Celebs everywhere are raving about intermittent fasting (IF), from Vanessa Hudgens to Jennifer Aniston. The buzzy, new trend might sound fancy, but it's actually pretty simple: IF involves restricting what you eat in a day to a certain time window.

The benefits of IF stretch far and wide—everything from weight loss to better concentration (and who couldn't use a brain boost?). But before you jump ahead to wondering what type of fasting schedule is right for you or whether you can combine IF with other diets like keto, let's get one thing straight: IF can still cause side effects, especially if you’re not doing it right.

“It’s important to figure out which style of IF works for you, whether that’s a shorter versus longer fasting window or only doing it so many days per week,” says Alyssa Koens, lead registered dietitian of weight loss coaching company Profile Sanford. “If you’re consuming too few calories or nutrients during fasting times, you could have side effects.”

More From Women's Health
 
preview for Women's Health US Section - All Sections & Videos

Since there aren't any official guidelines for exactly what IF routine is , it's mostly up to you to figure out what works best for your bod. That means lots of trial and error—and deciding if the potential negative side effects are worth it.

Do all intermittent fasting schedules cause side effects?

First, it's important to note that none of the benefits of IF have been heavily researched. The research is also scant in humans (compared to, say, the research in mice), and the studies that were conducted on humans mostly involved men, says Audrey Fleck, MS, RDN. It's also unclear if the root cause of the benefits (and side effects) is intermittent fasting itself, like how it affects your body on a cellular level, or simply the calorie restriction.

That being said, "There are always going to be some type of side effects," Fleck says. Yes, even for the 16:8 method, which is one of the more popular kinds of IF. "You're not eating, your body has to adapt to this [schedule]." She adds that it can take your body up to a month to adjust to intermittent fasting.

blue clock on white plate, intermittent fasting concept, ketogenic diet, weight loss, skip meal
ThitareeSarmkasat//Getty Images

What are the negative side effects of intermittent fasting, exactly?

As mentioned, intermittent fasting is still very much in the research phase—but there are some promising findings. 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.

This content is imported from poll. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

Short-Term Intermittent Fasting Side Effects

Right away, you might notice a few changes in your body and your digestion. These side effects can be milder, but it's still important to watch out for them. Here are some signs that might start to pop up in the first few days or weeks after starting IF:

1. Feeling hangry

    We’re not 100 percent certain that “hangriness” is a real word, but it’s 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 it’s hungry.

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

    In theory, if you’re consuming enough protein later in the day or night, you shouldn’t be starving first thing in the morning. But if you are, that’s a sign you need to make some dietary adjustments during your caloric intake period to avoid turning into a major crank—or 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 all—and 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 you’re excessively tired every day—or making dumb mistakes because you’re wading through brain fog—is a tip-off that you’re not eating the right foods during non-fasting hours or that fasting isn't fitting in with your lifestyle needs.

    “Pay attention to what you’re 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. Low blood sugar

    If you’re having persistent nausea, headaches, or dizziness during IF, that’s 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.

    Fleck adds that any IF regime that has you skipping breakfast can particularly affect your blood sugar. Starting your morning off on an empty stomach can disrupt the rest of your day and set your body up for intense cravings later on (more on that in a sec).

    4. Constipation

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

    It’s easy, she explains, for people to forget to drink water during fasting hours—but 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!).

    Long-Term Intermittent Fasting Side Effects

    Once you've tried IF for awhile, you probably have a better sense of how it's affecting you and your body. Still, it's a good idea to keep an eye out for these long-term side effects:

    1. 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 they’re 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 shouldn’t be the goal of any diet, says Koens: “You want to focus on forming a healthy, positive relationship with food.”

    Fleck explains that, for these reasons, it's a good idea to stay away from IF if you have a history of disordered eating. "I've seen it morph into becoming an eating disorder where there's fear about eating outside of windows and if you have that history of eating disorders and you do this, it will likely trigger past tendencies," she says.

    2. 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 doesn’t necessarily lead to a loss of nutrients, it tends to be harder to eat a well-rounded diet when you’re cramming a whole day’s 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.

    3. Changes in your menstrual cycle

    Here’s 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.

    4. Unhealthy diet

    Even if IF doesn’t 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 you’re 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.

    5. 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.

    6. Mood changes

    It would be weird if you didn’t 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, it’s important to remember that it is still a restrictive diet. Feeling obligated to follow it could have negative effects on your mood, especially if you’re becoming isolated from friends or family members due to your diet restrictions.

    If you’re 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.

    Is there research behind the long-term effects?

    To be honest: not so much. They're more like educated guesses based on what health experts already know about food, eating, and nutrition. "What we do know, at least for women, is that it's a strain on the body," Fleck says. "Maybe the body can adapt to it and you might feel good in certain way."

    "There's a lot of research behind under eating and under-nutrition. So that's where I think also we can base some of what we know," she adds.

    The bottom line: Be careful and pay attention to your body if you don't feel your best when doing intermittent fasting, and bring up any side effects with a doctor or nutritionist.