What You Can and Can’t Eat and Drink While Fasting: Your Go-To Guide

Key takeaways: 

• Different fasting methods (like intermittent, prolonged, and water fasting) have unique rules and benefits.
  

• Water, black coffee, unsweetened tea, and pure electrolytes are generally safe during fasting.
  

• Avoid snacks, juice, milk, alcohol, sweeteners, and gum to stay in a fasting state.
  

• The Fasting Mimicking Diet offers fasting benefits while still allowing minimal food intake.
  

• Break a fast gently with nutrient-rich, easy-to-digest foods like smoothies, soups, or cooked veggies.

Fasting is growing in popularity—and for good reason. A growing body of scientific research links various forms of fasting to benefits like weight loss, improved metabolic function, and even mental clarity. But whether you’re currently practicing intermittent fasting, trying the Prolon Fasting Mimicking Diet (FMD), or just starting to explore different approaches, one question tends to come up: What can I eat or drink without breaking my fast?

Knowing what constitutes "breaking a fast" is crucial because it directly affects the benefits you’re hoping to achieve. Consuming the wrong thing, even in small amounts, may interrupt the effectiveness of your fast and reduce your progress toward your health goals. 

In this article, we’ll break down exactly what you can and can’t consume during different types of fasts. Whether you’re new to fasting or looking to enhance the effectiveness of your regular fasting routine, we’ve got the insight you need to stay on track and keep your fasting goals firmly in sight. 

Understanding fasting

While fasting may have ancient roots as a spiritual or religious practice, research continues to demonstrate the health benefits of abstaining from food for a given period of time. These benefits include metabolic support, weight management and, for some fasting methods, triggering autophagy—the body’s cellular repair process. 

Depending on your goals, there are a variety of fasting methods to consider. However, not all fasts are created equal. 

Let’s break down some of the most common fasting methods.

Intermittent fasting (IF)

Intermittent fasting (IF) involves cycling between periods of eating and fasting. Rather than focusing on what you eat, IF emphasizes when you eat. 

Here are some of the most popular methods of IF:

Time-restricted eating (TRE) 

• The 16:8 Method: A more restrictive IF approach, this method involves fasting for 16 hours and eating during an eight-hour window each day (e.g., only eating between 12 p.m. and 8 p.m.).
  

• 12:12: A less restrictive IF method is the 12:12 approach, where individuals fast for 12 hours and then eat within a 12 hour window in accordance with their circadian rhythm (from sunup to sun down). According to Melanie Murphy Richter, MS, RDN, “The 12:12 method reflects a kind of metabolic common sense—one echoed in the world’s longest-living communities. By aligning your meals with the sun, you’re not just giving your body a break—you’re working with your biology. Our organs are designed to metabolize, process, and repair most efficiently during daylight hours. This gentle, sustainable rhythm is less about restriction and more about returning to the way we’re meant to function.”

The 5:2 method

This form of intermittent fasting involves eating your regular diet with normal caloric intake for five days each week, while limiting yourself to significantly reduced calories (about 20-30% of your usual intake) on two non-consecutive days of your choosing. Emerging studies suggest that the 5:2 method may help reduce oxidative stress and brain inflammation, potentially lowering the risk of cognitive decline and neurodegenerative disorders, while also supporting neuroprotection and enhancing mental clarity. Prolon’s 1-Day Reset offers an easy way to incorporate this approach as part of your weekly routine.

Alternate-day fasting (ADF)

In alternate-day fasting, individuals alternate between a day of normal eating and a day of either complete fasting or very minimal caloric intake (often under 500 calories). ADF is considered more rigorous than intermittent fasting and may lead to more rapid weight loss, though it can be harder to sustain long term. ADF can be practiced for varying durations, depending on individual goals and tolerance with some studies showing benefits after 8-12 weeks. 

Water fasting

Water fasting is a more extreme form of fasting that involves avoiding food and consuming only water for a designated period—anywhere from 24 hours to several days. While some people use it for health or spiritual reasons, extended water fasts should only be done under medical supervision.

Prolonged fasting

Prolonged fasting is any fast that lasts 72 hours or more. Studied for its remarkable health benefits, the Fasting Mimicking Diet (FMD) is a type of prolonged fast that allows for very low-calorie, carefully selected plant-based meals over a five day period and is designed to help your body enter a fasting state and trigger the same cellular benefits as prolonged water fasting (like cellular rejuvenation) without complete food deprivation. Three consecutive cycles of Prolon’s 5-Day FMD Program over three months have been shown to support healthy metabolic markers and reduced biological age.

