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Intermittent Fasting, Explained

Intermittent Fasting (IF) is an eating pattern that cycles between periods of eating and fasting. It has gained popularity as a tool for weight loss and improving metabolic parameters. Instead of focusing on what and how much you eat, you establish a period of time when you eat and a period of time when you don’t eat. Shortening the period of time when you eat (and prolonging the period of time when you don’t eat) may help reduce overall caloric intake without strict calorie counting. There are several types of IF. The three most popular types are Alternate Day Fasting, 5:2 Fasting, and Time-Restricted Feeding.

  • Alternate day fasting means fasting for 24 hours and then eating freely for the next 24-hours. Cycles between fasting and eating are repeated consecutively over the course of a week, so over the course of a month you end up fasting 3-4 days/week. Fasting days can be complete fasts (water, black coffee, tea without milk or sugar are allowed, and definitely encouraged in the summer months), or they can be days when you restrict calories to 500-600 calories/day.

  • The 5:2 diet is when you fast for 2 days (again, either water/coffee/tea or 500-600 calories/day) of the week, and 5 days of the week you eat normally. For example, you might restrict calories to 500-600 kcal per day on Sunday, eat normally on Monday and Tuesday, fast again on Wednesday, and eat normally on Thursday, Friday and Saturday.

  • Time-Restricted Feeding is the most popular type of IF. This IF method designates eating and non-eating periods of time every day. For example, you may eat normally between 10am and 6 or 8pm, and then fast from 6 or 8pm until 10am the next day. The eating period can be reduced to 6 hours over time. During the fasting period, you can drink water, black coffee, or tea.

The commonality between all of these IF strategies is that the fasting period is long enough to get your body to metabolically enter the “fasted state.” We enter the fasted state about 12 hours after our last meal. Since many of us are constantly surrounded by food and have very small, frequent snacks without even realizing it, it’s rare to be in this “fasted state”. However, the ability to cycle between the fed and fasted state was selected for over the course of evolution because food accessibility was often unpredictable. Thousands of years ago, we did not wake up to a breakfast buffet!

There are many benefits to IF. First, fasting is a type of stress because it causes dietary energy restriction. Low dose of stress stimulates DNA repair and may therefore prevent the development of some diseases[1]. Next, when you enter the fasted state, your body starts to “clean house” by breaking down and recycling unnecessary proteins and damaged cells[2]. Third, alternating between the fasting and fed states helps regulate your circadian rhythm, which drives your organs to work differently depending on the time of day. Regular cycling between the fed and fasted state keeps circadian and eating rhythms synchronized [2]. IF can also increase insulin sensitivity and reduce chronic inflammation.

IF has become a popular health and fitness intervention for more than weight loss [3]. Studies in cells and animals have demonstrated that IF has positive effects on brain and body function, and may even increase disease-free lifespan [4]. In humans, time-restricted eating was found to be better tolerated than other typical diets because it does not involve counting calories (and people in fact unintentionally ate fewer calories) [5].

Some criticisms of IF are that it can lead to low blood sugar, irritability, lack of focus, and loss of lean body mass. Furthermore, IF is not appropriate for people with type 1 diabetes, women who are pregnant, people with a history of an eating disorder, adolescents who are still growing, and elderly people who are susceptible to muscle loss.

How do you know if you should try IF? Since our bodies evolved to withstand periods of time without eating, it is probably safe for most people. But you need to decide if it is sustainable for you. As with any dietary change, IF will have the greatest effect if you also prioritize eating high quality food and getting regular exercise.


1. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39: 46.

2. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017;37. doi:10.1146/annurev-nutr-071816-064634

3. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res. 2014;164: 302–311.

4. Valter D. Longo MPM. Fasting: Molecular Mechanisms and Clinical Applications. Cell Metab. 2014;19: 181.

5. Gabel K, Hoddy KK, Haggerty N, Song J, Kroeger CM, Trepanowski JF, et al. Effects of 8-hour Time Restricted Feeding on Body Weight and Metabolic Disease Risk Factors in Obese Adults: A Pilot Study. Nutrition and healthy aging. 2018;4. doi:10.3233/NHA-170036

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