A British TV doctor known for his advocacy for intermittent fasting diets has disappeared while on vacation on a Greek island, with a search operation underway Thursday.

Michael Mosley, who regularly appears as a health expert on U.K. networks, went missing on the island of Symi, his agent confirmed to the Press Association. The 67-year-old vanished after going for a coastal walk on Wednesday afternoon, according to The Daily Mail, a paper for which Mosley is a columnist.

Mosley is known for his efforts to popularize the so-called 5:2 diet—where you eat normally for five days of the week and significantly cut calories in the other two—and more recently the “Fast 800,” which combines intermittent fasting with a low-carbohydrate, Mediterranean-style meal plan. Celebrities including Benedict Cumberbatch and Jimmy Kimmel have taken up the diet.

Mosley’s wife Clare Bailey, who is also a doctor, raised the alarm after he failed to return by 7:30 p.m. local time on Wednesday—around six hours after he set out on his walk, the Mail reports. Multiple agencies joined the search operation Thursday which included the use of drones and the support of volunteers.

Authorities fear Mosley may have fallen while on the walk, a police source told the Mail, adding: “Nothing at this stage can be ruled out.”

An appeal for information was shared in a Symi Facebook group on Wednesday along with a picture showing Mosley standing on a beach and holding a backpack. The person who created the post noted: “His name is Dr Mike Mosley and he is a familiar face for many British people as he has appeared on the BBC.”

Mosley also gained recognition for his documentary work, including a 2002 Emmy nomination for The Human Face, a short series on which he served as an executive producer about the science of human beauty with Monty Python star John Cleese.