Will Intermittent Fasting Slow Your Metabolism? [The Truth]

Intermittent fasting (or time-restricted feeding, as referred to by science), has become more prevalent in this past decade. For those following a ketogenic diet, it's almost inevitable that you'll eventually hear of and/or try intermittent fasting to optimize your results.

But one can only wonder what prolonged periods without food regularly might do to their metabolism.

Will intermittent fasting slow metabolism? According to the current scientific literature, intermittent fasting does not have any measurable effect on the metabolism, which includes resting energy expenditure and thyroid-stimulating hormone. 1

In this article, I'm going to cover what exactly goes into total daily energy expenditure (how many calories you burn daily), some of the current literature on IF, metabolism while EXTENDED fasting, things you can do to keep metabolism humming, and much more.

Table of Contents

Does Intermittent Fasting Slow Metabolism?

A common concern among people who want to try or already implementing intermittent fasting (IF) is whether or not it will slow down their metabolism, aka starvation mode.

After all, that would be counterproductive to your weight loss goals, so I get it.

Over the decades we've come across so many myths and misconceptions, to name a few:

Eat breakfast to “boost your metabolism.”
Eating small meals every 2-3 hours keeps your “metabolism burning.”
Don't eat after a specific time, because it will lead to fat.

I had these very same questions, which led me to go down the rabbit hole of intermittent fasting.

First, let me start by saying that starvation mode DOES NOT exist, nor can you permanently damage your metabolism, which I'll get to shortly.

While there are specific processes in your body that do begin to slow down (downregulate), it's not WHAT and WHY you may think.

How To Calculate Total Daily Energy Expenditure

So here's the deal, long story short, to effectively lose weight you must burn more calories than you consume.

Despite what some guru has led you to believe.

If you didn't believe that, you wouldn't need to worry about a slow metabolism, but you're here.

How many calories we burn throughout the day is what's referred to as your total daily energy expenditure (TDEE). Your TDEE consists of four main components:

Resting Metabolic Rate (RMR)
Thermic Effect of Food (TEF)
Physical Activity (PA)
Non-Exercise Activity Thermogenesis (NEAT)

What Is Resting Metabolic Rate (RMR)?

Your resting metabolic rate (RMR) is the number of calories (energy) you burn to maintain essential bodily functions, such as breathing, keeping your heart beating, and maintaining your body heat.

RMR is by far the most significant component of total daily energy expenditure 2 unless you happen to be a professional Ironman triathlete or Tour de France cyclist.

To summarize RMR, it's the number of calories you burn if you were to do absolutely nothing all day except sleep or lay in bed and not move.

RMR may be what most people consider to be their “metabolism.”

What Is The Thermic Effect Of Food?

The thermic effect of food (TEF) is the amount of energy (calories) required to eat, digest, absorb, and store food.

According to research, TEF accounts for nearly ~10% of our total daily energy expenditure (TDEE). 3

You can look at TEF as the number of calories it takes to digest the food you eat. Certain macronutrients, more specifically protein, require more energy to digest than carbohydrates or fat.

So one can argue that eating a more significant proportion of calories coming from protein would be an easy tool to increase this segment of your TDEE.

What Is Physical Activity?

Physical activity (PA) is the most self-explanatory of the four pieces of your TDEE.

Physical activity is any voluntary exercise you engage in. Lifting weights, going for a run, or playing a sport all fall under physical activity?

What Is Non-Exercise Activity Thermogenesis (NEAT)?

NEAT is all the activity you engage in daily that ISN'T physical exercise. NEAT includes activities like walking, tapping your feet when you sit, doing laundry, and even sex (that is unless you purposely engage in it for exercise).

Research shows that NEAT may vary by up to ~2,000 calories per day in individuals that are of the same height and weight. 4

To put that into perspective, a pound of fat has ~3,500 calories, so a little more than a pound of fat over two days.

This may explain the reason why you see someone who starts at the same height and/or weight as you lose weight slower or more rapidly than yourself.

They simply move around more throughout the day.

NEAT INTERMITTENT FASTING AND METABOLISM

NEAT also plays a significant factor in why you may believe that your metabolism is slowing down. Naturally, as you diet, your body decreases its spontaneous activity, thus reducing its average daily energy expenditure. 5

Research also shows that this adaptation to reduced NEAT can sometimes continue, even after we've stopped dieting, thus resulting in weight regain after you've returned to regular eating. 6

NEAT may ultimately be the culprit when one feels their metabolism has “slowed down” or their body is in “starvation mode.”

A good counterbalance may be to use a step tracker or other smart device to ensure a minimum daily movement or step count.

