I have been doing intermittent fasting for several years and I’ve been talking about its great benefits in length. But surely, any diet or eating habit can have its faults and its flaws. So when can intermittent fasting be unhealthy? Are there side-effects you should consider or more severe long-term dangers?
Intermittent fasting will induce short-term side-effects such as headaches or diarrhea. In the long term, according to certain studies and articles, it could disrupt REM sleep, cause pancreatic damages, and mess with menstrual cycles, among other risks. But most of the evidence scarce, for each study against fasting, there are several in favor.
I’ve read many articles citing studies or making hypotheses on the actual risks of intermittent fasting. Like any diet, there are side effects and actual dangers to health as you’ll discover in this article.
But like any dieting trend, people are also quick to jump on the train to discredit it.
I already did a full article debunking the biggest myths and so-called danger of fasting, in this article I won’t state the obvious, I will look at real actual specific dangers backed up by studies.
As much as possible I will try to give you a counterpoint with other studies and articles on the subject to argument on these dangers.
The Short-term Side Effects Of Intermittent Fasting
You could experience diarrhea at the beginning of taking on intermittent fasting or when doing a prolonged fast.
This will be especially true if you started fasting after having carbohydrates-heavy meals, but there are several other reasons why you might experience diarrhea.
One of the hypotheses is that when you start fasting, your insulin levels will go down. This big drop in insulin will tell your kidney to excrete a lot of water, leading to diarrhea.
Oversecretion of water and salt in the gastrointestinal tract will also happen during fasting, at least at first, which will impact your bowel movements.
Another thing that may affect your bowel movements can be increased consumption of coffee and tea. When fasting, apart from water, they are the go-to drinks.
It seems like caffeine has a laxative effect on the body. Many people experience this side-effect, including yours truly, even though studies aren’t that clear cut on the subject.
I found this study conducted in 1998, where they compared the effects of a 1 000 calories, coffee, and decaf on bowel movements.
While they all stimulated the colon, coffee was 23% more stimulant than decaf, which in turn affected the colon 60% more than plain water.
This side effect of caffeine, even though decaf has been found to be almost as effective, can go as far, for some people, as to cause diarrhea.
The last thing that could impact your bowel movements and cause watery stool is that you will lose tons of electrolytes each time you will go to the bathroom. Diarrhea often comes from low sodium and potassium levels.
But generally, fasting doesn’t cause diarrhea all by itself, it’s often linked to other eating habits, and these side-effects will occur mainly when you start doing intermittent fasting.
Diarrhea can also happen if you’re doing a prolonged fast, meaning several days, in the beginning, and right after breaking your fast. Make sure you eat the right foods when breaking your fast to prevent digestive problems of this type.
During intermittent fasting, there are several ways you can improve your condition and even prevent this side-effect:
- You could try getting psyllium husk in the morning, no more than 1 tablespoon, with at least a cup of water
- Replenish your electrolytes by drinking salted water when fasting or broth during your eating windows. You could even take sodium/potassium caps. Keeping your electrolytes balanced is very important
- Try switching from coffee to decaffeinated teas such as Rooibos to make sure it’s not messing with your bowel movements
- As always, drink plenty of water and stay hydrated
Diarrhea is one thing, but through other pathways, fasting can also cause short-term constipation. It mostly depends on people, but it happens often enough.
First, be sure you’re constipated. Fasting means eating less often, which means eating less. If you’re used to getting several bowel movements a day, you might be surprised.
You will experience fewer bowel movements as a result of fasting. Everyone’s digestive tracts have their own speed, it will move slower or faster depending on people.
If you already experience constipation a very few bowel movements throughout the week, this means your digestive tracts are slow.
When you’re eating all the time, 3 to 5 times a day with snacks, you’re always stimulating your digestive system. It helps everything moving constantly.
Fasting will give a rest to your digestive system, thus slowing down your digestive tract even more.
But don’t worry, if you don’t experience discomfort or pain, it’s a perfectly natural side-effect, you’ll simply go to the toilets less often.
