The Pros and Cons of Intermittent Fasting

There’s a lot of internet chatter and press coverage around a popular health trend called Intermittent Fasting (IF).  

Although fasting is an ancient religious, spiritual and health practice, it became repopularized in 2012 when Dr. Michael Mosley, often called the Sanjay Gupta of the BBC, filmed a documentary called Eat, Fast and Live Longer that followed his 9-week journey in which he lost 20 pounds and improved his glucose and cholesterol levels with intermittent fasting.   

The following year, Dr. Mosley published The FastDiet book with co-author Mimi Spencer, a 45-year-old journalist who lost 20 pounds in 8 weeks by practicing Intermittent Fasting herself.  The book became an instant bestseller in the U.K. and in the U.S.

Lots of patients have asked us what IF is, if it lives up to the hype, and if it’s safe to practice.

So let’s chat about it!   

What is IF?

Intermittent Fasting can be done several ways.  Some people practice 5:2 IF, where they eat whatever they want for five days a week, and fast two days a week.  The two fasting days are usually not consecutive, and they allow 600 calories a day for men, and 500 calories a day for women.

Other people practice a form of IF every day.  Some people do 16:8, where they fast for 16 hours a day, and have an 8-hour eating window.  The ratio of fasting : eating differs depending on people’s preferences, schedules and goals -- all the way to the very strict 23:1 ratio, where people eat a single meal every 24 hours.

What does fasting do for your body?

There’s a large body of research that shows myriad benefits of fasting.  

When your body goes into fasting mode, it rests from the work of metabolism, and instead rebuilds and repairs damaged cells.  

Fasting promotes weight loss, because the body begins to burn fat when it runs out of carbohydrates.  Fasting increases insulin sensitivity and decreases the risk of diabetes because the pancreas can rest when the body’s in fasting mode.  In addition, fasting lowers cholesterol and triglycerides.

Preliminary studies show that fasting protects against dementia, and promotes longevity.

Who would benefit from fasting?

Fasting can be an incredibly effective tool for reversing obesity, diabetes, hypertension and high cholesterol.  In addition to reversing disease states, fasting can also promote wellness and longevity in healthy patients.

Who would be harmed by fasting?

Pregnant women should never fast, because the body produces ketones when it’s in the fasting state, which can be harmful to a fetus.  Children also should not fast because their bodies need a constant supply of nutrients in order to develop properly. People who have a history of eating disorders should be careful with fasting, because it can trigger previous patterns of disordered eating.

Also, any patients with chronic health issues should consult their doctor before embarking on IF.  If they and their doctor decide that fasting is advised, these patients should be closely monitored during the process to prevent medical complications.

How can patients fast safely?

When fasting, it’s imperative to drink plenty of fluids to prevent weakness, dehydration, dizziness and fainting.  

It’s also important to make sure that the fasting is interrupted at healthy intervals, and that during the non-fasting times, the body gets all the macro- and micro-nutrients it needs to continue to thrive.  Otherwise, fasting can lead to malnutrition and starvation.

Patients who aren’t familiar with fasting should educate themselves before attempting it, and should gradually ease into the practice, since they could experience severe symptoms from abruptly changing their eating patterns.

How long should patients practice IF?

One of the points that proponents of IF make is that it’s not a diet; it’s an ongoing, intentional rhythm of eating.  If people practice IF for a limited time, even if they lose a significant amount of weight and reverse disease processes, that progress will be quickly erased if they go back to the way they were eating before.  In order for the health benefits to be sustainable, patients need to commit to it as an ongoing rhythm, not another quickly-abandoned health fad.


Our goal at GGUC is to help you feel as healthy and happy as possible!  Make an appointment online, or walk in to any of our six Bay Area locations to get the care you need.

Sarah Thebarge MMSc, PA-C Sarah Thebarge earned her physician assistant degree at Yale School of Medicine, and then studied journalism at Columbia School of Journalism. She has been a physician assistant and a freelance journalist since 2004. In addition to caring for patients at Golden Gate Urgent Care, Sarah frequently volunteers her medical skills in the developing world. Her writing has appeared in Huffington Post, USA Today and National Geographic, and her blog was featured on She is the author of the memoir The Invisible Girls and the upcoming book WELL: Healing our Beautiful, Broken World from a Hospital in West Africa. She currently lives in the Mission District of San Francisco.

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