Written and medically reviewed by Rich LaFountain, PhD
Have you ever been told that fasting “slows down your metabolism”? What about “having an intentional eating window means you’re starving yourself”?
If you’re a veteran, well-educated faster, you likely recognize these for what they are: fasting myths. Myths rarely arise from nothing; oftentimes they’re based upon someone’s lived experience or on a line of reasoning that, at least on the surface, seems rational. The reason they’re myths, however, is that they lack scientific evidence to back them up.
Here are eight of the most common fasting myths and the scientific evidence you can use to debunk them.
Fasting Myth #1: All Fasting Is the Same
Fasting protocols are highly varied and can fit into virtually any lifestyle. Different protocols also provide different health benefits. For many people, fasting for 16 hours and eating within an 8-hour window (16:8 time-restricted feeding, or TRF) helps them lose and maintain weight, align their circadian rhythm, and improve metabolic flexibility. Alternate-day fasting, on the other hand, involves a 24-hour fast that improves gut health by restoring health-promoting microbiome inhabitants. Longer fasts lasting up to 72 hours have been shown to maximize autophagy, prime the immune system, and may refresh the reward circuits in your brain. However, not every type of fast is for everyone; it’s up to you to experiment with different types of fasts to determine which best support your health goals while fitting into a sustainable, healthy lifestyle.
Fasting Myth #2: Fasting Is Unsafe
Fasting has been part of the human experience for as long as humans have existed. In fact, whether they know it or not, everyone fasts when they sleep! Other, more intentional forms of fasting are an important part of nearly all religions, and certain protocols have been used to support health for millennia. Contemporary research indicates that fasting can help to both prevent and treat health conditions including diabetes, cardiovascular disease, obesity, cancer, and neurodegenerative diseases.
However, these health benefits can only be accessed if you fast responsibly. Responsible fasting means appropriately selecting a fasting practice that is not too restrictive and still enables you to meet all of your nutritional and energy needs during your eating window. It also means not fasting at all if you are pregnant or breastfeeding — times when your body needs an ample, continuous supply of nutrients to function and stay healthy. Finally, fasting can be unsafe for individuals with an eating disorder. Anyone who has concerns about their mental and/or physical health when fasting should consult a dietician and other trusted healthcare professionals to determine what health and nutrition practices will work best for them.
Fasting Myth #3: Fasting Puts You in “Starvation Mode”
Fasting and starvation are different things. Fasting involves intentional avoidance of caloric food or beverages for a specific, known amount of time (usually hours or days, not weeks). This temporary break from caloric consumption allows your cells to switch to burning fat, rather than glucose, while your digestive tract has a chance to rest, repair, and recover. Starvation on the other hand, involves consuming no food for weeks, not days, and the endpoint is unknown. Although starvation is, at least initially, metabolically similar to fasting, true starvation is distinct in that it is usually coupled with chronic and/or acute stress, is often endured under crisis conditions, and is not voluntary.
If you’re fasting to enhance your general health, improve your metabolic flexibility, reduce fat, and live a better, longer life, then you’re unlikely to ever reach “starvation mode.” When you’re starving, your body slows its metabolic rate and is forced to pull protein from your lean tissues, primarily from your muscles. Intentional, responsible, fasting, on the other hand, provides your body an opportunity to benefit from using stored energy in the form of sugar and fat and can turn on autophagy to recycle and refresh components of your cells, including your immune system.
Fasting Myth #4: Fasting Slows Down Your Metabolism
You might have heard that eating small, frequent meals can boost your metabolism and improve weight loss. By that logic, fasting (which reduces meal frequency) must slow your metabolism. But that’s wrong! According to the research, meal frequency has a relatively low impact on your metabolism; instead, factors like total energy intake, diet quality, and meal timing play a larger role.
What does contribute to slower metabolism is caloric restriction paired with muscle loss. If you chronically eat fewer calories than your body needs — no matter if you’re eating those calories throughout the day or within a confined eating window — your metabolism will slow down. Furthermore, if you severely restrict your caloric needs (by 50% or less), research shows that the weight you lose will be more muscle than fat. This is bad news for metabolism in the long run, because muscle is more metabolically active than fat. In other words, if you want to lose weight, you’ll want to retain or even build more muscle, because that tissue will burn more calories—even at rest!—than fat. (Plus, more healthy muscle mass makes physical activity easier, and physical activity multiplies the number of calories you can burn!) Fortunately, study data demonstrate that responsible fasting and re-feeding can help you lose fat while preserving your hard-earned muscle, especially if you maintain strength training and exercise habits.
