Is intermittent fasting bad for you? Answered by SIMPLE

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Published on November 8, 2023 | Updated on May 3, 2024

You know what they say: something that seems too good to be true probably is too good to be true. 

So, whoever “they” are might have something to say about intermittent fasting. As an eating routine that focuses on when you eat, not what you eat, intermittent fasting is much less restrictive and prescriptive than most diets, and it’s still been linked to significant health benefits (like supporting heart health, lowering systolic blood pressure, and reducing inflammation and inflammatory diseases [1–3]) as well as weight loss and weight maintenance.[4,5] 

A food routine that lets you (literally and metaphorically) have your cake and eat it, too? Sounds pretty dang good. 

But is it too good? Is intermittent fasting bad — and if it is, why is fasting bad? Is fasting dangerous? Is fasting unhealthy? 

Here’s what you need to know about the dangers of intermittent fasting, as well as whether you + intermittent fasting = unhealthy. 

Is intermittent fasting bad for you? 

Remember those group projects from school where everyone was assigned a role based on their strengths or preferences, but inevitably, something wouldn’t go to plan? Maybe one person did the majority of the work. Maybe someone realized they weren’t as great at organizing as they thought. Maybe a series of unexpected events affected the group’s ability to deliver. 

Trying to calculate if fasting is bad for you is sorta like this scenario. Based on existing research and what you know about your body, you might be able to make an educated guess, but there’s no guarantee.[6] 

For many people, fasting — within certain parameters — is a generally safe experience, but it’s not safe, right, or effective for everyone. We’ll outline some specifics in the next section. 

And, just like the “if,” the elements of why intermittent fasting is bad can vary, too, though there are a few common negatives that we’ve listed below.   

That’s why it’s extra important to get medical advice from your primary care provider (not the internet!) if you’re thinking of fasting to meet a specific health goal or improve your health. It’s like that meme of the person trying to solve a complicated math problem: your primary care provider can see both the nitty gritty and the full picture of the situation, connect all the dots, and figure out what might be best for you. 

If you’ve gotten approval from your healthcare team to try fasting, you can call on us to show you how to create your ideal fasting routine. From advice on intermittent fasting for beginners to troubleshooting common intermittent fasting not working dilemmas, we’ll help you calculate your best-fit approach and navigate all the variables of your personal fasting experience. Start by telling us all about you and your goals through our SIMPLE quiz, and we’ll take it from there. 

Insufficient studies or research 

While extensive research has linked intermittent fasting to weight loss [4] and a wide range of health benefits, like reduced body fat, better heart health, lower systolic blood pressure, reduced inflammation, and improved metabolic rate [1,2,3,7,8], there are a few aspects of the research that leave people questioning the results. 

Firstly, more often than not, these studies focus on the short-term benefits of intermittent fasting (up to a year), and it’s still too early to say whether there’s any long-term effect. For instance, one recent longer-term study found the timing of meals may be less impactful on weight management than how many meals you eat.[9]  

Likewise, it’s difficult to prove fasting benefits and risks definitively because there are so many variables at play, like physical activity, preexisting health conditions, what you eat and drink, and other lifestyle factors, to name a few. Studies often focus on specific populations (like men or women or people from a certain location) and specific variables (like people who reflect a certain demographic or are living with certain medical conditions). This specificity means those results don’t necessarily hold true for everyone. 

Your health is never worth a gamble, and all this uncertainty can make it tough to know whether intermittent fasting would be helpful or harmful to you. 

Disruption of sleep 

More research is needed to determine how intermittent fasting affects sleep [10], but based on what we know so far, sleep disruption may be one of the most common fasting side effects.[11] 

Food and sleep already have a complicated relationship — how you sleep can affect your food choices and cravings just as much as what you eat can affect your sleep — so any eating pattern (or changes to it) has the potential to impact both daytime alertness and nighttime sleepiness.[12] The impact varies from person to person, but a fasting schedule may be particularly disruptive if you have your largest meal at night.[13] 

Another study, however, suggests insomnia as well as sleep quality and duration aren’t affected by intermittent fasting in adults carrying excess weight.[14] Some even assert that, over time, intermittent fasting may actually reinforce your circadian rhythms (your internal body clock that regulates your sleep-wake cycle), which would benefit sleep quality.[15]  

Bottom line? Sleep is another gray area when it comes to intermittent fasting. 

Calorie deficit 

Although intermittent fasting may not be ruled by calorie counting, it still fundamentally works by creating a calorie deficit since you take a complete break from eating for longer than you normally would. So, unless you’re living an all-you-can-eat-buffet-style fantasy or choosing very high-calorie foods during your eating window, chances are you’ll have a lower daily calorie intake. 

Such a deficit may support your goals if you’re using intermittent fasting as a weight loss method, but calorie restriction isn’t a good health choice for everyone. Calories are our friends — they provide the fuel our body needs to function. Rather than aiming for restriction or abundance and focusing on numbers, it should be about getting the right amount of calories for your body.

Plus, when you’re naturally eating less and less often, there’s more of a risk that you won’t get all the nutrients you need. One of the biggest intermittent fasting dangers is that, over time, malnutrition can cause some pretty gnarly side effects from your head — like hair loss and exacerbated mental health conditions [16,17] — to your toes — like reduced bone density and poor skin health.[18,19]

The process of deducing how many calories you need each day will never boil down to a simple equation, but we can help you pin down a rough estimate as a benchmark. You can use our SIMPLE calorie calculator or take our SIMPLE quiz for more tailored advice. 

Guilt and anxiety 

Another potential argument for why fasting is bad is because of the extra pressure it can create. Like any eating routine that involves making mindful choices — in this case, about both the timing and nutrient density of meals — intermittent fasting can be a helpful motivator to some but an unwelcome thought intruder to others. 

