These days our bodies are used to being nourished on a regular basis. However, that hasn’t always been the case. Fasting has existed for thousands of years and is often done out of necessity, when the food runs out.
Now, it is gaining more and more followers amongst people who are looking to optimize their health. But is it really beneficial to fast? How can we fast properly, and what are the potential risks of this practice?
Here is our rundown on the topic, developed with nutritionist Anthony Berthou.
Different types of intermittent fasting
First, we’ll tackle intermittent fasting which consists of alternating periods of fasting and periods of food intake. It is different from strict fasting, which we won’t discuss here.
There are different ways to practice intermittent fasting which we will discuss below. For each one of these methods, water and infusions are allowed. Tea and coffee can also be consumed by those who can’t go without them.
16-hour fasting
The idea is pretty simple, and many of us do it without even knowing. The practice consists of fasting 16 hours per day (this includes time asleep), and then eating during the following 8 hours in the form of 2 to 3 meals. For example, you can eat dinner at night, then sleep, and continue to fast until lunch the next day.
24-hour fasting
This consists of fasting for 24 hours. For example, once you have finished your dinner, you can fast until the next day’s dinner. You can start at breakfast or at dinner, as long as the fast lasts 24 hours.
You can opt for this method at an occasional frequency according to your preferences, or in a regular rhythm by practicing this fast once per week.
Every-other-day fasting
According to multiple studies, this fast would be the most effective method for longevity. Certain results observed in mice reveal a 20% increase in lifespan.
However, this fast is difficult to sustain in the long term and requires a strict nutritional regime. So, it is not well-suited for most people’s way of life. One alternative solution is to limit one’s intake to 500 calories per day on fasting days instead of fasting completely.
1 to 3 week therapeutic fasting
This fast stretches for a longer period of time and is done for therapeutic reasons, particularly alongside treatments for cancer or chronic inflammatory diseases.
It is essential that this type of fasting is done with the support and supervision of competent healthcare professionals. Many clinics are specialized in this work.
The fasting stages are designed to be the same duration for everyone; but it is necessary to consult supervising professionals and to seriously take into account all precautions before launching into this adventure.
Fasting when you want/when you can
This is the most intuitive kind of fasting which consists of listening to your body and fasting depending on how you’re feeling (after a heavy meal, for example). Contrary to popular belief, it is not problematic to skip a meal when our bodies don’t feel a need for it. Sometimes, it is even better to fast during a meal rather than eat a poorly prepared meal, like an industrial sandwich that is loaded with additives and is nutritionally uninteresting.
What happens in our bodies when we fast?
At the beginning of the fast, our bodies will use the nutrients consumed during our last meal (particularly carbohydrates) to give us energy. Then, once these nutrients are consumed, the liver uses the glucose that we have stored up in the form of glycogens in order to nourish our glucose-dependent organs, especially the brain (see our article on carbohydrates). Another part of the energy is also going to be drawn from fat stores. This is the most delicate phase of the fast during which people experience fatigue, significant hunger, and decrease in attention and performance.
For two to three days, the body will gradually activate an emergency function to adapt to this glucose deprivation: ketogenesis. From fatty acids, our liver also produces little molecules, ketones (or ketone bodies), which will become our alternative fuel to glucose. These ketones will then supply our bodies with energy, particularly for glucose-dependent organs like the brain and muscles. It is the use of this glucose alternative (the basis of the keto diet) which would explain some of the benefits of fasting.
The period of establishing ketogenesis, which lasts about 2 to 3 days, is generally a phase in which you feel better and regain some energy. This process is linked to the body’s production of ketone bodies, which can create a euphoric effect. Certain people, however, can react poorly to this step of fasting and experience ketoacidosis with symptoms that can include nausea and migraines. The body produces a lot of nitrogenous waste during this period, so it is very important to drink enough in order to push them out of the body (8.5 to 10.5 8-oz. glasses, or 2L to 2.5L, per day).
As such, it is necessary to consider all of the required precautions before attempting a fast. In fact, in the case of fasting, the body will also pull from muscle proteins to give you energy. This situation is all the more likely when the fast lasts a long time, and will thus lead to muscle loss. For thin people with little muscle reserve, their immune systems can be weakened and made more susceptible to infection, because the immune system consumes a large amount of protein (antibodies are proteins, and immune cells are made of proteins). So, it is vital to consume enough protein during these dieting periods (at least 1.2 grams of protein / kg of body weight / day [approximately 0.02 ounces / 1 pound of body weight / day], or around 90 grams for a 75 kg person [approximately 3.2 ounces for a 165 pound person]).
