Intermittent fasting is linked to health benefits such as weight loss and longevity.
But fasting doesn't have to mean going hungry, according to Bulletproof CEO Dave Asprey.
Research suggests you can eat some foods while fasting if you avoid protein and keep insulin low.
Intermittent fasting has skyrocketed in popularity in recent years, thanks to research showing it can help improve overall health, boost weight loss, stabilize blood sugar, and even help us live longer.
The diet limits your food intake to a specific window of time each day, without restricting overall calories.
Still, skeptics of fasting - intermittent or otherwise - contend that it's a glorified form of an eating disorder; a grueling disciplinary regimen that leaves people hungry and tired in the many hours they go without food.
That shouldn't be the case, Dave Asprey, CEO of the health and performance company Bulletproof, insists in his new book "Fast This Way."
In fact, he says, there is some research (coupled with his personal experience) that you can eat certain foods during your fasting hours, without sacrificing the supposed health benefits.
"The hallmark of fasting isn't no calories at all, it's that you don't raise insulin, and you don't activate your protein digesting machinery," Asprey told Insider.
The 2 standard rules of a fasting diet
There are two key principles of fasting that research has identified.
First, avoid too many carbs, since eating carbs can spike your insulin levels. One of the main benefits of fasting appears to be that it controls insulin, which won't work if you have a carb-heavy snack.
Second, avoid eating protein during your fast. While protein is an essential nutrient, digesting it triggers a cascade of hormonal reactions within the body, taking up a lot of energy. In contrast, when you're not digesting protein, the body begins a process called autophagy, in which it breaks down damaged cells. Research suggests this process can help prevent aging-related diseases and even extend our lifespan. That helps explain why fasting is linked to longevity.
It's generally accepted that coffee and tea, for instance, are permissible on a fast, even though they contain small amounts of calories.
But experts disagree on whether you can technically be fasting if you consume other nutrient-rich foods and drinks, like bone broth - or, indeed, Bulletproof coffee.
Asprey believes you can take liberties as long as you follow those 2 rules
For Asprey, as long as you don't break the key rules - avoiding excessive carbs in general and avoiding proteins in fasting hours - then consuming certain tupes of calories is totally compatible with fasting.
"We want the benefits, we don't want to suffer and be distracted," he said.
To that end, Asprey says that you should feel free to take fasting at your own pace, including breaking your fast to eat carbs or enjoy a nice meal once in a while.
"I skip breakfast six days a week, and once a week I have brunch with my family, because brunch is nice," Asprey said. "Doing the same thing every day, no matter what it is, probably isn't good for you."
The evidence for sneaking food into your fast
Intermittent fasting doesn't restrict overall calories - during your "feeding window," you can eat whatever you want, including junk food, Asprey said (although he doesn't recommend that).
But what makes Asprey's Bulletproof Fast unique is that you can also consume some calories during the supposed "fasting window," unlike traditional intermittent fasting.
The evidence for this comes from research on very low-calorie diets known as "fasting-mimicking diets," which studies suggest can replicate the benefits of fasting, without the hunger and fatigue.
One example of a highly-researched, fasting-mimicking diet comes from Valter Longo, an Italian biologist who has completed extensive research on fasting and biology, devising a diet that he says delivers the same benefits as a fast.
On Longo's diet, known as ProLon, you consume about 700 calories a day for five days, mainly in the form of healthy fats - flax seeds, nuts, olives - and some nutrient-dense, low-carb plant foods - kale, quinoa, dark chocolate, mushrooms, algae.
Asprey recommends that, during fasting hours, people drink his own Bulletproof coffee formula: coffee mixed with butter from grass-fed cows and a fatty acid supplement called MCT oil.
"The fats help you not feel hungry, and MCT oil helps you metabolize fat better," Asprey said.
There's some evidence this approach can work, since it can reduce hunger and help produce ketones, chemicals your body makes when no carbs are available. Ketones allow your body to burn fat for fuel, are linked to reduced inflammation, and may have protective effects against certain diseases, according to research.
There are risks to fasting, and evidence remains limited
Fasting may not be for everyone, and you should always discuss with your doctor before starting a fasting protocol. People with a history of eating disorders, for instance, should avoid any kind of fasting.
For others, the science of fasting is still debated, with some research suggesting it may not have benefits at all, so we don't fully understand how it works.
And some of the hallmarks of fasting, like lack of protein, can have major side effects, particularly long-term.
Fasting for more than a day at a time, and as long as several weeks, can have serious side effects such as fatigue, persistent hunger and loss of muscle mass.
Fasting for more than two days at a time can potentially stress your immune system, according to Dr. Miriam Merad, director of the Precision Immunology Institute at Mount Sinai School of Medicine who has published studies on the benefits of fasting.
"The safety issue is a tough one. The public can use our literature to starve themselves to death. What we are lacking is a good definition of what is too much fasting," Merad previously told Insider. "It's about balance and understanding the complexity of it."
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