What breaks a fast?

At its core, fasting is a deliberate period of abstaining from food and caloric intake to allow the body to shift from using glucose (sugar) to burning fat for energy. But what exactly “breaks” a fast? The answer depends on your goals, whether they’re weight loss, deep cellular rejuvenation, or digestive rest, but a few fundamental principles apply across the board.

Caloric intake

The most straightforward way to break a fast is by consuming calories. Even a small amount of food or drink that provides measurable energy—typically anything above one to five calories—can technically interrupt the fasting state. This includes snacks, full meals, and beverages that contain calories, like juice, soda, or milk. Even healthy options like smoothies or bone broth contain enough calories to disrupt the fasted state in most interpretations. 

While an FMD like Prolon’s 5-Day Program may appear to break the rule of calorie consumption because it includes food, the nutrition is the result of decades of scientific research and precise formulation. It’s designed to nourish your body while staying below key nutrient-sensing pathways—so you remain in a fasting state. Due to its unique scientific formulation, this level of precision cannot be replicated at home with simple calorie restriction or DIY fasting meals.

Insulin response

Even if a food or drink is low in calories, it can cause a spike in insulin and disrupt the metabolic advantages of fasting. Even if a food or drink is low in calories, it can cause a spike in insulin and disrupt the metabolic advantages of fasting. This is because insulin is released in response to the consumption of certain carbohydrates (especially glucose) and even some proteins, which can signal the body to switch out of a fasting state. For example, artificial sweeteners like sucralose or aspartame may have little to no calories, but in some people, they can still stimulate an insulin response—potentially interfering with fasting benefits. 

Digestive activation

In stricter fasting protocols, anything that activates the digestive system may be considered a fast-breaker. This can include certain supplements, flavored drinks, or even chewing gum. The idea here is that stimulating digestive enzymes or processes in the gut interrupts the body’s rest-and-repair state, particularly if your goal is to promote autophagy or support gut health. While Prolon’s 5-Day FMD does activate digestion, the body still remains in a fasting state. Additionally, it contains prebiotics, which are non-digestible fibers that act as food for beneficial gut bacteria, promoting their growth and supporting a healthy microbiome. This balanced gut environment is important for digestion and overall health.

What can I eat and drink during most fasts? 

Hydration is an important cornerstone of any healthy lifestyle, but it plays an especially important role in fasting. Staying hydrated while fasting may work to offset possible side effects like fatigue, headaches, and hunger. Dehydration has been shown to contribute to adverse health outcomes. When you're fasting, especially intermittently or for extended periods, it's important to know which liquids and supplements are generally considered safe and even beneficial to consume. Here are some options that can help support your body without disrupting your fast: 

• Water: Water is critical to maintaining your body's natural processes and can even help curb feelings of hunger, making it an essential when fasting. Both plain and sparkling water are acceptable during a fast as long as they’re free of added sugars, artificial sweeteners, or flavors. Many people don’t realize how much water constitutes the foods we eat, so during a fast, you should drink more water than normal to replace the water you’re missing from food— this is essential to avoid fatigue and dehydration. Two to three liters of water is typically recommended.

• Unsweetened tea: Most unsweetened teas are fasting-friendly and can provide a comforting, warm (or cold) beverage option. Black, green, and herbal teas are all generally acceptable—as long as they’re free from added sugars, sweeteners, or milk. Many teas offer additional benefits, such as antioxidants and mild appetite suppression, all while keeping you hydrated and in a fasted state. If you are drinking caffeinated teas, however, keep it to one 8oz cup, or 140 mg of caffeine total, per day. Exceeding this amount, or adding anything to it, could disrupt your fasting state

• Black coffee: Black coffee is widely accepted during fasting because it contains very few calories and won’t significantly impact insulin levels. It may even help suppress appetite, which makes it a popular choice for many people practicing intermittent fasting. However, it’s important to drink coffee in moderation and forgo adding cream, sugar, or flavored syrups, as these can add calories and break the fast. As mentioned above, make sure to limit your total daily caffeine intake to 8 oz, or 140 mg, of caffeine. 

