Every living thing requires fuel to function. Plants have it easy. They just soak up energy from the sun. But us humans and other critters? We’ve got to get our energy from what we eat.
To make sure we actually bother to find and eat food, evolution has made eating — and even just thinking about it — feel good! That’s why, back in the day, if we stumbled upon an orchard packed with juicy, ripe peaches, we’d stick around and stuff our faces. That way, we’d have enough energy stored up to get through if food were scarce for the next few days.
The idea of voluntarily going without food or intentionally limiting our intake — in other words, fasting — might have seemed crazy to our Paleolithic ancestors. (Unless they had a tummy ache from too many peaches, that is!)
But we know that for thousands of years now, fasting has been a cornerstone of spiritual practices all around the world. Adherents claim that abstaining from food for periods ranging from a day to several weeks brings them clarity of mind, lightness of body, and calmness of soul.
Now, modern science is showing that fasting can be a powerful addition to an overall healthy lifestyle. It can support weight management, metabolic health, and cellular repair. In fact, in laboratory and clinical trials conducted on animals, restricting calories is one of the few strategies that have been reliably found to increase longevity. (Our view on the use of animals in medical research is here.)
There are many different types (or “flavors,” if you prefer irony) of fasting. These include intermittent, water-only, and fasting-mimicking diets. Each method comes with its own set of benefits, risks, and challenges.
In this article, we’ll look at how fasting works. We’ll explore the science, identify and evaluate various fasting methods, and share tips for safe and effective fasting.
What Is Fasting?
Fasting refers to voluntarily abstaining from food and sometimes drinks for a specific period. It can range from short daily fasts (like skipping breakfast) to multi-day fasts.
As fasting has moved into the wellness mainstream, many varieties and “brands” have emerged. Here are some of the most popular.
Intermittent fasting (IF) is about giving your body regular breaks from digestion.
Many of us do this without realizing it; the first meal of the day is called “break fast” for a reason. However, IF can extend the period of fasting or adjust its schedule.
The key difference between all the various IF methods is how long you go without food. Popular forms of IF include:
- The 5:2 diet, in which you eat normally 5 days a week and restrict calories on 2 non-consecutive days (500 calories a day for women and 600 calories a day for men).
- Alternate-Day Fasting is what it sounds like: fasting every other day.
- One Meal a Day, or OMAD, involves fasting for 23 hours a day and eating during a 1-hour window.
- Time-Restricted Eating (TRE) limits eating to the same daily window, such as 10 a.m. to 6 p.m., without necessarily reducing total caloric intake.
Unlike some other IF approaches that involve full fasting days or significant calorie reduction, TRE aligns food intake with the body’s circadian rhythms and may be more sustainable for long-term use.
Read more about intermittent fasting here.
In addition to intermittent fasting approaches, other types of fasting involve more intensive restrictions and are often used for detoxification or therapeutic purposes.
When you water fast, you abstain from all food and caloric beverages, drinking only water for a period of anywhere from 24 hours to several days or more. Most experts recommend that water fasts of longer than three days be done only under medical supervision.
Juice fasting, also known as juice cleansing, allows fruit and/or vegetable juices while eliminating solid foods. The goal here is to give your digestive system a break while flooding your body with micronutrients.
Another variation is the mineral broth fast, where only bone or vegan broth is consumed for nourishment and gut support. Bone broth has gained popularity in recent years, but it does come with potential downsides.
Vegetable broth, especially when homemade, is a low-calorie, nutritious option that can be consumed during fasting, helping to curb cravings and provide hydration.
These methods are typically short-term and more restrictive than intermittent fasting.
What Happens in the Body During a Fast?
Why is fasting associated with improved health? What’s going on inside your cells and organs when you give your digestive system a vacation? It turns out that there are a bunch of good things that can happen.
Glycogen Depletion and Ketone Production
Your body has three basic sources of fuel to draw upon to keep you alive and moving. The first is the carbohydrates you eat, which are quickly converted into glucose and transported into your bloodstream, where they are carried to every cell in your body.
Using carbohydrates as fuel comes with two big benefits. First, they provide quick access to the energy you need. Second, glucose is the main source of energy for your brain.
The second source of fuel is glycogen, which consists of the carbohydrates that you don’t burn right away. Your biggest store of glycogen is in your liver, and it gets tapped when you need energy between meals.
The third source is your body fat. When you’ve used up your stored glycogen, which can occur between 12–24 hours of fasting, that fat starts to break down, not into carbohydrates or simple sugars, but into ketones, which your brain and body can use as an alternate fuel source to keep you going when food calories aren’t available. This metabolic shift — from glycogen to fat and ketones — is a fundamental aspect of fasting.
