Fact checked Before we hit “publish,” our science team needs to be 100% satisfied that we’re giving solid advice based on high-quality, reliable, scientifically-sound research. Learn more
Editorial guidelines At SIMPLE, we use our nutrition and wellness expertise to give you actionable content that helps you achieve your goals, overcome challenges, and increase your well-being. Learn more

    Intermittent fasting is an approach to eating that involves a repeated routine of “eating windows” and “fasting windows” (periods of time when you don’t eat). Rather than focusing on what you eat, it emphasizes when you eat. And quite a bit of scientific evidence backs it up as a health-promoting strategy.[1]  

    Unlocking the safety of intermittent fasting, no additional Google searches needed! Join us as we break down the “when over what” mantra backed by science.

    A flexible food routine where you can (quite literally) have your cake and eat it, too — and still reap benefits like weight loss and reduced inflammation?![2]

    Before you sign on the dotted line, you have to ask: is intermittent fasting safe? 

    While intermittent fasting is generally safe for many people, it’s definitely not right for everyone, and there are some people who should skip it entirely. 

    When it comes to your health, there’s no such thing as being overly cautious, and finding an eating routine that works for you isn’t a templated, paint-by-numbers situation. To help you read the fine print, sort through the terms and conditions, and understand how the fasting process may affect that unique body of yours, we’ve pulled together everything you need to know about intermittent fasting safety.

    Key takeaways

    • Intermittent fasting is generally safe, but it’s not health-promoting for everyone. 
    • The benefits and side effects of intermittent fasting — as well as how it works alongside things like physical activity — can vary because everybody is different, and not everyone experiences fasting in the same way.
    • Check in with your healthcare professional before starting any fasting-based food routine (or making other changes that may impact your health). 
    • Certain people should avoid fasting without medical approval. (More on the specifics a little later!)

    So, is going on an intermittent fasting diet plan safe? 

    Separating the fad diets and “miracle” plans from the real deal isn’t about “show me the money” (or the influencer endorsement / celebrity success story). It’s about “show me the evidence.”

    Generally speaking, intermittent fasting is safe for adults with a healthy BMI and adults living with overweight or obesity as long as they don’t have other health conditions or fall into certain risk categories (which we’ll outline later).[3,4] 

    But there’s nothing “general” when it comes to you and your body. What’s safe, effective, and sustainable for one person may not be for another, so “Is fasting safe?” sadly doesn’t have a universal answer. 

    Sigh. It’s never straightforward, is it? 

    But just because something is complex doesn’t mean it has to be complicated. You can take our SIMPLE quiz to learn more about different intermittent fasting approaches, get insight into the process, and see if it might be a safe choice for you. Heads up, though, we’re not a replacement for a medical care provider, so we also recommend consulting them before making any significant eating routine changes! 

    Is it safe to do intermittent fasting while working out? 

    If you’re wondering whether you can still get your sweat on while intermittent fasting, don’t worry! Fasting and exercise can be a dynamic duo as long as you stay hydrated and listen to your body.

    If you’re someone who has lunges for breakfast, uses kickboxing as a therapy sesh, or enjoys long walks to decompress, the idea of a fasting-based eating routine can be daunting. Fasting restricts energy, and you need energy for physical activity, so fasting must mean restricting exercise. Right?

    Not exactly. 

    It’s totally possible intermittent fasting and working out can work in tandem for the benefit of your health. While you definitely don’t want to overdo it on physical activity and underdo it on fuel intake — which leads to injury or illness — you can pair fasting and exercise in a safe, smart, and sustainable way. The key is to tailor the workout type, level, and length to your goals, needs, and well-being. 

    For example, pairing fasting with strength training may promote lean muscles and lower body fat percentages in people who are overweight if that’s what you’re aiming for.[5] Similarly, aerobic exercise may be useful for aiding weight loss in people with overweight or obesity.[6] 

    You should also keep that water bottle and a source of electrolytes (like unsweetened coconut water) handy. Dehydration is already a potential fasting side effect, much less when you’re in the midst of a sweat-fest. 

