What are the effects of not eating?

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Recently, buzzwords like ketosis and intermittent fasting have found themselves at the forefront of diet culture and you should consult your physician before doing anything regarding fasting. Additionally anyone who has an eating disorder, is in recovery from an eating disorder, or is at high risk for an eating disorder, should not attempt to try fasting unless it is medically necessary.

Fasting is defined as a coordinated set of metabolic changes that shift the body from digesting primarily glucose (sugar) to fats and amino acids (protein). An overnight fast of 8-10 hours is normal for most people, with many choosing to participate in longer fasts for health, religious, or cultural reasons. (11) While someone may feel excellent after fasting for 16 hours, others may not, and it is important to avoid intentional periods of painful food restriction. We encourage our members to work with their healthcare team to determine if fasting is a proper fit for one’s dietary needs.

At Within Health, our clinical experts understand that everyone’s body and metabolism are unique, and they encourage members to practice self compassion and mindful eating habits when fasting. This article will discuss the post ingestion timeline of what happens physically and chemically when someone fasts, and the effects of not eating on the body.

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Hours 0-4

During the first four hours after ingestion, the body metabolizes carbohydrates, fat, and protein into glucose, fatty acids, and amino acids. This is an anabolic, or growth, period that entails energy production and storage. (1)

As glucose enters the bloodstream, the pancreas secretes insulin to help transport glucose inside cells for immediate use. Anything not used immediately will find its way to the liver for short-term storage as glycogen. Long-term storage takes the form of adipose tissue throughout the body. (1)

During this first phase of digestion, one will typically feel full and satisfied because the body has readily accessible energy stores. This full feeling stems from the hormone Leptin, the levels of which will rise after a meal. Leptin will give way to the hormone Ghrelin as glucose depletes and the body becomes hungry again. (2)

Hours 5-12

As fasting persists, blood glucose and insulin levels steadily drop, and the body turns to the liver and skeletal system’s supply of glycogen for energy. (3) During these hours, there is a flood of Glucagon into the system via the pancreas. Glucagon is a catabolic hormone that initiates the breakdown of glycogen to maintain blood glucose levels. (3) As these stores deplete, the body will look elsewhere for more energy.

Sometimes, one can experience any of the following mild physical effects of not eating during these hours: (4)

  • Dizziness
  • Nausea
  • Fast heart rate
  • Problems with coordination
  • Headaches
  • Blurred vision and/or speech
  • Fainting

On the other hand, clinicians have noted many positive effects from fasting, such as: (12,13)

  • Increase vigilance
  • Improved mood, for example in the setting of depression
  • Increased sense of tranquility
  • Increased brain availability of serotonin, endogenous opioids, and endocannabinoids

Hours 12-24

With both glucose and glycogen depleted, the body begins to digest triglycerides which starts the metabolic state of ketosis. The free fatty acids and glycerol in these triglycerides undergo a conversion in the liver into ketone bodies and glucose, both of which the body uses as fuel. (5)

Ketogenic diets prioritize this time window because ketones replace glucose as the primary source of energy. In other words, the body is burning adipose tissue for fuel. The benefits of ketosis go far beyond weight management as well. Studies have shown improvements, with the aid of ketosis, in all of the following: (6,12)

  • Schizophrenia
  • Bipolar disorder
  • Major depressive disorder
  • Epilepsy
  • Mild cognitive impairment associated with Alzheimer disease
  • Rheumatic diseases
  • Chronic pain syndromes

Studies show that alternate-day fasting trials of 3 to 12 weeks in duration appear to be most effective at reducing body weight, body fat, total cholesterol, and triglycerides. This is with scheduled periods of eating in the absence of a diagnosed eating disorder (ED). For those who struggle with ED, Within Health recommends consulting with our team before embarking on that journey to ensure it is the best path for our members. (14)

Hours 24-72+

Any prolonged fasting of this kind should be done with the aid of a medical care professional.

The body will continue in ketosis and one may notice an uptick in both mental clarity and energy. Ketones have been shown to enhance antioxidants and decrease free radicals. (7) 

The more severe effects of not eating typically begin after 72 hours. (8) As the body continues the hunt for glucose, the next source stems from the digestion of proteins found in vital muscle tissue.

