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Does Intermittent Fasting Slow Your Metabolism?

Mitchelle Morgan

Updated on - Written by
Medically reviewed by Kathy Shattler, MS, RDN

does intermittent fasting slow metabolism

Diets like intermittent fasting and keto are popular because they boost your metabolism thus stimulating weight loss. When your metabolism is rapid or high, you burn more calories when sitting or moving about. As a result, if you eat fewer calories, you are more likely to lose weight.

So much so that when it comes to weight loss, the top priority is to increase the metabolism.

However, reducing weight isn’t the only advantage. It also allows you to have a better night’s sleep and more energy. You’ll be able to work more efficiently and feel more energized while maintaining a healthier weight.

There are several ways to boost your metabolism[1], such as intermittent fasting.

Intermittent fasting may help to lower blood pressure and halt the aging process by reducing weight. According to a Harvard study[2], it can also help prevent cardiovascular disease, improve metabolic health and lower cholesterol levels.

But does intermittent fasting lower or increase the metabolism rate? The answer may depend on other variables such as the length of the fasting window and whether you are male or female.

So, to find out, let’s get started.

Intermittent Fasting and Your Metabolism

Intermittent fasting, often known as IF, is a dietary pattern that incorporates intervals of fasting followed by intervals of regular eating.

The eating plan[3] used during the eating window may help in losing weight, lower your disease risk, and extend your life. Following a healthy meal plan and using fewer calories will help assure success.

It may be a healthier approach[4] to reducing weight than traditional calorie restriction for weight loss.

Surprisingly, intermittent fasting has the same or less reducing effects on the body’s metabolic rate as typical calorie-restricted diets.

Many people believe that intermittent fasting increases metabolism since it results in less lean body mass loss and more fat loss. However, science shows that the claim that metabolism slows only because of muscle mass and body fat is not yet definitive. More research on this is still needed.

One small study showed that on alternate days of fasting for a period of 22 days, resting metabolism stayed the same[5] primarily due to fatty acid oxidation during the fasted state.

Over time, the significant calorie restriction in intermittent fasting can drive your metabolism to plummet as your body begins to enter starvation mode. Adaptive thermogenesis[6] is the scientific word for the conservation of energy to prevent starvation. But study results are mixed relative to calorie adaptation during prolonged fasting and the adaptation during IF.

How Intermittent Fasting Affects Your Metabolism

Your metabolism health is affected by intermittent fasting in the following ways. Take note that contradicting facts indicate that it slows and fastens depending on various constraints like time.

All in all, here are the effects of intermittent fasting on metabolism:

Short-term Intermittent Fasting May Increase Your Metabolic Rate by 14%

Fasting could potentially slow metabolism[7] when used long-term since the body goes into the adaptive thermogenesis state. But some previous research has found that fasting for brief periods can speed up[8] rather than slow down your metabolism.

One particular research[9] of 11 healthy men revealed that a three-day fast boosted their metabolism by 14%.

The increase may be linked to the rise in the fat-burning hormone norepinephrine, which is stimulated by a decline in serum glucose and may be a hormonal signal for early starvation[9]. Thus the hormonal profile in long-term fasting is different than the profile seen in fasting with refeeding periods intermingled such as in IF.  More current, high-quality research is needed to determine how intermittent fasting affects metabolism.

Intermittent Fasting Promotes the Production of Fat-Burning Hormones

Hormones are the body’s natural messengers that help body cells, and fat cells communicate with one another. They move through your system to help complex functions like various metabolic processes run smoothly.

They also serve a significant role in weight management. Hormones also have a considerable impact on your desire to eat, the calorie quantity you consume, and the number of fat stores or the ones the body burns.

Some fat-burning hormones have been associated with significant improvements in the weight loss effectiveness of intermittent fasting. Such hormones can make it an effective weight-loss tool.

Human Growth Hormone

Skipping meals can trigger an increase[10] in human growth hormone (HGH) levels in the blood, an essential hormone for fat loss and muscle gain. Fasting has been proven to enhance quantities of human growth hormone in men by up to five times in some studies[11]. Increases in HGH concentrations in the blood stimulate fat burning while preserving muscle mass and providing other health benefits[12]. On the other hand, women don’t always reap the same advantages from skipping meals as men, and it’s unclear whether women will experience the same increase in HGH.

Insulin

Insulin is a hormone that plays a key role in fat metabolism. It instructs your system to store fat and prevents it from breaking down.

Having persistently high insulin levels can make losing weight much more difficult. Insulin resistance has also been connected to obesity,  heart disease, cancer[13], and type 2 diabetes[14]. Alternate day fasting, a form of intermittent fasting, produced greater effects in decreasing insulin levels[15] and insulin resistance than calorie-restricted diets.

Norepinephrine

The “fight or flight” response is aided by norepinephrine[16], a stress hormone that enhances wakefulness and consideration.