Think of fasting like a watering can hovering over your mental stress bucket. Even if there’s some evidence to suggest intermittent fasting can lighten that bucket by reducing levels of anxiety and depression [20], it can likewise pour additional challenges on top. 

For starters, it’s not the easiest of eating routines to stick to, especially at first when side effects like fatigue, headaches, brain fog, hunger, and irritability may pop up to say, “Hello, not nice to meet you.” So, it can cause stress, guilt, or even shame if you’re finding it challenging to maintain. 

For some people, it may also lead to a preoccupation with food and/or exacerbate disordered eating tendencies (which is why we never recommend fasting to anyone with past or present eating disorders).[21,22] 

No eating routine should ever get in the way of your physical, mental, or spiritual well-being, and with the kind of attentive focus it requires to practice safely and effectively, intermittent fasting may not be a good match for every headspace. 

Who is intermittent fasting bad for? 

Rather than asking, “Is fasting unhealthy?” “Is it bad to fast?” or “Is intermittent fasting safe?” try tacking these two words on to the end of these questions: “for me.” 

With any eating routine, you should always factor in your unique body, needs, goals, and preferences into your decisions. When it comes to things that could affect your health, you should also always speak with your healthcare provider before making any changes to your lifestyle, especially if you’re trying to achieve certain results.

So, when it comes to “Is fasting good for you?” calculate the benefits and risks with your doctor. 

Given that time-restricted eating can impact your blood sugar levels plus how many calories and nutrients you get, you should be particularly cautious about trying intermittent fasting if you: 

• have type 1 diabetes;
• are pregnant, breastfeeding, or trying to conceive; 
• are prescribed medication (like hypertensives or any medications that need to be taken with food);  
• have a history of or are currently diagnosed with disordered eating;
• have a BMI of less than 18.5; or
• are under 18 years old, or 80 years old or more. 

This list isn’t exhaustive, but it’s not absolute, either (for instance, in some cases, intermittent fasting and type 2 diabetes can walk hand in hand). However, if you identify with one of these categories, talking with your healthcare provider has to come before sketching any DIY-fasting plans. 

SIMPLE’s safety tips on intermittent fasting

If you’re giving fasting a go, there are tons of intermittent fasting tips you can try to avoid intermittent fasting mistakes and mitigate the risks.

And the good news is that these tips — while they may not guarantee a smooth fasting experience for everyone — are generally safe for anyone to test because they’re all health best practices. 

Our top recommendations for safe intermittent fasting include: 

• Ease into your new routine. Be the tortoise, not the hare. 
• Stay hydrated. Our body relies on water for loads of essential functions. 
• Prioritize nutrient-dense foods. Fruits, veggies, whole grains, lean protein, and healthy fats (like nuts and seeds) fuel your body best. 
• Limit less nutrient-dense foods. No need to cut them out completely — creating strict, arbitrary rules is a surefire way to add stress to your plate. 
• Practice good sleep hygiene. No caffeine after mid-afternoon, unplug from technology an hour or two before bed — do whatever gets you in that comfy sleep space. 
• Be kind to yourself. Kick any judgment to the curb and double lock the door behind it. 

SIMPLE’s expert opinion and final thoughts

There may not be a universal answer to “Is intermittent fasting bad for you?” or “Does intermittent fasting work for you?” but that’s because you’re a one-of-a-kind original, not a statistic or an average. 

The first step to figuring out if, when, and why intermittent fasting is bad for you is consulting your healthcare team. The more information they have to work with, the better, so be transparent about your reasons for intermittent fasting and any excitement, doubts, or fears you have about the process. The team at SIMPLE all agree that the safest and most sustainable routine is one that is medically approved and that you can stick to — without any knock-on effects on your health or well-being.  

If fasting is on the table for you, we’re ready to pull up a chair and help you strategize. Start with our SIMPLE quiz to unlock guidance on everything from planning your approach to planning your meals, contingency plans, and safeguards — and putting it all into action.  

Frequently asked questions about whether intermittent fasting is bad for you

Is 16:8 intermittent fasting bad for you?

Whether 16:8 intermittent fasting is bad for you depends on lots of factors and will vary from person to person. It can be a safe eating routine for many, but it’s essential to consult with your healthcare provider before starting any fasting schedule (or making any changes to your lifestyle that could affect your health).

When should you stop intermittent fasting?

When you should stop intermittent fasting should be based on your individual goals and how your body responds. Your health and well-being are always priority numero uno, so keep checking in with yourself and reach out to your doctor if you have any concerns.

Do you gain back weight after intermittent fasting? 

You can gain back weight after intermittent fasting if you fall into less health-promoting eating habits or lifestyle choices after fasting. Long-term success is supported by habits like choosing nutrient-dense foods, practicing regular physical activity, getting decent sleep, and minimizing stress.

How much fasting is too much?

How much fasting is too much varies among individuals. Excessive fasting may lead to adverse effects, so it’s crucial to find a sustainable fasting schedule that’s right for you and your unique body. We don’t recommend fasting beyond 18 hours at a time, as there is limited evidence to show any additional benefits in doing so. We recommend that you speak to your healthcare provider first — they’re there to help you know your body inside and out and keep you safe as you learn.  

Is fasting hard on your heart? 

Like the answer to “Is intermittent fasting unhealthy?” the answer to “Is fasting hard on your heart?” can vary based on many factors, like age, gender, fitness levels, and so on. Intermittent fasting may be safe for most people, but individuals with heart disease or heart conditions (or any other preexisting medical condition) should consult a healthcare professional to ensure it’s a safe choice.

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Published on | Updated on May 3, 2024