Everything You Wanted to Know About Intermittent Fasting from a Dietitian


As a dietitian, I often get asked about intermittent fasting. What is it? Are there any benefits to intermittent fasting (IF)? How long do I have to go without eating? Can I work out when I fast? Who should not do Intermittent Fasting?

We are a 24/7 nation. Food is available about every place you turn. We are bombarded with commercials about fast food and we have delivery companies that will deliver food anytime, anywhere from your favorite restaurants. Food is easily accessible all of the time. And most people find it difficult to fast. 

Over the years as portion sizes have increased, so has our waist size and risk for CVD and type 2 diabetes. In fact, an estimated 88 million Americans (more than 1 in 3) have prediabetes. And most of them don’t know they have it. Pre-diabetes puts you at an increased risk of developing type 2 diabetes, heart disease and stroke. 

Before considering intermittent fasting, check with your doctor, especially if you have been diagnosed with a chronic disease or if you are on certain medications that require you to take them with food. 

What is intermittent fasting?

It involves metabolic switching from one fuel source to another and reduction of cellular stress. The key to intermittent fasting is the timing of the fast. 

The science behind Intermittent Fasting is that when we eat carbohydrates they are quickly broken down into sugars which our cells use for energy. If we eat too many carbs or our cells cannot use what we have eaten, any extra is stored in our liver as glycogen and in our fat cells as fat. The hormone insulin made by the pancreas, helps get the sugar into the cells.

Between our meals, our insulin levels normally will go down and our fat cells will release the stored sugar to be used as energy.  When we let our insulin levels go down, we start burning fat. The goal of IF is to allow the insulin level to go down enough for a longer period of time to help burn off the fat to lose weight. 

How do I start intermittent fasting? How long do I go without eating? 

There are several ways to approach intermittent fasting.

  • You may eat during an 8-9 hour window and then fast the rest of the time. For example: Eat breakfast at 9:00 AM and have your dinner at 5:00 and then do not eat again until the next morning at 9:00. This will be about 16 hours of intermittent fasting, most of it while you are sleeping. This seems to be the easiest way to do intermittent fasting, since you are sleeping through most of the fast. (Unless of course, you are a sleepwalker who wears out a path to the kitchen during your nightly walks.)
  • There is the 5:2 intermittent fasting approach: where you eat regularly for 5 days of the week, then on the other 2 days, you eat one 500-600 calorie meal each day and fast the rest of the day. As a dietitian, I think 500-600 calories/day is too low of an intake. 

Going long periods without food (48-72 hours) is not any better for you and could actually encourage your body to start storing fat because it thinks you are trying to starve it.  

It can take your body several weeks before it gets used to intermittent fasting. Initially you may feel hungry and/or irritable, however people that can get through those weeks report feeling better from intermittent fasting. 

So during the fasting period, can I drink beverages?

Yes, you can drink water, carbonated water without calories, black coffee and tea without milk or sweetener. This does not mean that during your eating period you can pack in the calories and eat junk food or highly processed refined foods and sweets. Eat your normal 3 meals and a snack if that’s what you typically eat. 

Can I workout when I am fasting? 

When fasting your glycogen stores will become depleted and you will start burning fat as fuel, so during your workout you will be burning more fat. But you may also be breaking down the muscle you worked hard to build to use as protein for fuel. And you may have less energy to perform your workout or hit the wall sooner due to lack of available fuel. In my opinion, I would not exercise on the days you fast. 

What are the benefits of intermittent fasting?

A New England Journal of Medicine study found that intermittent fasting in the short term can lead to increased insulin sensitivity and reduced waist circumference and in the long term, increased longevity and decreased incidence of cancer, cardiovascular disease, diabetes and obesity. 

Source: Effects on Intermittent Fasting on Health, Aging and Disease; de Cabo R, Mattonson MP New England Journal of Medicine. nejm.org.  Dec. 2019

Metabolic Effects of Intermittent Fasting. Annual review of Nutrition. August 2017

The Obesity Code, by Jason Fung, MD

Intermittent fasting: Surprising Update- Harvard Health. www.health.harvard.edu 

Susan Cote is a Registered Dietitian/Nutritionist with a Master’s Degree in Dietetics and Nutrition. Susan was hired to start a Nutrition Program for the South Florida YMCA, where they will be offering nutrition and wellness classes for the YMCA members and employees of the Y, consultations and custom meal planning will also be available. She began her career working as a clinical dietitian with Baptist Health South Florida, then transitioned over to preventative and wellness nutrition. Susan taught culinary and nutrition classes at Johnson & Wales Culinary School and also worked for Florida Blue providing nutrition and wellness lectures for their corporate clients. Prior to becoming a Registered Dietitian, Susan worked for several years as a private chef aboard super yachts based out of Ft Lauderdale. She has a passion for great food and believes that everyone can fit healthy eating into their lifestyle without spending a fortune or a lot of time in the kitchen. Come and get a taste of our nutrition program at the South Florida YMCA. Email: nutrition@ymcasouthflorida.org.

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