    Most importantly, you want to remember your body may be a machine, but it’s not a robot: you can’t just plug and play whatever routine you had in mind, and “no pain, no gain” isn’t a mantra for your vision board.

    Check in with yourself: if you’re feeling dizzy, light-headed, or nauseous, it’s probably time to move from beast mode to (equally badass but more chill) cub mode. And remember to speak with your doctor if those symptoms don’t resolve after a while.

    Intermittent fasting side effects 

    If you’ve ever had the (dis)pleasure of trying on clothing that’s listed as “universal size” or “unisex,” you’re probably already well aware that when it comes to bodies, there is no such thing as a “one size fits all” approach. 

    The same goes for what happens within your body. Some people may experience one or two side effects from fasting, while others may encounter lots of adjustments. Likewise, some side effects may be more fleeting, while others are as persistent and pervasive as a 90s trend. (Hello, chokers and crop tops.)

    While there’s no guarantee how or to what extent your body will react, some of the most common intermittent fasting side effects can include: 

    • Hunger and cravings
    • Headaches
    • Lightheadedness / dizziness
    • Mood changes
    • Irritability / crankiness 
    • Poor sleep
    • Fatigue and low energy
    • Low blood sugar
    • Digestive issues 
    • Malnutrition 
    • Dehydration 

    If you notice any of these side effects, keep an eye on them, and don’t be afraid to speak with your healthcare provider if they persist or if something doesn’t feel right.

    Pros and cons of intermittent fasting 

    ProsCons
    No fully-off-limits foods Requires careful meal planning
    No calorie counting Difficult to manage around social events
    Offers an easy-to-follow structure  Exercise can be challenging
    Doesn’t require specialty ingredients Not appropriate / safe for everyone
    Inexpensive

    Pros 

    Intermittent fasting is all about when you eat, not what you eat, so it can be a more flexible, approachable eating routine for people tired of counting calories, restricting foods, or regularly skipping breakfast or dinner. 

    Since it doesn’t require any specialty foods or kitchen gadgets — you just eat as you normally would during your eating windows — your shopping bills won’t increase either. It’s also fairly easy to follow (in theory) since you just need to keep an eye on the clock and stick to whatever fasting / eating schedule you’ve set.

    Cons 

    While you can, to some extent, eat what you want during your eating window, you still need to plan meals carefully to ensure you’re getting all the nutrients you need for the day. Depending on your schedule and which intermittent fasting approach you choose, it can also be difficult to manage fasting windows around social commitments, school or work schedules, and other life events. 

    Physical activity can also be tricky since too much exertion without the proper fuel in your body can lead to injury and/or feeling unwell. The most significant con, though, is that this eating routine isn’t appropriate or health-promoting for everyone, so you need to be sure it’s safe for you before trying it.

    Intermittent fasting benefits 

    Remember what we said about the trials and tribulations of one-size-fits-all clothing? This process is no massive, tent-like rain poncho that somehow covers everyone.  

    As with the potential side effects of intermittent fasting, the benefits of intermittent fasting can vary significantly from person to person. Plus, there are so many different variables involved in a person’s health that some benefits may not always apply to everyone.

    That said, there are numerous research studies that suggest intermittent fasting can be beneficial in 

    • Reducing inflammation and inflammatory diseases [7];
    • Losing weight and maintaining your ideal weight [8,9];
    • Supporting heart health [10];
    • Promoting healthy brain aging [11];
    • Lowering systolic blood pressure [12];
    • Increasing your metabolic rate [13]; and
    • Lowering the risk of type 2 diabetes in people with preexisting insulin resistance (by reducing insulin resistance, improving insulin sensitivity, and decreasing cholesterol and blood glucose levels).[14,15] 

    If you’re unsure whether or not fasting is good for you specifically — and if so, which intermittent fasting approach might be a good fit — you can always take our SIMPLE quiz

    How effective is intermittent fasting? 