The transition from your body from burning fat for energy instead of food will also impact the regularity of your bowel movements.
Don’t panic if this happens, give it a few days, you’re body should adapt pretty quickly.
If you’re experiencing discomfort or pains though, you could try:
- Taking magnesium citrate which could help hydrate your colon
- Get bulletproof coffee when fasting. It will break your fast and prevent autophagy, but shouldn’t impact weight loss, and it can help get things moving. Check out my complete article on the subject
- As always stay hydrated and drink lots of water
- Try moving more and exercising
Headache, Lethargy, Brain Fog
One very common side effect of any type of fasting will be headaches, dizziness, fatigue, lack of focus, brain fog, etc.
This can become a pretty acute side effect when beginning fasting. If you just started intermittent fasting, it might appear during the first week.
It’s also very common when doing a keto diet, that’s usually why we refer to this as the “keto flu”.
But all in all, they’re very normal side effects which should be more of an inconvenience, experiencing flu-like symptoms, than a real problem.
Those side effects are often misinterpreted as hypoglycemia. And as I mentioned in this article, you shouldn’t suffer from hypoglycemia when fasting.
One of the main reasons they occur is the sudden drop in insulin levels, which will depend on the individual, signaling the kidneys to release excess water.
Insulin increases water retention, when you start fasting you might realize that you’re going to urinate more often. You might think it’s because you’re drinking more, but it’ll also come from your body releasing water.
With the body getting rid of water, you will also lose essential electrolytes such as sodium and potassium. And the reduced eating will also decrease mineral consumptions.
Much like diarrhea symptoms, there are two main ways to get rid of these effects:
- Get electrolytes, especially sodium, either trough caps or by adding natural salt (ideally pink Himalayan salt) to a cup of water, or by drinking a broth
- Stay hydrated, drink tons of water
I often mention this on my blog, but I experienced these effects first hand and quite strongly on the 4th day of a prolonged fast. I got up feeling tired and dizzy, having trouble just standing up.
All it took was one chicken broth, which tasted like the best food I ever had (and technically broke my fast, but well), and only a half-hour later I was woken up, sharp and energized.
If those ill-effects persist and become too strong though, it should not be taken lightly, you should contact your doctor and check your blood sugar. Get carbs or sugar as a quick fix.
Disrupted Sleep, Insomnia And Anxiety
There’s quite a lot to say about fasting, sleep and energy levels. So much so that I did a complete article on the subject.
Fatigue during the day when starting to fast will be a side-effect of not getting food in your system for a longer period than usual.
This means your body, which is pretty used to run on food, will have to start taping into your fat stores for energy.
But the body is a machine of habit and changing its whole machinery is not an easy task. During this change, your glycogen stores will get depleted and you might experience tiredness.
The problem is, it’s not only going to mess with your energy levels during the day, but it might also prevent you from getting adequate sleep.
Fasting will mess with your circadian rhythm, the internal body clock, which is regulated by sleeping habits, sure, but also heavily by eating habits.
Changing the way you eat drastically will cause irregularities in your circadian rhythm which could prevent you from getting to sleep.
Another side effect of fasting is that it will produce more of the counter-regulatory hormone adrenaline. It will also increase cortisol levels, which is known as the anxiety hormone.
Don’t forget that the body is ancient and comes from millions of years of evolution. The original fasting state was not because our ancestors were trying to lose weight, they simply didn’t have anything to eat.
So it makes sense the body would raise adrenaline levels, in order to keep you sharp and energized when your original goal was to hunt and gather food.
This could help you getting more focused during the day, but it’ll also leave you feeling jittery at time you’d rather be sleeping.
Once again, these symptoms will come from a change in your diet and shouldn’t take too long to wear off.
Sadly, there’s no magical cure to bad sleep and anxiety:
- Try going to bed at the same time each day, keeping consistent will reinforce your internal rhythm
- Avoid electronic devices and blue lights at least 30 minutes before going to sleep
- Try taking salt or magnesium baths before going to sleep
- If you’re brave enough, taking a cold water bath is a great way to help you sleep
This is kind of a strange side-effect because a lot of people have suffered from it when fasting, I included, but there is no strong evidence linking fasting and acid reflux.
