Nutrition & Health Info Sheets contain up-to-date information about nutrition, health, and food. They are provided in two different formats for consumer and professional users. These resources are produced by Dr. Rachel Scherr and her research staff. Produced by Marcela Radtke, BS, Anna M. Jones, PhD, Rachel E. Scherr, PhD.
What is intermittent fasting?
Intermittent fasting, also called time-restrictive feeding or intermittent energy restriction, refers to a dietary pattern that alternates between periods of normal energy intake and prolonged energy restriction [1, 2]. Intermittent fasting is a dietary approach that can differ in the amount of time spent in the fasted state per day or the number of fasting days per week.
Recently, intermittent fasting has become a popular trend in the diet and fitness industry and is gaining mainstream attention as an approachable weight loss method. Intermittent fasting was the number one diet-related Google search term in 2019 . However, much of the research on potential health benefits of intermittent fasting is with animal models, establishing a need for additional research in humans.
What are some of the different methods of intermittent fasting?
- The 16:8 Method: The 16:8 method allows an eating window of 8 hours and a fasting window of 16 hours . Individuals consume food throughout the eating period and refrain from food and calorie-containing beverages during the fasting period. Individuals participating in the 16:8 method often select an eating period from 10 am – 6 pm, 11 am – 7 pm, or 12 pm – 8 pm to ensure that they are sleeping through the majority of the fasting period to prevent discomfort from hunger .
- The Eat-Stop-Eat or 5:2 Method: The 5:2 method consists of normal energy intake for 5 days and fasting for 2 non-consecutive days out of the week [1). Depending on the method, fasting days may consist of a significant reduction in calories, to an intake of approximately 400-600 calories per day, or may prohibit any energy-containing food or beverage for the entire day.
- Alternate Day Fasting: In alternate day fasting, individuals restrict calorie consumption every other day to approximately 500 calories per fasting day and resume to normal eating patterns on the following day .
- Whole Day Fasting: Whole day fasting is similar to alternate day fasting, in that specific days of the week are designated as fasting days. During specified 24-hour windows, no calorie-containing food or drink is consumed, and normal eating resumes the following day. A week will typically include one or two 24-hour fasting days .
What are some reasons that people participate in intermittent fasting?
- Religious Practices: Intermittent fasting is a common practice in many different religions, such as Buddhist, Orthodox Christian, Muslim, and Jewish theologies . Religious-motivated fasting may include complete or partial energy restrictions for a specific period of time or prohibit the consumption of particular foods or beverages depending on the practice . For example, Ramadan fasting is the Islamic practice of a 28-30 day fast restricting all food and beverages from sunrise to sunset . In comparison, Orthodox Christian fasting practices occur every Wednesday and Friday throughout the calendar year, similar to the 5:2 method .
- Diagnostic or Medical Fast: A diagnostic or medical fast restricts food and calorie-containing beverages for improved accuracy and safety of the medical procedure. Specific procedures, such as colonoscopies and endoscopies, blood glucose testing, lipid panels, including cholesterol and triglycerides, or any procedure requiring general anesthesia all involve an extended period of fasting .
- Weight Loss: As intermittent fasting gains popularity as an effective weight loss strategy in diet culture media, many individuals looking to lose weight are turning to intermittent fasting regimens. Intermittent fasting has gained popularity as a sustainable approach to losing weight because it does not limit the consumption of specific food items, rather the time frame in which an individual consumes food . Some individuals find this method of dieting generally less restrictive than other diet plans because it does not involve tracking calories or amounts, and does not limit types of food. For this reason, some individuals find intermittent fasting to be an appealing strategy for weight loss and maintenance; however, many people still find it difficult to fast.
What are the possible health benefits of intermittent fasting?
The majority of research on intermittent fasting has been performed in animal models but as the topic continues to gain public interest, more research is being conducted with human subjects. The existing research regarding the health effects of intermittent fasting provide inconclusive results, with some studies supporting improvements in health-related biomarkers [4, 9, 10] and other studies disproving health claims and reporting potentially adverse health effects .
- Metabolic Syndrome: Metabolic Syndrome (MetS) refers to a multicomponent condition with the following characteristics: abdominal obesity, high LDL/low HDL, high blood pressure, and high plasma glucose [12, 13]. The common treatment for MetS is lifestyle modifications for weight loss. There have been multiple studies on intermittent fasting conducted in animal models showing reductions in weight, blood pressure, and serum lipids . However, there are limited studies in overweight or obese humans with MetS demonstrating these outcomes. Intermittent fasting may be an effective strategy for successful weight loss due to the general decrease in caloric intake during fasting periods while not overcompensating during eating periods. Consequently, when intermittent fasting occurs without decreasing overall caloric intake, it may decrease resting metabolic rate and result in weight gain .
- Blood Glucose: Intermittent fasting has demonstrated improvements in fasted and postprandial blood glucose levels and improved glucose tolerance in individuals with type 2 diabetes mellitus (T2DM) . Beneficial effects on pancreatic function and insulin-mediated glucose uptake as a result of intermittent fasting were demonstrated in human subjects, however, it is important to acknowledge the small, homogenous sample size that limits the generalizability of the results [16, 17].
Additionally, the majority of individuals with T2DM are categorized with a body mass index (BMI) in the overweight or obese range, and improvements in blood glucose markers often accompany weight loss . Therefore, the first treatment option prescribed by medical professionals is weight loss and increased physical activity to improve glucose homeostasis. Thus, as intermittent fasting may result in weight loss, improvement in biomarkers associated with T2DM may be related only to weight loss and not attributable to metabolic changes resulting from intermittent fasting specifically .