The benefits of fasting
Our digestive organs experience a great deal of strain everyday. They are often mistreated by an unbalanced or excessively rich diet. So, it seems natural enough to give our digestive system a bit of a break sometimes. Fasting’s numerous positive effects have been supported throughout various studies, however some risks must still be considered (see the following section).
Improved insulin sensitivity
Practicing intermittent fasting allows the body to release less insulin and therefore improve its sensitivity to insulin (see our article on carbohydrates). As such, fasting helps to prevent insulin-resistance and to improve diabetes management.
However, for people who suffer from diabetes, intermittent fasting is to be practiced with the support of a specialized health professional. It is true that fasting can lead to blood sugar imbalances outside of mealtimes, which requires adapting the proposed treatments.
Oxidative stress reduction
During a fast, our body’s oxidative stress slowly increases (see our article on antioxidants) but in a temporary manner. It will in fact stimulate our body’s adaptation abilities and will reinforce its protection by producing antioxidant enzymes. Creating mild oxidative stress allows the body to adapt and to produce more antioxidants.
Obesity weight loss
Different studies have revealed the effectiveness of intermittent fasting amongst people who are obese or overweight. Above all, it is the caloric restriction and better insulin sensitivity caused by fasting which would support such weight loss.
However, amongst people with so-called “normal” body sizes, fasting must not be done with the goal of getting slim (in fact, fasting has shown little efficacy in these cases).
Better cardiovascular health
Multiple studies have identified the benefits of fasting for reducing cardiovascular risks, which would be linked to a multitude of factors favoring strong cardiovascular health (visceral fat reduction, sodium level reduction, ketosis activation, stronger endogenous antioxidant protection, low-grade inflammation reduction, etc.).
Protection against neurodegenerative disorders
Fasting stimulates the production of neuroprotective proteins and encourages the cleaning of damaged molecules. In this way, it seems to protect neurons from neurodegenerative disorders, particularly Alzheimer’s and Parkinson’s diseases. It seems to also protect from general aging.
Benefits against certain types of cancer
Certain studies done on animals reveal a reduction in the growth and number of tumor cells when chemotherapy is combined with short fasting cycles for certain types of cancers.
The French Network for Nutrition and Cancer Research (Réseau National Alimentation Cancer Recherche) indicates, however, that the benefits of this type of diet have not yet been proven for humans, as most of its studies have been conducted on animals.
Cell cleaning and recycling (autophagy)
The discovery of autophagy was awarded the Nobel Prize in Physiology or Medicine, attributed to Yoshinori Oshumi in 2016. Autophagy is a process through which the cell optimizes the recycling of its components to facilitate its survival when it is lacking sufficient amounts of energy. New higher-functioning cells will be able to be generated to replace those that were destroyed.
The absence of calorie intake caused by a prolonged fast will contribute to the activation of the naturally protective biological process that is autophagy.
Is fasting a miracle solution?
While many studies have demonstrated the beneficial effects of fasting, a few nuances should be considered:
- First and foremost, the vast majority of studies have been performed on animals and cannot be assumed to present the same ways in humans.
- Also, the responses to fasting seem to differ according to genetic predisposition and the type of fasting practiced.
- Additionally, the benefits are in large part due to caloric restriction.
- Lastly, study results vary according to a person’s prior health status and according to their body size.
As such, the benefits linked to practicing fasting depend upon a multitude of factors and cannot be generalized. Fasting can be risky when it is done incorrectly or by those whose health is not suited for a prolonged fast.
Fasting is a broad and complex topic which still requires additional research. It is not a miracle solution and must be approached with precaution.
An easier intermediary solution to put in place which is less risky than total fasting consists of reducing one’s caloric intake to 500 calories / day (divided between two meals) at an occasional or regular frequency (1 to 2 nonconsecutive days per week, for example).
How do you practice intermittent fasting?