Generally, you suffer acid reflux when eating too much, rather than not eating at all. It comes from acid ingestion, which is also called dyspepsia, but no study linked fasting to dyspepsia.
What is consistent though is that most of the time, people suffering from acid reflux when starting a fast already suffered from it prior to the fast.
So if you already get acid reflux when eating normally, you could experience it again when fasting too.
And many people mentioned that intermittent fasting or low-carb diet got rid of acid reflux in the long term. Again, this is a side-effect occurring from switching from a normal eating pattern to fasting.
Depending on the way you ate prior to the fast and your overall eating habits, you will be more or less at risk of getting acid reflux.
One last thing, intermittent fasting and longer fasts tend to make the mucus layer covering your stomach walls grow thinner.
That’s why when breaking a fast, you should try choosing the right food, such as light and water-dense, which will not produce too much stomach acid.
As a preemptive measure when starting fasting or to lessen acid reflux, you can try:
- Adding a teaspoon of lemon juice to your water
- Adding a teaspoon of the famous Apple Cider Vinegar into your water, which has many great benefits
If you’ve ever tried fasting or keto, you must have experienced bad breath. When doing a prolonged fast, I can tell you it can become quite unbearable!
But there’s a really good news about this side-effect, if you’re breath smells bad, there’s a big chance you’re burning fat.
One of the side effects of your body burning fat for energy will be the release of ketones. Ketones are volatile odorous compounds and some of the smell gets released through the mouth.
During the process, and to make sure your breath comes from fat burning and not questionable dental hygiene, your tongue will also grow whiter.
You will also have a strange taste in the mouth, like acetone. Acetone is actually a by-product of fatty acids metabolism, so again, it makes perfect sense.
To summarize, bad breath when fasting is simply your body doing exactly what you want him to do: burning fat.
To get rid of bad breath, well I’d hate to be the guy to tell you to brush your teeth more often, but:
- Brush your teeth as often as possible if it really becomes a nuisance for you and your close ones
- Use a tongue scraper
- Drink a lot of water, this will not necessarily improve your breath but it can get rid of the acetone taste
The Risks For Health According To Science
For the purpose of this article, I dug deep into the Internet to find studies and actual mentions, backed up with science, saying implementing intermittent fasting or doing prolonged fast could have a negative impact on health.
Here are the dangers I found in blog articles, citing actual studies.
It Could Cause Pancreatic Damage And Diabetes
This first danger is really going to sound strange because it goes against everything you might have read about fasting.
This study conducted in 2006 on rats concluded that fasting increased oxidative stress and the production of free-radicals. The study was conducted solely on rats and had them fast for 72 hours.
Apparently prompted by these findings, a new study from 2018, conducted on Wistar rats, tried to see how this impacted the pancreas islet metabolism and insulin secretion.
Female rats were submitted to intermittent fasting for three-months, the abstract doesn’t say what type of IF was used, weight and food intake were recorded during this period, then the animals were killed and analyzed.
They found that IF successfully decreased overall food intake and body weight during the experiment. But also that the pancreatic islets they collected had increased insulin secretion.
The abstract also mentions the following:
In addition, impairment in AKT phosphorylation was observed in peripheral tissues indicating insulin resistance.Source
In short, they’re saying intermittent fasting can cause insulin resistance through a pathway called AKT phosphorylation.
I’m not a doctor and I tried reading about this signaling pathway to see how it impacts insulin receptors exactly, but I didn’t found much to explain this further.
Trying to find out more about this, I found this article from Medical News Today, saying the same thing: fasting could cause pancreatic damages and increase risks of type 2 diabetes.
While the article doesn’t link directly to the aforementioned study (at least I was unable to find the link) they mention the author, researcher of the study, Ana Cláudia Munhoz Bonassa.
