Fasting Diets

  • Medical Author:
    Erica Oberg, ND, MPH

    Dr. Erica Oberg, ND, MPH, received a BA in anthropology from the University of Colorado, her doctorate of naturopathic medicine (ND) from Bastyr University, and a masters of public health (MPH) in health services research from the University of Washington. She completed her residency at the Bastyr Center for Natural Health in ambulatory primary care and fellowship training at the Health Promotion Research Center at the University of Washington.

  • Medical Editor: Charles Patrick Davis, MD, PhD
    Charles Patrick Davis, MD, PhD

    Charles Patrick Davis, MD, PhD

    Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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Fasting diets introduction

Fasting in some form has long been part of many spiritual practices. For example, during the month of Ramadan, Muslims fast from dawn to dusk. In Jewish, Christian, Buddhist, and Mormon religions, followers fast on certain days of the week or month. Fasting for health and to lose weight is also nothing new, but a new crop of trendy diets has brought the practice back into the spotlight.

Diets like the 5:2 Diet, Lemonade Diet, and others claim to help people lose weight fast and detoxify the body. There are different types of fasting diets that vary in intensity and duration. There are water-only fasts (typically done under medical supervision in residential facilities). Intermittent fasting involves fasting for one to two days per week or for 12-14 hours each day. Modified fasts include liquid-only diets (such as before a colonoscopy), caloric restriction (typically 20%-40% less), and ketogenic diets (a high-fat diet that induces the same metabolic changes as fasting).

Research demonstrates that there is a role for fasting for weight loss, longevity, and some specific health conditions. Much of the data comes from animal research, but high-quality human research has demonstrated benefits, including lower rates of cardiovascular disease and diabetes, weight loss, and improvements in lab markers of metabolic health and aging. Caloric restriction and intermittent fasting are the only research-proven strategies to extend life span.

How does fasting work?

When dietary intake is restricted, the body must draw upon stored energy reserves to continue metabolic activities. Specifically, when carbohydrates are restricted, metabolism changes from the standard use of glucose for fuel to ketosis. Ketosis is the term for burning fatty acids (from stored body fat) for fuel. This is the fat-burning phase that decreases fat mass, inches, and body weight. Also, when fasting, inflammatory lab markers decrease; cholesterol, glucose, insulin, and other markers of potentially are unhealthy metabolism improve. Fasting is distinct from starvation in that starvation describes a state of chronic nutritional deficiency. When people fast for weight loss, they are drawing upon their excess reserves. When people follow modified fasts, they get most of the metabolic benefits of fasting but can still maintain nutritional adequacy by consuming some food.

What are intermittent fasting diets?

The 5:2 diet was recently the topic of a popular BBC documentary and book, and it's based on the principle of intermittent fasting (IF) or alternate-day fasting (ADF). Intermittent fasting means you eat normally at certain times and then fast or dramatically reduce your calorie intake at other times.

The 5:2 fasting plan calls for eating normally on five days of the week and fasting (eating no more than 500 calories for women and 600 for men) on the remaining two days. An alternative pattern of intermittent fasting is involves a 12-hour overnight fast between dinner and breakfast. The Bulletproof diet suggests an extended daily fast (14-18 hours), which is achieved by consuming only coffee enriched with grass-fed butter and coconut oil in the morning and consuming a healthy low-carbohydrate meal in the evening. This pattern induces ketosis. Alternate day fasting has been researched in animals and has variable benefits.

Research on intermittent fasting demonstrates fasting can increase life span, improve how the brain works, and protect against disease; but most of those claims are based on research conducted in animals like rats and mice. Among high-quality human studies, intermittent fasting appears to improve stress resilience at a cellular level, resulting in fewer spikes and troughs in blood sugar, cortisol, and other markers of oxidative stress.

Small studies in humans that suggest that an intermittent fasting diet may help promote weight loss and reduce risk factors for heart disease, diabetes, and other weight-related health problems. For example, a small 2012 study in Nutrition & Metabolism showed 30 obese women who followed a liquid intermittent fasting diet lost 8.8 pounds and 2.3 inches from their waistlines after eight weeks. (Excess body fat around the waist has been associated with a higher risk of heart disease.)

Research published in the Journal of the American Medical Association (JAMA) on caloric restriction (-25%) in humans suggest improvement in biomarkers of longevity. Studies in rodents demonstrate increased telomere length and other biometrics of anti-aging.

Medically Reviewed by a Doctor on 5/6/2016

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