Intermittent Fasting Metabolism Study

There have been numerous studies on Intermittent Fasting (IF) or Time-Restricted Eating (TRE), as cited by the scientific literature, in both sedentary and athletic populations.

In the context of weight and fat loss, IF's primary mechanism of action is its ability to control calorie intake. Meaning, by restricting your eating window, people seem to eat less food overall naturally.

In one study, participants were told to delay their normal breakfast time by 1.5 hours and move their normal dinner times up by 1.5 hours. This simple shift in eating times, and with no other instructions, resulted in less food eaten, and thus weight loss. 7

Another study had individuals eat within a 10-12 hour eating window of their choosing.

Again, no other instructions were given (they could eat whatever and whenever they wanted as long as it was in a 10-12 hour window). 8

Can you guess what happened? Subjects spontaneously ate less, and thus lost weight.

Just to reiterate, these subjects were also not following a ketogenic diet.

Wondering if you can do IF without keto?

In terms of metabolism, this study following individuals using a 16/8 feeding window found a slight reduction in T3, but no changes in thyroid-stimulating hormone (TSH) or resting energy expenditure.

It would seem as though no studies can show any meaningful impact when it comes to slowing the metabolism.

Does Extended Fasting Slow Metabolism?

Most people who do intermittent fasting (IF) usually engage in a 16/8 fast to feast eating protocol. However, some specific individuals like to take things a little further and do much-extended fasts.

We're talking about 24, 48, 72, and longer fasts.

But do extended fasts slow metabolism? Extended fasts do not seem to slow metabolism, but quite the opposite. Resting energy expenditure increases during the early stages of extended fasting. 9

You read that correctly.

Extended fasting, at least during the first 72 – 84 hours, research has shown an INCREASE in metabolism and plasma norepinephrine.

Norepinephrine plays a central role in the regulation of energy metabolism, which may explain the increase we see during short periods of starvation.

This might be a way of the body, kicking us into high-gear to find food. Our ancestral biology is telling us to go and hunt some food before we die of starvation.

What Are The Drawbacks Of Intermittent Fasting

Now that we've established intermittent fasting, even extended fasting, doesn't seem to slow down the metabolism, what are the potential downsides of IF?

Decreased Muscle Protein Synthesis (MPS)

If muscle gain is of any concern, intermittent fasting may not be the most “optimal” protocol. Notice the word optimal; I didn't say you couldn't gain muscle.

While you can gain muscle by eating all your meals in a small eating window, you won't be doing your body justice in terms of building muscle.

Muscle protein synthesis (MPS) is the process of building muscle mass.

Think of muscle protein synthesis as money going into your bank account, and muscle protein breakdown (MPB) as debits drawn from your bank account.

Building muscle requires more money (MPS) going into the account, then money drawn (MPB) from the account.

To effectively stimulate MPS to near maximal levels requires ~20 grams of protein, while going up to 40g may result in a ~10% increase.

Now, the problem with intermittent fasting is the limited amount of times we can stimulate this process. MPS, once stimulated, experiences a refractory period where the muscle can no longer respond to incoming amino acids, referred to as the muscle full effect. 10 11

During this period of ~ 3 hours, MPS can no longer be stimulated.

You can see how IF, and consuming only one or two meals would not be ideal for stimulating this process. In fact, it's entirely possible to lose muscle on keto if you're not careful.

Food focus

Some people decide to give intermittent fasting a try but like to eat small meals throughout the day.

You may have read that intermittent fasting is the holy grail for weight loss. Still, the reality is both IF and continuous energy restriction (smaller meals throughout the day) will generally provide the same results. 12

Given you keep the calories the same.

If IF stresses you out, causes you to continually watch the clock like a hawk waiting for your eating window, maybe it's time to try a different approach. I'd argue to say you're doing more harm than good as there is very little to no benefit when it comes to IF and weight loss.

However, if you enjoy condensing your eating window, enjoy larger meals less frequently, and eating in a time-restricted fashion allows you to adhere to your diet much better, then keep doing it.

May lead to binge eating

There were times that IF opened the door to overeating, at least for myself. I would go the whole day without eating, so by the time it was time to eat, I was RAVENOUS.

Intermittent fasting led to my appetite getting out of control… sometimes.

If this sounds like you, maybe IF isn't for you either.

This is the case for many people who utilize programs like Speed Keto which adopts the one meal a day (OMAD) schedule of eating.

How To Keep Metabolism High While Fasting

While some metabolism downregulation is expected due to weight loss, not so much intermittent fasting, there are things we have within our power to change.

There are a multitude of reasons why you may not be losing weight or hit a plateau. Want to find out more? Check out my article on the 11 Reasons Why You're Not Losing Weight On Keto.