If you choose to get into intermittent fasting, here are a few precautions to consider:
 | 1) Remember to stay hydrated. Drink 8.5-10.5 8-oz. glasses (2-2.5 liters) of water, herbal/regular tea per day to help eliminate nitrogenous wastes. |
 | 2) Eat enough protein. During the phases of food intake, be sure to maintain a satisfactory protein level by choosing fish, poultry, and vegetable proteins. |
 | 3) Make sure to adopt an alkaline-rich diet. Opt mostly for fruits and vegetables, limit your intake of salt, red meat, and cold cuts. |
 | 4) Do not jump into a fast against medical guidance. In following cases, it is essential to be accompanied by a health professional who specializes in fasting: eating disorders, kidney or liver deficiencies, chronic illnesses (diabetes, cancer, inflammatory or neurodegenerative diseases), auto-immune diseases, immunocompromised, pregnancy, etc. |
- Brandhorst S, Choi IY, Wei M, et al. A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan. Cell Metab. 2015;22(1):86-99.
- Chaix A, Zarrinpar A, Miu P, Panda S. Time-restricted feeding is a preventative and therapeutic intervention against diverse nutritional challenges. Cell Metab. 2014;20(6):991-1005.
- Garaulet M, Gómez-Abellán P, Alburquerque-Béjar JJ, Lee YC, Ordovás JM, Scheer FA. Timing of food intake predicts weight loss effectiveness. Int J Obes (Lond). 2013 Apr;37(4):604-11.
- Azevedo FR, Ikeoka D, Caramelli B. Effects of intermittent fasting on metabolism in men. Rev Assoc Med Bras (1992). 2013 Mar-Apr;59(2):167-73.
- Mattson MP. Energy intake and exercise as determinants of brain health and vulnerability to injury and disease. Cell Metab. 2012 Dec 5;16(6):706-22.
- Poon HF, Shepherd HM, Reed TT, Calabrese V, Stella AM, Pennisi G, Cai J, Pierce WM, Klein JB, Butterfield DA. Proteomics analysis provides insight into caloric restriction mediated oxidation and expression of brain proteins associated with age-related impaired cellular processes: Mitochondrial dysfunction, glutamate dysregulation and impaired protein synthesis. Neurobiol Aging. 2006 Jul;27(7):1020-34.
- Ahmet I, Wan R, Mattson MP, Lakatta EG, Talan M. Cardioprotection by intermittent fasting in rats. Circulation. 2005 Nov 15;112(20):3115-21.
- University of Southern California. 2012. “Fasting makes brain tumors more vulnerable to radiation therapy.” ScienceDaily.
- University of California, Berkeley. 2005. “Fasting Every Other Day, While Cutting Few Calroies, May Reduce Cancer Risk.” ScienceDaily.
- Lee C, Raffaghello L, Brandhorst S, et al. Fasting cycles retard growth of tumors and sensitize a range of cancer cell types to chemotherapy. Sci Transl Med. 2012;4(124):124ra27.
- Safdie FM, Dorff T, Quinn D, et al. Fasting and cancer treatment in humans: A case series report. Aging (Albany NY). 2009;1(12):988-1007. Published 2009 Dec 31.
- Zuccoli G, Marcello N, Pisanello A, Servadei F, Vaccaro S, Mukherjee P, Seyfried TN. Metabolic management of glioblastoma multiforme using standard therapy together with a restricted ketogenic diet: Case Report. Nutr Metab (Lond). 2010 Apr 22;7:33.
- Bauersfeld SP, Kessler CS, Wischnewsky M, et al. The effects of short-term fasting on quality of life and tolerance to chemotherapy in patients with breast and ovarian cancer: a randomized cross-over pilot study. BMC Cancer. 2018;18(1):476.
- de Groot S, Vreeswijk MP, Welters MJ, et al. The effects of short-term fasting on tolerance to (neo) adjuvant chemotherapy in HER2-negative breast cancer patients: a randomized pilot study. BMC Cancer. 2015;15.
- Kroeger CM, Klempel MC, Bhutani S, Trepanowski JF, Tangney CC, Varady KA. Improvement in coronary heart disease risk factors during an intermittent fasting/calorie restriction regimen: Relationship to adipokine modulations. Nutr Metab (Lond). 2012 Oct 31;9(1):98.
- Belkacemi L, Selselet-Attou G, Hupkens E, Nguidjoe E, Louchami K, Sener A, Malaisse WJ. Intermittent fasting modulation of the diabetic syndrome in streptozotocin-injected rats. Int J Endocrinol. 2012;2012:962012.