All in all, I saw two articles on the Internet talking about these potentially harming effects, both citing the same study, conducted on rats, with no details about the type of time-restricted feeding induced.
For now, we can agree the evidence on the subject is pretty weak. While there are several studies, conducted on humans, saying the exact opposite: fasting may reduce insulin resistance and can help reverse or improve diabetes.
Here are a few:
- Time-restricted feeding and risk of metabolic disease: a review of human and animal studies
- The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women
- Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus
- The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis
- Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings
Let me end by mentioning many specialists, such as Dr. Jason Fung, helping treat type 2 diabetes with low-carb diets and fasting and getting pretty impressive results.
This doesn’t mean researches shouldn’t be done on the subject. If new findings come at hand, reinforcing this danger, I’ll do my best to mention it here.
But for now, these risks are pretty slim when compared to the many studies stating the complete opposite.
It Increases Cortisol Levels And Stress
Cortisol is a steroid hormone that’s secreted by the adrenal glands. Its original function is to activate during the “fight or flight response”.
This means the original scenario is: you’re chilling on a rock, after hunting for food, and a bear attacks you. Cortisol levels will go way up and increase energy production, for you to be able either to fight or run for your life.
Generally, cortisol goes up and they back down to its original baseline, once the danger has passed.
Only, this is the original scenario, in our everyday life, where we’re constantly under stress, cortisol could keep elevated for hours.
And one of the side effects of intermittent fasting, or any kind of fasting, is that it’s a stress to the body. So, just like exercise, for example, it will raise cortisol levels.
Cortisol, when elevated, will constrict blood vessels and increase blood pressure to increase the delivery of oxygen to the blood and energy levels.
But it will also provide the body with glucose, by converting proteins in the liver through a process called gluconeogenesis, resulting in increased blood sugar.
So, while any type of fasting should decrease blood sugar, logically, elevated cortisol levels can reverse this, especially with people suffering from deregulated blood sugar levels.
This means if you’re suffering from adrenal dysfunction, intermittent fasting could potentially worsen the effects.
The original scenario doesn’t really paint an accurate picture of the way we live today, stress and cortisol levels are almost always more elevated than they should be.
Cortisol plays an important role to in the development of cardiovascular disease, by raising blood pressure and blood sugar.
Another side effect of cortisol is that it will reduce inflammation in the body, which can be great to help improve diseases or depression.
But constantly elevated levels of cortisol can lead to cortisol overload, which can suppress the immune system, leaving you more subject to common allergies or colds.
Cortisol effects on health are still being researched and in excess, like almost anything else, cortisol can be bad for your health.
But elevated cortisol levels are not necessarily a bad thing like I mentioned it can help fight inflammation and keep you energized and focused.
These are common side effects of fasting or sport.
And if you’re suffering from adrenal dysfunction, deregulated blood sugar levels, elevated stress, maybe you should not be fasting.
What I would say is that in 99% of the cases, the stress induced on the body by fasting is fine.
But any side effects that could lead you to believe that elevated cortisol levels are messing you up would be a very good indication to stop fasting and talk to your doctor.
Fasting will increase cortisol levels, and with it, can induce unwanted side-effects.
The last thing I would like to mention is reducing stress and cortisol levels won’t happen overnight and elevated levels will not just come from your diet. Fasting can play a role, yes, but it most certainly will not be the main cause.
Meditation, controlled breathing, relaxation, yoga, changing other habits, or even quitting your job will also have a big impact on your stress and cortisol levels.
It Could Dicrease REM Sleep
I already mentioned earlier, when talking about short-term side-effects of starting intermittent fasting, it will mess up with your circadian rhythm, which will cause disrupted sleep.
But your body adapt, after a while, your new eating habits will set in and you should get back to sleeping normally, no?
According to certain articles, not really, fasting appears to be messing up with REM sleep, even in the long term.
But when looking for more information and sources on this, I mainly found studies specifically looking into the effect of fasting on sleep during Ramadan.