• Electrolytes: Electrolytes like sodium, potassium, and magnesium are especially important during longer fasts. Supplementing with pure electrolyte products (those without sugar, artificial sweeteners, or significant calories) is generally acceptable and can help prevent headaches, fatigue, and muscle cramps. However, be cautious about traditional sports drinks, which often contain sugars and additives that will break your fast.

What food and drink should I avoid while fasting?

To get the full benefits of fasting, it’s important to avoid consuming these items: 

• Snacks or “small bites:” While it probably goes without saying that most foods with significant caloric value will probably break a fast, even a small snack is likely to have the same effect. Just a handful of nuts or a spoonful of peanut butter may impact your insulin response and metabolism. “What matters most isn’t just the presence of calories—but how those calories interact with the body’s nutrient-sensing pathways,” says Murphy Richter. “Foods high in protein or sugar, even in small amounts, can switch off the metabolic quiet that fasting creates by triggering insulin and digestive activity. That’s why most traditional fasting protocols avoid even a bite. What makes the Fasting Mimicking Diet so unique is that it leverages specific ratios of macronutrients—it’s low in protein, moderate in fat, and rich in complex carbs—to essentially fly under the radar of these nutrient sensors. It’s like feeding the body without ‘waking it up.’”

• Soda: Both regular and diet/zero calorie sodas offer no nutritional value and are not recommended during fasting. While the research is still ongoing as to whether artificial sweeteners in diet sodas pose a true risk to fasting benefits, strict fasting protocols tend to prohibit them.
  

• Fruit juice: High in sugar and calories, fruit juice can disrupt fasting. For a fruity taste, opt for sugar-free, calorie-free sparkling water instead.
  

• Milk and dairy products: While nutritious, drinks like milk, kefir, and yogurt are calorie-dense and should be avoided during fasting periods.
  

• Bone broth: Bone broth contains calories and nutrients that can trigger the body’s metabolic process and is generally not recommended during standard IF or prolonged fasts.
  

• Alcohol: Alcohol is generally considered off-limits during any type of fast as it is packed with empty calories and can interfere with the body’s fasting processes.
  

• Chewing gum: Just the act of chewing can stimulate digestive enzymes and gastric acid production, signaling to your body that food is coming and can therefore potentially disrupt the metabolic rest that fasting is meant to provide. Even sugar-free gum isn't entirely without concern; some brands contain small amounts of calories or sugar alcohols, which, although minimal, can still technically break a fast depending on how strict your fasting protocol is.
  

Specific guidelines for different types of fasts

Each type of fasting comes with its own set of guidelines, benefits, and precautions that are essential to understand before getting started. Here’s a closer look at the fasting rules for popular fasting protocols.

Time-restricted eating (TRE):

While the daily TRE style of fasting is somewhat flexible, it’s crucial to remember that during the fasting window, calorie intake must remain at zero or near-zero during those hours. This means no snacks, high-calorie drinks, or anything that could stimulate insulin production. Water, black coffee, herbal teas, and other non-caloric beverages are generally acceptable. 

Water fasting: A strict water fast means consuming only water—no coffee, tea, supplements, or flavored beverages. It's one of the most rigorous forms of fasting and should not be undertaken lightly. This method also carries increased risks such as dehydration, electrolyte imbalance, and fatigue. Medical supervision is strongly advised, especially if fasting for more than 24 hours or if you have underlying health conditions.

Fasting Mimicking Diet: The FMD is ideal for those who want the benefits of fasting without complete food restriction. This protocol is designed to provide nourishment while not activating the body’s key nutrient-sensing pathways. This allows your body to enter a fasting-like state while still consuming food. Backed by decades of scientific research and breakthrough nutrition technology,  Prolon’s 5-Day Program is a day-by-day plan that provides your body with essential nutrients in the form of nourishing soups, teas, and more so to help you avoid the risks of dehydration while unlocking the benefits of a prolonged fast. 

Common fasting questions and misconceptions

When starting a fasting routine, it’s normal to have questions—especially about what does or doesn’t “break” a fast. Let’s clear up a few of the most common concerns:

Can I have a squeeze of lemon or lime in my water during a fast?

