Every Intermittent Fasting Beginner Should Know About These 4 Side Effects
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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 is an eating regimen, explains Amy Lee, M.D., Chief Medical Officer of Lindora Clinic. There are various types and you have to find one that is right (realistic) for you, she adds. “A popular one is called 16:8 which is fasting for 16 hours a day and only eating during the 8 hours or what they call ‘eating window.’ There is one called the 5:2 diet which is eating ‘normally’ for five days and then for two, you attempt to cut back on calories…Next, there is something called alternate day, which means you fast every other day, taking only minimal calories. Another is called the 14:10, which is fasting for 14 hours and the eating window is around 10 hours. Finally, another regimen can be as simply committing a couple meals a day to purely low calorie vegetables and fruits and one meal a day with a bigger meal with more macronutrients.”
The commonality to all these regimens results in a “caloric deficit”; which is the key element of a dietary regimen, explains Dr. Lee. “The portion of food you eat daily plays a huge role in your body’s way of weight loss, maintenance or gain.” Before you start an IF regimen, “accept the fact that you should continue to stay active and consider some form of movement regardless of what IF regimen you end up with,” says Dr. Lee. She adds that most regimens call for a restricted regimen of 500 calories for females and 600 calories for males.
“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. You 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.
Because you are undergoing prolonged hours of fasting or not taking any calories in, you must be careful with dipping of your blood sugar especially for those who have prediabetes, type 1 or 2 diabetes, or known history of hypoglycemia, says Dr. Lee. “Most people experience mild symptoms when the blood sugar dips which could be as simple as fatigue, or dizziness. However, the more severe cases could be passing out or falling.”
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.
During meals, make sure to fuel up on healthy, satisfying foods. 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 an easier time managing their blood sugar. 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.
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.
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.
5. You may experience anti-aging and anti-inflammatory benefits.
There is data that shows that IF is anti-inflammatory and anti-aging, says Dr. Lee. Intermittent fasting increases the levels of antioxidants that can help prevent the body’s cells from being broken down by free radicals, which are molecules that can cause damage to cells. Additionally, fasting can reduce the chronic inflammation that occurs as people age.
One study in particular found that the alternation of fasting and refeeding periods is accompanied by positive effects on risk factors for aging, autoimmunity, cardiovascular disease, neurodegeneration and cancer.
When should you see a doctor about IF side effects?
Depending on which regimen you end up picking to try, you should consult a physician to make sure that you do not have any risk factors to cause symptoms that may be dangerous, advises Dr. Lee. “If you have conditions that affect [blood] sugars in general, you must make sure your medications are revised and followed. Also, those with unstable blood pressure should also take into consideration the risk of dehydration and water fluctuation.”
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