Scientists think they've discovered a fourth type of fuel for humans - beyond carbs, fat, or protein

• Ketones could super-charge the body in a way that's unlike any other source of fuel.
• San Francisco-based startup HVMN recently launched a drink made of pure ketone ester to harness its performance-boosting qualities.
• The company partnered with Oxford University to leverage $60 million-worth of scientific research on elite athletes.

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The nutrition label on a shot-sized bottle of this clear, odorless liquid defies traditional explanation. It contains 120 calories - roughly the equivalent of a hearty slice of bread - yet it has no fat, no protein, and no carbohydrates.

Those calories instead come from ketones, an ingredient that Geoff Woo, cofounder and CEO of San Francisco-based human performance startup called HVMN (pronounced "human"), likes to call "the fourth macronutrient."

"It's not a fat, it's not a protein, it's not a carb, but your body gets fuel from it," Woo told Business Insider.

With that in mind, Woo launched his company's first ketone product, a 2.2-oz bottle of ketone ester called Ketone. The drink, now available for pre-order, promises improved athletic ability, energy, and a heightened sense of focus. To make the product, HVMN leveraged more than a decade and $60 million-worth of scientific research through an exclusive partnership with Oxford University.

Ketone could boost performance 'unlike anything we've ever seen'

Most of the food we eat contains carbs, from fruits and starchy vegetables to potatoes and pasta. In fruit, carbs come from naturally-occurring sugars; in potatoes, veggies, and pasta, they come from starch. They're all ultimately broken down into sugar, or glucose, for energy.

When robbed of carbs, the body turns to fat for fuel.

In the process of digging into its fat stores, the body releases molecules called ketones. A high-fat, low-carb diet (also known as a "ketogenic diet") is a shortcut to the same goal. Instead of going without food, someone on the diet tricks the body into believing it is starving by snatching away carbohydrates, its primary source of fuel.

This is why,as long as you're not eating carbs, you can ramp up your intake of fatty foods like butter, steak, and cheese - and still lose weight. The body becomes a fat-melting machine, churning out ketones in its drive to keep running.

If you could ingest those ketones directly, rather than starving yourself or turning to a keto diet, you could essentially get a super-power.

"You could run up a wall, but you don't want to," is how Brianna Stubbs, lead researcher at HVMN and a postgraduate student at the University of Oxford, put it.

In studies with athletes, it appears that combining ketones and carbs produces what Stubbs called a "stacking effect."

That performance boost is "unlike anything we've ever seen before," said Kieran Clarke, a professor of physiological biochemistry at Oxford and the scientist leading the charge to translate her work on ketones and human performance into HVMN Ketone.

This is an energy drink that goes far beyond caffeine

In a small study published in July 2016 in the journal Cell Metabolism, Clarke gave an early version of HVMN's ketone drink to a group of elite cyclists (some of whom were former Olympians) and compared how they performed on a 30-minute cycling exercise to two other groups who were either given either a carb-rich drink or a fat-rich drink.

The high-performing cyclists on the ketone drink went an average of 400 meters further than the best performers who'd had the carb or fat drink. They likely didn't even feel a difference, Clarke said.

"It's not like caffeine or anything, it's not a stimulant. If you're not watching what you're doing, you think, 'Oh I'm doing alright, everything feels normal,' but then you look down and all of a sudden you see, 'Oh, wow, I've gone a lot further than usual!' You'll find on a rowing machine, for example, you're going a lot faster and you didn't even realize it," Clarke said.

A bottle of HVMN Ketone delivers 25 grams of beta-hydroxybutyrate (BHB), one of the substances the body naturally produces during a fast or a period of starvation.

Within an hour of consuming it, the drink can raise ketone levels to a level similar to what you would see after at least seven days of fasting, Woo said. That's based on two small studies published in the journal Frontiers in Physiology in which adults were given drinks containing either ketone ester or ketone salts, a supplement that combines ketones and sodium.

When we (Erin Brodwin, a science correspondent, and Melia Robinson, an innovation reporter) tried it out for ourselves in October, we used meters to measure our blood glucose and ketone levels before and after drinking it. To our surprise, we saw immediate and measurable results.

During the hour before and the hour after we drank the ester, Erin's ketone levels rose from 1.2 mmol/l to 4.2 mmol/l. Melia's ketone levels rose from 0.6 mmol/l, a low-level state of ketosis, to 6.0 mmol/l, a deep state of ketosis that can typically only be achieved through fasting. Most people maintain a non-existent level of ketosis of 0.1 mmol/l, but we started with higher levels because one of us happened to be trying a fast while the other was eating a low-carb diet.

But did those shifts in numbers reflect a difference in performance? It's hard to say.

Two people do not make for a sufficient sample size in a study of the drink's effects, and it's impossible to separate our perception from any placebo effect. Still, both of us noticed some improvement in focus, and we both skipped our usual 3 p.m. coffee - a change we didn't notice until hours later.

"[Ketone] sort of like makes life easy," Clarke said. "Rather than making you feel as though your heart is racing or you're exhausted ... you have this energy. Energy you just don't normally have."