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The Canyon Ranch Guide to Weight Loss: The Pros & Cons of Intermittent Fasting

Jan 9 2023
By Dr. Stephen Brewer
6 min read
A copy of Dr. Brewer's recent Weight Loss book

Does Intermittent Fasting work as a solution for weight loss? Our Medical Director, Dr. Stephen Brewer, shares the pros and cons of this diet trend.

In an excerpt from his new book, The Canyon Ranch Guide to Weight Loss: A Scientifically Based Approach to Achieving and Maintaining Your Ideal Weight ($24.95), Stephen Brewer, MD, Medical Director Canyon Ranch, discusses the pros and cons of Intermittent Fasting.

There is always a new diet craze. Everyone wants to jump on the bandwagon with the newest diet that is guaranteed to help a person lose weight. As I said earlier in this book, if one has been out of control in what they eat and not exercising on a regular basis, any program is often at least temporarily a good program. That is why so many new fad diets seem to work. If we follow the guidelines of a new diet or exercise, we are at least doing something rather than not doing anything to address our weight. Let’s look at one of the more popular diets and discuss the pros and cons of each.

Intermittent Fasting
At the time of the writing of this book, one of the more popular diets for weight loss is Intermittent Fasting. The premise of how this diet works is that if you are not eating carbs, your carb storage dries up, and therefore you must burn up fats for energy. Small studies have been mixed as to the efficacy of utilizing intermittent fasting to lose weight.

There are basically two types of Intermittent Fasting: one is alternative day fasting and the other is time-restricted fasting. Alternative day fasting may consist of a 24-hour fast followed by a 24-hour eating period that can be done several times a week. One type of alternate day fasting is the 5:2 diet. With this diet there are 2 fast days mixed in with 5 nonrestrictive days. Some 5:2 diets aren’t a complete fast on the 2 “fasting” days but are a very restrictive caloric diet. In his book The 5/2 Diet, Dr. Michael Mosley has men eating only 600 calories and women eating only 500 calories on those 2 days. With time-restricted fasting, an individual eats meals only during a specific time frame, from 4 hours (example: 2 p.m. to 6 p.m.) up to 8 hours (example: 11 a.m. to 7 p.m.), and the rest of the time a person does not eat. The difference between a purely caloric-restricted diet and Intermittent Fasting is that during the feeding phase of Intermittent Fasting (IF) there is no calorie restriction.

What are the Pros and Cons of an Intermittent Fasting diet?


  • There are no food restrictions on non-fasting days.
  • It’s easy to follow.
  • One does not have to count calories unless their specific diet allows 600 calories for men and 500 calories for women on the fasting days.


  • It is not recommended for individuals under the age of 18, those that could potentially become pregnant, or are breastfeeding.
  • One must be reminded to drink fluids during the fasting phase. Many of us equate food and drink, and when we are not eating, we often do not drink enough.
  • Fasting can be stressful on the body so there is a concern of increased cortisol production, which we learned in the medication chapter, can lead to weight gain.
  • Overeating and binge eating are two potential side effects of intermittent fasting. Therefore intermittent fasting is not a diet for an individual with a history of an eating disorder.
  • Fatigue is a common problem on the fasting days. This can lead to decreased physical activity, which is not what you want to do when you are trying to lose weight.
  • There has been a concern that the swings in eating may affect a person’s mood.

Additional negative aspects of Intermittent Fasting

  1. “Hangry” (One can become irritable and bad-tempered when they are hungry and haven’t eaten for a prolonged time.)
  2. Brain fog
  3. Developing food obsessions about when you can eat
  4. Hypoglycemia
  5. Alopecia (hair loss)
  6. Changes in the menstrual cycle
  7. Constipation
  8. Possibility of unhealthy dietary choices during non-fasting time
  9. Concern about developing a binge-eating disorder (BED)
  10. Sleep disturbances
  11. Decrease in REM sleep
  12. Sustainability is unlikely or not advised

One of the proposed benefits of intermittent fasting that was noted in the media was that this diet increased longevity. According to studies of rodents, conducted by the National Institute of Aging, when mice are put on a program that severely restrict diets, many show an expansion of lifespan and decreased serious diseases, especially cancer. We should note that this was a rodent study, and no long-term human studies have been done to prove or disprove the efficacy of fasting.

While the practice of intermittent fasting is not new, much of the research investigating the benefits of this routine of eating is relatively recent. For that reason, it is hard to tell if the benefits are long-lasting. Researchers often comment that long-term studies are needed to determine if the eating plan is even safe to follow for more than several months. For now, if you decide to do Intermittent Fasting, please consider a discussion with your healthcare provider because of the safety issues associated with it.

This was excerpted from The Canyon Ranch Guide to Weight Loss: A Scientifically Based Approach to Achieving and Maintaining Your Ideal Weight ($24.95; SelectBooks). Reprinted with Permission by the publisher, SelectBooks, Inc. Find it here.

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About the Expert

A headshot of Dr. Stephen Brewer

Stephen C. Brewer

MD, ABFM, Medical Director

Dr. Brewer knows from experience that the key to balancing a busy professional life with personal wellness is to find a plan that works with your lifestyle. He is passionate about helping our guests discover that balance for themselves.

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