Intermittent fasting (IF) has tons of benefits, including weight loss, preventing diabetes, and reducing your risk of cancer. Many people who follow IF say that it’s helped them savor their meals more and understand the difference between hunger and cravings. It has also helped them break through weight loss plateaus.
“IF makes you feel good because you’re getting rid of the inflammation,” says, Wendy Scinta, M.D., president of the Obesity Medicine Association and a member of Prevention‘s Medical Review Board. “I follow the 16:8 diet and find that when I prescribe IF to patients who want to lose 100 pounds and can’t seem to lose the last 15 pounds, IF helps them get there.”
But IF isn’t for everyone (Dr. Scinta doesn’t recommend it for people who have a history of disordered eating or pregnant women), and it’s important to understand the side effects that come with it.
No matter what type of intermittent fasting method you’re interested in following, here are side effects you should know.
1. Newbies may feel hypoglycemic.
At first, you may experience hypoglycemia, a condition caused by very low blood sugar levels. This can lead to headaches, increased heart rate, dizziness, and nausea, according to Dr. Scinta. Oh, and bad moods—no one’s happy when they’re restricting food. “When you don’t eat, your body will first burn the glycogen (stored glucose) in your liver and muscles (hence feeling irritated at first), then it will begin to burn fat for fuel,” says Frances Largeman-Roth, R.D.N., nutrition and wellness expert, author of Eating in Color and creator of the FLR VIP program, says. But as your body becomes more keto adaptive and learns to run on fat instead of glucose, Dr. Scinta says hypoglycemia becomes less of a concern.
However, if you continue to feel dizzy or lightheaded over time, Largeman-Roth says to eat something—even if it’s a small snack. “Losing weight is never a good enough reason to pass out,” she says.
And make sure to fuel up on healthy, satisfying foods during meals. Lean protein, fruits and vegetables, whole grains, and healthy fats, such as avocados, nuts, and extra-virgin olive oil will keep your blood sugar levels balanced during your fast and provide the nutrients your body needs to function properly.
Dr. Scinta says she often finds that people on IF struggle to get enough protein, so remember to eat regularly, including snacks, when you’re not fasting. “You should aim to get at least one gram of protein per kilogram of weight daily,” she says.
2. You’ll crave carbs and processed foods less.
Dr. Scinta says that many people who follow IF have a better time at keeping their blood sugar levels balanced. Because IF forces you to stop eating at a certain time, you’ll fuel up on more satisfying foods, like lean protein and fiber, to stay full during your fast. “What I’ve found with IF is that it’s helped me watch my carb intake,” Dr. Scinta says. “You’re not only eating as much, but you’re not eating as much of the bad stuff.”
IF also promotes satiety through the production of appetite-reducing hormones. A 2019 study from Obesity suggests that IF can help decrease ghrelin levels—the hormone that stimulates hunger—in overweight adults and improve people’s ability to switch between burning carbs for energy and burning fat for energy.
“There are folks who eat at night due to boredom or stress, not because they’re actually hungry. Putting guardrails on the times they can eat may help them avoid eating when they don’t need to be,” Largeman-Roth says.
Dr. Scinta and Largeman-Roth also advise people to stay hydrated while fasting because people tend to confuse thirst for hunger.
“When people fast in the morning, they drink a lot of coffee, which is a diuretic, and forget to drink water,” Dr. Scinta says. “Every function in the body requires water, so staying hydrated is incredibly important,” Largeman-Roth says. “We get about 20% of our water intake from the food we eat, so when we fast, we’re losing a significant source of hydration,” she says.
3. You’ll improve your insulin sensitivity.
A 2018 study in Cell Metabolism found that men with prediabetes who followed IF improved their insulin sensitivity, even though they didn’t lose weight. How does it work, exactly? Whenever you eat, your body releases the hormone insulin to move sugar from your bloodstream into your cells for energy. But people with prediabetes don’t respond well to insulin so their blood sugar levels stay elevated. Increasing the time between meals can help because your body releases less insulin.
However, Dr. Scinta says that people who are on insulin-dependent medications should consult with their doctor before following IF because it can affect the effectiveness of their treatment. “People with type 1 or 2 diabetes are on these medications to lower their glucose, so they need to have consistent meals to prevent spikes in their blood sugar,” Dr. Scinta says.
4. Your workouts may take a hit.
Following IF and working out is totally safe, but you’ll need to make some adjustments to your schedule so that you’re not running on empty. Say you’re following the 5:2 diet: Doing low-impact workouts instead of more intense ones, like weight lifting, running, and HIIT, on days when you’re limiting calories can help your body adjust to the new demands. As your body gets used to burning fat for fuel, the intensity of your workouts won’t be as much of a concern.
That said, the last thing you want to do is pass out during your HIIT class, so Dr. Scinta recommends timing your workouts at the beginning or end of your fast. This way, you can enjoy a pre- or post-workout snack. Foods that are easy to digest, like a smoothie, low-fat yogurt, and peanut butter with toast work better pre-workout, while foods with a higher carb-to-protein ratio, such as a bowl of oatmeal, are ideal for post-workout.
For this reason, health experts advise following the 16:8 diet over 5:2 and other intermittent fasting methods if you’re very active.