Fasting is an age-old practice, but intermittent fasting — a dieting plan that oscillates between defined periods of eating and not eating — became ultra-popular only recently following a slew endorsements from celebrities and tech moguls.
Fasting enthusiasts claim it helps with focus, weight loss, and energy.
There's some evidence to back that up. A small study published in the journal Obesity found that participants who ate between the hours of 8 a.m. and 2 p.m. had lower appetites and reduced body fat.
But, like any diet, intermittent fasting can give rise to extreme eating habits. Twitter CEO Jack Dorsey, who subscribes to a fasting routine, eats a single meal on weeknights followed by a weekend-long deficit — what some nutrition experts consider a sign of an eating disorder.
In some cases, the negative side effects of fasting could outweigh any potential benefit. Here are a few signs that an intermittent fasting routine is unsafe or unhealthy.
Dieting in general can give rise to orthorexia, a disorder that involves an obsession with healthy eating. Some of the signs of orthorexia include the need to talk about your diet all the time and a preoccupation with your next meal.
One overarching sign is when your diet starts to become inflexible, said Alissa Rumsey, a New York City-based dietitian. That includes altering or canceling social outings because they don't align with your eating habits.
There's some preliminary evidence that intermittent fasting can improve sleep by keeping you from waking up in the middle of the night. When people start their fast earlier, their eating window also tends to expire well before they go to bed. This helps them avoid nighttime snacking, which can improve quality of sleep.
But Rumsey said intermittent fasting can also disrupt your sleep cycle or lead to restless nights. Multiple studies have shown that fasting can decrease your amount of REM sleep, which is believed to improve memory, mood, and learning capacity.
Jack Dorsey has claimed that his fasting diet keeps him more focused and alert, but some nutritionists attribute these feelings to his body going into starvation mode.
"Animals who are starved shouldn't feel playful," Jennifer Gaudiani, an internal-medicine doctor, told Business Insider in April. "They should feel concerned and focused. They may interpret that initially as productive, but it's the brain saying, 'I don't have enough food.'"
In the long term, Rumsey said, intermittent fasting can lead to decreased alertness because the body doesn't consume enough calories during a fasting window to provide adequate energy. Fasting could also lead to fatigue, difficulty concentrating, or dizziness, said Rachael Hartley, a registered dietitian and nutritionist in South Carolina.
When people give themselves a time window for eating, they can start to feel guilty about breaking their fast too early or eating too late, said Rumsey. Any sort of anxiety or shame surrounding your diet can be a warning sign of disordered behavior, she said. That includes talking down to yourself when you're fasting — another sign of orthorexia.
Early research has found that intermittent fasting may reduce the risk of diabetes, cancer, and heart disease, but Rumsey said depriving yourself of food for an extended period of time can increase levels of cortisol, the body's stress hormone.
"Even if there are potentially some positive health benefits, they're going to be negated by the increase in stress," Rumsey said.
High cortisol levels are also associated with fat storage, which isn't ideal if you're trying to lose weight.
Intermittent fasting can cause some people to go into a calorie deficit, which can result in hair loss and irregular or missed periods. People on an intermittent fasting diet may also feel colder than normal as a result of low blood sugar.
"Intermittent fasting veers into disordered eating when it starts to affect one's health," said Hartley. That includes a change in mental and social behavior, such as an increase in anxiety and depression or a decreased ability to socialize.
"We all have a natural overnight fast when we sleep," said Rumsey. "I typically don't recommend any type of specific fasting outside of that because really what you're doing is overriding your body's intuition."
Instead, she said, people should listen to their body's natural cues about whether they're hungry or full.
"Getting hangry is a real thing," said Rumsey. "People can become really irritable."
Though intermittent fasting isn't necessarily unhealthy, people with a personal or family history of an eating disorder should steer clear of the diet. The practice also isn't ideal for those who are highly active, since they require more fuel.
"I would encourage people to be very careful exercising while fasting," said Hartley. "Food provides our body with energy for movement, so exercising while fasting may affect performance and also lead to an unhealthy energy deficit."
While Hartley said that no body type is particularly suited to intermittent fasting, she recognized that some people may only experience hunger pangs during a typical 8-hour fasting window. If people are able to fast "in a flexible way that isn't negatively affecting their life or health, that's great," she said.
In general, though, she advises people to eat a meal or snack every 3 to 4 hours.
"Food is our fuel," she said. "Why try to drive a car on an empty tank?"