Does Intermittent Fasting Slow Metabolism?

When it comes to food, there is no shortage of debate and discussion, especially once you get onto the topic of weight loss. People often majorly disagree about the best way to lose weight along with what foods you should be eating.

One aspect of this debate is connected to intermittent fasting. This is a specific type of diet that is effective for weight loss, at least for some people.

The approach is basically a form of autophagy diet and, as a result, has an emphasis on hunger.

For example, one of the most popular versions of intermittent fasting is the 16:8 diet, where you only eat for 8 hours each day. Even though it sounds tough, many people find the diet surprisingly easy. But, one of the most comment questions is: Does intermittent fasting slow metabolism?

After all, we’ve all been taught that skipping meals is a bad idea because it slows down your metabolism, making weight loss harder. At the same time, the idea of grazing (eating lots of little meals throughout the day) was promoted specifically because eating more frequently is supposed to help with metabolism.

So, what’s the reality?

Does intermittent fasting slow metabolism? And, if it does, is this a diet type that you should avoid entirely?

The Concept of Intermittent Fasting

Intermittent fasting comes in a range of forms but, as the name suggests, these revolve around fasting to some degree or another. For example, with the 16:8 form, you’re basically fasting for 16 hours each day.

The site James Clear has powerful illustrations showing what the different styles look like. 

That’s not as hard as it sounds, as those 16 hours include the time that you sleep. However, you are still essentially missing one meal (which often means that you’re skipping breakfast).

Another form is 5:2, which refers to days, rather than hours. Here, you heavily restrict calories for two days of each week and eat as you please for the rest of the week.

In many cases, this can mean consuming less than 500 calories on those fasting days.

There are other variations of the diet out there too, like alternate day fasting, which I’m not going into here.

Regardless of the version, intermittent fasting diets focus on the idea that short-term fasting is beneficial for health. After all, the pattern of having constant access to food is relatively recent and our bodies may not be well adapted to how frequently we eat.

For example, many people eat three full meals a day, in addition to snacks between them. Essentially, this means that the body never gets a break from metabolizing and dealing with food.

Supporters of intermittent dieting argue that fasting can help with weight loss and health benefits, partly because it can change the pattern of chemicals and hormones that are released. In particular, the approach has been linked to increased lifespan and to decreased risk of disease (1,2).

For example, the site Body and Soul talks about how intermittent fasting can help. 

Beyond all of this, intermittent fasting is practical for weight loss – as you don’t have to worry about counting calories or avoiding specific types of food. Instead, most of the focus is on when you eat, rather than what you eat.

Short-Term Fasting and Metabolism

The conventional idea is that skipping food, or fasting, will slow down your metabolism because your body has to make the resources that it has last longer.

In the long-term, this is true.

If you dramatically reduce your calorie intake over an extended period of time or go a long time without food, your metabolism will decrease (3,4)

Biologically, that pattern makes sense. After all, metabolism is a reference to how fast your body burns fuel. It’s not likely to be as fast if your reserves are low.

However, a short-term fast isn’t really the same thing. Indeed, some research has shown that this practice can even increase metabolism (5,6,7).

One reason for this may be that short-term fasting releases a hormone called norepinephrine, which may increase fat burning (8,9).

In fact, short-term fasting has been linked to multiple hormones.

One example of this is insulin. Having too much insulin can make it much harder to lose weight because insulin effectively tells the body to store fat (10,11,12). Research shows that intermittent fasting can help to lower insulin levels (13,14,15).

Another relevant hormone is human growth hormone, which can help to promote the loss of fat (16). Research suggests that this hormone can dramatically increase during fasting, and also helps to maintain muscle mass (17,18).

Additionally, the intermittent fasting approach isn’t likely to have the same impacts as going on a very low calorie diet anyway. For example, a 16:8 version of the diet only means you’re going a little longer without food than you would anyway. Likewise, a 5:2 version only has two low calorie days each week.

Intermittent fasting is a method for weight loss. As a result, it can potentially lead to the loss of lean muscle as well as fat.

This issue is common across weight loss methods, particularly those that dramatically cut down calories or protein intake.

Muscle mass is very important for health and for metabolism.

In particular, having a greater amount of lean muscle can play a key role in increasing metabolism and is also connected to longevity (19).

Indeed, one common recommendation for improving metabolism is to work on increasing lean muscle. Doing so may involve getting more protein in your diet or working out (particularly via resistance exercises).

Now, multiple studies have suggested that intermittent fasting has a similar effect to calorie restriction on lean body mass (20,21,22). This means that intermittent fasting can have a negative impact on your body composition but no more so than basic calorie restriction. As a result of this, intermittent fasting could potentially lower metabolism via that mechanism.

However, some studies do indicate that intermittent fasting may be better at preserving muscle than other types of diet (23,24,25).

More research, particularly long-term studies, is needed before we know for certain whether intermittent fasting is better at preserving lean mass than calorie restriction.

However, losing weight without decreasing muscle mass is achievable for most diet types, including calorie restriction and intermittent fasting. More than anything, this is simply something you need to plan for.

As I mentioned before, getting more protein and doing resistance exercises can majorly help you to maintain lean muscle. Indeed, research even supports this outcome for the 16:8 variation of intermittent fasting (26).

Weight Loss and Metabolism

One study found this in relation to people who lost weight via the TV program Biggest Loser. Even years after the show, participants still had a lower metabolic rate than you would expect for their size (30).

So, there is some degree of metabolism change as you lose weight, regardless of the diet type you use. That impact is likely to be greater if you have more weight to lose.

In this post, my original question was: Does intermittent fasting slow metabolism?

The basic answer is yes, but no more so than other weight loss methods. Indeed, the effect on metabolism may be minimal if you can also maintain lean muscle.

Nevertheless, there have been relatively few studies specifically focusing on intermittent fasting and metabolism and no long-term studies.

As such, there is still a lot that we don’t know.

But, even with the limits of current research, it’s clear there is no need to avoid intermittent fasting because of the potential impacts on metabolism. Certainly, intermittent fasting remains a powerful option for weight loss and many people do find it effective.