Fasting Myth #5: Fasting Makes You Hungrier
It may sound counterintuitive, but the more often you eat, the hungrier you’ll feel. This is because eating (carbohydrates in particular) causes a fluctuation in blood glucose and insulin levels: They rise when you eat and plummet thereafter. It’s this glucose plummeting that most often leads to the sensation of hunger — your body wants stable fuel!
The good news is that you do have a stable fuel source available: fat. Most people’s bodies have more than enough fat onboard to tide them over until they can eat again; the key is tapping into it. Fasting shifts your metabolism to burn fat, and when this happens, your hunger pangs subside. Furthermore, fat metabolism produces ketones — an important fuel for the brain and other organs when glucose levels are low — which also decrease hunger.
Fasting Myth #6: Fasting Leads to Nutritional Deficiencies
Responsible fasting is not a causal factor in nutritional deficiencies. In fact, there are many people who eat throughout most of their waking hours who are nutritionally deficient or malnourished! In most cases, it is perfectly doable to consume all the vitamins, minerals, and nutrients you need within a compressed eating window. The key is to eat a varied diet of high-quality foods and to fast responsibly.
In this context, responsible fasting means paying close attention to whether you are consuming all of the nutrients you need during your eating window. The more restrictive or tighter the eating window, the more difficult it is to successfully hit your nutritional requirements and the higher your likelihood of developing a deficiency if you follow this level of restriction over a long period of time. Protein provides a good example: For many people, the optimal amount of protein they should aim to consume is 1 gram of protein per pound of body weight. Spread throughout an 8-hour eating window, a 150-pound person only needs to consume about 40 grams of protein per meal to hit this target. However, if you are adhering to a shorter eating window, you have less time to consume all that protein, and research suggests your body can only make use of a given amount of protein at any given meal — meaning that loading 150 grams of protein into a single meal won’t cut it.
Now, this doesn’t mean you can never restrict your eating window. If you’re interested in accessing benefits of more restrictive fasts, such as autophagy or gut rest, you’ll want to limit those fasts to 1 or 2 times each week at most. On the other days, ensure your eating window lasts at least 6 to 12 hours so you have ample time to consume all the nutrients you need.
Fasting Myth #7: Fasting Causes Muscle Loss
Fasting beyond 12 hours encourages your body to “flip the metabolic switch” and prioritize burning fat over other sources of fuel. This is innate — you don’t jump straight to breaking down muscle, or the human race wouldn’t have lasted very long! Traditional dieting, on the other hand, encourages eating many small meals or snacks throughout the day so your body can make due in a forced energy deficit. When your body is receiving a continuous stream of food, even if it’s a minimal amount, it neglects flipping that metabolic switch. As a result, your body will dial back energetically costly endeavors like building and maintaining muscle, which eventually leads to loss of muscle mass.
Ultimately, it’s chronic caloric restriction — not fasting — that overstresses the body and leads to muscle loss. For this reason, research suggests that calorie-restriction diets are not sustainable in free-living humans. Intermittent fasting, on the other hand, promotes metabolic flexibility, turns on your fat-burning machinery, and eventually increases levels of ketones on your body — which research shows will help protect against muscle breakdown. Therefore, fasting coupled with high-quality nutrition, adequate protein intake, and an active lifestyle contributes to healthy muscle maintenance rather than muscle loss.
Fasting Myth #8: Fasting Impairs Brain Function
Not only is “fasting impairs brain function” a myth, but recent research developments demonstrate that fasting is an adaptive stress that improves brain function and is an important tool for prevention of age-related cognitive health declines.
In the day to day, fasting improves brain function because it stimulates fat metabolism and ketone production, both of which are more stable energy sources than glucose (which you get from food). Due to this increase in stable energy from fats and ketones, many people find that fasting provides them greater mental clarity. Fasting has also been shown to increase brain-derived neurotrophic factor (BDNF), which is a key regulator of cognitive function and brain health.
Conclusion
Fasting is a powerful, inexpensive approach to improving health that can be adjusted to fit your specific health and lifestyle needs. When done responsibly, it does not slow your metabolism and put you in “starvation mode,” lead to nutritional deficiencies, or deplete your muscle mass. In fact, when you find the routine that works best for you, fasting can actually eliminate hunger pangs, improve your brain function, and so much more.
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