Understanding Intermittent Fasting

intermittent fasting 16:8Susan Bowerman M.S., RD, CSSD, CSOWN, FAND – Sr. Director, Worldwide Nutrition Education and Training, Herbalife Nutrition


Intermittent fasting is the most recent diet trend that’s grabbing headlines lately. Advocates will often share stories about weight loss, better health, and even more energy, but it’s important to know the basics before trying something new.

Intermittent fasting, or IF, is a dietary pattern in which periods of eating are alternated with periods of fasting. In reality, most of us do this to some extent every day – we eat during our waking hours, and we fast while we are asleep – but current trends in IF involve either longer stretches without eating or a more restricted calorie intake during the fasting period.

Potential Benefits of Intermittent Fasting

A review article from Johns Hopkins University School of Medicine claims that intermittent fasting may offer some potential benefits, including:

  • Boosting verbal memory
  • Improving blood pressure and resting heart rates
  • Promoting the loss of body fat, particularly abdominal fat, thus helping reduce the risk of diabetes and obesity
  • Reducing chronic inflammation in the body, which has been linked to the development of many common chronic diseases.

IF is thought to be beneficial because certain aspects of the body’s repair processes are done more effectively in the fasted state.

Since most of us spend many hours in the fed state (usually 12 hours or more), some researchers believe that intermittent fasting could optimize the lifespan – or the “health span,” which refers to the period of life during which you are healthy.

How to Get Started on Intermittent Fasting

Generally speaking, there are two major approaches to intermittent fasting: time-restricted fasting and periodic fasting. Regardless of approach, the overall quality of the diet is key, since total calorie intake is generally going to be less than with a conventional eating pattern. That means that every calorie really counts and needs to be nutrient-packed.

Time-restricted eating (TRE)

In time-restricted eating (TRE), all your eating episodes are compressed into a shortened time interval, usually about 8 hours, but you eat every day. For example, in a 24-hour day, all your food intake would take place between, say, 10 AM and 6 PM.

Periodic fasting

With a more periodic approach to fasting, some form of fasting takes place at least once a week. Some people fast completely once or twice a week; others may fast every other day.  Another method calls for a very low-calorie diet twice a week with normal eating during the other five days (5:2 fasting). Generally speaking, about 500-600 calories are taken in on the two low-calorie days.

The main difference between these two approaches is the regularity with which you eat. With time-restricted feeding, you are eating fairly normally, but within a shorter time span over a 24-hour period – and you’re not fasting on any days.

With periodic fasting, there are days where you consume significantly less food (only about 500 calories) for a couple of days per week, or there may be up to three days during the week where you skip eating entirely.

Is one fasting method better than the other?

Most people find TRE easier to follow since it doesn’t require days of very-low-calorie intake – or no intake at all. One advantage of TRE is that it generally leads to modest calorie control without the need to count calories, simply because there are fewer hours during which you can eat.

This obviously would reduce, for example, late-night snacking for many people – which could result in significant calorie savings. On the other hand, some people adopt periodic fasting because they find it easier to just skip food altogether a couple of times a week.

Advice for Beginners

Here are some useful general tips if you’re looking to try IF for the first time:

  • Don’t assume that since you’re fasting part of the time, that you can eat whatever you want (in whatever amount) the rest of the time. It’s a common mistake, and it won’t help you get the results you’re after.
  • No matter which method you follow, the quality of your calories is really important. You need to pack in all the nutrients your body needs, but you have fewer calories with which to do that, so every bite really counts.
  • Chose nutrient-dense foods to make sure you’re getting significant nutrients per calorie. This includes lean proteins, low-fat dairy foods, vegetables, fruits, whole grains, and beans. Protein powders can be helpful in meeting protein needs when you’re taking in fewer calories and can be added to smoothies, yogurt, oatmeal, and soups.
  • On days where you restrict calories significantly – or skip eating altogether – you may have low energy, headaches, lightheadedness, or even some digestive discomfort. If you do choose to skip food for an entire day, make sure you consume plenty of fluids.
  • After a full fast, start the next day with light, healthy but easy-to-digest foods (like yogurt, smoothies, or eggs) and then ease your way back into your regular diet. And make sure you stay well-hydrated.