How popular is fasting?
Americans keep gaining weight and we’re always searching for ways to stop. Between 2000 and 2018, the prevalence of obesity in U.S. adults jumped from 30.5 percent to 42.4 percent, according to the Centers for Disease Control and Prevention (CDC).
Hoping to lower the numbers on the scale, 43 percent of Americans followed a specific diet or eating plan in 2020, up from 38 percent just a year before. The most popular option was intermittent fasting.
Why do people fast?
Many cultures around the world fast for spiritual reasons, but fasting as a health measure has become popular because research suggests it may offer metabolic benefits and possibly lengthen lifespan.
And because you’re usually eating only within a specific window, you tend to consume fewer calories than you would if you were eating around the clock, says Julie Upton, RD, a registered dietitian and cofounder of the nutrition news company Appetite for Health, in San Francisco. That, of course, can leads to weight loss.
According to a study, published in 2019 in The New England Journal of Medicine, the biggest benefits of fasting may come from what’s known as metabolic switching. That’s when the body changes from using energy from glucose that’s stored in the liver to ketones stored in fat. (Sound familiar? It’s the concept behind the keto diet.) Researchers believe ketogenesis, an increase in ketones in the blood, may trigger cellular signals that slow aging, reduce inflammation, and promote better blood sugar control.
Scientists are still trying to get a handle on how intermittent fasting impacts health over the long-term. And they’ve barely scratched the surface in terms of what happens when people fast while already at a healthy weight. Some of the latest studies have been observational (they rely on participants’ diet logs) or performed on mice. So the findings are often considered preliminary.
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“I recommend the most gentle form of intermittent fasting—time-restricted eating—for those looking for a way to support weight loss and improve metabolic health,” says Leigh Merotto, RD, a Toronto-based registered dietitian with a focus on metabolic health, digestion/gut health, and sports nutrition.
But, she adds, that’s only after “other measures have been tried first, such as improving general eating habits, moving more, or trying other stress-management strategies.”
To be sure: Fasting is not for everyone. It’s not recommended for people with diabetes who are taking medication to control their blood sugar, people with seizure disorder, or women who are pregnant or breastfeeding, for example. It’s always a good idea to check with your doctor before you start any new eating plan.
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Most popular types of fasting
Intermittent fasting is any eating plan that restricts when you do—and don’t—eat. There are several different intermittent fasting schedules:
- Alternate-day fasting. Fast every other day.
- Time-restricted eating. Eat within a restricted time range each day. For the popular 16:8 method, you fast for 16 hours and eat within an eight-hour window (say, 10 a.m. to 6 p.m.). For the 14:10 method, you fast for 14 hours and eat within a 10-hour window.
- Eat Stop Eat. Fast for up to 24 hours once or twice a week. On the other days, eat as you normally would.
- Modified fasts or 5:2 fasting. A spin-off of alternate-day fasting, eat as you normally would five days of the week and fast for two days. Here’s more about the different alternate-day fasting schedules.
Eating, drinking, and fasting
On fasting days, most people choose to consume only about 500 calories or 25 percent of their normal caloric intake. That means very little food, but lots of water and some other zero-calorie beverages. Some people forget that drinks can contain calories too, Merotto says.
Any drink (or food, for that matter) with macronutrients—carbs, fat, or protein—would push the body out of a fasting state, she adds. Because these nutrients contain calories, they promote an insulin response which alerts your body that you are no longer fasting.
“The basic idea of intermittent fasting is that you want to have periods where you are not consuming calories in the form of foods or beverages,” says Katherine Brooking, MS, RD, cofounder of Appetite for Health. “Your beverages should be calorie-free or very close to it,” like with coffee, cucumber water, lemon water, or tea.
Feel free to consume caloric beverages such as smoothies, juices, alcohol (in moderation), and shakes on your non-fast days and in your eating window. Just be sure to include the calories in your overall calorie count. (Learn more about how alcohol can affect your fast.)
What you can drink during fasting periods
Merotto gives a thumbs up to the following:
Water “It’s naturally calorie-free, and should be consumed during a fasting portion to support optimal hydration, for energy, and digestive health,” she says.
Lemon water If you crave some flavor with your hydration, “lemon juice has relatively no carbs compared to other fruit juices, so it can be used as a way to flavor water without breaking a fast,” says Merotto.
Apple cider vinegar Some experts don’t allow apple cider vinegar because it has calories (3 per tablespoon). Merotto gives it a green light, but recommends mixing it with water or unsweetened tea, as the high amount of acidity can damage the enamel on your teeth. (Lemon water can also be too acidic for your tooth enamel.)
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Unsweetened tea or coffee Herbal or decaf teas are okay if you brew them in water and add no other mixers. If you choose caffeinated varieties, don’t overdo it, Merotto warns. “Too much caffeine can cause poor sleep and irritability,” she says. “This is especially true if consumed on an empty stomach,” which is what you’ll have when fasting.
Bone broth This one is actually a “maybe.” “Bone broth does contain small amounts of fat and calories which could break a fast,” says Merotto. “But on longer fasts—like the 24-hour fast—some fat may be okay as it can keep the body in a ketosis state.” Limit yourself to very small amounts.
Skip these drinks when fasting
Merotto gives a thumbs down to these beverages during fasting windows:
Sweetened tea or coffee Once you add sugar, flavored syrup, milk or cream, these drinks—decaf or caffeinated—will officially break your fast.
Alcohol Beer, wine, cocktails, and spirits are a “hard no” for Merotto. These all contain calories and should be consumed only in moderation, if at all, during the non-fast portion. If you’re following a calorie-limited fast, note that alcohol will replace some of your food calories.
Diet soda or soda “While most diet sodas are technically ‘calorie free,’ we don’t have enough evidence as to whether they should be consumed during a fast,” says Merotto. “There is some evidence that artificial sweeteners such as sucralose and aspartame often contained in diet sodas may cause a rise in insulin response due to a chemical reaction in the brain similar to when actually sugar is consumed.” More research is needed, but neither diet sodas nor their sugar-laden regular soda counterparts have been shown to be supportive of weight loss goals, she says.
Protein shakes or smoothies Protein supplements and powders contain calories and will trigger an insulin response, breaking a fast, Merotto says. “They can, however, be a good way to break a fast as protein is very satiating, and can act as a quick and convenient recovery drink for those who work out in the mornings before ending their fast.”