Over the last several years, intermittent fasting has become increasingly popular for its promises of improved health and weight control. The idea is that it's easier to sharply restrict calories a few days a week or to limit eating to a shortened "eating window" each day than it is to moderately cut calories at every meal, every day.
Proponents claim that extended fasting periods (beyond the normal time between meals) promote cellular repair, improve insulin sensitivity, increase levels of human growth hormone, and alter gene expression in a way that promotes longevity and disease protection. But are there any risks?
Before weighing intermittent fasting side effects, it's important to know that there are several forms of intermittent fasting, and the evidence of their long-term effectiveness and safety is not yet known. The most common forms include:
- Alternate day fasting (or adf fasting), which requires fasting every other day
- Modified alternate-day fasting, which requires you to only eat 25 percent of your usual intake every other day
- Periodic fasting, which requires you to limit food to about 500 to 600 calories a day on only two days per week
- Time-restricted eating, which limits your daily "eating window"
Some plans may cause more side effects than others, but overall, it's important to discuss the following intermittent fasting side effects with a medical professional before choosing a plan that works with your lifestyle.
#1. Intermittent fasting may make you feel sick.
Depending on the length of the fasting period, people may experience headaches, lethargy, crankiness, and constipation. To decrease some of these unwanted side effects, you may want to switch from adf fasting to periodic fasting or a time restricted eating plan that allows you to eat everyday within a certain time period.
#2. It may cause you to overeat.
There's a strong biological push to overeat following fasting periods because your appetite hormones and hunger center in your brain go into overdrive when you are deprived of food.
"It's human nature for people to want to reward themselves after doing very hard work, such as exercise or fasting for a long period of time, so there is a danger of indulging in unhealthy dietary habits on non-fasting days," says Dr. Frank Hu, chair of the department of nutrition at the Harvard T.H. Chan School of Public Health.
A 2018 study found that two common effects of calorie-restricted diets—a slowed metabolism and increased appetite—are just as likely when people practice intermittent fasting as when they cut calories every day. And in studies of time-restricted eating, evidence is accumulating that eating that misaligns with a person's circadian rhythm (your body's natural daily pattern) may led to metabolic trouble.
#3. Intermittent fasting may cause older adults to lose too much weight.
While intermittent fasting shows promise, there is even less evidence about the benefits or how fasting might affect older adults. Human studies have looked mostly at small groups of young or middle-aged adults, for only short periods of time.
But we do know intermittent fasting could be risky in some cases. "If you're already marginal as far as body weight goes, I'd be concerned that you'd lose too much weight, which can affect your bones, overall immune system, and energy level," says registered dietitian Kathy McManus, director of the Department of Nutrition at Harvard-affiliated Brigham and Women's Hospital.
#4. It may be dangerous if you're taking certain medications.
If you want to give intermittent fasting a try, make sure to discuss it with your doctor first, says Dr. Eric Rimm, professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health. Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes. Some people who take medications for blood pressure or heart disease also may be more prone to imbalances of sodium, potassium, and other minerals during longer-than-normal periods of fasting.
Dr. Suzanne Salamon, associate chief of gerontology at Harvard-affiliated Beth Israel Deaconess Medical Center, expresses another concern: "People who need to take their medications with food — to avoid nausea or stomach irritation — may not do well with fasting."
How to reduce intermittent fasting side effects
Easing into an intermittent fasting plan can help your body adjust, according to McManus. "Slowly reduce the time window for eating, over a period of several months," she advises.
You should also:
- Continue your medication regimen as recommended by your doctor
- Stay hydrated with calorie-free beverages, such as water and black coffee
- Choose a modified fasting plan approved by your doctor if you need to take medication with food.
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