Yes, a small squeeze of lemon or lime is generally considered acceptable during most fasting practices. It adds minimal calories (typically less than 5) and is unlikely to interfere with key fasting benefits like ketosis or autophagy. Just be sure to avoid sweeteners or anything with added sugars.

In more strict forms of fasting—such as therapeutic or medically supervised water fasts—even lemon or lime may be excluded. Always follow the guidelines specific to your fasting approach.

Do medications or supplements break my fast?

This depends on the type of supplement or medication:

• Usually fasting-safe: Electrolytes (like magnesium, sodium, and potassium), water-soluble vitamins (such as B-complex and vitamin C), and most capsules without added sugars or fillers are typically safe to take during a fast.
  

• May break a fast: Protein powders, gummy vitamins, or any supplement containing sugars, carbs or significant calories can disrupt your fast.
  

If possible, take your supplements during your eating window if advised. Do not change or discontinue necessary medications. For any voluntary supplements or medications, consult your healthcare provider before pausing your routine.

Does brushing my teeth break my fast?

No, it doesn’t. The tiny amount of calories from toothpaste (if any) is negligible and won’t impact your fast. 

How to break a fast safely: what to eat and what to avoid

Breaking a fast isn’t just about eating again—it’s about how you eat and what you eat. Easing back into eating is essential to avoid digestive discomfort, blood sugar spikes and energy crashes.

Best foods to break a fast

Here are some ideal choices to reintroduce nourishment:

• Smoothies: Blended fruits, veggies, and protein can be a gentle and hydrating way to wake up your digestive system. Use simple ingredients and avoid adding too many high-fiber items all at once. Add a scoop of L-Protein for an added dose of plant-based protein that also supports healthy aging.
  

• Soups with easily digestible proteins and carbs: Think bone broth with soft-cooked rice, lentils, or chicken. Warm, liquid-based meals are soothing, high in protein, and are generally easy on the stomach.
  

• Cooked vegetables: Lightly steamed or roasted veggies (like carrots, zucchini or squash) are easier to digest than raw ones and provide essential nutrients without overwhelming your system.
  

• Fermented foods: Yogurt, kefir, or other probiotic-rich options can help restore healthy gut bacteria, which may be affected during extended fasting.
  

• Healthy fats: Nuts or avocado offer satiating healthy fats and essential nutrients, making them great choices as you return to a daily eating plan.
  

• Dates: Dates are a low-glycemic dried fruit that are a traditional and effective way to break a fast. They offer quick-digesting natural sugars and a touch of fiber, making them ideal for a gentle blood sugar boost.

Foods to avoid when breaking a fast

To prevent digestive distress and sluggishness, steer clear of:

• High-sugar foods: Candy, pastries, and sugary drinks may cause a sharp spike (and crash) in blood sugar levels.
  

• Fatty foods: Fried foods or greasy meals are harder to digest and may cause nausea or indigestion.
  

• High-residue or tough-to-digest foods: Raw cruciferous vegetables, legumes, and dense whole grains can be too harsh on the digestive tract immediately post-fast, especially in large amounts. Start with softer, cooked vegetables or low-fiber options before reintroducing these foods gradually.

To break a fast safely, ease into eating with gentle, nutrient-rich foods while avoiding heavy, sugary or high-fiber items that can disrupt digestion and energy levels.

Conclusion: navigating the fasting landscape

Fasting is a powerful, science-backed tool for supporting weight management, metabolic health, and healthy aging. Whether you’re practicing intermittent fasting, exploring longer fasts, or experiencing the benefits of the Prolon 5-Day FMD, understanding the core principles behind fasting is essential to getting the results you’re looking for while keeping your body safe and supported throughout the process.

No matter where you are on your fasting journey, remember: The best results come from consistency, and clarity on the rules and what you’re looking to achieve. With the right knowledge and tools, you can unlock a healthier, more vibrant you.

*Before starting any fasting regimen, especially one involving dietary changes or extended periods without food, it’s always wise to consult with your healthcare provider to ensure it aligns with your individual health needs and goals.

Sources: 

Nature.com: "Fasting-mimicking diet causes hepatic and blood markers changes."

NIH.gov: "Dehydration during fasting increases serum lipids and lipoproteins."

NIH.gov: "Effect of artificial sweeteners on insulin resistance among type-2 diabetes mellitus patients."

NIH.gov: "The effects of intermittent fasting on brain and cognitive function."