However, simple mistakes like eating or drinking the wrong foods or beverages while fasting may negate many of the benefits people seek.
This article covers which types of foods break a fast, the best foods to end your fasting period intentionally, what you can safely eat or drink without disrupting fasting, the effects of dietary supplements on fasting benefits, and more.
First, a short refresher on how intermittent fasting works, the most common styles of fasting, and the health benefits of fasting.
Fasting refers to the practice of intentionally avoiding any food and all calorie-containing beverages for any reason, including health, weight loss, spiritual, or religious purposes.
Intermittent fasting is a popular trend of scheduling periods of fasting anywhere from once per day to once or more each week, mainly to improve health and lose weight.
Although “intermittent fasting” is technically a misnomer for regular daily fasting, many people use the term that way anyway.
Most of the time, people fast and avoid food for anywhere from 4-12 hours at a time not counting sleep.
With an additional 8 hours of fasting during sleep factored in, that makes typical fast periods around 12-20 hours, with eating windows (which are exactly what they sound like) of at least 4 hours, but sometimes as long as 6 days, in between.
There’s a lot of variation in how people fast, but you can find examples of the most common patterns in the next section.
Some people also fast for prolonged periods of 24-36 hours or longer. The frequency of such extended fasts can range anywhere from a few times a week (like alternate day fasting, discussed below) to a few times a year.
The favorable effects of fasting are not altogether different from how the keto diet works.
When you avoid food, especially carbohydrates, your body depletes your glycogen stores and increases fat oxidation (fat-burning) as an alternative way to fuel your muscles, brain, and other organs[*].
After 24-72 hours of using fat for fuel, your liver begins producing ketones, which are high-energy molecules with beneficial anti-inflammatory properties[*].
Keep in mind that fasting is a flexible practice, and there are numerous styles of fasting designed for different purposes. For example, fasting primarily for fat loss isn’t identical to fasting in hopes of increasing longevity.
In the next section, you’ll find an overview of the most popular ways people organize their fasting schedules.
- Intermittent fasting: Avoiding food for a set period of time, typically less than 24 hours, followed by an eating window, and often once each day. Daily 16/8, 18/6, and 20/4 intermittent fasting are among the simplest and most popular fasting strategies.
- One meal a day (OMAD) fasting: Eating a single meal each day and fasting for the remainder of the time.
- Alternate day fasting (ADF). The practice of fasting for a full day, then eating freely the next day. For example, you’d eat nothing on Monday, eat all day Tuesday, not eat Wednesday, and so on. Some variations allow a single meal before bed on fast days, while others do not, making the typical fasting length 24-36 hours (counting sleep).
- Extended fasting: Irregular fasts lasting longer than 36 hours. Popular lengths include 2-day fasts, 3-day fasts, and 7-day fasts. Some people even fast for several weeks, but you should consult a doctor’s opinion before trying such extreme fasts.
- Fat fasting or fasting-mimicking diet (FMD). Eating zero carbs, little or no protein, and moderate amounts of fat for a set period of time to imitate a fast period. A fasting-mimicking diet most often uses a 5-day period with half to one-third of your usual calories[*]. Fat fasting is easier than cutting out food entirely and is one way to ease into more intense fasts. Your appetite may be reduced compared to full-on fasting, which lets you fat fast for longer. That said, you’ll also miss out on some of the benefits of a true fast.
If you’re interested to begin fasting, try a few different lengths and styles and find a rhythm that works for you.
There’s no single best way to fast for everyone. It’s about what feels sustainable for your unique physiology as well as appropriate for your goals and lifestyle.
- Fasting enhances fat oxidation (fat-burning) and the effects are synergistic with other fat loss practices like exercise and the ketogenic diet[*].
- Like the keto diet, the lack of carbohydrates during fast periods appears to help reduce blood sugar levels and increase insulin sensitivity[*].
- Autophagy from fasting helps remove and recycle old and defective cells, which may help treat or reduce the risk of some cancers[*].
