What to Know Before You Try Intermittent Fasting
There's more than one way to eat. Many people subscribe to the three-meals-per-day schedule, but food scarcity, cultural and religious norms, and personal preferences result in various eating habits. In recent years, intermittent fasting has become a popular eating plan and one you often see championed by athletes and celebrities. But how does it work, exactly, and should you try it? We consulted a registered dietitian to find out.
WHAT IS INTERMITTENT FASTING?
"Intermittent fasting is a diet that follows time-restricted eating, with or without a calorie deficit," says Cara Harbstreet, MS, RD, of Street Smart Nutrition. She notes that cycling between periods of eating and fasting can involve various iterations. "It can look like eating only during a specific block of time during the day or combining days of fasting with unrestricted eating throughout the week."
Many plans call for a 12-hour eating window followed by 12 hours of fasting. Stricter plans suggest eating all your meals within an eight-hour window and fasting for 16 hours -- for example, consuming all your calories between 10 a.m. and 6 p.m. Some people eat whatever they want in this narrow window, but it's always advised to eat plenty of nutrient-dense foods, regardless of what plan you follow.
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SHOULD YOU TRY IT?
People fast for different reasons, and certain religions and philosophies have prescribed it for centuries. But current interest in intermittent fasting can be linked to the recent research on its potential health benefits, including improvements in longevity, insulin sensitivity, weight loss and heart health. These possible benefits are compelling, but Harbstreet notes that these claims are rooted in studies involving mice, with limited or inconclusive data involving humans, so more research may be necessary to fully understand intermittent fasting's impact.
If weight loss is your goal, note that a 2017 study of 100 adults found the weight-loss benefits of intermittent fasting to be comparable with a calorie-restricted diet. But different plans work for different people.
"Some people may be drawn to an intermittent fasting diet because it doesn't prescribe what or how much to eat, and therefore can be adopted or personalized to best meet someone's specific needs, taste preferences or schedule," says Harbstreet.
She also says certain people may benefit from intermittent fasting, including those with demanding job schedules and limited time to cook or prep meals. Narrowing down that eating window decreases the time you have to spend in the kitchen.
However, Harbstreet prefers intuitive eating plans that don't include food restrictions, as restrictions can be harmful to someone's relationship with food. "Periods of fasting can impair a person's ability to listen to their body and respond appropriately to sensations of hunger and fullness," she says. In particular, those with an eating disorder, for example, or someone who is pregnant or breastfeeding are not good candidates for an intermittent fasting diet.
Also, practicing fasting while traveling, attending social gatherings, or celebrating holidays and special occasions may be difficult and ultimately not sustainable. "If one's dieting practices come at the expense of their social relationships and mental or emotional health, any potential physical benefits may be minimized or negated," says Harbstreet.
THE BOTTOM LINE
Intermittent fasting is generally regarded as a safe eating plan, provided you hit your body's nutrient targets, but it's not for everyone. "The best fasting strategy I can recommend is to prioritize getting a full night's sleep," says Harbstreet. Throw in bedtime and morning routines, and most people naturally fast for at least 8-10 hours by default.
Originally published September 2021, updated February 2023
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