    Intermittent fasting works by creating a calorie deficit. That means you’ll be primed to burn fat instead of sugar to use as fuel, which, in the most basic terms, can be a health-promoting process for lots of different internal systems and body stuff. (And if you’re wondering, “Does intermittent fasting slow metabolism” like calorie reduction, don’t worry — that doesn’t seem to happen so much with intermittent fasting.) 

    However, things like physical activity, preexisting health conditions, what you eat and drink, and other lifestyle factors can impact its effectiveness. Plus, how you define “effective” will depend on your goals, needs, and approach.

    Ultimately we believe the most effective intermittent fasting method — and the one that is safe and provides more lasting results — is the one that is right for your unique body.

    Who should avoid intermittent fasting? 

    So, fasting is like that mystery grab-bag prize: even if you know roughly what you might get, you don’t know exactly what’s in store until you’ve already committed. (And how many candles and gag gifts can one own?)

    But is intermittent fasting safe for everyone to try, at least?

    Unfortunately no. While we always recommend speaking with your healthcare provider before making any decisions that might impact your health, there are some people who should avoid intermittent fasting entirely unless they have permission and supervision from a medical professional. You shouldn’t attempt intermittent fasting without this official approval if you:

    • Are pregnant, breastfeeding, or trying to conceive; 
    • Have a history of or are currently diagnosed with disordered eating;
    • Have a medical condition (like diabetes);
    • Are underweight (your BMI is less than 18.5); 
    • Are under the age of 18, or 80 years old or more; or
    • Are prescribed medication (for example, anti-hypertensives, diabetes medications, or any that need to be taken with food).

    10 tips on doing intermittent fasting safely

    During your eating window, nourish your body with nature’s bounty. Nutrient-dense superstars, like carrots, cauliflower, and beets, provide the perfect fuel to keep you energized and on track with your intermittent fasting goals!

    Ready to put a fasting-based food routine to the test? Our experts have shared their top tips for safe intermittent fasting. 

    1. Know why you’re fasting in the first place. There are heaps of different fasting approaches out there, and the key to finding the best possible fit for you lies in aligning the approach to your specific goals (and also your nutritional needs, lifestyle, budget, schedule, and all the other things that make you “you”). 

    2. Run your plan of action by your doctor. Even if you’ve already gotten the green light to try intermittent fasting, it’s always good to double-check the specifics of your approach. 

    3. Start slowly. Safe, effective intermittent fasting that leads to long-lasting results is always a gradual process, so there’s no need to approach it Fast and the Furious style. We recommend starting with a time-restricted eating approach that extends the natural fast you undergo while sleeping (like intermittent fasting 14:10 or intermittent fasting 16:8). We don’t recommend fasts longer than 18 hours without medical supervision, either — they’re higher-risk, and “more” isn’t “better” when it comes to fasting. 

    4. Plan your eating and fasting windows carefully. The safest eating routine is one you can maintain without sacrificing your well-being in the process. Pick an approach that can work around your schedule, daily routines, social events, and other commitments. While intermittent fasting for specific purposes (like weight loss) may require some slight routine adjustments, you never want your eating habits to prevent you from living life.

    5. Focus on nutrient-dense, protein-rich foods. Just because you’re eating less or less often doesn’t mean your body needs less nutrition. Eating health-promoting foods like fruits, vegetables, whole grains, lean and plant-based protein, calcium-fortified dairy, and nuts / seeds not only ensures you’re getting all the nutrients you need but also keeps you fuller for longer and fends off pesky potential side effects. We’re also huge fans of having an intermittent fasting meal plan, so before your next trip to the grocery store, you may want to learn more about what to eat during intermittent fasting and what breaks a fast

    6. Become BFFs with your water bottle. Staying hydrated is extra important while you’re fasting because water helps your body with a whole lot of valuable processes, like removing waste, burning fat, and regulating blood pressure. If you’re not a fan of plain H2O, you can jazz it up with fruits, herbs, or spices. Some other low-calorie drinks like black coffee, diet soda, and plain, unsweetened green tea also contain a decent amount of water, but you’ll want to be conscious of caffeine and the potential side effects of artificial sweeteners. (Check out our guide on what you can drink while fasting to learn more.)