NEAT

As mentioned earlier, a significant proportion of our daily calories that IS in your control comes from non-exercise activity thermogenesis (NEAT).

People of the same body weight and height can vary by as much as 2,000 calories per day, which is nothing to scoff at.

NEAT is one of the biggest culprits, if not THEE biggest culprit when it comes to hitting a stall or plateau in your weight loss efforts.

The best way to keep your metabolism high while fasting is simply to MOVE MORE!

I'm a big fan of keeping a daily step count, but if that causes anxiety for you, that may not be ideal for you.

This is the part where I say, every little thing adds up, and that's because it does.

The best way to increase your NEAT is by building in small habits a little at a time.

Park further when you go to the store or the office
Take the stairs instead of the elevator
Walk (when you can) instead of driving
Take your dog out for a walk
Do some chores you've been putting off
Pick up the hobby you've always wanted to

The simple act of staying active throughout the day will burn more calories than hopping on that treadmill for 20-30 minutes, and more enjoyable at that.

Pedometer intermittent fasting metabolism slowdown

Eat more protein

Another tip is to increase your protein intake.

You're almost always better eating TOO MUCH protein than not enough protein.

Not only is protein considered the most satiating macronutrient 13, it also burns the most calories through digestion (thermic effect of food).

Just to illustrate the different macronutrients and their effect on metabolic rate:

Protein ~15-30%
Carbohydrates ~5-10%
Fats 0-3%

And while you won't be consuming extremely high protein intakes, chronic consumption of a diet high protein has proven to be relatively safe with no harmful effects on any measure of health. 14

Of course, if you have a pre-existing disease or impaired kidney function, this would not be ideal.

How much protein should you eat? A good rule of thumb is to eat 1 gram per pound of desired body weight. Meaning, If you weight 200 pounds but wish to drop down to 180, then you would consume 180 grams of protein per day.

Ideally, you would eat 1 gram per pound of lean body mass per day (lean body mass is your total weight – your fat weight).

Include resistance training

Aside from the numerous health benefits resistance training provides, having more muscle equates to a faster metabolism.

During periods of calorie restriction (dieting for weight loss), weight from both fat AND muscle tissue is lost.

Our goal during periods of weight loss should be not only to lose weight, but ONLY lose fat.

This means we want to spare all of the muscle we currently have or possibly increase it.

If you've never lifted weights before, you're missing out on a prime opportunity to lose both fat AND gain muscle, known as body recomposition.

Beginners who undertake weight training have a high probability of accomplishing both tasks in the short term, but most end up shortchanging themselves by trying to diet without exercising.

Additionally, including any type of exercise is often one of the predictors, those who successfully manage to KEEP the weight off employ.

It's said that 6 out of every 7 people who are overweight are able to lose weight at some point during their lifetime. Up to ~95 of these people end up regaining the weight.

What's more alarming is that one to two-thirds of these same individuals end up heavier than when they started.

Intermittent Fasting For Weight Loss

The number one reason that intermittent fasting has been shown to aid in weight loss is by causing individuals to eat less over time.

There is no inherent benefit to intermittent fasting over spreading out your meals throughout the day unless it helps you maintain a caloric deficit.

What Supplements Help Increase Metabolism

Very few supplements on the market are worth taking, at least when it comes to fat loss.

Wondering if you can take fat burners on keto and/or IF? The answer is yes, but let's hold onto that thought for a moment.

Most supplements, especially ones claiming to burn fat, are useless.

The real mechanism most supplements work is to help lighten your wallet.

Just take a look at my shark tank keto diet pill review, which is scamming hundreds of thousands of people.

And that's the truth.

Want to know what the most effective supplements are?

In short, the only ones that are likely to be useful and worth considering are:

Caffeine
ECGC (Green Tea Extract)
Yohimbine HCL (If you're already lean)

You'll have to read the other articles if you're interested in more information about these specific compounds and how they may aid in fat loss.

If you're looking for fat burners specifically, then check out this article: Can I Take Fat Burners On Keto?

If you want to know more about any effective supplement in general, then check out this article: The Best Keto Supplements Of 2020.

The Takeaway

Research has shown that intermittent fasting does not have a detrimental effect on metabolism. In fact, short periods of extended fasting have shown that metabolism increases.

However, metabolism does downregulate slightly through other mechanisms during periods of weight loss.

First, you're carrying a lighter body, so you burn fewer calories doing the same activities.

Secondly, your spontaneous movements (NEAT) generally goes down.

Lastly, to combat these adaptations, it's suggested that you remain active throughout the day and engage in resistance training as muscle is more metabolically active than fat.