- Johnson JB, Summer W, Cutler RG, Martin B, Hyun DH, Dixit VD, Pearson M, Nassar M, Telljohann R, 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 Radic Biol Med. 2007 Mar 1;42(5):665-74.
- Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J, Jebb SA, Martin B, Cutler RG, Son TG, Maudsley S, Carlson OD, Egan JM, Flyvbjerg A, Howell A. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011 May;35(5):714-27.
- Klempel MC, Kroeger CM, Varady KA. Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet. Metabolism. 2013 Jan;62(1):137-43.
- Varady KA, Bhutani S, Klempel MC, et al. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutr J. 2013;12(1):146. Published 2013 Nov 12.
- Carlson O, Martin B, Stote KS, Golden E, Maudsley S, Najjar SS, Ferrucci L, Ingram DK, Longo DL, Rumpler WV, Baer DJ, Egan J, Mattson MP. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women. Metabolism. 2007 Dec;56(12):1729-34.
- Halberg N, Henriksen M, Söderhamn N, Stallknecht B, Ploug T, Schjerling P, Dela F. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol (1985). 2005 Dec;99(6):2128-36.
- Harvie M, Wright C, Pegington M, McMullan D, Mitchell E, Martin B, Cutler RG, Evans G, Whiteside S, Maudsley S, Camandola S, Wang R, Carlson OD, Egan JM, Mattson MP, Howell A. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr. 2013 Oct;110(8):1534-47.
- Donati A, Recchia G, Cavallini G, Bergamini E. Effect of aging and anti-aging caloric restriction on the endocrine regulation of rat liver autophagy. J Gerontol A Biol Sci Med Sci. 2008 Jun;63(6):550-5.
- Kume S, Uzu T, Horiike K, Chin-Kanasaki M, Isshiki K, Araki S, Sugimoto T, Haneda M, Kashiwagi A, Koya D. Calorie restriction enhances cell adaptation to hypoxia through Sirt1-dependent mitochondrial autophagy in mouse aged kidney. J Clin Invest. 2010 Apr;120(4):1043-55.
- Wohlgemuth SE, Seo AY, Marzetti E, Lees HA, Leeuwenburgh C. Skeletal muscle autophagy and apoptosis during aging: effects of calorie restriction and life-long exercise. Exp Gerontol. 2010 Feb;45(2):138-48.
- Gong Z, Kennedy O, Sun H, Wu Y, Williams GA, Klein L, Cardoso L, Matheny RW Jr, Hubbard GB, Ikeno Y, Farrar RP, Schaffler MB, Adamo ML, Muzumdar RH, Yakar S. Reductions in serum IGF-1 during aging impair health span. Aging Cell. 2014 Jun;13(3):408-18.
- Longo VD, Mattson MP. Fasting: molecular mechanisms and clinical applications. Cell Metab. 2014;19(2):181-192.
- Galluzzi L, Pietrocola F, Levine B, Kroemer G. Metabolic control of autophagy. Cell. 2014;159(6):1263-1276.
- Mccue, Marshall. (2012). Comparative Physiology of Fasting, Starvation, and Food Limitation. 10.1007/978-3-642-29056-5.
- Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46–58.
- Gotthardt JD, Verpeut JL, Yeomans BL, et al. Intermittent Fasting Promotes Fat Loss With Lean Mass Retention, Increased Hypothalamic Norepinephrine Content, and Increased Neuropeptide Y Gene Expression in Diet-Induced Obese Male Mice. Endocrinology. 2016;157(2):679–691.
- Li L, Wang Z, Zuo Z. Chronic intermittent fasting improves cognitive functions and brain structures in mice. PLoS One. 2013;8(6):e66069. Published 2013 Jun 3.
- Mattson MP. Energy intake, meal frequency, and health: a neurobiological perspective. Annu Rev Nutr. 2005;25:237-260.
- Martin, B., Mattson, M. P., & Maudsley, S. (2006). Caloric restriction and intermittent fasting: two potential diets for successful brain aging. Ageing research reviews, 5(3), 332-353
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine. 2017;177(7):930.
- Faris “Mo’ez Al-Islam” E., Kacimi S, Al-Kurd RA, et al. Intermittent fasting during Ramadan attenuates proinflammatory cytokines and immune cells in healthy subjects. Nutrition Research. 2012;32(12):947- 955.