It has several different impacts on your system, such as increasing the metabolism of fatty acids released from your fat cells. Increased norepinephrine levels usually result in more fat being accessible for your body to metabolize. The quantity of norepinephrine in your circulation increases when you fast[17].

Does Fasting Slow Down Your Metabolism?

When fasting is done long-term, it eventually slows down your metabolism. But when used with shorter fasting and eating windows, research shows that the metabolism either stays the same or increases. But how does fasting affect the metabolism rate compared to a calorie-restricted diet?

Intermittent fasting metabolism slowing is less than when you restrict calories in your diet. A small group of men and women[18] who had been practicing IF for over a year were measured for their metabolic rate throughout the study. The metabolic rate at the end of the year study was 10% higher than predicted and did not produce adaptive thermogenesis.

Here is a practical example based on the TV series – The Big Loser:

The participants observed a calorie-restricted diet and did a lot of activity to shed weight.

Most of them had regained virtually[19] all of the pounds they had lost six years later. On the other hand, their metabolic rates had not increased and remained roughly 500 calories below what would be expected given their body size.

Other studies support that intermittent fasting boosts metabolism in the short term. IF is more inclined to boost the metabolism rather than slow it down.

According to a study from the University of Vienna[20], a three-day fast can boost your metabolism by 14%. A rise in norepinephrine, which helps produce more fat accessible for burning, is thought to be the source of the metabolic jump.

Whereas a three-day fast might appear extreme, a shorter fast can achieve the same norepinephrine variation. Even 12-hour fasts, according to research[21], boost metabolic processes and aid weight loss.

Lowering your metabolism could potentially become permanent in the plateau state due to the starvation mode.

A starving state[22] is your natural immune defense against starvation, which can occur when calories are restricted beyond what your body requires. You may feel lethargic, grumpy, and fatigued if you go into hunger mode. Worse, it has the potential to lower your BMR. Furthermore, calorie restriction-induced starvation may permanently lower metabolism[23].

When your metabolism slows, you may experience a plateau effect, in which you no longer appear to be losing weight. A calorie-restricted diet is more likely to lead you to regain the weight you’ve lost if you reach a plateau. And this is what happens to the participants of ‘The Big Loser’ six years later.

Is Intermittent Fasting Right for You?

An intermittent fast may help you lose weight in the short term, but it is not the best for prolonged use, especially if you do not follow a healthy lifestyle. One of the effects is to gain weight that you once lost due to the slowed-down metabolism.

The weight loss diet of alternating fasting and eating periods may not be for everyone. If you belong to any of the following groups, then intermittent fasting is not for you:

A woman that is on their menses or is trying to conceive

The reduction in calorie intake when you are on intermittent fasting has proven beneficial in men, but it is not the same for women.

The eating pattern improved insulin sensitivity in men but made blood sugar control[24] in women worse.

The human studies on this are still unavailable. However, the animal[25] experiments showed that mice put under this eating plan became emancipated, missed their cycles, and masculinized.

On that note, a woman in these two states, and women in general, need to approach intermittent fasting to burn fat with caution. They should get a dietitian’s advice on the best fast routine to follow if they want to use metabolism fasting in this pattern.

Diabetic individual

Diabetic patients may be at risk from intermittent fasting.

Blood sugar levels might dip dangerously low if you use insulin or oral hypoglycemic medicines and suddenly consume much less than usual. Hypoglycemia is the medical term for low blood sugar that often accompanies unpleasant symptoms[26] like confusion, blurred vision, and irritability.

High blood sugar is another risk of intermittent fasting for people with diabetes. Hyperglycemia is the medical term for this condition. Hyperglycemia can occur if you consume more than usual, especially after the fasting window.

It can be safely done with the supervision of a medical practitioner if it is to be used to curb weight gain.

Below are the other individuals who intermittent fasting is not good for:

  • Someone with an eating disorder.
  • Someone with low blood pressure.
  • Someone who is underweight.
  • Someone on medication that needs nutritional replenishment.
  • Pregnant and nursing mums.

Bottom Line

When you reduce your food intake deliberately using intermittent fasting, it may help in various ways. Notably, when you restrict calories, you first lose weight mostly because the body turns to metabolize the fat stores.

Briefly, your metabolism rate elevates but drops with continuous fasting. Thus, it may lead to weight gain and other health complications if you do not switch to a healthy lifestyle – consuming a healthy diet and exercising.

Intermittent fasting is beneficial, but you must use it cautiously, preferably with the supervision of licensed medical practitioners. And if you have eating disorders, are a nursing or pregnant woman, or other risk groups mentioned, do not use it.


+ 26 sources

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Mitchelle Morgan

Medically reviewed by:

Kathy Shattler

Mitchelle Morgan is a health and wellness writer with over 10 years of experience. She holds a Master's in Communication. Her mission is to provide readers with information that helps them live a better lifestyle. All her work is backed by scientific evidence to ensure readers get valuable and actionable content.

Medically reviewed by:

Kathy Shattler

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