Intermittent Fasting

The Promise

Why cut back every day if you could drop pounds by watching what you eat only a couple of days a week? That's the logic behind intermittent fasting, a weight loss approach that's become more popular over the past few years.

There are different versions, but the general idea for all of them is that you eat normally some days of the week and drastically reduce your calories on other days.

Some plans encourage you to skip food entirely for up to 24 or 36 hours at a time. On others, such as the Every Other Day Diet and the 5:2 Fast Diet, you can have some food but only get about one fourth of your regular calories.

Some research shows that intermittent fasting works. In one study, overweight adults who cut calories by 20% every other day dropped 8% of their body weight within 8 weeks. They also had less inflammation.

The possible secret behind the diet's health-boosting benefits: Fasting puts your cells under a mild stress. Scientists think that the process of responding to this stress, on your low-calorie days, strengthens cells’ ability to deal with stress and potentially fight off some diseases.

What You Can Eat and What You Can't

You can eat mostly what you want on days when you don’t fast. But to lose weight and get the nutrients you need, you should stick to healthy foods and limit treats like dessert and processed foods.

On fasting days, you’ll eat very little food or none at all.

For example, the Every Other Day Diet says to eat no more than 500 calories during each fast day.

Another program called the 5:2 Fast Diet involves eating 5 days a week and fasting for the other 2 days, when women can get no more than 500 calories and men no more than 600. That’s a quarter of the amount you likely eat on the days when you don’t fast. Whether you eat those calories in one sitting or spread them across micro-meals throughout the day is up to you.

Level of Effort: Hard

Limitations: It’s not easy to skip most of your calories a few days a week and rely mostly on water, coffee, and tea to keep you feeling full. You’ll need a balanced meal plan to eat in moderation on your so-called "feast" days, despite their name. You can indulge in a treat occasionally, but that's about it if you want to see results.

Cooking and shopping: You can continue your regular cooking and shopping, as long as you stick to mostly healthy foods.

Packaged food and meals? No.

In-person meetings: No.

Exercise: How much you exercise is up to you. But obviously, you’re not going to have a lot of energy for that on your fasting days. The creators of the Every Other Day diet studied people doing cardiovascular exercise (like biking) while on the alternate-day fasting plan and found that they were able to maintain muscle mass while fasting.

Does It Follow Restrictions/Preferences?

You choose what food you eat, so you can make it work with food restrictions -- whether you're vegetarian or vegan, high- or low-carb, avoiding fat, etc. But it's worth noting that you could have side effects like fatigue, weakness, and headaches.

What Else You Should Know

Cost: None beyond your shopping. In fact, because you will eat much less 2 to 4 days per week, your grocery costs should go down.

Support: There are several books and websites detailing variations on the basic idea of fasting a few days a week. So though there's no single destination for support, there are plenty of resources once you've decided which version of the plan appeals most to you.

What Laura Martin, MD, Says

Most of the intermittent fasting diets recommend cutting back to 500-600 calories on fasting days. In general, for many people this would be medically safer and easier than not eating at all on those days.

Remember to drink enough on fasting days to prevent dehydration. And you’ll need to eat a healthy diet on days that you don’t fast.

Does It Work?

Several studies looking at intermittent fasting diets do show at least short-term weight loss after following the diet for several weeks.

Will the weight loss will last over a longer time? That's not clear.

Is It Good for Certain Conditions?

Some research shows that this type of diet may curb symptoms of asthma and improve cholesterol levels, which could make heart disease likely.

Also, some studies, but not all, show improvement in the body’s use of insulin.

If you have medical conditions, talk with your doctor before you try intermittent fasting.

This diet is not recommended for children, pregnant women, people with eating disorders, and some people with diabetes.

The Final Word

Following an intermittent fasting diet that recommends eating 500-600 calories on fasting days may work and be healthy for some people.

WebMD Diet A-Z Reviewed by Melinda Ratini, DO, MS on July 27, 2016

Sources

SOURCES:

Johnson, J. Free Radical Biology and Medicine, March 2007.

Harvie, M. International Journal of Obesity, May 2011.

Varady, K. The American Journal of Clinical Nutrition, July 2007.

Halberg, N. The Journal of Applied Physiology, December 2005.

Heilbronn, L. Obesity Research, March 2005.

Heilbronn, L. The American Journal of Clinical Nutrition, January 2005.

News releases, National Institutes of Health.

The Every Other Day Diet.

The 5:2 Fast Diet.

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