Is this extreme intermittent fasting the key to a long life?

Intermittent fasting — which involves restricting when you eat, most commonly consuming food within an eight-hour window and fasting for the other 16 hours — is all the rage these days, with celeb fans including Jennifer Aniston, Chris Pratt and Kourtney Kardashian.

But some emerging research points to benefits of fasting for an entire day or more — a method known as prolonged fasting. 

Studies show this extreme form of fasting can boost immunity and brain health while reducing the risk of Type 2 diabetes and certain chronic diseases. 

But how many hours of not eating should you shoot for? And, on the other hand, how long is too much? 

Meet the fasting fan

Chris Rhodes, who holds a Ph.D. in nutritional biochemistry and is the CEO of Mimio Health, told The Post that, when it comes to health benefits, the “real magic begins around the 36-hour mark” between meals. 

“At around 36 hours, you’re going to be totally depleted of your glycogen [a form of glucose] stores and entering the fasting state,” Rhodes said. 

That’s when your metabolism enters ketosis, in which the body burns fat for energy instead of glucose — improving your insulin sensitivity, lowering your blood pressure and giving your brain a boost, he added.

This also kicks off a biological chain reaction that scientists believe may slow aging and ward off chronic disease.

“That is going to trigger the process of autophagy, which is that cellular cleanup and recycling mechanism,” Rhodes said. 

The process will continue to ramp up in the hours that follow, when the immune system starts to undergo a dramatic reset. He says it can have “profound effects” on inflammation and immunity.

According to Rhodes, shorter fasts — like skipping breakfast or eating once a day — show “improvement in some metabolic markers,” but you’re not getting a lot of additional benefits.

“It takes around 20 to 24 hours for your body to fully deplete those glycogen stores,” he said. “Even at that point, that’s kind of when you’re just starting to kick off a lot of these benefits.” 

That being said, he cautions against fasting past the 72-hour mark — especially without medical supervision — since this is when the body is in danger of shifting into starvation mode. 

“This is the point during long term fasting where the body can start to damage itself and become less functional over time,” Rhodes said.

Once your “feeding day” — a k a the day you get to eat again — arrives, he recommends making sure you’re getting enough protein, fiber and nutrients in to make up for the days you were running on empty. 

“Prolonged fasting is not as well studied but it does seem to have similar benefits as ADF. However, it is more dangerous and should not be done without physician supervision.”

Dr. Christopher Choi

“Eat the rainbow,” Rhodes said. “Focus a lot on vegetables that are going to be high in fiber that are going to really fill you up without adding too much of a big calorie load.”

He also suggested avoiding processed meat and processed foods that “are going to be taxing on your liver and not going to support your detoxification.”

So how do you suppress your appetite for that long?

Of course, for beginners of prolonged fasting, the hardest part might be fighting hunger pangs. 

That’s why Rhodes launched Mimeo Health, which offers supplements that — in addition to boosting mood, performance and longevity — can help suppress appetite. 

They recently completed a double-blind, placebo-controlled trial that showed taking the supplements for eight weeks helped people control their appetite, reduced their cravings and enhanced their satisfaction from meals when they did eat.

His other suggestions include drinking lots of tea and water, chewing gum, smelling cinnamon sticks and just generally keeping your mind occupied. 

A little bone broth is OK too. As long as you’re not exceeding roughly 200 calories a day, you’re in the clear, he said. 

Rhodes also recommends beginners take it slow. “Start by skipping breakfast, then get yourself up to a 16-hour fast,” he said. 

If you want to go further than that, work your way up to one meal a day, then every other day and so on.

“Make sure that it’s something that you can do safely and that it’s something that you want to do as a lifestyle and incorporate it from there,” he added.

It’s not easy. Rhodes admitted that when he first started fasting as a lifestyle, he had to quit when he got into a relationship and his girlfriend found it weird. 

The downsides — and possible dangers — of prolonged fasting

It’s also not for everyone. 

Obesity expert Dr. Christopher Choi told The Post that “both alternate day fasting (ADF) and prolonged fasting over 36 hours are generally considered healthy.” 

ADF, in particular, “has a lot of evidence supporting its benefits, including weight loss, lower blood pressure, improvement in insulin sensitivity, cholesterol improvement and metabolic parameters,” as well as increased lifespan, he said. 

“Prolonged fasting is not as well studied but it does seem to have similar benefits as ADF,” he said.  “However, it is more dangerous and should not be done without physician supervision.”

As far as downsides go, he noted that it can be difficult to maintain on a long-term basis, and it can cause irritability, weakness and fatigue. 

“Prolonged fasting may even cause hypoglycemia and hypotension,” Choi said. “Prolonged fasting is not generally practiced. People who have diabetes, hypertension or are pregnant may not be able to participate in fasting.” 

Who shouldn’t do prolonged fasting?

Then there are psychological factors to take into consideration. 

“People who have a history of eating disorders are not a good fit,” for any type of fasting regimen, clinical psychologist Renee Solomon told The Post. 

“This makes them become even more obsessive and disturbed by their eating disorder. It is too regimented for them and focuses on losing weight in a way that is not balanced for them,” she added.

Solomon noted that women with hormonal issues are also not good candidates for fasting, as it “can impact periods, ovulation, and possibly lead to infertility.”

Finally, she cautioned that “people with thyroid conditions should also not do any type of fasting, as it can negatively impact their thyroid condition, and neither should children or teenagers, as it can delay their growth, puberty and brain development.”

Rhodes believes most people can benefit from fasting at least once, even if it’s a short one.

“I think that it really helps break you out of this mindset that we have been fed in American society since we were kids: You need to eat all the time, you should not ever feel any kind of hunger, hunger is bad,” he said.

He added: “If you want to break that societal cycle, if you want to just try something new, it can be really helpful and really powerful.”