What is it? Coffee, butter and coconut oil.

How much does it cost? £10 for the coffee, £17 for some Octane oil – a coconut-derived fat – and some butter, which you’ll also need to make it.

What does it promise? Fans claim that it gives you improved mental and physical performance, and encourages weight loss, too. It’s a high-calorie, high-fat drink that, in theory, keeps you feeling full for hours after you drink it. Octane oil is a medium-chain triglyceride (that is, a fat that’s easily converted into energy for increased performance), which acts as an appetite suppressant. If you drink this coffee for breakfast and only eat between 2pm and 8pm, you’ll apparently lose a pound a day.

What’s it actually like? To make? Easy. You brew the coffee, then blend it with the oil and butter. To drink? Fine. The butter makes it creamy and rich, and blending it transforms it into a pleasantly elaborate almost-latte. But as a health drink? I made Bulletproof coffee twice a day for a week – and drank it with my regular diet – and I remain fat and lazy, so the jury’s still out.

Best and worst bit Even though I couldn’t feel any benefits, the coffee itself actually tastes quite nice. Unless you accidentally make your Bulletproof coffee with salted butter. Pro tip: do not use salted butter. Also, side-effects of medium-chain triglyceride consumption can include diarrhoea and vomiting.

Is it worth it? As a drink, yes. As a weight-loss aid, only if you’re really, really committed.