However, Aisling Pigott, a registered dietician, and spokesperson for the British Dietetic Association (BDA), is critical.

“Bulletproof coffee is not something I’d ever recommend, because it’s introducing extra calories and extra fat in a way that isn’t providing any other nutritional value,” she says.

“There is no benefit to adding butter to your coffee. With a Bulletproof coffee-based breakfast you’re missing out on what you’re getting from a food-based breakfast – there is no protein, vitamins or minerals.”

Mr Asprey says he is unconcerned by such criticism, and points to other health professionals who say that diets high in fat – and low in sugar – are good for you.

“I had arthritis in my knees, I’d been diagnosed with chronic fatigue syndrome, I had cognitive dysfunction, I was at high risk of stroke or heart attack, I was pre-diabetic and I felt terrible all the time,” he says. “I undid all of that.”

He adds that unlike more modern diets, his drink is based on ancient Tibetan custom. “I’m okay being part of a 4,000-year-old fad,” he says.