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  • Marie Belzile-Davidson, MS, RDN, LD

The Pros and Cons of Intermittent Fasting

How many different diets and ways of eating can you think off of the top of your head? I am willing to bet it is seemingly too many, right? There is low-fat, keto, paleo, Whole30, low-carb, vegetarian, vegan, pescatarian, DASH, Mediterranean, raw, etc. How about intermittent fasting - did that come to mind?

Intermittent fasting is a hot topic in the world of dietetics. It has been for a few years now and it does not look like it is going away any time soon. What is it though? Is it beneficial? If so, for who? How do you properly do it? We will walk through all of these.

Fundamentally, intermittent fasting is eating within a limited window. That is it. You may also hear it referred to as "time-restricted eating." There are seemingly infinite ways to intermittent fast and not all ways are created equal. Here are just a few of the many intermittent fasting variations out there:

  • 12 or 14-hour overnight fast: Not eating for 12-14 hours between the time you have your last meal of the day to the time you have your first meal the following day (exp: eating dinner at 6 pm then not eating breakfast until 6 to 8 am the next morning).

  • alternate day fasting: Eating normally one day but only a minimal amount of calories the next day and continuing to alternate between the two.

  • 5:2 eating pattern: Eating normally five days per week then restricting to no more than 600 calories per day for the other two days.

  • periodic fasting: Restricting caloric intake for several consecutive days then eating normally all other days (exp: restricted intake for five straight days per month).

Some of the aforementioned forms of intermittent fasting are quite intense while others (such as the 12-hour overnight fast) likely happen naturally for some individuals. Is any form of intermittent fasting beneficial though?

Benefits and Drawbacks of Intermittent Fasting

I first want to say that fasting overnight for roughly 12 hours (between dinner and breakfast) may be beneficial for most adults, regardless of biological sex - it allows your digestive tract time to rest. Why does it need rest? Well, this gives your migrating motor complex time to do its job - it is essentially the cleaning crew of your intestines that pushes through any food that has been left behind to prevent bacteria from growing where it should not. This rest also allows your intestinal cells to repair or renew themselves. These cells are crucial to overall health - they are the ones that come into contact with the food you eat and absorb all the nutrients you need from your snacks and meals. If they are injured and are not given time to repair themselves, they cannot properly do their job. They are also the cells that keep your food in your digestive tract (where it should be). Without proper rest and renewal, the barrier these cells form can start to break down and leak (more on leaky gut here).

Ok, moving on to some additional potential benefits of intermittent fasting. Some research studies have shown that intermittent fasting can help people achieve weight loss, can prevent some diseases, can slow aging, reduce markers of inflammation (C-reactive protein), help with blood sugar level management, may help build exercise endurance, and can support healthy immune function. That sounds pretty great, right?

We need to put this into context though - many of the clinical studies from which this data has been gathered have been short (a few months or less) and were mostly conducted on men. We cannot directly apply this to women and assume the same results. We also cannot assume that the same results achieved within a few months will have the same positive impacts long term. That means, there is more research that needs to be done if we are to better understand how intermittent fasting impacts women and impacts all humans on a long-term basis.

While research has shown us some benefits, there are some drawbacks to intermittent fasting that should be considered. Extended fasting periods (I am talking past 12 to 14 hours between dinner and breakfast the next day) can stress the body and raise cortisol levels. Reminder: too much stress is unhealthy and can cause unnecessary inflammation (more on that here). If you are someone that already experiences a lot of stress in daily life, intermittent fasting may not be the right call. Additionally, prolonged periods of not eating may slow your metabolism and could lead to overeating during the times you are not fasting. This could result in net-zero health benefits or even worsened health (exp: unnecessary weight gain, increased inflammation, skewed hunger cues, sluggish metabolism, etc.).

There are benefits and drawbacks to all things in life, and dietary choices are not exempt from that. Overall, the entire person needs to be taken into account when determining if intermittent fasting may be more helpful or harmful. Additionally, more research is needed that includes women and looks at the long-term impacts of intermittent fasting. Research is continuously being conducted so keep an eye out for more information to come out on intermittent fasting over the next several years.

What is happening when I fast?

Our bodies have survival mechanisms that allow us to adjust to periods of fasting. It makes sense - food was not as readily available for our distant ancestors as it is today.

Just keep in mind that while our bodies have a backup function for periods of fasting, that does not mean it is something we should try to function off of regularly. Sure, early humans may have periodically fasted but they lived in a much different society than us - we cannot directly apply how they lived to our current world.

So what happens when we do not eat a sufficient amount of calories? Our bodies first turn to stored sugar in the form of glycogen (we have some in our liver and muscles). Why? Our body's preferred fuel source is carbohydrates. The liver uses its glycogen to supply sugar to the rest of the body but muscle glycogen is different - it is only used by your muscles and cannot supply the rest of your body with energy. The average person has enough glycogen in their liver to last through 12 to 16 hours of fasting.

Side note: this means your liver only has the capacity for enough glycogen to get you through a fast from dinner to breakfast the next morning (12 to 14 hours later). If you like to get your movement in during the early hours of the morning (a.k.a. before you have breakfast), consider having a small snack with carbohydrates and possibly a little protein 10 to 15 minutes before you start your exercise. This will give your body some of the carbohydrates it needs to fuel your movement and will take some stress off of your fasted body.

Once those liver glycogen stores have been emptied, if your body does not get food from you eating, it will start to use fat as its energy source. The fat stores in your body are metabolized into biochemicals known as ketones. These ketones can be used as a source of fuel. Using ketones rather than carbohydrates as fuel is known as ketosis. This is your body's "backup" system to keep things running when the carbohydrate tank is empty.

When our bodies shift to a different fuel source, other changes that happen throughout the body. I will spare you a biochem lesson but these changes are partially behind some of the benefits that have been seen in research on ketosis. Some benefits include protection against Alzheimer's, decreased inflammation, and a decrease in seizures for individuals with epilepsy. Again though, there are benefits and drawbacks to all things in life. Too many ketones can be harmful and most of the research done on ketosis has been short-term. We have yet to have a better understanding of the long-term impacts of ketosis.

Before you start intermittent fasting

As with any major dietary change, be sure to discuss what is right for you and your needs with a registered dietitian. If you have been following along with our blogs, I may sound like a broken record by now - nutrition is highly individualized. You are unique!

Generally speaking, there are certain health conditions, medications, and stages of life that may make intermittent fasting more dangerous. These include type 1 diabetes, diuretics for high blood pressure or heart disease, being under 18 years old, being pregnant or breastfeeding, and more.

If you are interested in learning more about intermittent fasting and whether or not it may benefit you, consult a registered dietitian to help you figure out what is best for your unique needs!


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National Institutes of Health National Institute on Aging. (2018, August 14). Calorie Restriction and Fasting Diets: What Do We Know? Retrieved from

National Institutes of Health NIH Research Matters (2015, July 13). Health Effects of a Diet that Mimics Fasting. Retrieved from

National Institutes of Health NIH Research Matters. (2017, September 26). Calorie restriction slows age-related epigenetic changes. Retrieved from

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NIH Intramural research program. (2018, March 13). Intermittent Fasting Boosts Endurance in Mouse Marathoners. Retrieved from

NIH National Center for Advancing Translational Sciences. (2018, August). NCATS-Supported Study Shows Eating Before 3 p.m. Can Improve Health. Retrieved from

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