The conclusions are almost the same every time: while Ramadan doesn’t impair cognitive functions, it can reduce REM sleep.
This comes from a disruption in the circadian rhythm, which I already talked about, but also because of the schedule imposed by Ramadan.
When doing Ramadan, you’re supposed to eat at night, so often people will go to bed right after breaking their fast.
Often, that’ll cause body temperature to go up. Diurnal body temperatures tend to increase when eating right before going to bed.
This will not only impair your ability to fall asleep but according to the studies I mentioned, it will decrease the quality of REM sleep.
Another thing, your body prefers to metabolize food at certain times a day. Hormones related to food metabolism usually work against hormones promoting sleep.
This study shows that increasing insulin downregulates levels of melatonin and vice versa. Melatonin is the sleep-promoting hormone, helping you to fall asleep.
Melatonin work is made easier when insulin levels are low in the body. Which is what promotes intermittent fasting.
As you can see, most of the researchers I was able to find on disrupted REM sleep with fasting generally are about Ramadan. This is a practice that lasts for around 30 days, this can hardly be considered a long-time habit.
I was unable to find studies on long-term fasting and REM sleep. I’ve read a few times that prolonged fasting could also deregulate REM sleep, but was unable to find researches on the subject.
To conclude on this topic, there is a disruption in sleep patterns and cycles when starting intermittent fasting, but I found no evidence that REM sleep was affected in the long-term.
If you want more on the subject, check out my full article on fasting, sleep, and energy.
It May Increase LDL Cholesterol Levels
This one is a tricky one, to better understand the whole thing, let’s first talk quickly about cholesterol and triglycerides.
There are three big markers when it comes to the risk of cardiovascular diseases: HDL cholesterol (‘good’ cholesterol), LDL cholesterol (‘bad’ cholesterol), and triglycerides (cells in which fat is stored).
First off, I used a very important word here: they are markers. They’re not the cause, lowering cholesterol by itself is not an effective way to reduce cardiovascular risks.
High markers come from somewhere else, often insulin resistance and metabolic syndrome. To cure it, you need not take lowering cholesterol medication, but rather treat the source of it.
If you want to know more about cholesterol and triglycerides, they’re purpose in the body and how fasting and keto can help with that, check out my complete article on fasting and arteries clogging.
Now, I’m not saying cholesterol levels have no effects on your health, I’m saying they can be a warning sign that you should check the root cause of (alimentation, smoking, drinking, etc.).
And when I’m saying it’s a tricky question, it is. Because researchers are not crystal clear on the subject, certain studies say fasting increases all cholesterols, other that it reduces it.
We’ll start with this study conducted in 1999 on 10 non-obese patients who fasted for seven days.
Total serum cholesterol increased and LDL cholesterol raised for 6.6% during the fasting week. They also concluded that:
The increases in serum cholesterol, LDL and apo B were associated with weight loss. Fasting did not affect serum concentrations of triacylglycerol and HDL cholesterol.
But if we look at one of the graphs, the effect of fasting on LDL cholesterol is pretty obvious:
Total cholesterol and LDL go up (as well as Apo-B for Apolipoprotein B, a lipoprotein synthesized by the liver) while IGF-1, for insulin-like growth factor, goes down.
In this article of HCP-live, they mention that fasting for 24 hours seems to raise both LDL and HDL cholesterol.
Benjamin Horne (no relation to Twin Peaks), one of the researchers, when quoted in the article says:
There were some fairly substantial changes [during the fast] which indicated that the liver stopped taking up cholesterol and excreting it so that you would have it for energy
And even though there’s no link to the study in the article, I was able to find it. It’s a systematic review not specific to cholesterol but it takes a pretty deep look into intermittent fasting.
While they do see elevated cholesterol on short-term fasting in this 2013 trial they mention, another study they mentioned, that I was unable to find online, states that the fasting regimen seems to reduce LDL after 6 weeks.
Horne, the researcher, has done a lot of different studies on the subject and the only article I saw mentioning LDL going down, with Horne attached to it, is this one from Medical News Today.
