Frequently Asked Questions About Intermittent Fasting

What is “intermittent fasting”?

“Intermittent fasting” (IF) is a term used to describe a technique of fasting and feeding on a periodic schedule.  It’s most frequently used as a weight loss approach, but may have other benefits as well such as improved fat metabolism and improved insulin resistance among others.

While IF can be used in conjunction with other dietary restrictions, it places no particular constraints on what kinds of foods are eaten, or how much is eaten.  In practice, however, eating reasonable portions of healthy foods is obviously a good idea in conjunction with IF.

Technically speaking, in particular when used in research contexts, “intermittent fasting” may sometimes refer to more specific techniques involving longer fasting periods of a day or more.  In popular usage, however, “intermittent fasting” is used to refer to any schedule of fasting and feeding, regardless of how long the fasting period lasts.  This can sometimes be confusing, but on this site, we will use the terms mostly interchangeably and specify explicitly if we are referring to a particular fasting schedule.

What is “time restricted feeding”?

“Time restricted feeding” is a more technical term for techniques in which the fasting and feeding periods are part of a day, for example, 20 hours of fasting and 4 hours of feeding (as opposed to full or multiple day fasts).  As mentioned in the previous question, we will use the term “intermittent fasting” to refer to the more general concept on this site, and generally avoid using “time restricted feeding” unless we are making a specific distinction between the two techniques.

Are “intermittent fasting” and “time restricted feeding” the same thing?

As described in the definitions above, they are technically two different things based on the duration of the fasts involved, but in popular usage they are often used interchangeable.  On this site, we will use the term “intermittent fasting” more generally to refer to any technique involving periods of fasting and feeding 

What are the different kinds of intermittent fasting?

There are many different variations on intermittent fasting schedules.  A popular and effective variation is to use a daily feeding and fasting schedule, for example, 16 hours of fasting and 8 hours of feeding.  Or 20 hours fasting and 4 hours of feeding.  A more “intense” variation involves fasting for one or more days at a time, such as 5 days of eatings and 2 days of fasting.

There’s no one-size-fits-all to intermittent fasting, and the variation one chooses should be based on lifestyle, health considerations, personal preferences,  and personal goals.  But the general consensus is that a minimum fasting period of 16 hours is needed to reap the core benefits of intermittent fasting.

What do people mean by 16:8 or 20:4 or 5:2 or OMAD?

People often refer to their personal IF schedules using these numbers & terms.

If they use two numbers that add up to 24 (e.g., 20:4), they’re referring to a daily schedule in which the first number represents the number of hours fasting, and the second number represents the number of hours of feeding.

If they use two numbers that add up to 7 (e.g., 5:2), they’re referring to a weekly schedule in which the first number represents the number days feeding and the second is the number of days fasting (not necessarily consecutive).  Note that the order of the numbers is the opposite of the daily schedule.

OMAD refers to “one meal a day”, sometimes also described as 23:1, for having a single feeding session (one meal) over every 24 hour period.

If there are so many varieties, which is the best kind of intermittent fasting for me?

It really depends on your goals.  Generally speaking, a 16:8 daily schedule is considered the minimum to gain the full metabolic effects of the fasting period.  If your goal is weight loss, many people have more success with shorter feeding windows.

Regardless of which routine you end up on, it’s advisable to ease into intermittent fasting gradually.  So starting with 16:8 or even 14:10 (which is technically shorter than the recommended fasting period) is a great way to get started easing into any kind of IF routine.  Over time you can modify the fasting window as your body adapts.  Jumping immediately into a OMAD routine would not be recommended–start with a smaller fasting window and work your way up.

Is intermittent fasting effective for losing weight?

Yes – many studies have established that IF is effective for losing weight in a variety of different patient populations.  Among others, in this systematic review, researchers reviewed 27 different randomized controlled trials which all showed weight loss in obese and overweight patients compared to baseline.

If you’re skeptical of intermittent fasting because you’ve seen a recent flurry of headlines claiming “no benefit to time-restricted eating“, I implore you to look past the headline and read the content of the research.  Or, read our recent write-up of headlines and the research to understand what it really said.

Are there other benefits to intermittent fasting?

Yes, there are a number of benefits that are attributes to different kinds of fasting.  This review paper details several benefits of different kinds of fasting including:

  • Weight loss
  • Lower insulin levels
  • Lower fasting glucose levels
  • Reduced cholesterol and triglyceride (TG) concentrations
  • Improved gut microbiota 
  • Improved Sleep quality
  • … and much more

Note that this list of benefits is drawn from a variety of different studies involving different experimental designs (patient populations, fasting schemes, etc). Don’t assume that any one particular intermittent fasting routine will impart all of these benefits, but you can use the list as a launching point into further research and to help find a fasting routine that works best for your health and fitness goals.

Are there any dangers to intermittent fasting?

Many research studies have found intermittent fasting to be a safe and effective way to lose weight in a variety patients and  experimental settings.  Still, it’s advisable to consult a medical professional before starting with intermittent fasting or any other major dietary changes.

The information on this site is not intended as medical advice.

What kinds of things are okay to consume during a fast?

Generally speaking, the only things okay to eat during a fast are zero calorie, unsweetened beverages such as water, (black) coffee and tea.  Most people avoid artificial sweeteners as well, as they may cause changes in blood glucose.

Be sure to hydrate well during your fast.  A great deal of water we consume comes from foods we eat, so you’ll have higher hydration needs when fasting.  Similarly, you’ll want to take care to get sufficient minerals when fasting, since you won’t be getting these from food consumption either.

What kinds of things will break a fast?

Anything other than the unsweetened, zero calorie beverages above are considered to be breaking a fast, yes, including a zero calorie breath mint.  

That said, you probably won’t negate the full effects of fasting if you slip up and consume a couple of calories.  

Can I exercise effectively and do intermittent fasting at the same time?

Yes!  Many studies indicate beneficial effects of exercising when fasted.  And ff you’re curious, you can read about my personal experiences with intermittent fasting and marathon training.

However, because fasting can effect energy levels and other metabolic changes, it’s advisable to ease into an exercise and intermittent fasting routine.  Listen to your body and be careful not to overdo it–take it easy until you know how your body will respond. And as always, be sure to consult with a medical professional if you have any concerns, and before you start a new dietary or exercise routine.

Disclaimer

You should consult a medical professional before trying intermittent fasting or making other dietary changes.  The information on this site is not intended as medical advice. All content, including text, graphics, images and information, contained on or available through this web site is for general information purposes only.