Fasting and Autophagy: What the Evidence Actually Shows

Autophagy is a natural process in which you break down and recycle damaged and worn-out bits of your cells that are surplus to requirements. Fasting is known to trigger the autophagy process. 

In this article, we’ll go through what autophagy is, what the science tells us about when and how fasting activates it, and what it may (and may not) mean for your health.

Few biological processes have made the journey from obscure cell biology to mainstream wellness like autophagy has. It now appears on supplement labels, wellness apps, and across social media. 

But who says it is triggered? How do they know it's triggered? Even if it is triggered, is it even good for your health? Maybe stopping autophagy is better than increasing it? Can you ever have too much autophagy? What does the evidence show?

What is autophagy? (your cells' built-in recycling system)

The word comes from the Greek meaning “self-eating”. It definitely exists. Autophagy is a vital process where your cells identify, break down, and recycle parts that are broken or are not needed anymore. It’s best to think of it as your cell's waste disposal and recycling system, one that has been operating long before you were born.

What exactly is getting cleared out? 

• Damaged proteins that have been made incorrectly or no longer function as they should. 
• Worn-out but once useful parts of your cell, like the parts that make energy or the proteins in your cell. 
• Waste products, viruses and other harmful debris that shouldn’t be inside your cell.

Your cells are efficient. They don’t just throw away all of the old and unwanted material. They break it down to its basic units (like amino acids, lipids, sugars) and use these parts to make new things that your cells actually need. So it’s not just a clean-up system: it is a recycling plant and supply store as well.^[1]

Autophagy isn’t turned on or off; it is always active, running in the background all the time. Even if you're eating normally, autophagy is working away in the background, keeping your cells healthy at a normal pace. However, when put under stress (which includes extended fasts), it speeds up how quickly it works.^[2]

But whether deliberately increasing it via fasting produces meaningful health benefits is a separate question.

Does fasting-induced autophagy improve human health?

When autophagy rates decrease, things tend to go wrong in your cell. And when normal autophagy rates are restored, things seem to improve. However, this doesn’t automatically mean you want to increase autophagy all the time. 

Autophagy clears toxic proteins that collect in your cells, which have been linked to serious brain diseases (e.g., Alzheimer's, Parkinson's and Huntington's disease).^[3]

Autophagy is also thought to play an important role in cancer. In healthy cells, autophagy can take care of tumours by clearing faulty or damaged parts of the cell and the DNA. However, some researchers have also suggested that autophagy can actually help keep cancer cells alive once they’re established. 

So the “fasting cures cancer via autophagy” is an oversimplification and lacks evidence². Autophagy helps keep your metabolism working as it should, so it has been linked to metabolic diseases like type 2 diabetes, fatty liver disease and obesity. It has also been linked to ageing, as autophagy rates tend to decline as you age. 

So it is associated with the build-up of cellular damage that comes with age. However, definitive proof that fasting produces meaningful improvements in health or healthy ageing in humans by restoring normal rates of autophagy has not been proven.

How fasting triggers autophagy: the mechanism

Fasting is widely recognised as a natural trigger of autophagy — this is true. When you eat, your body releases a hormone called insulin, which goes on to tell your body to “grow and make things”. So when insulin is released, your body dials down autophagy rates.^[4]

When you eat, your body has enough nutrients coming into its cells, so it doesn’t need to break down and recycle old parts. Why would it bother doing this when it can just use the new incoming ones?

So, when you fast, your insulin levels drop, and the autophagy processes are activated. As you continue to fast, the energy levels in your cells start to fall. This sets off another alarm within your cells, telling them that they need to increase autophagy rates again. 

However, if your cells start getting really low on energy, they'll act to protect themselves and preserve their energy, so they’ll actually shut down the autophagy process itself. So the evidence suggests it is nowhere near as straightforward as: low energy + low insulin = more autophagy.

When does fasting-induced autophagy start?

The short answer is we don’t exactly know. No switch is activated at exactly 16 hours into a fast. Your body isn’t a stopwatch. From animal and cell studies (i.e., not humans), indicators that autophagy is underway have been measured after 24 hours of fasting, but only in some organs, not all of them.^[5] 

Measuring autophagy in humans is difficult, so the moment at which it becomes more active has not been precisely measured across different parts of the body. 

By triangulating different pieces of evidence, we can say that autophagy probably starts increasing after 16-24 hours in most people. But no known autophagy switch is turned on in everyone at exactly 16 hours. 

Research has shown that a group of obese people who followed a time-restricted eating pattern for 6 months increased autophagy more than those who followed a normal eating pattern or who followed a standard calorie-restricted diet!.^[6] 

So intermittent fasting did seem to increase autophagy, and they seem to coincide with improved health markers (e.g., weight loss, reduced inflammation and cardiometabolic health)

Can you have too much autophagy?