They mention this study in the article, saying that, during fasting days on alternate-day fasts, cholesterol went up slightly, just like it had in many other studies.
But they found out that over a 6-week period, the cholesterol levels decreased by about 12%. Benjamin Horne states that:
Because we expect that the cholesterol was used for energy during the fasting episodes and likely came from fat cells, this leads us to believe fasting may be an effective diabetes intervention.
In this other study conducted on 35 obese women, who followed alternate-day fasting both with high-fat and low-fat diets, they concluded that alternate-day fasting increased LDL cholesterol particle size during the 10 weeks of the trial.
At first, when reading the study, I thought they meant it increased LDL. When in fact results have shown the opposite, LDL reduced by 5% on a high-fat diet and 3% on the low-at diet.
LDL particle size actually is another marker that can be important since larger particles correlate positively with HDL and negatively with triglycerides. In short, the larger the particle, the higher good cholesterol, and lower triglycerides.
As we can see with all this researches, if there’s no clear answer as of yet, there seem to be a pattern:
Fasting seems to increase cholesterol levels in the short-term, even on a 7-day water fast, but over the long term, LDL cholesterol levels go down, while LDL particles increase, which is good.
I’m no researchers and there’s certainly more to it, but it seems pretty clear that fasting will not increase your risks of cardiovascular diseases, just by slightly elevated cholesterol levels for a short period.
It Could Turn Into An Eating Disorder
There are many articles out there claiming that intermittent fasting could turn into an eating disorder.
The first main argument made is that when deprived of food for a long period, people tend to binge eat when they’re finally entering their eating window.
This study for example is pretty straightforward on this subject: Regular eating, not intermittent fasting, is the best strategy for a healthy eating control.
First, it states that many studies on the subject only mention short-term fasting (weeks or months) and many of the so-called benefits have been seen in animals, more so than humans.
And like many other studies out there, there’s some truth to that.
An experiment can only be conducted for so long, and experimenting on animals with a metabolism close to ours is the easiest way to do researches on a subject.
As a reference, they mention a study called Effects of Intermittent Fasting On Health, Aging, and Disease which is mainly about the benefits of fasting.
But this study concludes with:
It remains to be determined whether people can maintain intermittent fasting for years and potentially accrue the benefits seen in animal models. Furthermore, clinical studies have focused mainly on overweight young and middleage adults, and we cannot generalize to other age groups the benefits and safety of intermittent fasting that have been observed in theseSource
Which goes in the direction the initial study is talking about. It then cites other studies saying that fasting, or delayed eating, will then increase the risks of binge eating disorders.
Here’s what it then says in the abstract:
Moreover, the procedure of regular eating, adopted by the “enhanced” cognitive behavior therapy for eating disorders (CBT-E), results in a rapid decease in the frequency of binge-episodes in patients with bulimia nervosa and binge-eating disorders. These data indicate that regular eating, not intermitting fast, is the best choice for adopting a healthy eating control and avoiding the development of unregulated and, in some cases, disturbed eating behavior.Source
To summarize, if you’re suffering from bulimia nervosa or bing-eating disorder, you should adopt regular eating, instead of fasting.
If you’re already suffering from an eating disorder, of course, the best thing would be to listen to your doctor and not engage in a new eating pattern that could increase your eating disorder.
But what about regular people doing intermittent fasting? Are there actual studies saying it could create an eating disorder in healthy people?
Well not really, they again cite other studies (with no access to their actual sources, both from the same researchers), saying that adopting a regular eating pattern of 3 + 2 + 0 is the best way to avoid eating disorders.
This cognitive pattern means the patient should plan in advance 3 main meals (breakfast, lunch, and dinner) and 2 snacks (mid-morning snack and mid-afternoon snack) every day.
But again, they’re talking about people already suffering from an eating disorder:
Randomized and cohort studies have shown that regular eating procedures result in a rapid decrease in the frequency of binge-episodes in patients with bulimia nervosa and BED [binge-eating disorder].Source
I would argue anyone without an eating disorder who’s strictly following the 3 + 2 + 0 seems to be also suffering from a type of eating disorder: it’s called “eating all the time”.
