Many diet trends have come and go, but intermittent fasting (IF) seems to be here to stay. That's not only because many people have found success with weight loss through IF but also due to the many health benefits it's been linked to, from less inflammation to better concentration. Before you jump in, though, there are intermittent fasting side effects to consider.
“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, RD, nutritionist in chief of weight loss coaching company Profile Sanford. “If you’re consuming too few calories or nutrients during fasting times, you could have side effects.”
There are always going to be some type of side effects, says Audrey Fleck, RDN. Yes, even for the 16:8 method, one of the more popular IF schedules. "You're not eating, your body has to adapt to this [schedule]." And it can take your body up to a month to adjust to intermittent fasting, she adds.
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.
Meet the experts: Alyssa Koens, RD, is a nutritionist in chief of weight loss coaching company Profile Sanford. Audrey Fleck, RDN, is a nutritionist who specializes in integrative and functional nutrition for women.
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 can cause negative side effects, and you shouldn't hop onboard 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 you need to stop IF and talk to your doctor or a nutritionist before continuing.
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 proper word, but it’s definitely a real sensation. It describes 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. Teaching your body to go 16 hours without food takes some practice, and this model of eating may not work for some people’s bodies.
A 2021 study published in JAMA found a negative correlation between hunger and mood, which is pretty close to a clinical measurement of hanger, according to Morgan Goodstadt, RD, the founder of Good Nutrition. Study participants in a 2022 study published in the journal Nutrients also reported feeling hungrier, have more cravings, and experiencing lower mood and energy on fasting days versus eating days.
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 that rings true for you, that’s a sign you need to make some dietary adjustments during your feeding 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 is not ideal—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 consuming food in the a.m. 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. Diabetics should avoid any kind of fasting diet: IF can cause you to become hypoglycemic, a dangerous condition for anyone with insulin or thyroid problems.
Any IF regime that has you skipping breakfast can particularly affect your blood sugar, Fleck adds. 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 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 a while, you probably have a better sense of how it affects you and your body. Still, it's a good idea to keep an eye out for these side effects that can show up when you're no longer an IF newbie.
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. Having orthorexia means you focus so much on “correct” or “healthful” eating that it actually has a detrimental effect on your overall well-being, according to The National Eating Disorders Association. A 2022 study published in the journal Eating Disorders found men and women who follow IF score significantly higher in the Eating Disorder Examination Questionnaire compared to those who are eating a normal diet.
“You want to focus on forming a healthy, positive relationship with food,” says Koens.
It's therefore a good idea to stay away from IF if you have a history of disordered eating, Fleck notes. "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 adds.
2. Hair Loss
Seriously? Yup. Sudden weight loss or a lack of proper nutrients, especially protein and B vitamins, can cause hair loss, Koens says. Although some studies suggest calorie restriction can lead to hair loss, more research needs to be done to fully demonstrate this relationship, Goodstadt notes.
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 food into a handful of hours. If you notice more hair than usual is falling out in the shower every day, re-evaluate the composition 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, as noted in a 2021 study published in Experimental Gerontology and a 2019 study published in the journal Obesity.
Women who have excessively low body weight are prone to a condition called amenorrhea, or the absence of menstruation, per Mayo Clinic. 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 the calories it needs to function.
If you stop getting your period and think it's linked to intermittent fasting, stop fasting and speak with your gynecologist right away.
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.
In Goodstadt's experience, she often hears "‘I get to eat whatever I want between fasts,’ which suggests a relatively unbalanced diet during these eating windows,” she says.
If you often fall into this trap and take fasting as license to chow down on highly processed foods, 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
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, Koens says.
However, there is some research pointing to the opposite effect. A 2018 review in the journal Nature and Science of Sleep shows 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, so you definitely don't want to miss out on quality shuteye.
If you notice you can't fall or stay asleep after you've started an IF eating plan, again, take a break from the diet 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.
So, is there anyone who should avoid intermittent fasting?
If you fall into any one of these camps, IF is properly not right for you:
- Anyone who has diabetes and is at risk of hypoglycemia
- People with a history of an eating disorder
- Individuals on medications that require regular eating intervals
- Pregnant and breastfeeding women
Adjusting to a new dietary pattern may cause some mild symptoms, but seek medical attention if any symptoms become severe, particularly changes in blood pressure, dizziness, fainting, or nausea. Disruption of your sleep or menstrual cycle should also prompt you to stop IF.
Most studies on IF range from 12 weeks to a year, so there isn't much research on the benefits of long-term fasting, Goodstadt says. Many IF studies also have relatively high dropout rates, which suggests that doing IF for long periods of time can be difficult. “Even just from a practical perspective with my clients, IF can be very restrictive and difficult to follow on a regular basis. Creating such strict rules around food can be challenging to maintain and also create unnecessary stress around eating and food,” she notes.
Think about the “why” if you're curious about trying intermittent fasting, Goodstadt says. There may be other diets that feel more sustainable and can achieve the results you want.
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.
Ashley Martens is a Wellness Writer based in Chicago, Illinois. With a digital marketing background and her knowledge of general nutrition and a lifelong passion for all things health and wellness, Ashley covers topics that can help people live happier and healthier lives.
