Simple’s guide to the dangers of intermittent fasting

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Published on November 22, 2023 | Updated on June 17, 2024

Thinking about testing the waters of intermittent fasting?

If you’re a look-before-you-leap sort of person — always a good idea when it comes to actions that can affect your health! — you’re probably poised at the edge of the pool, scouting for any potential lurking dangers of intermittent fasting. 

(And don’t worry: by “danger” we don’t mean like a sneak shark attack. More like a potential hazard to approach with caution!) 

Before you dive in, here’s what you need to know about the risks and side effects of intermittent fasting.

(Heads up: if you’re new to intermittent fasting, you may want to dip your toes into our overview of intermittent fasting for beginners first!)

Risks vs. benefits of intermittent fasting 

If we’re comparing intermittent fasting to swimming, the whole “different strokes for different folks” adage reaches a new depth. Though there may be some suggested lanes to use as guidelines, no intermittent fasting journey is exactly the same.

The same is true for the risks of fasting. While there may be some general potential dangers of intermittent fasting, the risks are a lot like someone standing above you and throwing things randomly into the pool while you’re paddling away. Sometimes it’s impossible to avoid a certain roadblock, sometimes you can swerve around it, and sometimes it doesn’t even affect you at all. 

We’ve outlined some of the common risks below, but with so many “maybes” impacting your experience with potential intermittent fasting dangers, it can be easy to get stuck treading water or find yourself caught in a riptide. 

So, if you’re considering intermittent fasting, there’s one golden safety rule to remember: always talk to your healthcare provider first. They’re like your swim coach and lifeguard all in one, and they’re best placed to help you be safe, prepare for what to expect, and adjust your approach along the way to accommodate any obstacles. 

When you’re ready to get in the water, you can also use us as your floaties: we’re on hand (or arm / leg) to add extra lift to get through any unexpected waves. Try us on for size by taking our Simple quiz. 

Uncertainty regarding safety 

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).[1,2]

Unfortunately, much like if you’ve ever heard someone attempt “Bohemian Rhapsody” at karaoke, just because something works beautifully for one person doesn’t mean it’s a sweet symphony for you. 

While there is extensive existing research into the short-term potential benefits [3] and risks of intermittent fasting,[4] one of the intermittent fasting dangers is that we’re still currently missing longer-term studies that examine how safe intermittent fasting is in the long run. 

It’s also difficult to prove the impacts and safety of this type of eating routine definitively because so many variables are involved (like physical activity, preexisting health conditions, what you eat and drink, and other lifestyle factors). Whenever studies are carried out, even they are limited in what they can control within the context of one experiment, they 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). 

All of that boils down to a heck of a lot of uncertainty when it comes to whether a fasting-based eating routine would be safe for you and, if so, for how long. 

Healthy aging

Chronic inflammation is one of the hallmarks of aging and is associated with an increased risk of chronic disease and mortality in the older population.[5,6] There’s quite a bit of research that links intermittent fasting to reduced inflammation,[7] particularly in overweight individuals, but one of the risks of intermittent fasting is that some research connects continually skipping meals with an increased risk of early mortality.[8] While the relationship between meal skipping and mortality involves lots of factors, we always recommend consuming a balanced diet during eating windows and avoiding long fasting windows in order to steer clear of possible nutrient deficiencies that would make you more at risk of certain health conditions.[9] 

Some studies also suggest intermittent fasting may be associated with an improved immune system and autophagy (when your body starts flushing old / damaged cellular material and regenerating healthier, fresher material).[10] While cell generation is broadly accepted as a sign of healthy aging [11], there are still a lot of questions around how the process works, how it helps, and how we can use it — and if a fasting-based eating routine could be a safe, feasible way to trigger it.[11,12,13] 

Eating disorders 

Another of the biggies as far as fasting risks go surrounds disordered eating. Although bringing more conscious awareness to your eating patterns may be a helpful perspective or motivation for some people, for others, it could encourage a preoccupation with food, calories, or eating-related habits. 

This focus may lead to a more negative relationship with food as well as developing disordered thoughts or behaviors around eating or exacerbating disordered eating tendencies.[14,15,16] That’s why we always emphasize that fasting isn’t suitable for anyone with a history of disordered eating or those who have an active eating disorder. 

Diabetes

Intermittent fasting may support better metabolic health, manage symptoms related to diabetes, and lower your risk of developing type 2 diabetes by reducing insulin resistance, improving insulin sensitivity, and decreasing cholesterol and blood glucose levels.[17,18]

However, one of the major intermittent fasting risks is that fasting with diabetes is like surfing: it can be health-promoting and safe for some, but it’s definitely not something everyone should try.[19] Particularly for folks who use insulin or take medications for low blood sugar, intermittent fasting could cause some serious waves in blood sugar levels, which can increase the risk of hyperglycemia and hypoglycemia. 

So, regardless of what type of diabetes you’re living with, you should always get the okay from your doctor or endocrinology specialist before attempting any intermittent fasting schedule. 

Heart disease

By lowering inflammation, cholesterol, blood sugar, and systolic blood pressure,[20] intermittent fasting may support cardiovascular health, increase our heart’s resistance to possible pathogens, and reduce the risk of heart disease.[21,22,23] 

There is some recent research, though, that suggests prolonged intermittent fasting (greater than 18 hours) may actually be associated with heart disease and related mortality.[8] More studies are needed to understand the extent to which fasting can impact heart health and how other health and lifestyle factors may influence that impact. 

Gallstones

Gallstones stem from high cholesterol or a build-up of bile in your gallbladder,[24] and when you lose weight quickly or avoid eating for a long period of time, those bile juices can start accumulating and forming blockages like hair in the shower drain.[25]  

Some people are also more likely to get gallstones, like women and people living with diabetes or overweight, so when you combine those factors with fasting, you may increase your risk further.[26] 

Who should avoid intermittent fasting altogether? 

“Can fasting be dangerous?” is like asking if swimming can be dangerous. It sure can be, but the risk level is very much dependent on the swimmer and the situation in question.

To figure out whether intermittent fasting is dangerous for you, phone your doctor (not a friend, the internet, or your know-it-all neighbor) for medical advice. After all, it’s literally their job to know your body inside and out, so work with them to make sure fasting is fit for your purpose.

While everyone who’s considering intermittent fasting should have these conversations with their healthcare team, due to the effects fasting can have on blood sugar levels, your relationship with food, and body weight management, you should avoid fasting or be particularly cautious 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. 

When to consult a healthcare professional 

Our bottom line — forever and always — when it comes to your health is that it’s never worth a gamble. No matter how lucky you think you are, those chips should only be cast strategically and after expert guidance.

With their view of both the seed and the forest when it comes to health and well-being, your healthcare team will be best placed to answer questions like:

• What are the dangers of fasting?
• If, when, how, and/or why might intermittent fasting = dangerous territory?
• Is intermittent fasting good for you?
• Is intermittent fasting bad for you? 
• Is intermittent fasting safe for you?
• Does intermittent fasting work?
• How can you avoid intermittent fasting mistakes? 
• What intermittent fasting side effects can you expect? 
• Is there a world where fasting may also lead to weight gain?
• What can you do if you hit an “intermittent fasting not working” brick wall?

If you’re experiencing any persistent or severe side effects (even seemingly “minor” ones like headaches, hunger, dehydration, or irritability), your doctor is there to help. Keep an eye on things, and if you have any concerns that your eating routine isn’t giving you the nutrients you need or something just doesn’t feel quite right, it’s worth reaching out. 

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Published on | Updated on June 17, 2024