Is intermittent fasting safe? — the complete guide by Simple

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Published on May 29, 2023 | Updated on June 13, 2024

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]  

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.

So, is going on an intermittent fasting eating routine 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 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 

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 schedule 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 schedule 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 does, 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 schedule — 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 extremely active
• have a body mass index (BMI) <18.5; 
• are under 18 or 80 years or older; 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

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 schedules out there, and the key to finding the best possible fit for you lies in aligning the schedule 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 a fasting schedule 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 intermittent fasting schedule 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 schedule 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. 

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Published on | Updated on June 13, 2024