- Malinowski B, Zalewska K, Węsierska A, et al. Intermittent Fasting in Cardiovascular Disorders-An Overview. Nutrients. 2019;11(3):673. Published 2019 Mar 20.
- Harris L, Hamilton S, Azevedo LB, Olajide J, De Brún C, Waller G, Whittaker V, Sharp T, Lean M, Hankey C, Ells L. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep. 2018 Feb;16(2):507-547.
- Li G, Xie C, Lu S, et al. Intermittent Fasting Promotes White Adipose Browning and Decreases Obesity by Shaping the Gut Microbiota. Cell Metabolism. 2017;26(4):672-685.e4.
- Santos HO, Macedo RCO. Impact of intermittent fasting on the lipid profile: Assessment associated with diet and weight loss. Clinical Nutrition ESPEN. 2018;24:14-21.
- Peterson CM. Intermittent Fasting Induces Weight Loss, but the Effects on Cardiometabolic Health are Modulated by Energy Balance. Obesity. 2019;27(1):11–11.
- Steven S, Taylor R. Restoring normoglycaemia by use of a very low calorie diet in long- and short-duration Type 2 diabetes. Diabet Med. 2015 Sep;32(9):1149-55.
- Overland, J., Toth, K., Gibson, A. A., Sainsbury, A., Franklin, J., Gauld, A., & Wong, J. (2018). The safety and efficacy of weight loss via intermittent fasting or standard daily energy restriction in adults with type 1 diabetes and overweight or obesity: A pilot study. Obesity Medicine, 12, 13-17.
- Chia-Wei Cheng, Fasting-mimicking diet promotes Ngn3-driven β-cell regeneration to reverse diabetes, Cell, 2017;168(5):775-788
- Choi IY, Piccio L, Childress P, et al. A Diet Mimicking Fasting Promotes Regeneration and Reduces Autoimmunity and Multiple Sclerosis Symptoms. Cell Rep. 2016;15(10):2136–2146.
- Di Francesco A, Di Germanio C, Bernier M, de Cabo R. A time to fast. Science. 2018;362(6416):770.
I have been eating only one meal per day, 5 days a week, on weekends 2 smaller meals for over 3 years now, just dinner and drinking only water during the day, so essentially doing the 24 hour fast over 70% of the time. Being in my 60s and never in my life being overweight, I have no detectable health issues. My diet consists of mostly vegetables with beef, dairy, fish, nuts and fruit. I tend to avoid breads, potatoes, and any processed foods. This has been working very well for me, but after reading this article I may try doing some more extended fasting, as I would like to increase a healthy lifespan
Great article! Thank you!
As a 70-year-old, I have tried both intermittent and longer (up to five days) fasts in the past three years. Although they both help keep the weight down and clear out the system, as one gets older, insulin resistance seems to be the greatest challenge which we all face as we get older, particularly if overweight.
Personally, I would point any person interested in trying to understand their metabolism to have a look at a professional collective of medically-qualified collaborators who contribute to Low Carb Down Under on You Tube: an outstanding source of enlightened discussion free of the influence of Big Food and Big Pharma!
Great article. I love that it touches on intermittent fasting to prolonged fasting but also states how the data was collected (i.e. animal studies) and that the benefits mostly are from reducing caloric intake. Though studies have used animal (e.g. mice) fasting has been practiced by humans for thousands of years mostly surviving in the form of religious rituals or pactices (for example many Episcopalians or Christians fast for Lent). Reduced calorie diets are promoted such as Yogis in India advising only one meal a day (admittedly this also falls under intermittent fasting as well).
One fasting subject they have not touched apon is fasting and exercise (or physical exertion) which has been studied far less therefore far less data is available. But something to consider perhaps, especially for diabetics as hypoglycemia and ketoacidosis are real life threatening conditions.
In conclusion, great article!
This is great! Fasting is physically beneficial for your body as well as a spiritual practice for those who mediate and pray! It’s great! Thanks for the insight.
Great article! I would love to hear more about insulin resistance.
As most studies have been performed on males, I believe there is lack of evidence fasting is beneficial for woman health, and hormonal balance, could you please touch on this?
The article mentions different types of fasting and benefits of fasting, but it is unclear to me whether it is believed that any benefits are derived only from extensive vs intermittent fasting. Can you provide any clarity on this?