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Comparing 3 Methods of Intermittent Fasting

Research shows they can help some people lose weight and stay healthy

By Karen Asp

No doubt you’ve heard about intermittent fasting, which is getting buzz for good reason. Growing evidence supports the health benefits of this pattern of eating in which you alternate periods of eating with periods of fasting.

Intermittent Fasting
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Studies show that intermittent fasting not only helps people lose weight, it may also improve brain and heart health and even increase exercise performance, says Mark P. Mattson, chief of the laboratory of neurosciences at the National Institutes on Aging, who has conducted several studies on intermittent fasting.

One big mistake people make when doing intermittent fasting? "They think they can eat whatever they want during their feeding window."

Research is also ongoing in how intermittent fasting might work against specific diseases, including cancer and cognitive impairment.

Interested in trying it? Here’s what you need to know before you put it into action:

Three Methods of Intermittent Fasting

At first glance, intermittent fasting might seem simple, but it’s actually a little more complicated than it sounds. That’s because there are three ways to do it:

The 5:2 fast: With this method, you eat normally for five days during the week. Then for two non-consecutive days, you generally eat less than 600 to 800 calories per day. This type of diet is called “fasting mimicking.”

Time-restricted feeding (TRF): There are several versions within this category. “With TRF, you limit your feeding window to a certain time period, the most common being eight, ten or twelve hours,” says Dr. Ian K. Smith, who writes books about nutrition and diet and who served two terms on President Obama’s Council on Fitness, Sports and Nutrition.

Here's how TRF fasting times would look: 16:8, in which you fast for 16 hours and feed for eight hours; 14:10, in which you fast for 14 hours and feed for 10 and 12:12, where you fast for 12 hours and feed for 12. For instance, with the 12:12 approach, you might stop eating at 7 p.m. and start eating again at 7 a.m.

The 24-hour fast: The premise of this daily diet is simple: “You basically eat dinner every day and then restrict your calories the rest of the day,” says Dr. Richard Firshein, who specializes in integrative, anti-aging and precision medicine.

The 24-hour fast is another example of a fasting mimicking diet in which you consume under 150 to 200 calories for both breakfast and lunch so you’re eating about 400 calories until dinner. After that, you eat a regular dinner.

While Safe for Most, Some Should Be Cautious

What you may not realize is that you’re already doing a form of intermittent fasting. “Every night you go to sleep, you’re technically fasting,” Smith says, adding that when you officially engage in intermittent fasting, you’re simply increasing the fasting period around your sleep.

This is one reason intermittent fasting is generally safe for most people. “For the vast majority of people who are healthy without medical issues, intermittent fasting won’t be counterproductive or deleterious to them.” Smith says.

However, there are some people who should be cautious —  if you have a health issue such as low blood sugar, diabetes or low or high blood pressure. You also need to be cautious if you’re underweight, have a history of eating disorders or you’re taking medications.

“If you’re seeing your doctor for a specific health issue, then you should definitely review (with your doctor) all of the risks before you do intermittent fasting,” Firshein says.


Intermittent fasting is not a good idea for children or pregnant or breastfeeding women either.

Intermittent Fasting: Give It Time

If you’re interested in trying intermittent fasting, consider which method will fit best into your lifestyle and goals for weight and fitness. And regardless of which type you choose, proceed slowly.

“Studies show that it can take up to a month for people to adapt to going each day or a couple of days a week not eating much food,” Mattson says. If you’re used to eating three meals a day plus snacks and you decide to start skipping breakfast, you’ll be hungry in the morning for several weeks. After a month, however, that hunger usually will dissipate, Mattson says.

With TRF in particular, it’s a good idea to gradually shorten your feeding window over a two-month period, especially if you’re shooting for an eight-hour feeding window, he notes.

If you want to try the 5-2 method, start with one day a week for one month. If that goes well, move to two days a week, as long as they’re not on consecutive days, Mattson says.

Eat Healthy and Stay Hydrated

One big mistake many people make when doing intermittent fasting? “They think they can eat whatever they want during their feeding window,” Smith says. Yet nothing could be further from the truth.

If good health is a goal, it’s always important to eat a healthy diet during the feeding window. This includes fruits, vegetables, nuts and fish, while avoiding refined sugars, high fructose corn syrup and saturated fat, Mattson says.

And while you won’t be eating during the fasting window, you should still make sure you’re staying hydrated. Water (even sparkling water or water infused with fresh citrus), coffee and tea are perfectly fine to drink during these periods, Smith says.

Of course, before you begin fasting, check with your doctor and consider the pros and cons. And while intermittent fasting may not be for everybody, there’s no doubt that eating healthier is the key to health for everybody.

Karen Asp Journalist Karen Asp specializes in health, fitness, nutrition, pets and travel. A contributing writer for VegNews and pet columnist for Better Homes & Garden, she writes regularly for numerous leading publications, including O, Reader’s Digest, SELF, Women’s Health, Woman’s Day and Real Simple. She’s also the author of Anti-Aging Hacks. Visit her at or via Twitter and Instagram at @karenaspwriter. Read More
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