- Thanks to its ability to increase insulin sensitivity and promote autophagy, fasting is beneficial for your cardiovascular health and central nervous system and may reduce the risk of heart disease and degenerative neurological conditions[*][*].
- Similar to calorie restriction, fasting is proven to increase the lifespan of some animals by up to 30%, but life extension results in humans are inconclusive so far[*].
As we just covered, according to its very definition, genuine fasting requires you to avoid all foods and any beverages that contain calories.
The mechanisms of fasting benefits, such as ketosis, autophagy, and calorie restriction, are very sensitive to disruption from calories and especially carbohydrates[*].
To reap the full health and longevity benefits of fasting, your fast periods should contain as close as possible to zero calories from any source.
But for achieving weight loss and at least some of the health benefits of fasting, less strict versions of fasting (such as the fat fasting or fasting-mimicking diet discussed under “Intermittent Fasting Plans,” above) can also work.
Keeping in mind that true or authentic fasting requires zero calorie intake outside of your designated feeding windows, here’s what to remember if you’re considering other versions (such as fat fasting):
- Fats have the least impact on your metabolism during fast periods, and will not kick you out of ketosis from fasting or the keto diet.
- Eating dietary protein doesn’t end ketosis either, but it does activate cellular pathways that could negate some of the health and longevity benefits of fasting[*].
- Carbohydrates raise insulin and blood sugar, end the state of ketosis and activate growth pathways that run counter to the benefits of fasting[*]. They’re the macronutrient with the most impact on fasting-related metabolic pathways.
As a general rule, consuming more calories or more food has a more profound effect.
Taking a few small bites of any food would have a minimal impact on fasting, whereas eating multiple servings, regardless of macros, would inevitably end your fasting window.
Ingredients to Avoid During Fasting Periods
If you’re engaged in strict fasting, watch out for these sneaky sources of hidden carbs and calories, many of which can show up unlabeled or in so-called “zero-calorie” foods:
- Alcohol in any form
- All sugar alcohols (including erythritol, sorbitol, xylitol, maltitol, and isomalt)
- Fruit juice or juice concentrates
- Fructose, sugar, sucrose, maltodextrin, dextrose, or any type of corn syrup
- Isomaltooligosaccharides (IMOs)
Additionally, some artificial ingredients that are calorie-free could still have adverse effects during fasting by activating growth pathways, triggering insulin release, or damaging your gut microbiome.
Read labels carefully and steer clear of the following:
- Artificial sweeteners, especially saccharin and sucralose (Splenda)[*][*][*]
- Artificial colors and flavors[*]
- Preservatives and other chemical additives[*]
Next, you’ll learn the best food choices for intentionally ending your fast periods.
The biggest considerations for which foods to eat at the start of your eating window relate to your goals and reasons for fasting.
If your main goal is fat loss, the best foods to break a fast are nutrient-dense whole foods that are low in carbohydrates and contain plenty of protein and healthy fats.
Pastured meats, wild-caught seafood, low-carb organic veggies, and healthy fats are all excellent choices.
In other words, most ketogenic foods are appropriate for breaking your fast if you want to lose weight. They can help you feel full, avoid overeating, and stay in ketosis as you wrap up your fast.
People who are fasting for longevity or general health reasons should break their fast with similar foods.
However, the longer fast periods (often 24 hours or longer) that people use for longevity bring an additional caveat: the best foods for breaking a longer fast must also be as gentle as possible on the stomach.
When ending extended fasts, avoid high-calorie meals, calorie-dense foods, or large servings of protein or fats at first. Instead, opt for gentle, soothing, nutrient-dense foods that are easy to digest.
Bone broth is the perfect example, and light soups or cooked vegetables are also excellent options on an empty stomach. Follow these selections up with the keto-friendly foods recommended above.
Finally, if you’re an athlete or exercise hard and follow the cyclical or targeted keto diet, it may be appropriate to break your fast with carbs to refuel your muscles and support optimal performance for training or competition.
