Everything You Need to Know About Fasting

a plate has a fork and a knife set at the 12 and 3 position to mimic clock hands with food inbetween the utensils

This article is repostedfrom The Health Hub with permission.

Voluntary fasting is a practice that has been undertaken for thousands of years, but only recently has it exploded onto the scene as something that might offer unique health benefits.

But does it?

Over the last 10 years several books have been written about the importance of fasting for health and fasting has become a popular trend. People have promoted various types of fasting from daily 14-16 hour fasts to fasts that last days.
Below we are going to examine the health benefits of fasting and discuss whether these benefits warrant incorporating fasting into your life.

I also want to qualify this article by saying the information I discuss applies to fasting as a health practice, not fasting for spiritual reasons. There may be spiritual reasons for fasting that are outside of the scope of this discussion.

Types of Fasting

So, let’s start by defining what fasting. Fasting consists of going a period of time without eating. There are many ways that people practice fasting.

Some people follow what is called a 16:8 protocol, which consists of going for 16 hours between dinner and breakfast. Others follow a 6-hour eating window, and another approach called the one meal a day approach has become popular in recent years.

This isn’t practiced very often in the public, but many studies have utilized an alternate day fasting approach where someone completely refrains from food on one day and then eats on the alternate days.

There has also been interest in extended fasts that go beyond 24 hours. Some people have promoted 3 and 5 day fasts and sometimes even longer with various health claims attached to them. For example, this poorly cited article from Eric Berg claims that prolonged fasting regrows brain and “increases mitochondria”.

In this article we are going to discuss the claims made around fasting and discuss some common myths and misconceptions regarding fasting.

Health Benefits of Fasting 

First, I want to start off by saying that there are numerous documented health benefits of various fasting protocols. For example, a 2021 study that was an umbrella review of meta-analyses of randomized controlled Trials demonstrated beneficial outcomes for body mass index, body weight, fat mass, low-density lipoprotein cholesterol, total cholesterol, triglycerides, fasting plasma glucose, fasting insulin, homeostatic model assessment of insulin resistance, and blood pressure (1)

This is clear and when people are referring to health benefits of fasting, I agree there are many documented benefits from various types of fasting protocols.

Fasting works…

But the important question is, is there something uniquely beneficial about the fasting protocol or are the benefits simply attributable to the fact that fasting causes people to eat less which leads to weight loss?

Because everything that I just stated above has also been shown for ANY type of calorie restricted weight loss diet (2). When people who are carrying excess body fat reduce their calorie intake, they lose weight and usually various aspects of their health improves - cholesterol improves, insulin sensitivity improves, and blood pressure improves…

So, what we really want to know is there any unique benefits to fasting that are separate from the reduction in calories that it can help to create?

Is Fasting Uniquely Beneficial to Health?

To answer this question, we must look at studies that compare diets that are equated in calories but modify the fasting window in some way. Then we can determine if fasting is a unique strategy to improve health or simply a tool that can help some people to eat less calories which is what is serving to improve health.

There have been a few compilations of studies (called meta-analyses) on this topic. One meta-analysis published in 2020 reviewed 28 studies and concluded that fasting produces similar outcomes to calorie restriction without fasting (3). Another study published in 2022 included 43 randomized controlled trials and concluded the same. Essentially no differences between fasting and calorie restriction groups (4).

As of now, there really isn’t any strong evidence to support unique benefits of fasting.

And if you have heard other people talk about fasting and you are wondering about the effect of fasting on “autophagy” or growth hormone, let’s discuss that…

Mechanist Speculation Regarding Fasting

Autophagy is a process where your cells break down unused proteins to provide fuel and building blocks for essential processes during low energy states. Some people label this as “cellular cleaning” and say that it reduces disease risk.

But there is no evidence that upregulating autophagy has any specific health benefits. Autophagy is really upregulated during starvation, but does that make starvation healthy? Of course not.

This process is not something that just gets turned on during a fast. It is turned up and down depending on our energy needs and availability. Any form of calorie restriction causes an increase in autophagy as well as exercise (5).

This is what is called mechanistic speculation. It is claiming health benefits based on a short term change in a specific mechanism that occurs within the body that hasn’t even shown to be related to health outcomes.

There are a lot of people making claims about intermittent or prolonged fasting based on this type of mechanistic speculation.

Fasting is a Tool To Achieve Calorie Restriction

It is important to understand that these things are simply a tool to restrict calories and there hasn’t been any evidence that demonstrates their unique benefit to health in any way.

There is one minor caveat that I want to add.

There have been some minor benefits shown with a approach to fasting called early time restricted eating. This is when someone eats more of their food in the early part of the day and cuts off their dinner early, vs. later eating patterns that are of equal calories.

Benefits of Early Time Restricted Feeding

There was a study published earlier this year in the Journal of Clinical Endocrinology and Metabolism looked at 12 randomized controlled trials and found that early time restricted feeding showed metabolic advantages to eating approaches that provided calories later in the day (6).

This type of fasting may be an ideal approach. HOWEVER, the benefits of this eating structure are minor, compared to other variables (total calories, protein, fiber, quality of food, etc.) and I highly recommend doing what works best for you in the end.


So quick, review...

Fasting has lots of health benefits. However, as of now there isn’t any good research showing these benefits of fasting are independent of the lower calorie intake that it can help some people achieve.

If you enjoy fasting and it helps you to manage your calories and you have a healthy relationship with food then I think that it can be a good tool.

However, there are many people who are chasing fasting for health benefits that haven’t really been shown to be there and there are others who have a pattern of disordered eating habits who are using it as a way to restrict their eating.

I do not think that fasting is a useful tool in these scenarios.

If you do want to give fasting a try speak with your doctor and make sure there are no health concerns for you as fasting can have various unintended side effects.

Curious to learn more? Listen to the in-depth discussion on intermittent fasting and more at the Nutrition Science Podcast.

Citations and Sources:

      1. https://pubmed.ncbi.nlm.nih.gov/34919135/
      2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5497590/
      3. https://pubmed.ncbi.nlm.nih.gov/32428841/
      4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9108547/
      5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6950580/
      6. https://pubmed.ncbi.nlm.nih.gov/36702768/
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