And it’s what allows many of the health benefits of fasting to occur, such as cellular repair (autophagy), improved insulin sensitivity, and potential anti-aging effects, as we’ll see.
Cellular Autophagy and Detoxification
Fasting triggers autophagy, a process where damaged cells are broken down and recycled. Think of an old house with a fireplace as the only source of heat. If the inhabitants can’t get more firewood, they’ll start looking for other sources of wood to burn. They’ll prioritize broken chairs, rickety shelves, and other damaged items, which can later be replaced by new ones.
In this way, autophagy (which comes from Greek words for “eats itself”) helps clear out dysfunctional proteins and promotes regeneration. There’s some evidence that this may reduce risk factors for neurodegenerative diseases like Alzheimer’s and Parkinson’s.
Improved Insulin Sensitivity
When your insulin sensitivity is high, your body’s cells respond efficiently to insulin, allowing sugar to enter for energy and keeping your blood sugar levels balanced. If you’re insulin resistant, the sugar has trouble opening the door to each cell. This leads to high blood sugar and forces your body to produce even more insulin, which, over time, can develop into type 2 diabetes.
Water-only fasting has been shown to lower insulin levels, helping the body become more insulin sensitive over time. A 2018 study found that TRE improved insulin signaling and glucose metabolism.
Those improvements aren’t necessarily long-lasting, though. What you eat after a fast (known as the “refeeding” phase) is crucial in retaining the benefits of the fast. A 2022 study found that overweight adults who underwent prolonged water-only fasting followed by a plant-based refeeding diet experienced sustained improvements in weight, blood pressure, cholesterol levels, inflammation, and markers of fatty liver disease.
Anti-Aging and Longevity Effects
Direct evidence that water-only fasting extends human lifespan is still scarce, but intriguing research suggests some promising effects. Prolonged fasting appears to kickstart powerful processes in the body, boosting cellular repair, lowering inflammation, and sharpening metabolic health. A study involving 12 healthy volunteers who undertook a 7-day water-only fast observed significant changes in approximately 3,000 proteins in their blood, impacting every major organ in the body. These sweeping changes could very well set the stage for a longer, healthier life.
A 2023 study found that a single 36-hour water-only fast led to significant changes in the plasma lipidome. No, that’s not a stadium in a dystopian sci-fi series. Your plasma lipidome refers to all the “fatty” vehicles that travel through your bloodstream, including cholesterol, triglycerides, and a bunch of others that you may not have heard of because they can’t afford a good public relations firm. Specifically, certain lipids associated with inflammation and metabolic dysfunction decreased, while others linked to cellular signaling and longevity increased.
Fasting has also been shown to significantly reduce levels of IGF-1, a growth hormone that, at high levels, is linked to aging, inflammation, and cancer progression.
Weight Loss and Fat Burning
Remember that when you run out of carbs and glycogen, your body starts burning its fat to produce ketones. That process, called ketosis, occurs in both IF and prolonged fasting. Over time, it can significantly reduce body fat, particularly visceral fat (the stuff that surrounds your organs and is much more harmful than subcutaneous fat, or the fat that’s just under your skin). (For more on the pros and cons of a ketogenic diet, see our article here.)
Cardiovascular Health
Water fasting has been shown to lower blood pressure, cholesterol, and triglycerides.
A 2024 study conducted at the Buchinger Wilhelmi Clinic (a fasting clinic in Germany) observed that participants who underwent a 12-day medically supervised water-only fast lowered their total cholesterol, LDL (“bad”) cholesterol, and triglycerides. Notably, the most artery-clogging particles decreased and remained lower even one month after fasting.
Enhanced Mental Clarity and Neuroprotection
BDNF (brain-derived neurotrophic factor) is a protein that helps keep your brain cells healthy and supports learning, memory, and overall brain function. Think of it as a kind of “fertilizer” that allows your brain cells to grow stronger and better connected.
Fasting increases the production of BDNF. And while research on humans is limited, animal studies suggest that intermittent fasting may reduce oxidative stress and inflammation in the brain. Animal studies also suggest that IF may reduce risk factors for neurodegenerative diseases like Alzheimer’s and Parkinson’s by improving how the brain uses energy, reducing inflammation, and helping nerve cells survive longer.
