," Weiss said.

But they both caution that any single reading should not be taken as an absolute indication of success or failure in low-carb eating.

"People shouldn't think that, 'Oh, okay, I just ate one bite of carbs and then 10 seconds later it will reflect in the ketone level,'" Wu said. "There's many things, from the environment, to what you ate, to when you ate, to if you exercised - just so many things that can affect every single breath."

Plus, the keto diet is not for everyone. Some initial evidence from small-scale studies in athletes and others suggests that going keto can significantly change cholesterol levels in the body, and it's not clear yet whether this effect is good or bad.

"I think the vast majority of people who go on this diet will have no trouble with their cholesterol," Weiss said. "But I'm not going to tell the people that do have trouble with their cholesterol that it's not a problem."

For that reason, the Keyto app promotes a "heart healthy" approach to the keto diet.

"Some of the sort of traditional bacon, butter, and eggs keto may not be optimal for everybody," Weiss said.

Instead, he added, "the fats that you find in things like nuts, or avocado, or olive oil, and fish like salmon or trout," are the ones to prioritize.

This advice aligns with other recent studies that suggest eating lots of processed foods and meats isn't great for our long-term health or longevity.

Unfortunately, the healthiness of each individual food a person eats isn't something the smell of their breath can reveal.