I could not find any actual evidence that fasting could be the root cause of creating an eating disorder.
This article mentions fasting as a way for people suffering from anorexia nervosa to increase weight loss. And that’s definitely true.
Fasting is a great way to lose weight, but when overdone it can be an actual risk to your health and a tool to enhance an already existing eating disorder.
But just like eating all the time will lead to obesity. Just like eating too much sugar will lead to greater risks of cardiovascular disease.
What I’m saying is that any type of diet, any type of eating habits, could potentially turn into an eating disorder when used inappropriately.
If you’re suffering from any eating disorder, follow your specialist regimen.
And if you’re afraid you will develop an eating disorder when fasting, I’ve been doing intermittent fasting since 2017 and I still think it’s great.
I’ve never felt that no eating for a day or even 7 days equals to an eating disorder. Here are my two cents on how to approach fasting, or any other diet for that matter:
- Don’t guilt yourself for not following your diet all the time
- Don’t punish yourself when eating too much or binge eating
- Don’t overthink the way you eat
As for actual eating disorders, read about it, if you think you’re developing one, talk to your doctor about it.
I just want to end with this, I mentioned earlier that people are prone to binge eat after having spent several hours without eating.
Check out this 2002 study on the effect of fast on energy compensation, conducted on 24 healthy subjects, who underwent a 36-hour fast.
After the fast, they measured calorie intake during the next day. And they ate around 20% more calories on that day, 2,914 calories against 2,436 for the baseline.
There was some over-eating on the next day, but when calculated over two days, the net calorie deficit was still at around 1,958.
The study then concludes:
These data suggest that a 36 h fast, which generated a negative energy balance of approximately 12 MJ, did not induce a powerful, unconditioned stimulus to compensate on the subsequent day.Source
You should always take any study with a grain of salt, but I’ll leave you with my own experience of several years of intermittent fasting. And if you ever fasted, you might now that:
- After a fast, you will very rarely be binge eating because hunger will disappear pretty quickly when you eat
- You will feel full sooner
- Even if you find yourself really hungry, chances are you are not going to eat as much as the regular 3 meals and 2 snacks eating pattern mentioned above
To summarize, I’m not saying that fasting doesn’t come with any disadvantages or dangers. It could potentially lead to an eating disorder if your head is in the wrong place.
But I’m pretty sure we can say the same about calorie restriction, low-fat diet, low-carb diet, etc. Any diet taken too far can lead to restrictive eating habits and bad eating patterns.
Just don’t tell me the way most people eat, meaning every 2 hours, is healthier than any of those diets.
It Could Disrupt The Menstrual Cycle
There’s few evidence of this, but I’ve read this danger several times in different articles and I would like to acknowledge it.
Evidence is scarce not because it’s not a real thing, but more so because hard studies haven’t been conducted and published on the subject.
As it’s often the case with fasting, one of the few studies I found on the subject is a study on how Ramadan affects menstrual cycles.
Over the 80 female college students that participated in the study, 30% had abnormal cycles during Ramadan.
Not only that, but 11.3% also suffered abnormal cycles 3 months prior to Ramadan, and 16.3% 3 months posterior. And on average, women fasting for more than 15 days where more subject to disrupted cycles.
But we can all agree that Ramadan is not exactly intermittent fasting, it involves certain practices that are not a part of IF, such as eating early in the morning and right before bed.
Of course, as I already mentioned, intermittent fasting will mess with your circadian rhythm at first, since eating patterns play a big role in our internal body clock.
Changing eating patterns will affect your internal rhythm, which could disrupt sleep, promote insomnia, but also potentially mess with women’s menstrual cycles.
As with sleep, there aren’t that many studies on the subject, and as with sleep, I will go out on the limb here and say: if you’re experiencing abnormal cycles at first, your body will adapt after a while and everything will get back to normal.