Inflammation Reduction
Researchers discovered that people who fasted for 24 hours once a week over 6 months had higher levels of a protein called galectin-3 than those who didn’t fast. Galectin-3 helps repair cells and may reduce inflammation in the body.
IF helps reduce inflammation by lowering the activity of NF-κB, a protein that triggers harmful immune responses. More sluggish NF-kB means lower levels of inflammatory molecules, which can compromise gut health and tip your immune system into imbalance.
In a 2023 study, participants showed a remarkable drop in inflammatory activity within their immune cells after a 36-hour water fast, compared to when they had eaten a regular meal. Not eating gave their immune system a chance to calm down and reset.
Other fasting studies have found relief from chronic conditions like arthritis, IBS, and autoimmune flare-ups.
A 2025 study put 20 middle-aged volunteers on a medically supervised 10-day water fast, followed by a 5-day refeeding period. They experienced weight loss, a metabolic shift into fat-burning mode, improved fat processing in their bodies, and even a reduction in amyloid beta proteins, which are linked to brain plaques.
In this and some other studies, a marker of inflammation called CRP has been seen to rise during fasting. Researchers theorize that the rise may be due to a temporary increase in stress-related hormones like adrenaline.
On the other hand, a different inflammatory biomarker called ESR (erythrocyte sedimentation rate) was found to decrease significantly. And periodic fasting has been shown to clinically improve symptoms of rheumatoid arthritis, suggesting that fasting typically leads to decreased inflammation over time.
And because visceral fat itself releases inflammatory chemicals, the reduction in belly size through fasting is also likely to lower inflammation levels in the long run.
Is Fasting For Everyone?
We’ve seen that water fasting, in particular, offers a powerful, evidence-backed way to support detoxification, weight management, metabolic health, and longevity. But it isn’t for everyone. The following groups should probably attempt water fasting only under medical supervision, if at all:
- People with a history of eating disorders
- Pregnant or breastfeeding women
- People with chronic illnesses (especially diabetes, kidney disease, or clotting disorders such as thrombosis)
- Anyone taking medications that may be affected by fasting, such as blood sugar-lowering drugs, blood pressure medications, or those requiring food for proper absorption
Do You Need Supplements While Water-Only Fasting?
Fasting for less than 24 hours typically does not require supplementation, as long as you are healthy and maintain adequate hydration. During short fasts, your body’s electrolyte levels generally remain stable, and drinking a normal amount of water will support your basic physiological functions.
That said, in certain situations, you may have to supplement to maintain your electrolyte balance. These include intensive physical activity or heat exposure, and if you have trouble retaining electrolytes or fluid.
Engaging in intense exercise or being in a hot environment can lead to increased sweating, which means electrolyte loss. Supplementing with electrolytes like sodium, potassium, and magnesium can help maintain hydration and prevent muscle cramps and fatigue.
I’d strongly recommend not scheduling your water fast for the day you run an ultramarathon in Death Valley in July, though! All kidding aside, please save your intense workouts for days when you’re not water fasting.
If you have a health condition that affects your electrolyte balance or ability to retain fluids, such as kidney dysfunction or heart failure, consult with a healthcare provider to determine if you should supplement when fasting.
If you do decide to add electrolyte supplements to your water during fasting, opt for products that are free from added sugars and calories to avoid breaking the fast. Plain water, herbal teas, and zero-calorie electrolyte solutions can be good ways to stay hydrated without interrupting the fasting state.
Longer or Repeated Fasts
Fasting for more than 48 hours or engaging in repeated fasting cycles can increase the risk of deficiencies in key electrolytes, such as sodium, potassium, and magnesium, as well as certain vitamins, particularly if your diet lacks nutrient-dense foods.
Extended fasting depletes essential electrolytes because you’re excreting them, but not replenishing your supplies from food. Sodium and potassium levels, in particular, can drop rapidly during the early stages of fasting.
That’s because decreased insulin levels cause the kidneys to excrete more sodium, leading to a loss of both sodium and potassium through urine. Such losses can, over time, result in symptoms like fatigue, muscle cramps, and dizziness.
Vitamin Deficiencies and Refeeding Syndrome
Prolonged fasting can also lead to deficiencies in vital vitamins, including B vitamins like thiamine (B1). If you just pick up eating normally after an extended fast, you run the risk of something called refeeding syndrome: a potentially life-threatening condition characterized by sudden shifts in fluids and electrolytes.
This syndrome is associated with deficiencies in phosphate, potassium, magnesium, and thiamine, and can lead to complications such as cardiac arrhythmias, muscle weakness, and neurological issues.
