The Silicon Valley execs who don’t eat for days: ‘It’s not dieting, it’s biohacking’

The last time any food passed Phil Libin’s lips was a day ago, when he ate yakitori at a restaurant in San Francisco’s Mission district. He’ll next eat in three days time on Thursday evening, when he has a reservation at one of the fanciest sushi restaurants in town. In the intervening four days it’s just water, coffee and black tea.

Over the last eight months the former CEO of Evernote and current CEO of AI studio All Turtles has shunned food for stretches of between two and eight days, interspersed with similar periods of eating. He’s lost almost 90lbs and describes getting into fasting as “transformative”.

“There’s a mild euphoria. I’m in a much better mood, my focus is better, and there’s a constant supply of energy. I just feel a lot healthier. It’s helping me be a better CEO,” he said over a cup of black coffee – one of many that day – at All Turtles’ Soma office. “Getting into fasting is definitely one of the top two or three most important things I’ve done in my life.”

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.



Phil Libin, CEO of All Turtles, regularly fasts for between two and eight days. Photograph: All Turtles

Intermittent fasting gained popularity 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.

Proponents combine fasting with obsessive tracking of vitals including body composition, blood glucose and and ketones – compounds produced when the body raids its own fat stores, rather than relying on ingested carbohydrates, for energy.

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

Geoffrey Woo, CEO of biohacking and nootropics company HVMN (pronounced “human”), led a seven-day fast at his company at the start of 2017, along with more than 100 members of WeFast, a community he founded dedicated to intermittent fasting.

Some participants wore continuous glucose monitors, usually worn by diabetics, to check blood glucose levels in realtime using pinprick probes inserted into the skin. They also measured blood elevated ketones, to check that their body was indeed using fat as fuel, a metabolic state known as “ketosis”.



Geoff Woo after seven days of fasting Photograph: HVMAN

“Ketones are a super-fuel for the brain,” said Woo. “So a lot of the subjective benefits to fasting, including mental clarity, are down to the rise in ketones in the system.”

Woo and others in the company wanted to quantify the impact of fasting on productivity, so they combined the physiological tracking (of ketones and blood sugar) with a piece of software called Rescue Time that measures how productive individuals are at work.

“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.” Woo said.

At 5’11” and 165lbs, Woo doesn’t need to lose weight, although he did drop 12lbs over the week. “I’m focused on longevity and cognitive performance.”

Woo now does a weekly 36-hour fast and a quarterly three-day fast. It’s given him a better grasp of his own sense of hunger.

“We conflate the need to eat with the need to socialize, walk around, take a break and mull on things,” he 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.

It was Le Meur who introduced Libin to fasting over coffee in December 2016. At the time Libin weighed around 260lbs, the heaviest he’d ever been, and Le Meur was two-and-a-half days into a fast.

“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.



People in San Francisco eat lunch in the Soma neighborhood. Like meditation and mindfulness before it, fasting has become the latest trend in self-improvement. Photograph: Ramin Talaie for the Observer

There is a mounting body of scientific research exploring the effects of fasting on the body. 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. Extended fasting can also increase people’s susceptibility to infection and worsen already damaged kidneys.

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.”

Woo’s WeFast community now has more than 6,000 members across a Facebook page and Slack channel. Participants discuss the latest research on fasting and share tips and results. WeFast also convenes offline: there’s a monthly meet-up in San Francisco where members “break fast” at a restaurant.

The membership skews, said Woo, towards Silicon Valley engineering types in their twenties and thirties.

“In Silicon Valley and other competitive global markets more people are looking at any technique to gain productivity,” he added.

We all know that eating a big unhealthy lunch can lead to an afternoon productivity slump, but when people with an engineering mindset start digging into the science of why such “carb comas” happen they can start to control it.

“Instead of hacking computer chips, they can hack their own bodies,” he said.

For Libin, fasting is another Silicon Valley trend in the same vein as meditation, which gained popularity a few years ago with the creation of apps such as Headspace.

“There’s a general culture here where people believe all problems are solvable,” said Libin. “You want less stress and anxiety? There’s meditation. You want to live 40 years longer? You can probably do that with fasting.”

“Around 80% of what people do here turns out to be nonsense but there’s a lot of willingness to try.”

Now the self-described “foodie” saves himself for gourmet dining rather than swiftly snaffled sandwiches. “I don’t have any boring meals any more. Every time I put food in my mouth it’s unique and special.”

So he’ll eat processed carbohydrates like bagels, but only in New York (“the bagels in San Francisco suck”) and ramen when he’s in Tokyo.

Since coming out as a faster earlier this year, Libin has been inundated with requests from people seeking his advice on how to get started, but he doesn’t think it will ever be mainstream in the way meditation has become.

“It seems way too extreme,” he said. “No-one grew up being told that meditation was super bad for you. Everyone grew up hearing fasting was dangerous and super-difficult.”

Furthermore, no one makes money when people don’t eat.

“In this society usually things that work against every entrenched economic interest are hard to take off.” said Libin.

“You need to be a weirdo like me to get into this.”


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