10 Intermittent Fasting Side Effects That Might Mean It’s Not A Great Fit For You

But since there arent any formal guidelines for how many calories you should be consuming during IFor for what kinds of foods you should be filling up on during non-fasting hoursit can be hard to hit your IF groove without some bumps in the road first...assuming its even the right choice for you at all. And there are some signs to watch out for that might mean it's just not the best fit for you and your lifestyle.

But first: If you're wondering why IF is so popular in the first place, there may be potential benefits of intermittent fasting.

There are several potential positive effects of IF , including weight loss, better appetite control, and lower insulin levels. The main issue is that none of those benefits have been heavily researched in any way. The research is also the scantest in humans (compared to, say, the research in mice).

It's also unclear if the root cause of the purported benefits is intermittent fasting itself (e.g. how it affects your body, say, on a cellular level) or simply the calorie restriction. So its kind of trial and error at this point to see whether an IF eating style works for you and your body.

On the other hand, there are a bunch of possible negative side effects with IF.

As mentioned, the upsides of intermittent fasting are still very much in the research phasebut there are some promising findings there. However, there's also plenty of anecdotal evidence that IF does come with some possible negative side effects, and you shouldn't start an IF eating plan without hashing those things out with your doctor first.

Here are 10 red flags to watch out for. And if you notice any of these side effects, that means stop IF and talk to your doctor or a nutritionist before proceeding.

1. Feeling hangry

Were not 100 percent certain that hangriness is a real word, but its definitely a real sensation. This is the feeling of grouchiness, grumpiness, or overall irritability that comes with not being able to eat when your body is telling you its hungry.

As WH previously reported, teaching your body to go 16 hours without food takes some practice, and some peoples bodies might not ever be happy eating within a restricted window.

In theory, if youre consuming enough protein later in the day or night, you shouldnt be starving first thing in the morning. But if you are, thats a sign you need to make some dietary adjustments during your caloric intake period to avoid turning into a major crankor it's a sign that you're just not vibing well with fasting. For some people (e.g. those who work out a ton), not eating for long periods may just not be ideal for them at alland that's definitely something worth considering. Don't force it.

2. Fatigue or brain fog

Ever found yourself yawning over and over mid-morning, only to realize you never got around to eating breakfast? Since not eating breakfast is typically how most people do IF, realizing that youre excessively tired every dayor making dumb mistakes because youre wading through brain fogis a tip-off that youre not eating the right foods during non-fasting hours or that fasting isn't fitting in with your lifestyle needs.

Pay attention to what youre fueling your body with, says Koens. You can eat what you want on IF, but you should still be fueling it with good food that will make you feel healthy and strong. And if you just feel *way* better eating breakfast most days, listen to your body.

3. Food obsessions

Being on any kind of restrictive diet can affect your relationship with food, says Koens. While some people like the rigidity of IF, others may find themselves focusing way too much on when they can eat and how many calories theyre getting.

Spending an excessive amount of time thinking about the quality or quantity of your food every day can lead to a type of eating disorder called orthorexia. According to The National Eating Disorders Association , having orthorexia means you focus so much on correct or healthful eating that it actually has a detrimental effect on your overall well-being.

That shouldnt be the goal of any diet, says Koens: You want to focus on forming a healthy, positive relationship with food.

4. Low blood sugar

If youre having persistent nausea, headaches, or dizziness during IF, thats a red flag that indicates the diet may be throwing your blood sugar out of whack. As WH previously reported, diabetics should avoid any kind of fasting diet for this exact reason: IF can cause you to become hypoglycemic, a dangerous condition for anyone with insulin or thyroid problems.

5. Hair loss

Seriously? Yup. Koens says that sudden weight loss or a lack of proper nutrients, especially protein and B vitamins, can cause hair loss.

An important point: While IF doesnt necessarily lead to a loss of nutrients, it tends to be harder to eat a well-rounded diet when youre cramming a whole days worth of eating into a handful of hours. If you think more hair than usual is falling out in the shower every day, reevaluate the nutrition content of your daily meals and speak with your doctor about whether IF is really a wise move for you.

6. Changes in your menstrual cycle

Heres another side effect of sudden weight loss (which can be a result of IF): Women who lose a dramatic amount of weight or are consistently not getting enough calories every day might find their menstrual cycles slow down or even stop completely.

Per the Mayo Clinic, women who have excessively low body weight are prone to a condition called amenorrhea , or the absence of menstruation. Sudden weight loss or being underweight can disrupt your typical hormone cycle and cause missed periods; so while you might be rejoicing in the way IF has helped you shed pounds, you could also be depriving your body of calories it needs to function.

If you stop getting your period and think it's linked to intermittent fasting habits you're practicing, stop fasting and speak with your gynecologist to troubleshoot.

7. Constipation

All backed up? IF could be to blame. Any diet can cause an upset stomach if youre not getting enough fluid, vitamins, protein, or fiber, says Koens, who emphasizes the importance of staying hydrated all day long.

Its easy, she explains, for people to forget to drink water during fasting hoursbut going 16 hours a day without enough fluid is a recipe for (gastrointestinal) disaster. So if you've started an IF diet and can't seem to get your bowel movements to happen regularly (or at all), it's time to hit pause on your plan and speak with a nutritionist or MD about what's happening (er, or not happening in this case!).

8. Unhealthy diet

Even if IF doesnt trigger a serious disorder like orthorexia, it could still bring about some pretty unhealthy eating habits. In addition to not getting the proper nutrients, you could also find yourself making mess nutritious choices during non-fasting hours.

The main worry is setting off binge-eating behavior, because you are so hungry youre eating 5,000 calories [and going way over your daily amount], Charlie Seltzer , MD, weight-loss physician and certified personal trainer, previously told WH.

If this sounds like you, you may be better off working with an RD to find a plan that doesn't force you to restrict your eating hours and instead focuses on fueling your body with proper nutrients around the clock, not in a specific window.

9. Sleep disturbances

Koens says that many people report improved sleep patterns while doing IF, possibly due to the way IF helps curb late-night snacking habits, and in turn, an inability to fall asleep because your stomach is busy still digesting that 10 p.m. nosh.

However, there is some research pointing to the opposite effect. A 2018 review in the journal Nature and Science of Sleep points to evidence that diurnal intermittent fasting (meaning daytime fasting) causes a decrease in rapid-eye movement (REM) sleep. Getting enough REM sleep has been linked to all kinds of health benefits , including better memory, cognitive processing, and concentration, per the Harvard Business Review. It's unclear why exactly.

If you notice you can't fall asleep or stay asleep after you've started an IF eating plan, again, hit pause and talk to a pro to make sure you're not hurting your health.

10. Mood changes

It would be weird if you didnt experience any moodiness or *ahem* hangriness during IF, at least in the beginning. And while some people feel a serious boost of energy or motivation once they adjust to fasting, its important to remember that it is still a restrictive diet. Feeling obligated to follow it could have negative effects on your mood, especially if youre becoming isolated from friends or family members due to your diet restrictions.

If youre feeling down, anxious, or discouraged about IF, it's crucial to stop and get in touch with a registered dietitian, psychologist, or nutrition coach right away. They may be able to help you create a fasting schedule that better suits your mind and body.

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