If so, your best bets are whole food sources of carbs like sweet potatoes, rice, quinoa, oatmeal, dates, raisins, bananas, or sprouted-grain bread paired with complete animal proteins such as meat or whey protein.
If you’re engaging in strict fasting, which is the correct approach for health and possible longevity benefits, you won’t eat anything while fasting. Period.
Otherwise, there are a few circumstances where at least small amounts of certain foods could be acceptable or useful.
One such situation is if you’re unaccustomed to extended fast periods and you’re struggling to meet your intended duration of fasting.
For example, if you’re 9 hours into a 12 hour fast and you begin feeling hungry and irritable, eating a minimal amount of an appropriate food could allow you to be more comfortable for the next 3 hours until the fast is over.
Over time you could become more comfortable with fasting and go the full 12 hours without eating anything.
Another reason could relate to weight loss. If your main reason for fasting is to reduce your body weight, using small amounts of keto-friendly foods to decrease hunger, prevent irritability, and make it through your fast can have the net effect of helping you achieve your weight loss goals.
While you’ll theoretically receive fewer health benefits compared to a true fast, you’ll still become healthier as long as you lose excess body fat in the process.
Here are the top foods to extend your fast periods with the least impact on weight loss and other beneficial effects of fasting:
- Watery, fibrous, crunchy raw vegetables (such as celery, broccoli, or cauliflower) for hunger or cravings
- Very small amounts of keto-friendly low-carb fruits, especially if you’re experiencing brain fog or irritability
- Healthy fats, especially MCT oil for its benefits on fat loss and ketosis
- Collagen protein, which may help suppress appetite but is less likely to activate growth pathways during fasting due to its amino acid profile[*]
Keep reading to learn what you should and shouldn’t drink during your fasting periods.
Although some purists argue that water is the only acceptable drink while fasting, there’s no strong evidence to support this claim.
Their thought process holds that anything other than water could potentially disrupt the benefits of fasting, but even if that’s the case, any effects from non-caloric beverages are likely to be minor compared to eating food.
Any drink that doesn’t contain calories (particularly from carbs) is fine. Ideally, you should also avoid the artificial sweeteners and other chemical additives mentioned earlier in this article.
Here are some of the best drinks that won’t break your fast period:
- Springwater, mineral water, or sparkling water (optionally, add lemon or lime)
- White, black, oolong, pu-erh, or green tea (skip the honey or sugar)
- Herbal teas or tisanes
- Black coffee with nothing else added
Overall, dietary supplements during fast periods have some of the same tradeoffs as eating or drinking while fasting.
Some supplements can potentially offer minor advantages, while others are detrimental.
If you prefer to play it safe and ensure you get the most extensive health benefits from fasting, save your supplements for eating windows only.
Here are the supplements that have the fewest downsides while fasting:
- Electrolytes, especially if you’re sweating a lot
- Beta-hydroxybutyrate exogenous ketones
- MCT oil powder
- Collagen protein
In contrast, supplements like BCAAs, creatine, and whey protein should definitely be avoided during fasts, because they can activate growth pathways that could potentially counteract fasting benefits[*][*][*][*][*].
Lastly, avoid taking vitamins and other antioxidants within your fasting window. Antioxidants have the effect of reducing damage to cells, which ironically may negate some benefits of fasting[*].
The simplest and most effective approach to fasting is to fast strictly, consuming zero calories from food or other sources until you complete your fast
That means you can drink water, black coffee, unsweetened tea, and similar beverages.
On the other hand, while eating likely diminishes some of the benefits of fasting, some foods likely have less impact and could allow you to function better while fasting or enable you to extend your fasts for weight loss purposes.
Ending your fast is relatively simple. Most people should simply eat healthy, keto-friendly whole foods to break their fasts.
If you’re engaging in prolonged fasts for longevity or other benefits, try easing into your eating period with easily digestible foods like bone broth or cooked vegetables.