While that sounds alarming, it’s much more of a danger for people who have been suffering from prolonged malnutrition, whether from lack of access to food or inability to digest it. In medically supervised therapeutic fasting, most participants are far from malnourished.
Importance of Diet Quality
The risk of nutrient deficiencies during fasting is higher if your regular diet is poor, consisting of highly processed foods that are low in micronutrients. Consuming a diet rich in whole, unprocessed foods, such as vegetables, fruits, nuts, seeds, whole grains, and legumes, can help replenish vital nutrients and mitigate any risks associated with prolonged fasting.
Recommendations
- Monitor Electrolyte Levels: If you plan to fast for more than 48 hours, consider monitoring your electrolyte levels and consult with a healthcare professional about the potential need for supplementation.
- Reintroduce Food Gradually: After prolonged fasting, reintroduce food slowly and focus on nutrient-dense meals to reduce the risk of refeeding syndrome. After a longish fast, your taste buds may have changed; you might find that a small plate of steamed zucchini is the most delicious thing you’ve ever tasted!
- Stay Hydrated: Ensure adequate water intake during fasting periods. In his book Fasting and Eating for Health, Dr. Joel Fuhrman advises that while one quart of water per day is generally enough for most people, some may need to drink two or more quarts. That said, drinking gallons of water, especially if you’re sweating, could be a recipe for throwing off your electrolyte balance.
- Consult Healthcare Providers: Before starting an extended fasting routine, especially if you have underlying health conditions, seek medical advice to make sure you fast safely.
By paying attention to electrolyte and vitamin intake and choosing a balanced diet when you do eat, you can support your health during fasting practices.
What about Salt After Fasting?
Salt may be risky post-fast if:
- You consume a large quantity very quickly, especially along with high-carb foods.
- You have underlying kidney, heart, or blood pressure issues.
- You’re already depleted in potassium or magnesium.
Salt is beneficial and often recommended in the following post-fast scenarios:
- Added in small amounts to rehydration drinks (e.g., homemade electrolyte solutions).
- Used moderately in meals when breaking a fast (e.g., soups or broths).
- Balanced with potassium and magnesium, either from whole foods or supplements.
Clinical fasting centers (like TrueNorth Health Center) often use low-sodium refeeding protocols in the first day or two, gradually reintroducing sodium over time.
Loss of Muscle Mass
One downside of long water fasts is muscle loss. When you don’t eat for a long time, your body may start converting its muscle tissue into energy. Therefore, some of that weight loss is likely to be due to muscle loss.
What is a Fasting Mimicking Diet?
The Fasting Mimicking Diet (FMD) is a type of eating plan designed to replicate the physiological effects of fasting, while still allowing you to eat. (Talk about the best of both worlds!)
Developed by Dr. Valter Longo and popularized through the ProLon Fasting Mimicking Diet, the FMD allows limited calorie intake while still triggering many of the beneficial metabolic processes associated with water-only fasting.
Pros of the Fasting Mimicking Diet:
- Weight Loss and Fat Reduction: Clinical studies have shown that participants following the FMD for five days a month over three months experienced significant reductions in body weight, abdominal fat, and BMI.
- Improved Metabolic Health: The FMD has been associated with enhanced insulin sensitivity, lower blood glucose levels, and reduced blood pressure, thereby contributing to better metabolic profiles.
- Potential Longevity Benefits: In a pair of fascinating clinical trials involving 86 participants, researchers tested the effects of 3 monthly cycles of an FMD, where each cycle consisted of 5 consecutive days of very low-calorie, plant-based meals, followed by about 25 days of regular eating. They found that, after completing the three cycles, participants’ biological age — measured by a validated composite of blood biomarkers linked to mortality risk — dropped by a median of 2.5 years!
- A fasting-mimicking diet study was found to downregulate IGF-1 (insulin-like growth factor 1), which, as we’ve seen, is associated with aging and cancer risk.
- Caloric restriction and fasting-mimicking diets have been shown in animal models to extend lifespan.
Cons and Considerations:
- Short-Term Side Effects: Some individuals may experience hunger, fatigue, headaches, or digestive discomfort during the five-day regimen.
- Cost and Accessibility: Commercially available FMD meal kits, such as ProLon, cost money (though depending on your shopping habits, they might cost less than your regular food budget over the same period).
- Not Suitable for Everyone: The FMD is not recommended for people who are pregnant, breastfeeding, underweight, have eating disorders, or have certain medical conditions. Consultation with a healthcare provider is advisable before starting the diet.
