Food can be broken down into three macronutrients: fat, protein, and carbohydrates. Now, according to scientists at Oxford University and the National Institute of Health (NIH), there could be a "fourth macronutrient" called ketones.

Ketones aren't anything new, but you won't find them naturally occurring in the food we eat. Instead, the substance is produced in the liver as part of a biological process to help us cope with starvation. If we don't have enough fuel from food, the body digs into its internal fat supplies, breaking it down into ketones.

Normally, you have to reach starvation mode to produce ketones – a state that can be triggered by the ketogenic (or keto) diet, which involves a very strict low-carb meal plan and may include feelings of euphoria. Soon, however, this magical substance will be available commercially. A ketone drink is set to hit the market and will be sold by the San Francisco based start-up HVMN, apparently pronounced "human".

The colorless liquid is zero-carb, zero-fat, and zero-protein, yet it packs in 120 calories. It is being sold as "a clinically validated superfuel" that will "improve training, recovery, and performance".

But here's a warning: Even Geoff Woo, co-founder of HVMN, admitted it tastes like "a combination of a liquor shot with nail-polish remover" in an interview with The Atlantic. Plus, at $99 for three 2.2-ounce bottles, it probably isn't within most people's pay-packets.

Does it work? Two writers at Business Insider tested the drink and reported improved focus, but they also admitted it was hard to tell if it was thanks to the ketone drink or simply a placebo effect.

There is, however, some scientific research to back its claims. A 2016 study, published in the Journal Cell Metabolism, tested the effects of a ketone drink on elite level cyclists. Researchers from Oxford University compared the performance of the ketone drinkers to those who had been given a drink containing either fat or carbohydrates. Those on ketones were able to achieve an extra 400 meters (1,312 feet) per half hour and were reportedly less tired and achy post-workout.

The actual idea to create a ketone drink began when the Defense Advanced Research Projects Agency (DARPA) offered scientists millions of dollars in funding to come up with an extremely energy-efficient food soldiers could take into the battlefield. The challenge was accepted by biochemists at Oxford University and the NIH, who went on to invent a ketone ester drink. They call ketones "a fourth type of fuel for humans".