How Long Should You Fast?


If you’re researching ways to lose weight - you have surely come across "fasting." If you’re like most people, the idea of fasting can be daunting. After all, who wants to go for long periods without eating? However, there are many different types of fasts and fasting diets. Understanding the different types of fasting diets, the benefits of fasting, and how to get the most out of a fast can help you lose weight! 

What Are the Benefits of Fasting?

Throughout history, fasting has been used - often for religious reasons - to show the willing abstinence or reduction from food. In recent years, it has become an increasingly popular dietary trend. This is because fasting can help trigger several different bodily functions that help burn weight, boost metabolism, and more. What are some of the other health benefits of fasting?

  • An increase in cognitive function
  • Reduced risk of obesity and chronic disease
  • Reduced inflammation
  • Improved overall wellbeing
  • Lowered chances of developing metabolic diseases

Fasting can be challenging, but it can pay off in the long run. Understanding the benefits can help you better understand why fasting has boomed in popularity. 

How Long Should Intermittent Fasting Be?

There are different types of fasting methods. Depending on which fasting diet you’re following, your fasting window may be 16-hours, or it could be an entire day. Knowing the different intermittent fasting rules can help you find the diet that best fits your lifestyle. Below are some popular ways to do intermittent fasting:

  • The 16/8 Method - As you may have already gathered from the name, this method has a 16-hour fast window and an 8-hour eating window
  • The 5:2 Diet - On the 5:2 diet, you usually eat 5 days of the week and only consume 600 calories the other 2 days
  • Eat Stop Eat - This diet is similar to the 5:2 diet. Instead of eating 600 calories twice a week, however, you will practice a 24-hour fast twice a week
  • Alternate-day Fasting - On this diet, you will fast every other day. Some versions of this diet include alternate 24-hour fasts or a significant calorie restriction
  • The Warrior Diet - This diet was created by Ori Hofmekler. You will eat only small amounts of fruits and vegetables throughout the day, with one large meal at night

The diet that works best for you will coincide with your lifestyle, schedule, and eating patterns.

What Are the Benefits of Fasting?

Fasting has been shown to have many health benefits. One of the main benefits is a process known as autophagy. This is the body’s way of replacing old, damaged cell organelles with newer, healthier ones. You can help induce this process through fasting, caloric restrictions, or nutrient restrictions. As you can imagine, fasting can help. But what are some of the other benefits of fasting?

  • Promotes blood sugar control
  • Inflammation reduction
  • Improved heart health
  • Improved brain function
  • Weight loss
  • Improved longevity

While fasting can have its benefits, it can also have drawbacks as well. Speak with your doctor before starting a fasting diet to ensure fasting is safe for you. 

What Happens to Your Body When You Fast for 36 Hours?

Some fasting diets can be rather extreme. Any fast over 36-hours long is often referred to as an extended fast. Fasting for this long flips a metabolic switch in your body, going from sugar-burning to fat-burning. This is exactly why fat is stored inside of our body. 

Our bodies store fat to provide energy when food is scarce. The more fat your body stores, the longer your body can fuel itself without food. Studies have found that fasting for this long can help increase weight loss, lower blood sugar, induce ketosis and autophagy, and reduce the risk of type 2 diabetes. 

How Often Should You Fast for 48 Hours?

It should be noted that research regarding fasting is still in its infancy. However, we know that fasts, even as long as 48-hours, provide numerous health benefits with minimal side effects. While they are beneficial, you should only fast this long every so often. 

Most experts recommend only fasting for 48-hours once or twice per month. You can reduce some of the adverse side effects - such as dizziness, exhaustion, and sluggishness - by staying hydrated, drinking coffee, and keeping yourself busy. Benefits of 48-hour fasts include cell repair, weight loss, and insulin sensitivity. 

How Often Should You Do a 24 Hour Fast?

A 24-hour fast provides many of the same benefits as a 48-hour fast. While experts only recommend a 48-hour fast a couple of times a month, you can fast for 24-hours 1-2 times per week. Now, what you should do outside of your 24-hour fasting window is still up for debate. 

Some 24-hour fasting diet plans require you to follow strict caloric restrictions even on days you’re not fasting. Other diet plans allow you to eat whatever you want during your eating windows. What you should do is somewhere in-between. Eat healthy, nutritious foods during your eating window but don’t feel restricted. 

When You Should Stop Fasting

As we mentioned earlier, fasting doesn’t come without its drawbacks. Understanding when you should stop a fast is incredibly important. If you are fasting, be sure to keep a snack on hand at all times in case you feel faint. Signs that you should stop your fast immediately or seek medical help may include:

  • Tiredness that prevents daily tasks
  • Weakness 
  • Unexpected sickness
  • Unexpected discomfort

Feeling tired or irritable during a fast can be expected - especially if you are new to fasting. This doesn’t mean you need to cut your fast immediately. However, you should monitor your health and know when it’s time to cut the fast. 

What Happens Each Hour When Fasting?

Our body is a well-oiled machine. Each minute that passes by during a fast affects each part of that machine. If you’re keeping up with the hour-by-hour fasting, here’s what you may notice:

  • 4-8 hours - All the food you ate before the fast has left the stomach, and blood sugars fall
  • 12-hours - Food consumed before the fast has been burned, your digestive system goes to sleep, and your body begins the healing process
  • 14-hours - Your body is now burning fat for energy instead of sugar
  • 16-hours - Fat burning increases
  • 18-hours - Natural human growth hormone increases dramatically
  • 24-hours - The autophagy process begins, and ketones are released into the bloodstream
  • 36-hours - The autophagy process sees a 300% increase
  • 48-hours - The autophagy process sees a 30% increase, and the immune system begins to reset and regenerate
  • 72-hours - Autophagy maxes out

Supplements May Help

A healthy lifestyle extends beyond your diet and fasting. Pairing your healthy lifestyle with exercise and a regular supplement regimen can help enhance the positive effects of dieting and fasting. Spermidine supplementation can help your body induce both ketosis and autophagy. Furthermore, autophagy provides many of the same benefits of fasting without the need for caloric restriction.


  • Don Moxley - Director of Applied Science

    Don Moxley is the Director of Applied Science at Longevity Labs. Moxley draws upon his career as an athlete, a sports scientist, and an instructor to lead and educate on the science of autophagy and longevity.