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Experts / March 2021
Diana Kelley, Reebok Contributor

What to Know About Intermittent Fasting

This increasingly popular diet claims the timing of your meals is everything when it comes to weight loss and health benefits—but does it actually work?

You probably know someone in your life who has tried voluntary fasting in recent years to lose weight or feel healthier. Admirable, you might think. But not for me. Willing yourself to not eat for a lengthy period of time sounds challenging, to say the least. But intermittent fasting is actually something most people do already—they just don’t know it. For instance, if you finish dinner around 7 p.m. and don’t eat or drink again until your 7 a.m. breakfast, you just did a circadian rhythm fast.
Circadian rhythm fasts, which follow a pattern of 12 hours of eating and 12 hours of no eating, are one type of intermittent fasting. Other types of intermittent fasting include a routine of 16 hours of no eating and eight hours of eating, modified fasting like the 5:2 diet (five days of normal eating followed by two days of eating 500 to 600 calories) and alternate-day fasting (ADF) which means 24-hour fasts every other day and eating normally in a 12-hour window the days in between.
If it sounds a bit complicated, it is—part of a trending approach to health and fitness that involves quantifying and measuring your every move for optimal wellness. But it’s also designed to be a diet that people can stick with longer than a 10-day crash plan. “I think intermittent fasting is popular now because most people are looking for sustainable strategies; things that they can do for a long period of time, as opposed to doing a cleanse for two weeks,” says Cynthia Thurlow, a functional nutritionist in Washington, DC. As people’s understanding of the food-health connection has grown, so has interest in intermittent fasting. “It's gaining popularity as we're starting to appreciate and understand that extending our eating times drives diabetes, and obesity and metabolic syndrome,” she adds.
Increasingly, research is backing up the benefits of intermittent fasting as a tool for weight loss and improvements in diabetes markers, brings legitimacy to the concept, Thurlow says. What’s more, evidence suggests intermittent fasting might also help you sleep better.

You Fast, You Snooze 

If you ever had a late dinner that landed heavy in your belly and made it hard to sleep, you can understand why limiting your meals to certain hours could benefit your zzzs. And in fact, a recent study published in BMC Nutrition found that participants who fasted about 16 hours daily during a three-month period reported better sleep quality and improved overall health. “Intermittent fasting beneficially impacts sleep because while you're fasted, your liver will secrete glycogen overnight to keep your blood sugar stable,” says Thurlow. That prevents the spikes and dips that happen while your body processes an evening snack and which can disturb your sleep.
On the other hand, if you’re fasting too long or not getting enough nutrition during your eating hours, you also may find your sleep disturbed. “There’s a little bit of a delicate balance,” says Thurlow. “But when you find the sweet spot for fasting, you will be able to sleep well because you'll have better digestion.”
“I think the effect intermittent fasting has on sleep depends on the type of fasting the individual is following, the timing of the last meal of the day, and the macronutrient distribution of food, especially at the evening meal,” agrees San Diego-based nutritionist EA Stewart, R.D. “Ideally, I suggest all of my clients stop eating three hours before bed time, or two hours at the minimum to allow for digestion and to lessen the chance of reflux.”
Your evening meal should contain some healthy fats, a little protein, and complex carbs to help stabilize blood sugar levels, says Stewart. You’ll need to cut out alcohol if you’re following this plan, too.

Benefits and Drawbacks of Intermittent Fasting 

While weight loss is probably the main reason so many people are fasting right now—with better sleep as a great side effect—fasting science shows a number of other benefits as well. For instance, following an alternate-day fasting routine that’s paired with exercise can reduce fat mass and waist circumference and improve cholesterol levels, according to research. Other studies find that modified intermittent fasting, like the 5:2 diet where you eat about 500 calories two days a week, can reduce inflammatory markers and improve insulin and lipid profiles.
Still, some studies find participants have less energy and more irritability and hunger when following a 5:2 diet or other intermittent fasting diets. If you’re able to take it easy and lounge in sweatpants during your fasting days or hours, that’s one thing. But most people need to feel on it all week long, and feeling sluggish only adds more stress to their day.
Another drawback: Many people squeeze in their workouts before work, and fueling for those efforts is key. (One study found eating breakfast led to more physical activity earlier in the day in a group of overweight adults.) Whether you’re running or hitting the weights hard, it’s wise to adjust your fasting schedule to accommodate those nutrition needs (even if it means sticking with a 12:12 schedule instead of 16:8).


Starting an Intermittent Fasting Plan

Slow and steady wins when you’re first exploring intermittent fasting. “I don't encourage anyone to go from an eight-hour fast to going to an 18-hour fast because that will set many people up for failure,” Thurlow says. “I suggest starting with 12 hours and slowly opening up that window.” Some people add on an extra 30 minutes every week; others add 30 minutes to their fasting window each day until they reach their desired fast length.
As you begin your intermittent fasting journey, check in with yourself each day, Thurlow suggests. Ask yourself if you feel energized, mentally clear and have a positive outlook. If you feel fine, play with that fasting window and try to extend it, she suggests. If you’re finding your energy low, cut back a little on the hours you fast. Remember, the guidelines for intermittent fasting are just there as parameters. Your success hinges on taking those recommendations and tailoring them to a diet that works best for you.

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Experts / March 2021
Diana Kelley, Reebok Contributor