Fasting consists of voluntarily abstaining from caloric food for a set period, during which your body shifts from burning dietary calories to burning stored energy. The core of any fast is this metabolic switch: once you stop eating, your blood sugar and insulin levels drop, your liver burns through its stored carbohydrates, and your body begins breaking down fat for fuel. Everything else, from the specific time window to what you’re allowed to drink, varies by the type of fast you follow.
What Happens Inside Your Body
About three to four hours after your last meal, your body enters an early fasting state. Blood sugar and insulin begin to decline, and your liver starts converting its stored glycogen (a form of carbohydrate) into glucose to keep your brain and muscles running. This glycogen supply is limited. By roughly 12 to 18 hours without food, those stores are mostly depleted, and your body ramps up a process called lipolysis, where fat cells are broken down into smaller molecules your body can use as fuel.
If the fast continues, your liver begins converting those fat molecules into ketone bodies, primarily one called beta-hydroxybutyrate. This is the hallmark of the metabolic switch from sugar-burning to fat-burning. Blood ketone levels between 0.5 and 3 mmol/L indicate your body has entered nutritional ketosis, the state where stored fat becomes your primary energy source. Insulin plays a gatekeeper role here: it powerfully suppresses ketone production, which is why the fast only works once insulin drops low enough to get out of the way.
Hormonal Changes During a Fast
Fasting triggers significant hormonal shifts beyond the drop in insulin. One of the most striking involves human growth hormone (HGH), which supports fat metabolism and muscle preservation. A study published in Frontiers in Endocrinology found that during a 24-hour water-only fast, people who started with low baseline HGH levels saw a median increase of 1,225%, with some individuals experiencing increases as high as 20,000%. Those who started with higher baseline levels saw a more modest median rise of about 50%. The takeaway: fasting produces a real and measurable surge in growth hormone, though the magnitude depends on where you start.
Fasting also affects a cellular cleanup process called autophagy, where your cells break down and recycle damaged or dysfunctional components. Animal studies suggest autophagy ramps up significantly between 24 and 48 hours of fasting. Researchers haven’t yet pinpointed the exact timing in humans, but autophagy is one of the reasons extended fasts attract interest beyond simple weight loss.
Common Fasting Protocols
Most people who fast follow one of a few well-established schedules. The differences come down to how long you fast, how often, and whether you eat anything at all during the fasting window.
- 16:8 (daily time-restricted eating): You eat within a six- to eight-hour window each day and fast for the remaining 16 to 18 hours. This is the most popular approach because it fits naturally into a routine. Skipping breakfast and eating between noon and 8 p.m. is a common version.
- 5:2 (modified fasting): You eat normally five days a week and restrict yourself to one 500 to 600 calorie meal on the other two days. This isn’t a complete fast, but the severe calorie restriction on those two days produces many of the same metabolic effects.
- OMAD (one meal a day): You eat a single meal within roughly a one-hour window and fast for the other 23 hours. This is a more aggressive version of daily time-restricted eating.
- Extended fasts (24 to 72 hours): These involve going a full day or more without caloric food, consuming only water, and sometimes electrolytes. Extended fasts push the body deeper into ketosis and are more likely to trigger significant autophagy, but they carry greater risk and aren’t suitable for everyone.
What You Can and Can’t Consume
A strict fast means only water. But most intermittent fasting protocols allow non-caloric beverages: black coffee, plain tea, and water are generally considered safe because they don’t raise insulin enough to interrupt the metabolic switch. Adding cream, sugar, or milk introduces calories and can trigger an insulin response, which defeats the purpose.
Artificial and natural sweeteners occupy a gray area. Stevia, for example, has been studied extensively. A meta-analysis found that stevia consumption has no significant effect on insulin concentration or long-term blood sugar markers. However, there’s a hypothesis that non-nutritive sweeteners may still interact with sweet taste receptors in the gut and influence hormones like GLP-1 that are involved in insulin signaling. In practice, a splash of stevia in your coffee is unlikely to break a fast in a meaningful way, but purists avoid all sweeteners to eliminate any possibility of a metabolic response.
Bone broth, diet sodas, and supplements with fillers or calories are more clearly disruptive. If your goal is autophagy or deep ketosis, anything beyond water, plain coffee, and plain tea introduces uncertainty.
What Fasting Does Not Consist Of
Fasting is not the same as starving or simply eating less. Calorie restriction means eating smaller amounts throughout the day, keeping your body in a constant fed state with insulin always somewhat elevated. Fasting, by contrast, creates a distinct metabolic period where insulin drops low enough to unlock fat stores and trigger hormonal changes that calorie restriction alone does not produce.
It also doesn’t require complete immobility or rest. Light to moderate activity during a fast is generally well tolerated, and some people find that exercise in a fasted state feels easier once their body adapts to burning fat. Intense or prolonged exercise during longer fasts is a different story, since your body’s access to quick-burning glucose is limited.
Finally, fasting is not a single experience. A 16-hour overnight fast and a 72-hour water fast are fundamentally different in terms of what your body goes through. The 16-hour version nudges you into early fat-burning. The 72-hour version pushes you into deep ketosis, significant autophagy, and hormonal territory that shorter fasts never reach. The protocol you choose should match what you’re actually trying to accomplish.