What Is the “Fasted State” and What “Breaks” Your Fast?

December 13, 2019 | By Fastwell Team | Fasting

These days there is a lot of buzz about fasting. Almost everyone has a friend who regularly skips breakfast, eats one meal a day or will even go for days without food. But, what exactly happens in the body when you fast? And, what “breaks” your fast?

We talked with top experts (both researchers and practitioners) and poured over dozens of publications to understand the physiology of fasting. To put it simply, we learned that the “fasted state” is really a set of biological mechanisms that are activated when you stay below thresholds for intake of macronutrients and calories. 

With this understanding of what fasting is and how it impacts the body, Fastwell has designed its products to have the right thresholds of calories, macronutrients and micronutrients to help you optimize your experience and outcomes with fasting. Our products help you fast safely.

Without further ado, here are the details about these biological mechanisms and the thresholds that govern them.

What does the body do when fasting and how do you get into the “fasted state”?

The body enters ketosis

The first thing to happen during fasting is the body switches from burning glucose to burning fat. This happens after a period of about 12-24 hours of carbohydrate restriction (<20 g), which causes blood insulin to drop and ketone bodies to rise. A ketone level of 0.6 to 3.0 mmol/l is considered “nutritional ketosis”. To accurately test your ketone levels, you can use a blood ketone monitor.

  • The time it takes to reach ketosis depends on the individual. Some people reach ketosis in a few hours while others may take 24 hours or longer.
  • Low carb diets, like a ketogenic diet help support ketosis, but fasting is more powerful to get you into ketosis.1 Pairing fasting with a low carb diet is a great way to get the benefits of fasting and low carb.
  • Exercise and intake of healthful fats can also help speed up the shift into ketosis.2

Autophagy kicks in

Autophagy is a process by which our bodies eliminate damaged cellular components. It’s hypothesized that autophagy is activated after 24 hours of consuming less than ~700 calories and less than 30 g of protein (or 35 g of plant-based protein).

  • Autophagy helps clear the body of malfunctioning cells that could eventually become cancerous.3
  • In a ground-breaking Nobel Prize winning experiment,4 it was shown that fasting induces autophagy, which can prevent the development of tumors in malfunctioning cells in mice.

Stem cell rejuvenation occurs

Stem cells play an important role in cell renewal and healing. As we age, stem cell rejuvenation slows down which contributes to declining organ function. Improving stem cell turnover can be protective against diabetes and cancer. Several animal studies have given indication of stem cell rejuvenation during fasting.

  • Prolonged fasts have been shown to promote pancreatic stem cell rejuvenation in diabetic mice. The reprogramming of the pancreatic cells led to the restoration of insulin secretion and a complete reversal of diabetes.5
  • Fasting may promote intestinal stem cell rejuvenation in older mice, which could reduce the risk of intestinal cancers.6

Human Growth Hormone levels increase

Growth hormone helps maintain tissues and organs throughout life, unfortunately production begins to slow in middle age. Fasting has been shown to be one of the best ways to increase HGH naturally.7 See a great breakdown of the research on increased HGH through fasting here.

Microbiome health improves (potentially)

Microbiome health is becoming more understood as a key factor for metabolic health. It is hypothesized that fasting helps the microbiome by allowing gut bacteria to rest from digestion. This thorough article on microbiome and fasting quotes “Germ Guy” Jason Anthony Tetro, a Visiting Scientist at the University of Guelph as saying “We are just now starting to determine whether there is any impact on the human microbiome during fasting.”

How does fasting affect disease risk markers?

We worked with Dr. Krista Varady of University of Illinois-Chicago (one of the top researchers of fasting!) to understand how the biological mechanisms of fasting might impact metabolic health and chronic disease. (See sources 8-12)


  1. https://thefastingmethod.com/power-comparison-fasting-vs-low-carb-fasting-26/
  2. https://www.medicalnewstoday.com/articles/324599
  3. White E: The role for autophagy in cancer. J Clin Invest 2015, 125:42-6.
  4. Komatsu M, Waguri S, Ueno T, Iwata J, Murata S, Tanida I, Ezaki J, Mizushima N, Ohsumi Y, Uchiyama Y, Kominami E, Tanaka K, Chiba T: Impairment of starvation-induced and constitutive autophagy in Atg7-deficient mice. J Cell Biol 2005, 169:425-34.
  5. Cheng CW, Villani V, Buono R, Wei M, Kumar S, Yilmaz OH, Cohen P, Sneddon JB, Perin L, Longo VD: Fasting-Mimicking Diet Promotes Ngn3-Driven beta-Cell Regeneration to Reverse Diabetes. Cell 2017, 168:775-88 e12.
  6. Mihaylova MM, Cheng CW, Cao AQ, Tripathi S, Mana MD, Bauer-Rowe KE, Abu-Remaileh M, Clavain L, Erdemir A, Lewis CA, Freinkman E, Dickey AS, La Spada AR, Huang Y, Bell GW, Deshpande V, Carmeliet P, Katajisto P, Sabatini DM, Yilmaz OH: Fasting Activates Fatty Acid Oxidation to Enhance Intestinal Stem Cell Function during Homeostasis and Aging. Cell Stem Cell 2018, 22:769-78 e4.
  7. https://www.healthline.com/nutrition/11-ways-to-increase-hgh#1
  8. Anton SD, Moehl K, Donahoo WT, Marosi K, Lee SA, Mainous AG, 3rd, Leeuwenburgh C, Mattson MP: Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring) 2018, 26:254-68.
  9. 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.
  10. Patterson RE, Sears DD: Metabolic Effects of Intermittent Fasting. Annu Rev Nutr 2017, 37:371-93.
  11. Rothschild J, Hoddy KK, Jambazian P, Varady KA: Time-restricted feeding and risk of metabolic disease: a review of human and animal studies. Nutr Rev 2014, 72:308-18.
  12. St-Onge MP, Ard J, Baskin ML, Chiuve SE, Johnson HM, Kris-Etherton P, Varady K, American Heart Association Obesity Committee of the Council on L, Cardiometabolic H, Council on Cardiovascular Disease in the Y, Council on Clinical C, Stroke C: Meal Timing and Frequency: Implications for Cardiovascular Disease Prevention: A Scientific Statement From the American Heart Association. Circulation 2017, 135:e96-e121.
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