This is not by any means a study, but I found this blog post where Megan, the author, states that IF seems to mess with the cycles during the first two months, after coming back to normal.
She even noticed that after six months, her cycle became like clockwork, when she, beforehand, often suffered from irregularity.
Not only that, but she also stopped suffering from premenstrual syndrome (PMS), which to quote her is “a bloody miracle if there ever was one”.
One article from a website called the Fasting Method may not be all you need to discard fasting as a danger to your menstrual cycles.
I also have 4 women friends doing intermittent fasting, for several months or even several years, and we sometimes talk about the effects of fasting, they never mentioned problems in this area.
One of them being my girlfriend with which I started IF in 2017, I would say I’m pretty sure she never suffered from disrupted cycles due to IF.
To summarize, it is known that circadian rhythm, through sleeping and eating patterns, has an impact on hormone secretion and that it can modulate the menstrual cycle.
So, again, fasting can disrupt your rhythm and first, but after a while it will come back to normal.
It could even strengthen your internal body clock and improve functions related to the circadian rhythm, such as sleep and menstrual cycles.
Who Should Not Do Intermittent Fasting?
I often am a bit condescending when people say, and they always end up saying it: “Fasting looks great, but I could never do it!”.
That’s where I generally insert my pitch that fasting can be achieved by anyone, that it’s easy, that it has tons of benefits and very few actual dangers to be concerned about.
But there are definitely some people who should not do fasting in any shape or form, or at least avoid it as a daily habit.
This is not linked to any danger I might have mentioned here, I would say doing any type of calorie or food restrictive diet is not advised if you’re in one of the following categories:
- You’re suffering from any type of eating disorder
- You have a history of disordered eating
- You are already underweight
- You are suffering from adrenal dysfunction
- You are suffering from deregulated blood sugar levels
- You are pregnant
Any condition in which energy intake and meal regularity play an important role in improving or sustaining balance means you should stick to regular eating.
I have also read articles saying if you’re chronically stressed or already suffering from a great deal of stress, you should avoid intermittent fasting.
I wouldn’t necessary agree, you shouldn’t go all in on a whim, but if you plan on doing it step by step, you should be fine.
Intermittent fasting could rid you of other, more stressful, side effects of eating too much or constantly: stomachache, bloating, fatigue from digestion, etc.
In any case, if you’re new to intermittent fasting and to dieting and sport in general, take it slow. It will cause additional stress on your body and mind, so better not to rush things.
But in the long run this might be a great way to alleviate stress and improve your overall condition.
I would like to end by saying: much ado about nothing, wouldn’t you agree?
On this blog, I’m used to dissecting subjects, reading articles, studies, and giving you my own opinion on things. And more and more, I feel like we’re making a big deal out of a pretty small one.
Yes, they’re dangers to fasting, very light ones, as well as many dangers to eating too much or all the time, or to eating zero-carb, or low-fat.
Any diet comes with its culprits and no amount of studies will be able to give you the full spectrum of dangers and benefits your current diet has.
Because everyone is different, so we’re taking a general look, and say: you should be fine.
For centuries people weren’t eating three times a day, for some periods, they weren’t eating every day, and they survived, they didn’t crush under the stress or developed heart diseases overnight.
You’re afraid fasting can be a danger to your health? If it’s a real actual fear, then don’t do it, find another way to get healthier and achieve your goal.
If you start obsessing over each and every side effects, this will have a negative impact on your health.
If not, why not simply give it a shot? It’s not for everyone and I’m not only talking about health but try implementing it, give yourself a month or two to try it.
And if you feel any unwanted side effect, if you feel depressed or stressed, then stop it, it may not be right for you.
But by experience and after having convinced other people to try it, I’ve never had a negative feedback on intermittent fasting.
This doesn’t mean one of my friends may not stop after discovering it’s causing him or her excess stress, or that it’s messing up with her menstrual cycle, or something else.
But I’m pretty sure the alternative, putting more weight, feeling more depressed about food, hating yourself, is such a great solution.