Intermittent Fasting Vs. Extended Fasting

If you’re interested in the benefits of fasting, it’s important to distinguish between an intermittent fast and an extended fast. 

An intermittent fast is part of your daily or weekly routine. It shouldn’t be something you suffer through. 

An extended fast is a more serious intervention. It generally requires medical supervision and can be challenging and uncomfortable even for veteran fasters.  

In this article, we’ll compare and contrast intermittent fasting (IF) and extended fasting (EF). First, let’s define these terms. 

What Is Intermittent Fasting?

Intermittent fasting is the practice of taking regular breaks from calories. These breaks can span anywhere from 12 hours (overnight fasting) to 36 hours with alternate-day fasting (ADF). 

Taking a break from calories doesn’t necessarily mean eliminating calories. Some intermittent fasting protocols—like 5:2—allow around 500 calories per day on “fasting” days. The other five days of the week, you eat normally. 

Other IF protocols like 16:8 and OMAD are practiced daily. You compress your feeding window to 12 hours, 8 hours, 4 hours, or one meal and abstain from eating the rest of the day. 

The act of compressing one’s feeding window is also called time-restricted feeding, and a growing body of evidence suggests that it may be beneficial for human health.[*] 

Humans, after all, evolved to oscillate between fasting and feeding. When we fast, we burn fat and activate cellular repair mechanisms.[*] When we feed, we rebuild and heal. 

Unfortunately, we aren’t oscillating these days. We’re feeding all the time. And you see this reflected in the surging rates of obesity and type 2 diabetes.  

This isn’t how we evolved to eat. Our ancestors went many hours without food as a matter of course. And sometimes, they went days or weeks. 

What Is Extended Fasting?

An extended fast begins around 36 hours and can continue for days, weeks, and even months. In one 1973 case study, a man was placed on a medically supervised fast for 382 days, losing 276 of his 456 pounds.[*] That’s about as extended as extended fasting gets. 

Extended fasting (also called prolonged fasting) isn’t a recent invention. Back in caveman times, food was often scarce. When the hunt failed and the fig tree shriveled up, you fasted. 

Our ancestors survived because they could access body fat to fuel their fasts. Even a lean Homo sapiens has many thousands of fat calories stored on their frame, and obese people (like our friend from the 1970s case study) have significantly more stores to run on. 

This doesn’t mean extended fasting is for everyone. It can be a stressful experience that causes insomnia, among other side effects. 

More on that later. Let’s talk about the good stuff now. 

Overlapping Benefits of IF and EF

Extended fasting is like strapping a rocket to intermittent fasting. It might accelerate the health benefits, but there will be more turbulence en route. 

That said, the benefits of intermittent fasting and extended fasting are similar. And the benefit most people care about is fat loss. 

#1: Weight loss

When you practice IF or EF, you typically eat fewer calories overall. And when you eat fewer calories, you find yourself in a state of negative energy balance—or, expending more calories than you consume. This is key for fat loss.[*] 

Another key for fat loss? Keeping the hormone insulin low. This is a normal consequence of fasting. Low insulin signals a breakdown of body fat (called lipolysis) that leads to increased fat burning.[*] 

Turning to the literature, one 2018 review found that multiple forms of intermittent fasting were effective for weight loss, even in non-obese people. And a 2019 study published in PLOS One found that extended fasting of 4 to 21 days resulted in significant average weight loss in 1422 volunteers.

#2: Ketosis

When you keep insulin low by fasting, it signals fat burning and ketone production in the liver. Ketones then fuel the brain and body. 

There are many benefits of ketosis, and one of them is cognition. Having more ketones in circulation appears to improve mental performance, especially in the aging brain.[*] 

#3: Potential diabetes treatment

Both intermittent and extended fasting have the potential to lower blood sugar and insulin levels. These are key therapeutic goals for the treatment of type 2 diabetes. 

The evidence is early but promising. For instance, Dr. Jason Fung has seen good results reversing type 2 diabetes with fasting in his Toronto clinic.[*] And a 2018 JAMA study found that a year of 5:2 fasting lowered average blood sugar (as measured by HbA1c) in people with type 2 diabetes.[*] 

#4: Autophagy

When you fast, you induce a cellular recycling program called autophagy. Autophagy (which means “self-eating”) is when your cells digest old parts and replace them with new ones. 

Autophagy has been linked to longevity in animals, but it’s notoriously difficult to measure in humans.[*] You need to biopsy the tissue in question, which isn’t exactly convenient.  

Bottom line? Extended fasting will likely stimulate more autophagy than intermittent fasting, but the magnitude (and downstream effects) of this benefit are unknown. 

IF Vs. EF: Differences

Beyond autophagy, there are a few differences between IF and EF worth mentioning. 

#1: Ease and comfort

Going 16 hours without food is one thing. Going 5 days is another entirely. 

In the former scenario, you may get a bit piqued, but you’re unlikely to have a miserable experience. During the extended fast, however, you might:

• Have insomnia (fasting increases alertness chemicals like orexin-A)[*]
• Feel weak, cold, and tired
• Experience excruciating hunger
• Be unusually grumpy
• Resent friends and family for eating dinner while you chew a piece of old xylitol gum

To be clear, not everyone has a bad experience with extended fasting. But the risks are there. 

#2: Medical supervision

Most people won’t need medical supervision to practice intermittent fasting. But with extended fasting, it’s recommended. Along with monitoring your symptoms, a clinician can monitor electrolyte levels, body weight, muscle mass, and other key biomarkers to ensure things go relatively smoothly. 

Quick note: Anyone using fasting (IF or EF) to treat a medical condition (like type 2 diabetes) should consult with their doctor first. This will ensure, among other considerations, that medications are handled properly. 

#3: Breaking the fast

Breaking a short intermittent fast doesn’t require a strategy. You just eat a normal meal. 

Breaking an extended fast, however, requires more care. Your digestive system needs time to fire back up.[*] 

It’s wise to break a prolonged fast with a small high-protein meal of, 200-300 calories. This not only supplies amino acids to shut down muscle loss, but it also gives your satiety hormones time to kick in. This will prevent overeating (and subsequent indigestion) after your fast.   

Intermittent or Extended Fasting?

If you’re new to intermittent fasting, don’t start with a heroic fast. For the uninitiated, going two or three days without food can be a miserable experience. 

Instead, start slow. Go from dinner to breakfast without snacks, and work your way up from there. Once you’re adapted to time-restricted feeding, you can venture into the longer stuff if you want. 

If the longer stuff doesn’t suit you, that’s fine. Longer fasts aren’t for everyone. 

Do what feels right for your body. That’s the important thing. 

Please note that as with any significant diet or lifestyle change, we recommend working alongside a registered health professional, especially if you are currently on any prescribed medications or undergoing any medical treatments. Fasting is not appropriate for everyone and should be avoided by those with a history of eating disorders, pregnant and breastfeeding women, and anyone under 18.