    7. Adjust your physical activity schedule, but don’t stop altogether. Like we said before, intermittent fasting and working out aren’t sworn enemies, and certain types of physical activity can actually enhance your results (not to mention the positive effects of physical activity on your overall health and well-being). But to combine the two safely, we recommend only doing high-intensity workouts during your eating window and only doing short, low-intensity workouts right before your eating window (toward the end of your fasting window). You can still do cardio, strength training, and longer workouts during your fasting window, but only if it’s on the earlier side of it. 

    8. Check in with your routine and body regularly. One of the coolest things about intermittent fasting is that it’s flexible and customizable, so you don’t have to grit your teeth through a routine that’s feeling too tight or too loose. Having regular progress and “how’s this feeling” check-ins can help you stay on track with your goals and needs (and address any “intermittent fasting not working” concerns) while also ensuring you catch any potential problems or safety issues early. 

    9. Practice self-compassion, not self-judgment. Particularly when you’re just starting out, intermittent fasting can be a rollercoaster. Respecting your limits and praising your milestones rather than dwelling on setbacks and stewing in guilt will keep your mental and physical self working in harmony. 

    10. Enlist a support group or hype squad. Even if no one in your household or social circle is on the intermittent fasting ride with you, you don’t have to go to the carnival alone. Having family and friends supporting you from the sidelines means you have not only extra cheerleaders and motivators but also safeguards in place. 

    Simple’s expert opinion and final thoughts 

    So, intermittent fasting — is it safe? 

    Overall, yes. Intermittent fasting can be a safe, effective way of improving your overall health and meeting your body’s nutritional needs under the right conditions. But everyone responds to fasting differently, and it may be a bumpy ride — or one you shouldn’t be on at all. 

    The team at Simple all agree: the safest intermittent fasting routine is one that is right for you, your body, and your nutritional needs. It should also be sustainable and promote gradual rather than fantastical “quick fix” results, especially for things like weight loss. 

    If you’ve gotten the all-clear from your doctor to give intermittent fasting a whirl, head to our SIMPLE quiz today. We’re here to help you hammer out the details about what plan might work best, how you can get started, and where to go next — all while prioritizing your safety and well-being. We’ve also got one heck of a community of people who have been there, done that if you’re looking for some extra support!

    Frequently asked questions about how safe intermittent fasting is 

    How long can you safely do intermittent fasting?

    How long you can safely do intermittent fasting depends on your body, goals, and fasting approach. Sticking to a consistent fasting schedule may not only be difficult to manage safely but also introduce more potential and severe side effects, including losing too much weight. Plus, everyone has different optimal fasting windows and timelines. We recommend starting slowly and checking in with your doctor regularly.  

    Is it safe to intermittent fast every day? 

    It is, in theory, safe to intermittent fast every day with certain time-restricted eating approaches, but other methods are based on having a certain number of fasting days and standard eating days every week, and some aren’t safe to do daily. Missing a few days of fasting every month isn’t going to affect results tremendously (and life’s too short to eliminate wiggle room), so we recommend listening to your body. If it’s signaling it needs a break, listen up and do what’s best for you.

    Are there negatives to intermittent fasting?

    There are some negatives to intermittent fasting as far as potential side effects are concerned, but everyone experiences fasting differently. Additional risks and concerns exist for people who are underweight, pregnant, or breastfeeding or people who have (or have had) an eating disorder and other health conditions, so it’s not safe to fast under these circumstances unless you have prior medical approval.

    How many days a week is it safe to do intermittent fasting?

    How many days a week it’s safe to do intermittent fasting not only depends on individual factors but also on what type of fasting approach you’re doing. Some approaches are safe in theory to practice daily (see the answer to the question above!), but others are based on having a schedule of “normal” eating days and fasting days. 

    Is 16-hour intermittent fasting safe?

