Understanding the Stages of Fasting

If you’re even remotely in touch with today’s health and fitness trends, you’ve probably heard of fasting. But what’s actually happening to your body during a fast? And how long do you have to fast to get the benefits? In this post, we’ll look at the effects of fasting from the time you finish eating, all the way up to 48 without food.


Before you experiment with fasting: quick disclaimer: you may want to consult with a health professional. This might be especially relevant if you’re pregnant or have a chronic illness.


1 Hour

About an hour after you finish eating, your blood sugar peaks as glucose from the food you just ate is swirling around your bloodstream. Your body deals with this extra glucose by secreting insulin, which helps store glucose as glycogen in your liver and muscles, or as fat in various parts of the body.


6-8 Hours

After 6-8 hours of not eating, whatever food you ate will have passed through the stomach and small intestine. People often notice an uptick in their energy levels at this point because their bodies no longer have to spend a bunch of resources breaking down food.


12 Hours

Around the 12 hour mark, you are officially in a fasted state. This is marked by a significant drop in insulin levels since there’s no longer any circulating glucose that the body has to deal with. When insulin levels in the body drop below a certain point, your liver starts converting fat into ketones to use as a source of energy. This further increases energy levels as ketones are a more efficient source of energy when compared to glucose.


Ketones also stimulate the release of Brain derived neurotrophic factor or BDNF in the brain. BDNF, in turn has been shown to stimulate neurogenesis, where new neurons and neuronal connections are formed. This is particularly exciting news when it comes to the study of long term memory and neurodegenerative diseases like Alzheimer’s.


20-24 Hours

All of the above effects continue as the fast is prolonged, but around the 20 hour mark, the body starts producing endogenous antioxidants. These antioxidants reduce inflammation in the body, which can prevent chronic disease.


Another process that begins shortly after is something called “autophagy”. Derived from the Greek language, it essentially means “self-eating”. Sounds scary doesn’t it? But keep in mind that your body is incredibly sophisticated. It doesn’t just start eating every cell in sight. It prioritizes breaking down the cells that are old and damaged. So the process is a kind of cellular cleansing, and might even be an effective means of cancer prevention. You shed the unnecessary baggage and emerge a leaner, cleaner version of yourself.


48 Hours

After 48 hours, levels of human growth hormone peak, stimulating lean muscle growth, greater physical and cognitive performance, and cellular repair. Now you might be wondering at this point: how can I be building muscle if I’m not eating any protein? The truth is, we need our muscles most in times of famine, and it’s absurd to think that our ancestors would have evolved muscles that required constant fuel when they might not know where their next meal is coming from. This cultural obsession we have with constantly eating isn’t backed by any real science, and probably does more harm than good in a lot of cases.


Beyond 48 Hours

It’s important to note that all of the effects we’ve mentioned so far continue as the fast is prolonged past the 48 hour mark, but that they exhibit diminishing returns. For example, most researchers agree that the fat-burning effects of fasting peak between 12-16 hours. In much the same way, Studies on autophagy and fasting reveal that the peak levels of autophagy occur between 24-36 hours of not eating, after which point there’s a significant drop off. Thus, it’s important to be clear about why you’re fasting in the first place. Are you trying to control your weight? Do you want to increase focus and brain health? Do you want to prevent disease? Or maybe improve longevity?


So there you have it. You now have a basic understanding of what’s going on at each stage of fasting. It’s my hope that you can use this knowledge to find the fasting window that works best for you. As always if you enjoyed this article, give it a like, and subscribe to our mailing list for updates on new posts, and exclusive insights into the world of biohacking.



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  2. Fung, Jason. “Longer Fasting Regimens – 24 Hours or More.” Diet Doctor, 29 Jan. 2020, www.dietdoctor.com/longer-fasting-regimens.

  3. H, Karen, et al. “The 5 Stages of Intermittent Fasting - LIFE Apps: LIVE and LEARN.” LIFE Apps | LIVE and LEARN, 22 May 2019, lifeapps.io/fasting/the-5-stages-of-intermittent-fasting/.

  4. Kim, Gene. “The Amazing Ways Intermittent Fasting Changes Your Body and Brain.” Business Insider, Business Insider, 20 Dec. 2017, www.businessinsider.com/fasting-diet-ketosis-brain-body-effects-2017-12.

  5. Prasad, Seema. “48-Hour Fasting Benefits And Risks: Everything To Know.” Medical Daily, 28 May 2019, www.medicaldaily.com/48-hour-fasting-benefits-risks-everything-know-435613.

  6. Rui, Liangyou. “Energy metabolism in the liver.” Comprehensive Physiology vol. 4,1 (2014): 177-97. doi:10.1002/cphy.c130024

  7. Wilhelmi de Toledo, Françoise et al. “Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects.” PloS one vol. 14,1 e0209353. 2 Jan. 2019, doi:10.1371/journal.pone.0209353

  8. Zhu, Yueming et al. “Metabolic regulation of Sirtuins upon fasting and the implication for cancer.” Current opinion in oncology vol. 25,6 (2013): 630-6. doi:10.1097/01.cco.0000432527.49984.a3

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