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Intermittent Fasting Side Effects

9 Common Side Effects of Intermittent Fasting and How to Deal With Them

You've heard so much about the benefits of intermittent fasting (IF), including weight loss, reduced inflammation, improved digestion, reduced bloating, increased mental clarity, better sleep, and getting a handle on sugar cravings.

You're ready to give it a try, but you need to be aware of some not-so-awesome side effects that you'll likely experience in the beginning. POPSUGAR caught up with Stephanie Ferrari, a registered dietitian, who said, "Think about it this way — people don't go from couch potato to triathlete overnight. Your body needs time to acclimate to any extreme changes. So you're going to experience some side effects when you suddenly stop eating for long periods of time." These can be unbearable at the beginning, but as long as you know how to deal with them, you'll be able to stick with IF and reap all the benefits.

Before starting any new diet plan, including intermittent fasting, be sure to check in with your doctor first.


When you're used to eating five to six times a day, your body comes to expect food at certain times. Stephanie said, "The hormone ghrelin is responsible for making us feel hungry. It typically peaks at breakfast, lunch, and dinner time and is partially regulated by food intake. When you first start fasting, ghrelin levels will continue to peak and you will feel hungry." At first, it will take serious willpower. Days three through five may feel the worst, but there will come a time when you reach the beginning of your eating window and you don't even feel hungry!

Dr. Luiza Petre, a nutrition and weight-loss specialist and board-certified cardiologist, suggests combating hunger in that first week or two by drinking tons of water to keep your belly full, help you feel more alert, and help satiate that habit of having to put something in your mouth. Within 30 minutes of waking up, pound at least 12 ounces. If you feel a pang of hunger, drink another 12 ounces or more. One thing intermittent fasting will teach you is that what you thought was hunger was probably thirst or boredom.

Drinking black coffee and tea can also curb hunger. Also get enough sleep, keep busy, and avoid strenuous workouts in the first couple weeks, since that can increase hunger. Eating enough the day before and getting your fill of carbs, healthy fats, and protein is also key in preventing hunger.


Stephanie said, "If I told you that you couldn't eat watermelon ever again, chances are, all you'd want to do is eat a slice of watermelon. During intermittent fasting, you're going extralong periods without eating. So chances are, you'll only be able to think about eating. That's when the cravings kick in. You'll also find that you're more likely to crave sweets and/or refined carbohydrates because your body is looking for that glucose hit."

Do whatever you can to not think about food, and be sure to indulge a little during your feeding window so you have the chance to satisfy those cravings.


As your body is getting used to this new eating schedule, dull headaches that come and go are pretty common. POPSUGAR caught up with Edward Vasquez, who has an intermittent fasting YouTube channel called Fledge Fitness which offers information, tips, and inspiration for IF. He said dehydration can be one factor, so make sure you're drinking tons of water during both your fasting and feeding windows.

Stephanie adds that headaches can also be caused by blood sugar levels decreasing and by stress hormones released by your brain while fasting. With time, your body will get used to this new eating schedule, but try to remain as stress-free as possible.

Low Energy

Your body is no longer getting the constant source of fuel you used to get from eating all day long, so expect to feel a little sluggish those first couple of weeks. Try to keep your day as relaxed as possible so you can exert the least amount of energy. You might want to give your workouts a break or just do light exercise like walking or yoga. Getting extra sleep may also help.


Feeling hangry is real, and it sucks. Stephanie says to expect to feel a little cranky when your blood sugar levels drop or you're dealing with the other side effects of IF, like cravings and low energy. You can deal with this by avoiding situations and people that might make you more annoyed and focusing on doing things that make you happy.

Heartburn, Bloating, and Constipation (Oh My!)

Your stomach produces acid to help digest your food, so when you're not eating, you may experience heartburn (this side effect isn't as common as the others). This could range from mild discomfort to burping all day to full-on pain. Time should cure this side effect, so just keep drinking water, prop yourself up when you sleep, and when you do eat, avoid greasy, spicy foods that could make your heartburn worse. If it doesn't go away, speak to your doctor.

Intermittent fasting can also cause constipation, which can cause bloat and discomfort. Stephanie suggests drinking tons of water to help.

Feeling Cold

Cold fingers and toes while fasting is pretty common, but for a good reason! When you fast, blood flow increases to your fat stores. Called adipose tissue blood flow, this helps to move fat to your muscles, where it can be burned as a fuel. Stephanie says that when your blood sugar decreases, that can also make you more sensitive to feeling cold. Combat coldness by sipping hot tea, taking warm showers, wearing extra layers, and avoiding being outside in the cold for prolonged periods of time.


Edward also brought up this negative side effect, and Stephanie agrees. People tend to overeat in the beginning of the IF journey, either because they heard calories don't matter (they do!) or because they are so excited about food that they overdo it. Planning out your meals ahead of time can keep portions in check.

You may also feel so famished by the time your fasting window ends that you eat really fast and end up eating way more than you normally would. Stephanie warns, "When the fasting period is over during IF, you need to be mindful about your first meal. You may feel like reaching for a slice a pizza (or four), but opt for the grilled chicken and salad instead. Your future self will thank you."

Bathroom Trips

Because you're drinking oceans of water to stay hydrated and fill you up, you're going to feel the need to run to the bathroom more often. We're talking maybe even twice an hour! Sorry to say there's no way around this. You definitely don't want to reduce your water intake, so just make sure you're always close to a bathroom.

How Long Will These Side Effects Last?

All of this sounds pretty bad, but I speak from experience — these symptoms generally only last a week, maybe three at most. The best way to alleviate the side effects is to ease into intermittent fasting — don't go from eating six meals a day to eating two. Dr. Petre adds, "Just give it some time and intermittent fasting becomes natural and healthy, with less appetite, more mental sharpness, and an enviable waistline in the long run."

Always Listen to Your Body

Stephanie says, "Intermittent fasting is not for everyone. For example, people with diabetes, pregnant or nursing mothers, and children should not practice intermittent fasting. People who are managing chronic illnesses should always check in with their doctor prior to starting any new diet or eating regime. Finally, anyone with a history of or risk for developing eating disorders should avoid fasting of any kind."

There is a time when these side effects shouldn't be ignored. Stephanie says, "IF might not be for you if you experience dizziness due to low blood sugar, if fasting is interfering with your ability to keep up with your responsibilities, or you develop an unhealthy obsession with food." You may need to cut your fast short and eat earlier than you planned, or you may need to stop fasting altogether. If you have any concerns or issues, it's always a good idea to consult your doctor.

Image Source: POPSUGAR Photography / THEM TOO
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