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Intermittent Fasting Side Effects

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

Weight loss is a huge reason people are drawn to trying intermittent fasting (IF), because it's sustainable and still allows you to eat your favorite foods. There are other benefits besides losing body fat, such as disease prevention and less inflammation, and many rave about improved digestion and reduced bloating. Clearer skin is another benefit, as well as increased mental clarity and better sleep. But these perks won't happen right away, and many people have to go through an adjustment period first, which includes some uncomfortable side effects.

POPSUGAR caught up with registered dietitian Stephanie Ferrari, MS, RD, of Fresh Communications, 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.

1. Hunger

When you're used to eating five to six times a day, your body comes to expect food at certain times. Ferrari 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!

Luiza Petre, MD, 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.

2. Cravings

"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," Ferrari said. "During intermittent fasting, you're going extra-long periods without eating. So chances are, you'll only be able to think about eating. That's when the cravings kick in." She added that you'll also find that you're more likely to crave sweets and/or refined carbohydrates because your body is looking for that quick glucose hit.

Do whatever you can to stay busy and keep your mind off food. And once your eating window opens, be sure to indulge a little so you have the chance to satisfy those cravings. With intermittent fasting, you're restricting when you eat, so you shouldn't be too restrictive with what you eat.

3. Headaches

As your body is getting used to this new eating schedule, dull headaches that come and go are pretty common. Dehydration can be one factor, explained Ferrari, so make sure you're drinking tons of water during both your fasting and feeding windows. Ferrari added 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. The first week or two, be kind to yourself, and try not to do too much. Taking rest when you can, doing some yoga, taking a walk, or doing something you love like drawing, reading, playing with your kids or pets, or listening to music can help relieve stress, which can also help alleviate headaches. Placing a warm compress on your head can help alleviate the discomfort.

4. 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 take a break from working out or just do light exercise like walking or yoga. Getting extra sleep may also help, or even taking a nap during the day. Remind yourself that this low energy is just temporary, and that within a couple weeks, you'll be experiencing an increase in energy and mental clarity.

If the low energy is unbearable or getting in the way of your daily tasks, you may have chosen an intermittent fasting method that is too extreme. If you went straight to 16:8, maybe take a couple weeks to ease into it with 14:10. Or if you tried doing 5:2, where you fast two days a week, maybe you'd do better with an intermittent fasting schedule that allows you to eat every day. Be flexible as your body acclimates.

5. Irritability

Feeling hangry is real, and it sucks. Ferrari said 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 may also feel more annoyed if you're used to using food to cope with stress or other emotions.

During the first few weeks of intermittent fasting, focus on doing things that make you happy. Find other, nonfood ways to cope with stress, like calling a friend, watching some funny TikToks, listening to a podcast, or exercising. With time, you'll realize that these other methods are actually more effective anyway.

6. 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 if you're not staying hydrated, which can cause bloating and discomfort. Ferrari suggested drinking tons of water and getting enough fiber during your eating window to prevent this issue, which can also help prevent headaches and make you feel more energized.

7. 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. Ferrari said that when your blood sugar decreases, that can also make you more sensitive to feeling cold.

Combat coldness by sipping on hot black coffee or green tea (with nothing in it, since clean fasting is recommended for maximum weight-loss benefits). You can also take warm showers, wear extra layers, have a blanket on your lap while working or lounging on the couch, and avoid being outside in the cold for prolonged periods of time. Doing some light exercise can also help warm you up — even a minute of jumping jacks, running up and down your stairs a few times, or a dozen push-ups can do the trick!

8. Overeating

People may overeat in the beginning of their IF journey because they heard they can eat whatever they want — but remember that calories still matter! 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. This is totally normal, and tends to regulate after a few weeks. If it doesn't, and you end up binge-eating every day, this is a sign that your fasting window is too long. That's why it's a good idea to work with a registered dietitian when you start intermittent fasting, so they can help you make tweaks as needed.

If overeating is causing digestive upset or weight gain, Ferrari said it's important to be mindful about your first meal. Meal prepping when doing IF will ensure that you have nourishing food planned out, which can prevent ravenously reaching for unhealthy choices.

9. Bathroom Trips

Because you're drinking oceans of water, unflavored seltzer, black coffee, or unsweetened tea to stay hydrated to prevent headaches and stay energized, you're going to feel the need to run to the bathroom more often. We're talking maybe even twice an hour, or more! Sorry to say there's no way around this. You definitely don't want to reduce your water intake, Ferrari urged, so just make sure your water bottle is always filled and that you're always close to a bathroom.

As your digestive system gets used to this new schedule of eating, some people may also experience diarrhea. This is not uncommon, especially if you overeat as soon as your window opens or you reach for foods you don't normally eat. Just be patient, as these issues usually resolve after several weeks.

Always Listen to Your Body and Know When to Quit

Remember that intermittent fasting is not for everyone. For example, "people with diabetes, pregnant or nursing mothers, and children should not practice intermittent fasting," Ferrari said. She added that people who are managing chronic illnesses should always check in with their doctor prior to starting any new diet or eating regimen. Finally, anyone with a history of or risk for developing eating disorders should also avoid fasting.

That being said, if you don't fall under those categories, you still might find that intermittent fasting doesn't work for you. There is a time when these side effects shouldn't be ignored. "IF might not be for you if you experience extreme 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," Ferrari said. 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 or a registered dietitian.

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