Intermittent Fasting (IF) refers to dietary eating patterns that involve not eating or severely restricting calories for a prolonged period of time. There are many different subgroups of intermittent fasting each with individual variation in the duration of the fast; some for hours, others for day(s). This has become an extremely popular topic in the science community due to all of the potential benefits on fitness and health that are being discovered.

What is Intermittent Fasting (IF)?

Fasting, or periods of voluntary abstinence from food has been practiced throughout the world for ages. Intermittent fasting with the goal of improving health relatively new. Intermittent fasting involves restricting intake of food for a set period of time and does not include any changes to the actual foods you are eating. Currently, the most common IF protocols are a daily 16 hour fast and fasting for a whole day, one or two days per week. Intermittent fasting could be considered a natural eating pattern that humans are built to implement and it traces all the way back to our paleolithic hunter-gatherer ancestors. The current model of a planned program of intermittent fasting could potentially help improve many aspects of health from body composition to longevity and aging. Although IF goes against the norms of our culture and common daily routine, the science may be pointing to less meal frequency and more time fasting as the optimal alternative to the normal breakfast, lunch, and dinner model. Here are two common myths that pertain to intermittent fasting.

Myth 1 - You Must Eat 3 Meals Per Day: This "rule" that is common in Western society was not developed based on evidence for improved health, but was adopted as the common pattern for settlers and eventually became the norm. Not only is there a lack of scientific rationale in the 3 meal-a-day model, recent studies may be showing less meals and more fasting to be optimal for human health. One study showed that one meal a day with the same amount of daily calories is better for weight loss and body composition than 3 meals per day (1). This finding is a basic concept that is extrapolated into intermittent fasting and those choosing to do IF may find it best to only eat 1-2 meals per day. 

Myth 2 - You Need Breakfast, It's The Most Important Meal of The Day: Many false claims about the absolute need for a daily breakfast have been made. The most common claims being "breakfast increases your metabolism" and "breakfast decreases food intake later in the day". These claims have been refuted and studied over a 16 week period with results showing that skipping breakfast did not decrease metabolism and it did not increase food intake at lunch and dinner (2). It is still possible to do intermittent fasting protocols while still eating breakfast, but some people find it easier to eat a late breakfast or skip it altogether and this common myth should not get in the way. 

Types of Intermittent Fasting:

Intermittent fasting comes in various forms and each may have a specific set of unique benefits. Each form of intermittent fasting has variations in the fasting-to-eating ratio. The benefits and effectiveness of these different protocols may differ on an individual basis and it is important to determine which one is best for you. Factors that may influence which one to choose include health goals, daily schedule/routine, and current health status. The most common types of IF are alternate day fasting, time-restricted feeding, and modified fasting. 

1. Alternate Day Fasting:

This approach involves alternating days of absolutely no calories (from food or beverage) with days of free feeding and eating whatever you want. Three weeks of a typical schedule with alternate day fasting may look like this: 

This plan has been shown to help with weight loss, improve blood cholesterol and triglyceride (fat) levels, and improve markers for inflammation in the blood (3). 

The main downfall with this form of intermittent fasting is that it is the most difficult to stick with because of the reported hunger during fasting days. 

2. Modified Fasting - 5:2 Diet

Modified fasting is a protocol with programmed fasting days, but the fasting days do allow for some food intake. Generally 20-25% of normal calories are allowed to be consumed on fasting days; so if you normally consume 2000 calories on regular eating days, you would be allowed 400-500 calories on fasting days. The 5:2 part of this diet refers to the ratio of non-fasting to fasting days. So on this regimen you would eat normally for 5 consecutive days, then fast or restrict calories to 20-25% for 2 consecutive days. Three weeks of a typical modified fasting 5:2 diet may look like this (using 2000 calories per day as an example):

This protocol is great for weight loss, body composition, and may also benefit the regulation of blood sugar, lipids, and inflammation . Studies have shown the 5:2 protocol to be effective for weight loss, improve/lower inflammation markers in the blood (3), and show signs trending improvements in insulin resistance (4).  In animal studies, this modified fasting 5:2 diet resulted in decreased fat, decreased hunger hormones (leptin), and increased levels of a protein responsible for improvements in fat burning and blood sugar regulation (adiponectin) (4).

The modified 5:2 fasting protocol is easy to follow and has a small number of negative side effects which included hunger, low energy, and some irritability when beginning the program. Contrary to this however, studies have also noted improvements such as reduced tension, less anger, less fatigue, improvements in self confidence, and a more positive mood (3). 

3. Time-Restricted Feeding:

If you know anyone that has said they are doing intermittent fasting, odds are it is in the form of time-restricted feeding. This is a type of intermittent fasting that is used daily and it involves only consuming calories during a small portion of the day and fasting for the remainder. Daily fasting intervals in time-restricted feeding may range from 12-20 hours, with the most common method being 16/8 (fasting for 16 hours, consuming calories for 8). For this protocol the time of day is not important as long as you are fasting for a consecutive period of time and only eating in your allowed time period. For example, on a 16/8 time-restricted feeding program one person may eat their first meal at 7AM and last meal at 3PM (fast from 3PM-7AM), while another person may eat their first meal at 1PM and last meal at 9PM (fast from 9PM-1PM). This protocol is meant to be performed every day over long periods of time and is very flexible as long as you are staying within the fasting/eating window(s). Below are two examples of a 3 day schedule in which the previously mentioned 16/8 plans could be implemented: 

Time-Restricted feeding is one of the most easy to follow methods of intermittent fasting. Using this along with your daily work and sleep schedule may help achieve optimal metabolic function. Time-restricted feeding is a great program to follow for weight loss and body composition improvements as well as some other overall health benefits. The few human trials that were conducted noted significant reductions in weight, reductions in fasting blood glucose, and improvements in cholesterol with no changes in perceived tension, depression, anger, fatigue, or confusion (3). Some other preliminary results from animal studies showed time restricted feeding to protect against obesity, high insulin levels, fatty liver disease, and inflammation (5).

The easy application and promising results of time-restricted feeding could possibly make it an excellent option for weight loss and chronic disease prevention/management. When implementing this protocol it may be good to begin with a lower fasting-to-eating ratio like 12/12 hours and eventually work your way up to 16/8 hours.

Common Question About Intermittent Fasting:

Is there any food or beverage I am allowed to consume while intermittent fasting? Unless you are doing the modified fasting 5:2 diet (mentioned above), you should not be eating or drinking anything that contains calories. Water, black coffee, and any foods/beverages that do not contain calories are OK to consume during a fasting period. In fact, adequate water intake is essential during IF and some say that drinking black coffee while fasting helps decrease hunger. 

If you just want the benefits:

Research on intermittent fasting is in it's infancy but it still has huge potential for weight loss and the treatment of some chronic disease (3).

To recap, here are the possible benefits of intermittent fasting:

Shown in Human Studies (3):

1. weight loss

2. Improve blood lipid markers like cholesterol

3. Reduce inflammation

4. Reduced stress and improved self confidence

5. Improved mood

Shown in Animal Studies:

1. Decreased Body Fat (4)

2. Decreased levels of the hunger hormone leptin (4)

3. Improve insulin levels (5)

4. Protect against obesity, fatty liver disease, and inflammation (5)

5. Longevity (6)