- Limited Long-Term Research: Although short-term studies are promising, further research is necessary to understand the long-term effects and benefits of FMD fully.
Do-It-Yourself Fasting Mimicking Diet (DIY FMD)
While ProLon offers scientific rigor along with ease and convenience, you can save money by approximating the protocol and availing yourself of its benefits by taking a DIY approach.
If you decide to do your own FMD, the ideal approach is to use fresh, whole foods, such as vegetables, fruits, and beans. You can put together your own low-calorie, low-protein, low-carb, high-fat diet designed to “trick” the body into a fasting state while still allowing you to consume food.
| DIY FMD Guidelines | |
| Guidelines | 1. Caloric Targets Day 1: 1,100–1,200 calories Days 2–5: 700–800 calories/day 2. Macronutrient Breakdown 3. Some DIY-Friendly Food Options |
| Sample DIY-FMD Meal Plan | |
| Days 1–6 Meal Plans | Day 1 (~1,100 kcal) Breakfast: Green tea or lemon water, 1 tbsp almond butter, 1 avocado with lemon, 1 small apple or ½ banana Lunch: Butternut squash & leek soup, baked sweet potato round, flaxseed cracker, sunflower seeds Dinner: Steamed broccoli & zucchini, ½ avocado, flaxseed in veggie broth
Day 3 (~800 kcal) Day 4 (~800 kcal) Day 5 (~800 kcal) Day 6 (Refeeding) Supplements & Tips: Multivitamin (no iron unless prescribed), electrolytes if needed, avoid sugar/salt (except minimal sea salt), use herbs & spices, monitor hydration & energy. |
Food Tips for Before and After Fasting
Before Your Fast
Think of your pre-fast meal as stocking your body’s pantry. Reach for slow-burning, complex carbohydrates first, such as oats, sweet potatoes, brown rice, beans, and other legumes. They trickle glucose into your bloodstream over several hours, keeping your energy levels stable as you move into the fast.
Pair those carbs with a dose of healthy fats, such as half an avocado, a spoonful of nut butter, or a small handful of almonds or walnuts. This helps dial down hunger hormones and makes you feel comfortably full.
Finally, pile on water-rich produce such as cucumbers, leafy greens, berries, and slices of citrus. Their fluid and electrolyte content tops off your hydration stores, so you’re not starting the fast on empty.
Breaking the Fast
When it’s time to eat again, treat your digestive system like a muscle that’s been resting: Warm it up gently. Begin with something light but nourishing, such as a clear vegetable broth, a blended green smoothie with banana and chia or flaxseeds, or a plate of steamed vegetables alongside quinoa or tender lentils.
Resist the understandable urge to celebrate with pizza, pastries, or other high-fat or sugary fare; those foods can swamp your stomach and send your blood sugar on a roller-coaster ride.
Take small bites, chew thoroughly, and sip water between mouthfuls. Pay attention to satiety cues rather than the clock or a calorie target.
Over the next few days, gradually widen the menu and portion sizes. Aim for roughly 40–60% of your usual calories on the first refeeding day, then 60–80% the following day.
By easing back in, you’ll help your body transition smoothly from fasting mode to full-power nourishment.
Is Fasting Right for You?
Fasting can be a powerful tool to support metabolic health, brain function, and longevity. However, it’s not for everyone. Those with medical conditions, a history of disordered eating, or who are pregnant or breastfeeding should consult a healthcare professional.
When approached with mindfulness and supported by a healthy diet, fasting may complement a healthy lifestyle and give your body the reset it needs.
Editor’s Note: If you want an easy way to experience the Fasting Mimicking Diet (FMD), you might want to try ProLon. It’s a thoughtful, science-backed approach developed over 25 years in partnership with leading research institutions. ProLon’s 5-Day Fasting Mimicking Program is designed to nourish your body while activating autophagy, the natural cellular renewal process linked to healthy aging, metabolic balance, and longevity.
Unlike traditional fasts, ProLon provides carefully calibrated, plant-based meals that keep you nourished while your body reaps the benefits of prolonged fasting. In just five days, many people experience fat-focused weight loss, improved energy, sharper mental clarity, preserved muscle mass, and rejuvenated skin — all without the strain of complete food restriction. Click here to learn more and take advantage of a special discount.
If you make a purchase through that link, ProLon will give you a discount and contribute a portion of the proceeds to support Food Revolution Network’s mission. (Thank you!)
Disclaimer: While the rest of ProLon’s program is vegan-friendly, their L-Nutra Bar contains honey, which may present ethical concerns for some folks.