- Oxidative Stress and Inflammation: Intermittent fasting has shown the potential to decrease markers of inflammation, such as TNF- α, Interleukin-6, and C-reactive protein in humans [20, 21]. Chronic inflammation may lead to the development or exacerbation of other health conditions, such as cancer and atherosclerosis . The exact mechanism of how intermittent fasting may reduce markers of inflammation has yet to be explained, however, both human and animal studies indicate that sustained calorie restriction may have protective health effects . Additional research is needed to determine the physiological effect of intermittent fasting on inflammatory pathways before health claims can be accepted.
What are the possible adverse effects of intermittent fasting?
The impact of intermittent fasting is a relatively new field of research; therefore, future research studies are necessary to determine the broader benefits and implications of the various fasting methods on the human body. Although research focuses on many of the beneficial outcomes of intermittent fasting, adverse effects have been observed .
Individuals participating in intermittent fasting may be at risk for nutrient and electrolyte deficiency if they are not consuming adequate water or nutrient-dense foods during the eating period. Binge eating may occur after fasting periods, and some individuals may compensate with higher caloric intake during their eating window . If this trend of overeating continues, weight gain may occur because the individual is consuming more calories than they are expending, regardless of the calorie deficit experienced during the fasting period .
In addition, some individuals have reported fatigue, headache, depression, and irritability resulting during the fasting period [10, 25]. There is concern from health professionals regarding potential risk of developing disordered eating patterns, increased susceptibility to infections, or dehydration when fasting incorrectly .
- Patterson RE, Laughlin GA, LaCroix AZ, Hartman SJ, Natarajan L, Senger CM, et al. Intermittent Fasting and Human Metabolic Health. J Acad Nutr Diet. 2015;115(8):1203-12.
- Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects-A Narrative Review of Human and Animal Evidence. Behav Sci (Basel). 2017;7(1).
- Year in Search 2019. Google Trends. https://trends.google.com/trends/yis/2019/US/ Accessed January 24, 2020. .
- Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015;73(10):661-74.
- Persynaki A, Karras S, Pichard C. Unraveling the metabolic health benefits of fasting related to religious beliefs: A narrative review. Nutrition. 2017;35:14-20.
- Trepanowski JF BR. The imapct of religious fasting on human health. Nutr J. 2010;9.
- Ljungqvist O, Soreide E. Preoperative fasting. Br J Surg. 2003;90(4):400-6.
- Obert J, Pearlman M, Obert L, Chapin S. Popular Weight Loss Strategies: a Review of Four Weight Loss Techniques. Curr Gastroenterol Rep. 2017;19(12):61.
- Wegman MP, Guo MH, Bennion DM, Shankar MN, Chrzanowski SM, Goldberg LA, et al. Practicality of intermittent fasting in humans and its effect on oxidative stress and genes related to aging and metabolism. Rejuvenation Res. 2015;18(2):162-72.
- Horne BD, Muhlestein JB, Anderson JL. Health effects of intermittent fasting: hormesis or harm? A systematic review. Am J Clin Nutr. 2015;102(2):464-70.
- Soeters MR, Lammers NM, Dubbelhuis PF, Ackermans M, Jonkers-Schuitema CF, Fliers E, et al. Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. Am J Clin Nutr. 2009;90(5):1244-51.
- Cornier MA, Dabelea D, Hernandez TL, Lindstrom RC, Steig AJ, Stob NR, et al. The metabolic syndrome. Endocr Rev. 2008;29(7):777-822.
- Grundy SM, Brewer HB, Jr., Cleeman JI, Smith SC, Jr., Lenfant C, National Heart L, et al. Definition of metabolic syndrome: report of the National Heart, Lung, and Blood Institute/American Heart Association conference on scientific issues related to definition. Arterioscler Thromb Vasc Biol. 2004;24(2):e13-8.
- Longo VD, Mattson MP. Fasting: molecular mechanisms and clinical applications. Cell Metab. 2014;19(2):181-92.
- Barnosky AR, Hoddy KK, Unterman TG, Varady KA. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res. 2014;164(4):302-11.
- Halberg N, Henriksen M, Soderhamn N, Stallknecht B, Ploug T, Schjerling P, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol (1985). 2005;99(6):2128-36.
- Carlson MG SW, Campbell PJ. Fuel and energy metabolism in fasting humans. American Journal of Clinical Nutrition. 1994;60:29-36.
- Johnson JB SW, Cutler RG, Martin B, Hyun D, Dixit VD, Pearson M, Nassar M, Maudsley S, Carlson O, John S, Laub DR, Mattson MP. Alternate Day Calorie Restriction Improves Clinical Findings and Reduces Markers of Oxidative Stress and Inflammation in Overweight Adults with Moderate Asthma. Free Radical Biology & Medicine. 2007;42(5):665-74.
- Brown JE, Mosley M, Aldred S. Intermittent fasting: a dietary intervention for prevention of diabetes and cardiovascular disease? Br J Diabetes Vasc Dis. 2013;13(2):68-72.
- de Azevedo FR, Ikeoka D, Caramelli B. Effects of intermittent fasting on metabolism in men. Revista da Associação Médica Brasileira (English Edition). 2013;59(2):167-73.
- Aksungar FB, Topkaya AE, Akyildiz M. Interleukin-6, C-reactive protein and biochemical parameters during prolonged intermittent fasting. Ann Nutr Metab. 2007;51(1):88-95.
- Libby P. Inflammatory Mechanisms: The Molecular Basis of Inflammation and Disease. Nutrition Reviews. 2007;65(12):140-6.
- Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.
- Johnstone AM. Fasting - the ultimate diet? Obes Rev. 2007;8(3):211-22.
- Fond G, Macgregor A, Leboyer M, Michalsen A. Fasting in mood disorders: neurobiology and effectiveness. A review of the literature. Psychiatry Res. 2013;209(3):253-8.
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