Is Fasting an Effective Therapy for Weight Loss?

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Dr. Randi Fredricks, Ph.D. is a therapist, researcher and author with a Ph.D. in Psychology and a Doctorate in Naturopathy. Dr. Fredricks works ...Read More

With numerous best-selling books touting the advantages of fasting for weight loss, many people are wondering if fasting is a safe and effective way to lose weight. With New Year’s resolutions approaching, this question seems more pertinent than ever.

Fasting is an ancient practice, going back thousands of years. Although fasting is often done for spiritual or religious reasons, it has a long history of being used as a treatment for obesity.


The practice of fasting is typically misunderstood, not only by lay people but by medical and mental health professionals as well. In the proper sense of the word fasting is abstaining from all food and drinking only water for a specific amount of time. Because the definition of fasting has become so misused, this type of fasting is also referred to as “water fasting.” In contrast, juice fasting or consuming any food or drink in addition to water is considered “calorie restriction.”

The majority of research suggests that water-only fasting is superior to juice fasting because it allows the body to rest and recuperate in a manner that juice fasting does not In fact, there are very few peer-reviewed studies showing the efficacy of juice fasting, whereas there are thousands of studies supporting water fasting. In this article, fasting refers to water only fasting.

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The first published study that successfully used fasting as a treatment for obesity was over one hundred years ago. In 1915, The Journal of Biological Chemistry ran an article advocating fasting as a medical therapy for obesity. By the 1960s, doctors were routinely fasting obese patients and publishing their results in medical journals.

The best-known of all research on fasting for obesity is a 1973 case study of a 27-year-old man referred to simply as “Mr. A. B.” At the start of the study, Mr. A. B. weighed 456 pounds. After fasting for 382 days, he lost 276 pounds. Although he drank only water, Mr. A. B. received potassium from days 93 to 162, sodium from days 345 to 355, and one multivitamin in pill form every day. At the beginning of the study, he was closely supervised, but during the course of the research he was feeling so good that he was allowed to go home. As with most fasting of this type, he continued to have a daily urinalysis and check of his vital signs. Additionally, blood tests were run every two weeks. As a testament to the potential outcome and long term effects of fasting as a treatment for obesity, Mr. A. B. had only regained only 16 pounds five years later.

Other research on whether patients who fast to lose weight can maintain their weight loss has been mixed. This is noteworthy because statistically most people tend to gain all the weight back that they lost during a diet — and then some. A study published in a German medical journal in 1976 followed 42 patients for almost 2 years after they had fasted to lose weight. Fourteen of the participants had actually lost more weight and nine weighed more than they did pre-fast.

A study published two years later in the International Journal of Obesity tracked 121 patients who had fasted to lose weight for over 7 years after the completion of their initial weight loss. Of the 121 participants who had originally fasted for between 30 and 60 days, 50 percent had reverted to their pre-fast weight at the 3-year mark whereas 90 percent had by the 7-year mark.

The reason for these mixed results may be due to the need to incorporate psychological counseling when using fasting as a treatment of obesity, much in the same way that is currently being done with bariatric surgery for weight loss. It is also worth noting that most experts concur that fasting to lose weight without changing your diet afterward is ineffectual much in the same way as it is with other weight loss methods. This is why the top fasting clinics typically have psychologists on staff who are a part of a psychoeducation program during treatment.


Berger, M., Granz, M., Berchtold, P., Krüskemper, G. M., & Zimmermann, H. (1976). Serial studies of the long-term effects of a total-fasting regimen (author’s trans). Deutsche Medizinische Wochenschrift, 101(16), 601-605. DOI: 10.1055/s-0028-1104121

Drenick, E. J., & Johnson, D. (1978). Weight reduction by fasting and semistarvation in morbid obesity: Long-term follow-up. International Journal of Obesity, 2(2), 123-132. PMID: 711359

Fredricks, Randi. (2012). Fasting: An Exceptional Human Experience. Bloomington, IN: Authorhouse. ASIN: B00BN9NUI6

Stewart, W. K., & Fleming, L. W. (1973). Features of a successful therapeutic fast of 382 days’ duration. Postgraduate Medical Journal, 49, 203-209. PMCID: PMC2495396

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