The basic truth is every body responds come fasting a small differently. Genetics, health, lifestyle…all these things play a part. However for a healthy and balanced person embarking top top a fasting journey, yes sir a basic timeline of events—a predictable set of metabolic responses together your rapid stretches from hrs into days. We’re breaking under that timeline here.


Between 0 and also 3 hours, your body is quiet going v the procedure of ingesting and storing the last thing you ate. After your body breaks under carbs, protein, and fat into glucose, amino acids, and fatty acids, her body metabolizes lock for power or stores them for later on use. Relying on the last point you ate before beginning your fast, a couple of different hormones can be at play in her bloodstream.

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If you’ve consumed a meal heavy in carbohydrates, you’ll have actually a many glucose in her bloodstream. That method a rise in insulin. Her pancreas secretes insulin to help shuttle glucose into cells whereby it can automatically be offered for energy production, protein synthesis in muscles, or stored for later use, either as glycogen (the temporary storage that glucose) or together adipose organization (long-term storage). Three hrs after her last meal, glucose and insulin have most most likely returned come “normal” pre-meal levels.

You’ll likewise experience a fluctuation in ghrelin and leptin. Ghrelin, the “hunger hormone” is what turns on your appetite. Leptin does the opposite. ~ a meal, your ghrelin level fall and your leptin levels rise, effectively transforming your appetite off, signaling to your mind that you’re full.

The 0–3 hour time framework is what we speak to an anabolic, or growth period since nutrient are easily accessible and the body deserve to burn them, construct muscle with them, or save them.


During the 4–24 hour phase, her body switches come the catabolic, or breakdown, state wherein stored nutrients are placed to use. As soon as blood glucose and insulin levels drop, you’ll suffer an uptick in glucagon—a catabolic hormone that stimulates the failure of glycogen (stored glucose) because that energy. Since glucose is still your body’s main fuel resource in this phase, her metabolism will certainly attempt to failure enough glycogen to store your blood glucose in the “normal” variety (about 70–120 mg/dL).

Toward the finish of this phase, you’ll most likely start depleting your glycogen stores, which means you need access to another fuel source. Her body will begin the move from glucose to ketones. Glucose is still her primary, preferred fuel source, yet when her glucose reserves are nearing empty, you’ll start using fat stores and also ketone body to comprise the difference. In between 12 to 24 hours, blood glucose levels will be diminished by around 20%.

The precise time the your human body starts moving from utilizing glucose come ketones for power depends on just how much glycogen you’ve gained stored away and how much energy you’re burn throughout the day. This move happens much faster if you exercising because you’ll burn with your glycogen stores more quickly.


Your body is switching right into fat-burning mode now—glycogen is considerably depleted, so you’ll produce and use ketone bodies for energy. With the malfunction of fat (a procedure called lipolysis), fat cell in the human body release complimentary fatty acids. PPAR-alpha (a regulator of fat management in the liver), which is crucial for ketogenesis, is activated and ensures those fatty acids room used.

Fatty acids take trip to the liver whereby they are transformed right into ketone bodies through the process of beta-oxidation. When we speak “ketone bodies,” we’re introduce to three distinct species of molecules: acetone, acetoacetate, and beta-hydroxybutyrate, or BHB because that short. Your body can use both acetoacetate and also BHB for power production. Blood ketone meters, which friend may have seen world use while fasting or top top a ketogenic diet, measure up BHB level in the blood. BHB levels can vary based upon the individual, but within 24–72 hours of fasting, you’re most likely to watch BHB levels increase to somewhere in between 0.5–2 mM; the variety for nutritional ketosis.

At this point, ketones end up being your primary fuel, but your brain still requirements a little of glucose to function. With none come be discovered in your blood, and also your glycogen stores completely tapped, your body makes glucose native non-carbohydrate sources choose fat, ketones, and amino acids with a procedure called gluconeogenesis. Yes, your body have the right to actually do sugar the end of protein and fat. During this step of fasting, friend produce around 80 grams the glucose per day making use of this process, many of i m sorry is supplied by the brain. The remainder of the body have the right to rely almost exclusively top top ketone bodies.

So those going on through ghrelin, the hunger hormone? This deep right into a fast, It in reality starts to decrease again. One study verified that also though ghrelin rises and falls in a cyclic pattern regarded circadian rhythm, complete ghrelin output decreased every 24 hours of fasting. So, by job 3, overall ghrelin output was lower than job 2 and also day 1. These outcomes may describe why overall hunger levels seem come decrease around the third day of a fast and beyond.


We’re currently in the prolonged fasting phase. After day 3, glucose and also insulin levels remain low, hunger remains suppressed, and you’re in a steady state the nutritional ketosis. You’ll likewise see a decrease in something referred to as IGF-1 (insulin like development factor). IGF-1 is a hormone affiliated in growth and development. When nutrients are restricted, her liver reduce its IGF-1 production, to add there’s a diminish in IGF binding proteins. Momentary decreases the IGF-1 activity in adults have been associated with less oxidative stress and may be critical part of anti-cancer and also anti-aging dietary interventions.

Prolonged fasting has also been shown to activate to move resistance to toxins and stress in both mice and human studies. A minimum that 72 hrs of fasting appears to be forced to suffer these benefits. Fasting because that 3+ days has actually been presented to decrease circulating insulin and also glucose through 30% or more, which can contribute to lessened risk of metabolic disease. The reduced levels of expansion hormone and also insulin in the bloodstream also seem to have actually benefits for immunity, inflammation, neurogenesis, and overall metabolic health.

BHB (one that the 3 ketone bodies) levels proceed to climb throughout your fast. At this point, they’ll most likely settle out approximately 1.5–3 mM. BHB levels above 2.0 mM might correlate with lower hunger levels together well, due to the fact that the human body is now producing sufficient quantity of ketones to fuel the body.


After fasting because that 5 or more days, glucose, insulin, and IG-F room all significantly reduced, and also you’re in a stable state the ketosis. Five days that fasting in humans has been displayed to cause over a 60% decrease in IGF-1 and also a far-ranging increase in IGF-1 inhibiting proteins. ~ 10 job of fasting, IGF-1 levels might be as low as those checked out in people with growth hormone deficiency—a populace associated with reduced risk of cancer, diabetes, and overall mortality.

Conversely, BHB level will proceed to rise. It’s necessary to note right here the difference in between ketosis (burning ketones for fuel) and also ketoacidosis (dangerously acidic blood from too many high ketone levels, usually just seen in alcoholics, diabetics, and also cases of excessive starvation). Normally, the presence of insulin speak the human body to stop boosting its production of ketones and also plateau them in ~ a safe level, but due to the fact that your insulin levels continue to be low throughout a lengthy fast, this feedback loop doesn’t occur. One study confirmed that BHB levels will ultimately plateau between 5–6 mM by work 20–25 that fasting. For this reason if you’re contemplating one extremely long fast, it’s worth consulting v a medical skilled to make sure you’re doing therefore safely.

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As you can see, over there is lot of of promising research on the metabolic benefits of fasting. Specifically which services you’ll experience depends on the term of your fast. Together always, we suggest working through a medical skilled to uncover the plan and also approach that’s ideal for your health and also goals.