If you're thinking of revamping the way you eat, one of the hottest ways to do it is skipping eating at all. Intermittent fasting is among the trendiest diet plans of the moment, with celebs, Silicon Valley execs and Instagram influencers saying it gives them better health, better focus, better energy, and maybe also trims down weight they've gained.
There's no one way that people do intermittent fasting. Some followers forego food for anywhere from 16 hours to a whole day; some go a whole weekend without food, others do alternate-day fasting. There are many approaches.
There has beenresearch on different aspects of intermittent fasting. But intermittent fasting hasn't yet been shown to be a long-term panacea for everything that bothers you, whether that's weight, focus, energy or something else. “There hasn’t been a long-term study establishing that intermittent fasting has good outcomes,” says Howard Steiger, a professor of psychiatry at McGill University whose research focuses on eating disorders.
That said, for people who are otherwise healthy—whether they’re at a normal weight, overweight or obese—“there is no evidence to show that [intermittent fasting] is harmful,” says Sai Das, an associate professor at Tufts University’s Friedman School of Nutrition Science and Policy. She adds, though, that “we cannot say across the board that it’s safe for everyone.”
In other words, if you do want to try intermittent fasting, proceed with caution. Here’s what you need to know before you start:
Intermittent fasting isn't risk-free.
Before getting into intermittent fasting, “everyone should really have a conversation with their physician,” Das says. That’s especially true if you’re at high risk for health problems or complications—that is, if you’re over 65 and/or have an existing medical condition.
Das also strongly recommends talking to your doctor if you’re on medication, since dosing schedules are often built around regular meal times. Ditto if your job involves handling heavy equipment or other tasks that might endanger yourself or others if you were to experience lightheadedness or the other side effects of low blood sugar.
You should avoid fasting altogether, though, if you have higher caloric needs—that is, if you’re underweight, younger than 18, or pregnant or breastfeeding, the other experts we interviewed said. Also avoid fasting if you have diabetes, since fasting could cause your blood sugar to plummet to dangerously low levels.
You absolutely shouldn’t fast if you’re susceptible to developing an eating disorder. Risk factors include having a family member with an eating disorder, perfectionism, impulsivity and mood instability, Steiger says. Dietary restriction can cause eating disorders in people with these risk factors, he adds, and studies have associated fasting with a higher risk of bulimia in particular.
You’ll feel hungry.
No surprise that you might notice your stomach grumbling during fasting periods, especially if you’re used to constant grazing. You can, however, keep hunger in check using certain strategies.
During fasting periods, avoid looking at, smelling, or even thinking about food, which can trigger the release of gastric acid into your stomach and make you feel hungry, says Yuan-Xiang Pan, an associate professor of nutrition at the University of Illinois at Urbana-Champaign. He suggests reading a book or going all-in on another mentally engaging activity, “so that you’re not sitting in front of TV thinking, ‘Man I want to eat something,’” he says. If you really need to fill your stomach, drink water, or plain iced tea or coffee, he adds.
You can set yourself up for less hunger while you're fasting by eating better when you're not fasting. That means “focusing on high fiber, moderate protein and healthy fats,” says Shelby Cox, a registered dietitian nutritionist and director of the Kendall Reagan Nutrition Center at Colorado State University.
You might overeat.
“Non-fasting days aren’t days when you can just totally splurge,” Das says. Otherwise, you might end up in a net calorie surplus, leading to weight gain. For some people, it can be hard to avoid splurging, since fasting may actually trigger binge eating. In a five-year study of 496 adolescent girls, fasting—which the researchers defined as not eating for a 24-hour period for weight control—strongly predicted binge-eating. A 2015 review also found that fasting significantly increased levels of the stress hormone cortisol, which may lead to cravings. It makes sense. “If you’re used to having three meals… maybe a snack in between, when you practice intermittent fasting, it’s a huge change,” Pan says. “That can cause increased stress levels in some people.”
If you’re a stress eater, try activities that lower cortisol, like meditating or listening to music. Also remember to fill up on nutritious, satiating foods during eating periods.
You might get dehydrated.
“Intermittent fasting is sometimes associated with dehydration because when you don’t eat, sometimes you forget to drink,” Pan says. Remember to pay special attention to your body’s thirst cues during fasting periods.
You might feel tired.
It’s normal to feel groggy, especially as an intermittent fasting newbie. Your body is running on less energy than usual, and since fasting can boost stress levels, it can also disrupt your sleep patterns, Pan says. Try meditating or other stress-lowering activities. If you have a regular fitness routine, schedule your workouts during eating periods, Cox says.
You might feel irritable.
“The same biochemistry that regulates mood also regulates appetite,” Steiger says, with nutrient consumption affecting the activity of neurotransmitters like dopamine and serotonin, which play a role in anxiety and depression. That means dysregulating your appetite may do the same to your mood. Again, stick to a nutritionally balanced, satiating diet during your eating periods, and remember to clock in enough sleep, which research has also linked to mood.
You might have an easier time getting drunk
It’s ok to imbibe while intermittent fasting, but not during or immediately after fasting periods. As you’ve probably already figured out firsthand, you’ll get drunker faster if you drink on an empty stomach.
That said, even if you do restrict your drinking to eating periods, “you’re displacing your opportunity for adequate nutrition”—which intermittent fasting already limits—"with alcohol,” Cox says. And if weight loss is your goal, low-nutrient, high-calorie alcohol “is not your best option.”
Bottom line: Cox encourages anyone considering intermittent fasting to talk to a doctor, “especially if they want to continue it on a long-term basis to make sure that’s the best option for them.”
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