Phil Libin’s last time eating time was a day ago, he ate Yakitori in SF mission district. His next eating time will be three days from now, in one of the best sushi restaurant in town. During the days in which he is not eating, he only drinks water, coffee or black tea.

It has been eight months now that the former CEO of Evernote and current of Al studio All Turtles is fasting for 2-8 days. He described it as “transformative” and lost also 90lbs so far.

Phil claims that getting into fasting is one of the top most important things he has done in his life. He says that it makes him feel much healthier, Focused, with much better mood and constantly energized. It also helps him to be a better CEO.

Libin is one of a growing number of Silicon Valley types experimenting with extended periods of fasting, claiming benefits including weight loss, fewer mood swings, and improved productivity.

Intermittent fasting becomes popular with the 5:2 diet, where people eat normally for five days a week and eat a dramatically reduced number of calories (around 500) on the remaining two days. However, Libin and others like him are pushing that idea further and with a focus on performance over weight loss. They are combine it with tracking of vitals like blood glucose and ketones that produced when the body raids its own fat energy.

This, they insist, is not dieting. It’s “biohacking”.

WeFast, a community dedicated to intermittent fasting has founded by
Geoffrey Woo, CEO of biohacking and nootropics company HVMN. The community consisnts of more than 100 members that share their experiences and feelings wile fasting.

Woo and others in the company create a software called “Rescue Time” that meant to measure the impact of fasting on the individual productivity.

“You would think that after seven days of not eating you would be totally distracted and hunting for food, but at around the two or three day mark hunger tapers off as ketone levels are elevating. You are feeding your brain and body with an alternative fuel source. We conflate the need to eat with the need to socialize, walk around, take a break and mull on things” Woo said.

Libin shares his experiences with a WhatsApp group, called “Fast Club”, made up of around 20 other CEOs and investors in the Bay Area. He would only mention members who had already outed themselves as fasters: Startup investor Y Combinator partner Daniel Gross and Loic Le Meur, cofounder of LeWeb tech conference and founder of Leade.rs, a startup that connects conference organisers with speakers.

When Libin first introduced to fasting he weighed 200lbs and sat with Le Meur over a coffee. He told him all about fasting while being into two-and-a-half days fasting himself.

“It sounded crazy,” said Libin. “So I went home and Googled and read a bunch about it because I wanted to prove to him why he was wrong. But I did the research and it seemed plausible.”

So he gave it a try.

“The first day I felt so hungry I was going to die. The second day I was starving. But I woke up on the third day feeling better than I had in 20 years,” he said.

Each year dozens of papers are published showing how fasting can help boost the immune system, fight prediabetes and even, at least in mice, slow aging.

However there is also evidence that fasting can be dangerous if not carefully supervised, risking heart failure if supplements of essential minerals like sodium, magnesium and potassium aren’t ingested.

San Francisco-based eating disorder specialist Shrein Bahrami was concerned that extended fasting was another fad that could be used as a cover for not eating.

“The hyper focus on tracking vital signs and food has become normalized, so it’s difficult to know when it’s become obsessive,” she said, but people with eating disorders typically feel a lot of shame and other negative emotions around food and body image, which doesn’t tally with the experience of people like Libin and Woo.

“If you are going to do an extended fast, which I recommend against, then consult a doctor,” she added.

Eight months in and Libin finds fasting easy and frequently attends “nice dinners” with friends where he will only drink water.

“People think it’s torture but it’s actually really pleasant. I get the social interaction, I can see the food and smell it. All of those things are pleasant,” he said. “I usually leave a dinner where I eat nothing feeling kind of full.”

Does he still have to split the bill? “I have done that, yes.”