    As with any eating routine, whether 16-hour intermittent fasting is safe depends on a lot of individual factors. If you aren’t someone who should avoid fasting altogether, intermittent fasting 16:8 is generally a safe and effective time-restricted eating approach since it simply extends the natural daily fast that happens when you’re sleeping. It’s a great place to start for beginners.

    1. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017 Aug 21;37:371–93.
    2. He Z, Xu H, Li C, Yang H, Mao Y. Intermittent fasting and immunomodulatory effects: A systematic review. Front Nutr. 2023 Feb 28;10:1048230.
    3. Martens CR, Rossman MJ, Mazzo MR, Jankowski LR, Nagy EE, Denman BA, et al. Short-term time-restricted feeding is safe and feasible in non-obese healthy midlife and older adults. Geroscience. 2020 Apr;42(2):667–86.
    4. Morales-Suarez-Varela M, Collado Sánchez E, Peraita-Costa I, Llopis-Morales A, Soriano JM. Intermittent Fasting and the Possible Benefits in Obesity, Diabetes, and Multiple Sclerosis: A Systematic Review of Randomized Clinical Trials. Nutrients [Internet]. 2021 Sep 13;13(9).
    5. Keenan S, Cooke MB, Belski R. The Effects of Intermittent Fasting Combined with Resistance Training on Lean Body Mass: A Systematic Review of Human Studies. Nutrients [Internet]. 2020 Aug 6;12(8).
    6. Armstrong A, Jungbluth Rodriguez K, Sabag A, Mavros Y, Parker HM, Keating SE, et al. Effect of aerobic exercise on waist circumference in adults with overweight or obesity: A systematic review and meta-analysis. Obes Rev. 2022 Aug;23(8):e13446.
    7. Wang X, Yang Q, Liao Q, Li M, Zhang P, Santos HO, et al. Effects of intermittent fasting diets on plasma concentrations of inflammatory biomarkers: A systematic review and meta-analysis of randomized controlled trials. Nutrition. 2020 Aug 12;79-80:110974.
    8. Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 2022 May;18(5):309–21.
    9. Ganesan K, Habboush Y, Sultan S. Intermittent Fasting: The Choice for a Healthier Lifestyle. Cureus. 2018 Jul 9;10(7):e2947.
    10. Yang F, Liu C, Liu X, Pan X, Li X, Tian L, et al. Effect of Epidemic Intermittent Fasting on Cardiometabolic Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Front Nutr. 2021 Oct 18;8:669325.
    11. Francis N, George & Anne Ryan Institute for Neuroscience, University of Rhode Island, 130 Flagg Road, Kingston, RI, 02881, US, Department of Psychology, Stony Brook University, 100 Nicolls Rd, Stony Brook, NY, 11794, US. Intermittent fasting and brain health: Efficacy and potential mechanisms of action. OBM Geriatrics. 2020 Jun 1;4(2):1–19.
    12. Wang W, Wei R, Pan Q, Guo L. Beneficial effect of time-restricted eating on blood pressure: a systematic meta-analysis and meta-regression analysis. Nutr Metab . 2022 Nov 8;19(1):77.
    13. Adafer R, Messaadi W, Meddahi M, Patey A, Haderbache A, Bayen S, et al. Food Timing, Circadian Rhythm and Chrononutrition: A Systematic Review of Time-Restricted Eating’s Effects on Human Health. Nutrients [Internet]. 2020 Dec 8;12(12).
    14. Albosta M, Bakke J. Intermittent fasting: is there a role in the treatment of diabetes? A review of the literature and guide for primary care physicians. Clin Diabetes Endocrinol. 2021 Feb 3;7(1):3.
    15. Yuan X, Wang J, Yang S, Gao M, Cao L, Li X, et al. Effect of Intermittent Fasting Diet on Glucose and Lipid Metabolism and Insulin Resistance in Patients with Impaired Glucose and Lipid Metabolism: A Systematic Review and Meta-Analysis. Int J Endocrinol. 2022 Mar 24;2022:6999907.