With an adequate amount of water, the human body can continue to survive anywhere from three weeks to 70 days by digesting proteins in the muscular system. (8,9) Survival is the body’s main concern at this phase, so it will divert energy to critical organ operation.

Deprivation of nutrients at this stage is largely reversible depending on the situation presented. If someone has gone many weeks without eating, to suddenly reintroduce food again could trigger refeeding syndrome. Treatment for this requires inpatient monitored feeding to correct any sudden electrolyte shifts. (10)

Under the proper care of a medical professional, the potential physical and mental benefits from fasting can be positive, when done in mindful ways. Intentionally restricting food for days at a time, when it causes significant emotional distress, is a sign that fasting should be avoided for the time being and likely is not something to be tried again.

Disclaimer about "overeating": Within Health hesitatingly uses the word "overeating" because it is the term currently associated with this condition in society, however, we believe it inherently overlooks the various psychological aspects of this condition which are often interconnected with internalized diet culture, and a restrictive mindset about food. For the remainder of this piece, we will therefore be putting "overeating" in quotations to recognize that the diagnosis itself pathologizes behavior that is potentially hardwired and adaptive to a restrictive mindset.


  1. Patricia, J. J., & Dhamoon, A. S. (2021). Physiology, Digestion. In StatPearls. StatPearls Publishing.
  2. Klok, M. D., Jakobsdottir, S., & Drent, M. L. (2007). The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review. Obesity reviews : an official journal of the International Association for the Study of Obesity, 8(1), 21–34. https://doi.org/10.1111/j.1467-789X.2006.00270.x
  3. Sanvictores, T., & Casale, J. (2021, July 26). Physiology, fasting. StatPearls. Retrieved May 5, 2022, from https://www.statpearls.com/ArticleLibrary/viewarticle/21631#ref_22140269 
  4. Kohleimer, M. (2003). 4: Absorption, Transport, and Retention. In Nutrient metabolism. essay, Academic Press. 
  5. Browning, J. D., Baxter, J., Satapati, S., & Burgess, S. C. (2012). The effect of short-term fasting on liver and skeletal muscle lipid, glucose, and energy metabolism in healthy women and men. Journal of lipid research, 53(3), 577–586. https://doi.org/10.1194/jlr.P020867
  6. Brandi Jones, M. S. N.-E. D. R. N.-B. C. (2021, October 7). Symptoms of not eating. Very good health. Retrieved May 3, 2022, from https://www.verywellhealth.com/not-eating-signs-symptoms-5201896 
  7. Masood W, Annamaraju P, Uppaluri KR. Ketogenic Diet. [Updated 2021 Nov 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499830/
  8. Bushak, L. (2014, September 17). Now entering starvation mode: What happens to your metabolic processes when you stop feeding your body. Medical Daily. Retrieved May 3, 2022, from https://www.medicaldaily.com/now-entering-starvation-mode-what-happens-your-metabolic-processes-when-you-stop-feeding-280666
  9. Barrell, A. (2020, March 17). How long can you go without food? survival, effects, and more. Medical News Today. Retrieved May 5, 2022, from https://www.medicalnewstoday.com/articles/how-long-can-you-go-without-food#water 
  10. Malnutrition. Johns Hopkins Medicine. (2021, August 8). Retrieved May 3, 2022, from https://www.hopkinsmedicine.org/health/conditions-and-diseases/malnutrition 
  11. Maughan RJ, Fallah J, Coyle EFThe effects of fasting on metabolism and performanceBritish Journal of Sports Medicine 2010;44:490-494.
  12. Fond G, Macgregor A, Leboyer M, Michalsen A. Fasting in mood disorders: neurobiology and effectiveness. A review of the literature. Psychiatry Res. 2013 Oct 30;209(3):253-8. doi: 10.1016/j.psychres.2012.12.018. Epub 2013 Jan 15. PMID: 23332541.
  13. Michalsen A. Prolonged fasting as a method of mood enhancement in chronic pain syndromes: a review of clinical evidence and mechanisms. Curr Pain Headache Rep. 2010 Apr;14(2):80-7. doi: 10.1007/s11916-010-0104-z. PMID: 20425196.
  14. Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015 Oct;73(10):661-74. doi: 10.1093/nutrit/nuv041. Epub 2015 Sep 15. PMID: 26374764.


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