If you want to lose weight, you’ve probably considered fasting. Currently, “intermittent fasting” is the most popular type, and involves cycling between periods of eating and periods of fasting. This method, studies have found, can not only help you lose weight but may have other health benefits as well.
On the other hand, you may have heard that if you go on any type of fasting diet, it may cause your metabolism to slow down, which could make you even more prone to weight gain in the future.
Is this true, and if so, should you skip this weight-loss method?
What is Metabolism?
Your metabolism is like your internal engine. Every day, your body converts food and drink into energy, much as a car might convert gasoline into power. It then uses that energy to keep the heart beating, lungs breathing, muscles moving, and brain thinking.
In general, metabolism involves two kinds of activities:
- Anabolism: Also called “constructive metabolism,” this type is about building and storing what the body needs. It is responsible for helping new cells grow, maintaining body tissues, and storing energy for future use.
- Catabolism: Also called “destructive metabolism,” this type is about breaking down. The cells break the bonds in molecules like carbohydrates and fats to release the energy in them, providing fuel for motion and heat.
Everyone has a “resting metabolic rate (RBR),” which is a measurement of the energy required to keep your body functioning at its most basic level—keeping your heart beating, your blood circulating, and your other organs operating as they should while at a resting state. This rate is measured in calories: How many calories do you need to maintain this basic level of operation?
Everyone’s RBR (sometimes called “basic metabolic rate” or BMR) is different and is affected by factors like weight, gender, age, and body composition (muscle vs. fat). A very large, muscular person, for example, will have a higher RBR than a smaller, thinner person.
The easiest way to determine your RMR is to use an RMR calculator. There are many online available. You enter your gender, age, height, and current weight, and the calculator gives you the basic number of calories you would need to consume each day to maintain that RMR. Keep in mind that this doesn’t include any extra calories you may need to support your daily exercise regimen or any other activities.
In general, the “faster” your metabolism, the more calories you can burn per day, while a “slow” metabolism requires fewer calories, storing the rest as fat. This is why some people can eat whatever they want without gaining weight, while others have to be super careful.
What Affects Metabolism?
There are a variety of factors that can affect your metabolism—speed it up, or slow it down. These factors determine how many calories your body burns and how many it stores as fat.
- Muscle mass: The more muscle you have, the more energy your body needs to make them work. That means you’ll burn more calories, and be less likely to store them as fat.
- Age: As we age, our metabolism naturally slows down. This is because of changes in hormonal and neurological processes, and can also be related to a loss of muscle.
- Body size: Those with larger bodies require more energy to make them operate.
- Gender: Men generally have faster metabolisms than women.
- Genetics: Your genes can affect your metabolism, as some families have naturally faster RMRs than others.
- Physical activity: The more active you are, the more energy your body needs to support that activity, which can increase your metabolism.
- Hormonal factors: Hormone changes and imbalances can affect your metabolism. This occurs during puberty as metabolism speeds up, and during menopause when metabolism often slows down. Hormonal issues like hypo- and hyperthyroidism can also affect metabolism.
- Environmental factors: The body uses the least energy when it’s perfectly comfortable. If it’s hot or cold, it has to rev up the metabolism to deal with these changes.
- Diet: What you eat affects your metabolism. Some foods slow it down while other foods help speed it up. Proteins, for instance, increase metabolism much more than fat.
- Drugs: Caffeine and nicotine can increase your metabolism, while other medications like antidepressants and steroids can slow it down.
There’s one more thing that should be on this list: calories consumed.
How Do Calories Consumed Affect Metabolism?
In general, if you significantly restrict calories, your metabolism will likely slow down to conserve energy. This is a survival mechanism and may kick in if you find yourself say, trapped on a deserted island with very little food. Your body will naturally slow down to help you survive.
It makes sense, then, that going on a diet—in which you decrease the number of calories you consume—may also slow down your metabolism. It’s a cruel trick of nature that when you cut back on calories to lose weight, your body adapts by slowing your metabolism down.
The bad news is that when you go back to eating your regular diet, the body often fails to pick back up again. Instead, it maintains that slower metabolism even though you’re eating more. That’s why you can suddenly regain the weight you lost and often even more.
Several studies have found this to be true, particularly during significant calorie restriction. In a 2006 study, for example, participants reduced their calories for four days. Results showed that they all experienced significant decreases in body weight, but also decreases in their BMR. The fewer calories they ate, the greater the decreases in their BMRs.
In a later study, researchers found that reducing calories resulted in drops in metabolic rate as well as decreased physical activity levels. And in 2017, researchers reported that 14 “Biggest Loser” competitors—all of whom had lost significant weight while on the show—still had slower metabolisms six years later than they’d had at the start of the show. “We found that despite substantial weight regain in the 6 years following participation in The Biggest Loser,” the researchers wrote, “RMR remained suppressed at the same average level as at the end of the weight-loss competition.” More specifically, RMR was 500 kcal/day lower than expected in these subjects.
The conclusion seems to be that the less energy (in the form of calories) you take in, the less energy your body produces to match. Calorie-restrictive diets slow metabolism, and when the dieter goes back to consuming a normal amount of calories, the sluggish metabolism is unable to burn the extra energy, and the weight piles back on.
Does Intermittent Fasting Reduce Metabolism?
So the question is, does intermittent fasting also reduce metabolism? Fortunately, it doesn’t seem to.
The magic of intermittent fasting is that you limit the number of calories you’re consuming only periodically and for a limited amount of time, usually between 12 and 36 hours. You’re not limiting calories for days or even weeks on end, as is required with many diets. Instead, you go through intermittent short periods of restricting calories, followed by a return to normal.
