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Intermittent Fasting by Susan Macario

Updated: Jan 9, 2020

A diet that tells you when to eat rather than what to eat? That sounds interesting. Intermittent Fasting is a pattern of eating, rather than a specific diet, and it is becoming popular. People are talking about it and writing about it—but what is it exactly? Intermittent Fasting is a dietary strategy that involves periods of not eating and periods of ad libitum, or “as-you-desire,” food consumption. Proponents of Intermittent Fasting proclaim that it is an effective weight loss method and that it can improve your overall health. What does the science say?

Despite being practiced for thousands of years by various cultures, the science on Intermittent Fasting is limited. Clinical trials are just beginning to investigate the topic. Consequently, the studies tend to be short in duration, affecting the validity of the analyses, especially when trying to assess the impact of fasting on the lifespan. Most of the fasting studies demonstrating any significant weight loss and improvement in risk factors for diabetes, cancer and cardiovascular disease have been done on animals, raising the question of whether the benefits are transferable to humans. Intermittent Fasting can be done by following any one of several different eating patterns. One of the more popular is called Alternate Day Fasting. As the name implies, the fasting takes place every other day. Calorie consumption on the fasting day may be between 0-500 calories. Another intermittent-fasting method is one that involves a daily 16-hour fast, with total calorie intake occurring in an eight-hour window. This is often referred to as the “skipping breakfast” method. Others may prefer to fast one day each week, taking in only water for that day of fasting. As you can see, the varied approaches to Intermittent Fasting allow each person to discover the pattern that works best for him/her. Clinical trials currently do not support the idea that Intermittent Fasting, specifically Alternate Day Fasting, is an effective method for weight loss or reduction of chronic disease risk factors. If Alternate Day Fasting is combined with a decrease in caloric intake, however, the health benefits may be measurable. Krista Varady, PhD, Associate Professor of Nutrition at the University of Illinois at Chicago, reviewed the available scientific literature on Intermittent Fasting and reported, “Further analysis of the mechanisms responsible for beneficial effects of Alternate Day Fasting is clearly warranted, particularly if these effects occur in the absence of negative energy balance.” Negative energy balance is when the number of calories taken in is less than the number of calories being expended.

Another dietary approach to health and wellness is calorie restriction. It is not an official pattern of Intermittent Fasting, but it has been shown to have a positive effect on body mass and disease prevention. People practicing calorie restriction decrease their total calorie consumption, either daily or more occasionally. It is possible to practice Intermittent Fasting without reducing your calorie intake because one can fast and then make up for any calorie deficit during the eating phase of the pattern.

During Islamic Ramadan, followers adhere to an eating schedule that coincides with the sunrise and sunset. For a full lunar month (28-30 days), they are essentially practicing Intermittent Fasting, eating only when the sun is down. According to John Trepanowski, a postdoctoral research fellow at Stanford, Ramadan fasting has elicited mixed results in relation to cardiovascular health, particularly with respect to lipid profiles. In addition, Trepanowski highlighted several potentially adverse effects of Ramadan fasting, such as increased gastric acidity, energy level imbalances, dehydration and negatively impacted sleep. Finally, a wide range of confounding variables, such as age, physical activity and food choices were difficult to control for in Trepanowski’s research on health-related biomarkers among Ramadan fasters, allowing for few definitive conclusions to be made.

Researchers at the Harvard T.H. Chan School of Public Health recently published a list recommending who should not experiment with Intermittent Fasting. The list includes individuals with the following conditions:

  • diabetes

  • eating disorders that involve unhealthy self-restriction (anorexia or bulimia nervosa)

  • use of medications that require food intake

  • active growth stage (such as adolescents)

  • pregnancy and breastfeeding 

The evidence for the benefits of Intermittent Fasting is not sufficient to outweigh the potential for physiological and psychological harm to the individual. There is also the potential risk of negatively affecting other family members, particularly children who are exposed to a parent practicing Intermittent Fasting.

Many non-scientific articles describing Intermittent Fasting have been published recently that glorify this eating pattern. It is important to note that most of these articles do not discuss calorie restriction.

Despite its current allure, Intermittent Fasting may not result in persistent health benefits. The method may prove more viable for long-term positive health outcomes if reduced calorie intake accompanies the Intermittent Fasting. But it is clear that further long-term study is needed before anyone chooses to make Intermittent Fasting a way of life. Until then, feel free to skip breakfast if you are not a natural breakfast eater. We may, one day, learn that it is an effective way of eating to benefit one’s health.


  • Varady, Krista A. “Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?” Obesity Reviews (2011), 12, e593-e601.

  • Varady, Krista A. & Marc K. Hellerstein, “Alternate-day fasting and chronic disease prevention: a review of human and animal trials.” American Journal of Clinical Nutrition 2007; 86: 7-13.

  • Trepanowski, John F. et al. “Impact of caloric and dietary restriction regimens on markers of health and longevity in humans and animals: a summary of available findings.” Nutrition Journal (2011), 10:107.

  • Trepanowski, John F. & Richard J. Bloomer. “The impact of religious fasting on human health.” Nutrition Journal (2010), 9:57.

  • "Diet Review: Intermittent Fasting for Weight Loss," Harvard T.H. Chan School of Public Health:

Susan Macario

Susan Macario is a lifelong fitness enthusiast. She discovered yoga four years ago and is fascinated with the science and physical aspect of the practice. With a background in biology and a love of sport, Susan enjoys discovering new ways to improve and maintain her health. A recent interest in minimalism has also sparked a desire to simplify her recipes (and her life!). Susan worked as a cardiac catheterization lab technician for a decade at the University of Rochester and then at the Palo Alto VA Hospital. While at Rochester, she also worked in the ICU, performing various cardiac function tests on post-cardiac surgery patients (both adult and children). She spent a year volunteering at Stanford's Health Improvement Program (HIP), assisting the researchers with data collection and study participant education. Currently, she works in education, tutoring students at her neighborhood elementary school and with a nonprofit after-school program in East Palo Alto.

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