The result is often an increased metabolism rather than a decreased one, which can not only help you lose weight but keep it off.
Studies show this to be true. In one experiment, researchers had non-obese participants fast every other day for 22 days. They measured their body weight, body composition, RMR, and more at baseline and after day 21 and day 22. Results showed that the participants lost weight and fat mass, but their RMR did not change significantly the entire time.
In another study, scientists had healthy participants fast for four days. They found that during that time, up to and including the fourth day, the RMR actually increased over baseline. This was thought to be because of hormones released during the fasting period that drove up the metabolic rate.
You don’t have to fast for an entire day for this method to work, either. Modern-day intermittent fasting often includes only 12-16 hours of fasting, and you get to choose when those hours kick in. Whereas other diets focus on what you eat, intermittent fasting is easier for many because it focuses only on when you eat.
Some common intermittent fasting plans include the following:
- 16/8: You restrict your daily eating to one eight-hour period. So you may allow yourself to eat from 9:00 in the morning to 5:00 in the evening, after which you will not eat anything more until 9:00 the next morning.
- 14/10: This is a variation on the option above, and involves 10 hours of allowed-eating time, with 14 hours fasting.
- 5:2: With this plan, you eat regularly for five days a week, then on the other two days, you limit yourself to one 500-600-calorie meal. You can choose any two days you like.
People find it easier to adapt to intermittent fasting both because it’s easier and because they usually feel better while doing it. In a 2018 review, scientists found that intermittent fasting was an efficient way to lose weight without regaining it down the road. In an earlier study, men assigned to a 16/8 intermittent fasting plan along with resistance training for eight weeks experienced decreases in fat mass while RMR remained unchanged.
Some studies even show that short-term fasts may actually boost metabolism rather than slowing it down. Harvard Health reports that just changing the timing of meals, but eating earlier in the day and extending the overnight fast, significantly benefited metabolism even in people who didn’t lose weight. And in a 2019 study, scientists found that going without food for a short period boosted human metabolic activity, even to the point of helping to reverse some effects of aging.
Tips to Help You Succeed with Intermittent Fasting
If you’d like to lose weight without reducing your metabolism, try intermittent fasting. You can choose any of the plans listed above or some variation of those, then follow these tips to help increase your odds of success:
- Stay hydrated—drink lots of water and calorie-free drinks like herbal tea.
- Distract yourself—to avoid obsessing about food, plan other distractions to keep your mind busy.
- Eat healthy—select nutrient-dense foods that are rich in protein, fiber, and healthy fats.
- Eat water-rich foods—these will help you feel less hungry. Choose soups and watery fruits and vegetables.
- Pace yourself—avoid strenuous activity on those days when you fast for longer periods (if you choose that sort of plan).
Note that not all diets are right for all people, so choose a system that works for you. If you are interested in Intermittent Fasting, make sure that you have a plan in place to ensure you are getting the nutrition you need in the time aloted.
Looking for more metabolism-boosting secrets? Click here for more information.
Fothergill, E., Guo, J., Howard, L., Kerns, J. C., Knuth, N. D., Brychta, R., Chen, K. Y., Skarulis, M. C., Walter, M., Walter, P. J., & Hall, K. D. (2016). Persistent metabolic adaptation 6 years after “The biggest loser” competition. Obesity, 24(8), 1612-1619. https://doi.org/10.1002/oby.21538
Ganesan, K., Habboush, Y., & Sultan, S. (2018). Intermittent fasting: The choice for a healthier lifestyle. Cureus. https://doi.org/10.7759/cureus.2947
Heilbronn, L. K., Smith, S. R., Martin, C. K., Anton, S. D., & Ravussin, E. (2005). Alternate-day fasting in nonobese subjects: Effects on body weight, body composition, and energy metabolism. The American Journal of Clinical Nutrition, 81(1), 69-73. https://doi.org/10.1093/ajcn/81.1.69
Kouda, K., Nakamura, H., Kohno, H., Okuda, T., Higashine, Y., Hisamori, K., Ishihara, H., Tokunaga, R., & Sonoda, Y. (2006). Metabolic response to short-term 4-day energy restriction in a controlled study. Environmental Health and Preventive Medicine, 11(2), 89-92. https://doi.org/10.1007/bf02898148
Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q. F., Battaglia, G., Palma, A., Gentil, P., Neri, M., & Paoli, A. (2016). Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine, 14(1). https://doi.org/10.1186/s12967-016-1044-0
Redman, L. M., Heilbronn, L. K., Martin, C. K., De Jonge, L., Williamson, D. A., Delany, J. P., & Ravussin, E. (2009). Metabolic and behavioral compensations in response to caloric restriction: Implications for the maintenance of weight loss. PLoS ONE, 4(2), e4377. https://doi.org/10.1371/journal.pone.0004377
Tello, M. (2020, February 10). Intermittent fasting: Surprising update. Harvard Health Blog. https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156
Teruya, T., Chaleckis, R., Takada, J., Yanagida, M., & Kondoh, H. (2019). Diverse metabolic reactions activated during 58-hr fasting are revealed by non-targeted metabolomic analysis of human blood. Scientific Reports, 9(1). https://doi.org/10.1038/s41598-018-36674-9
Zauner, C., Schneeweiss, B., Kranz, A., Madl, C., Ratheiser, K., Kramer, L., Roth, E., Schneider, B., & Lenz, K. (2000). Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American Journal of Clinical Nutrition, 71(6), 1511-1515. https://doi.org/10.1093/ajcn/71.6.1511