There's a new "miracle" diet every few months that swears to yield rapid results- cut carbs, miss meals, juice for days only. You've probably either given one a try yourself or seen someone close to you dive in headfirst. Do these extreme diets really benefit your body, or do they simply put your body into survival mode? From YouTube "self-experiments" to TikTok challenges, prolonged fasting has become one of the most contentious health trends of the decade. Its proponents argue that it improves cognitive function, activates robust repair processes, and even reverses aging. Critics counter that it approaches irresponsible, forcing the body into starvation mode with surprisingly little scientific evidence to support many of the more lofty assertions.So, what actually goes down when you go without food for three days? To find out, we must dissect how the body slowly changes gears hour by hour, what research really reveals, and why fasting for 72 hours is much more complicated than a speedy "detox hack."The First 6–10 HoursIn the first few hours after a meal, your body relies on glucose from digested food to power everything from your brain to your muscles. By the 6-to-10-hour mark, those reserves are nearly gone.This is when the pancreas steps in, secreting glucagon—a hormone that is meant to maintain blood sugar levels by drawing on stored glycogen in your liver. What happens? You might get lightheaded, cranky, or suddenly sensitive to hunger. For most folks, this stage is like missing a meal or two, but it's merely the start of the metabolic cascade.Repair Mode and Autophagy After 16–24 HoursPast 16 hours, the body begins to turn on itself for energy. Autophagy starts—cells actually break down and reuse damaged parts for fuel.Certain animal experiments propose this might sweep away toxic proteins and cut disease risk. A few aggressive assertions even attribute autophagy to suppressing cancer cells, but authorities warn that is a long way from being established in humans. The one thing that is certain, though, is that growth hormone release surges at this point, slowing muscle loss and keeping you going even as calories run out.At 24 hours, glucose stores are depleted mainly. Fat is now your energy supply, with ketones beginning to accumulate in the blood. For the fasting aficionados, this is the "deep repair" stage, although it comes with headaches, exhaustion, and hunger pangs for most beginners.]]> Ketosis After 48 HoursBy the 48-hour point, your body is solidly in ketosis, living off fat-based ketones as its fuel money. For some, this change introduces unexpected acuity. Small studies indicate that ketones can temporarily enhance concentration and cognitive functions—an evolutionary response to enable humans to search for food when there was none to be found.But the disadvantages are difficult to overlook. Energy lulls, dehydration, confusion, and crankiness are typical. Muscle protein is progressively metabolized for necessary amino acids. Hunger peaks at this point, pushing mental determination and physical endurance to the limit.Survival Mode After 72 HoursThree consecutive days without food marks the turning point. At this stage, fasting isn't any longer a matter of "burning fat" or "detoxing"—it's survival.A 2019 Queen Mary University of London study indicated that long periods of fasting have the potential to induce a multi-organ reprogramming, resetting the immune system and metabolism. This involves increased autophagy, ongoing dependence on fat reserves, and immune cell production changes.Not all have an equal "reset." For some, the cost is dire: fatigue, lightheadedness, electrolyte disturbances, and risk of heart strain. Unscheduled by doctors, danger soon surpasses possible gain.Is 72-Hour Fasting Safe?As Johns Hopkins Medicine says, prolonged fasting can have an unwanted effect if the body enters starvation mode, where it holds on to fat instead of burning it. Rather than aiding weight control, this can decelerate metabolism and make it more difficult to sustain weight loss over time.Side effects are also true. According to Healthline, symptoms include nausea and irritability all the way to severe fatigue and brain fog while in ketosis. For patients with diabetes, heart disease, or deficiencies, the dangers can be life-threatening.Should You Still Try It?If fasting for 72 hours is so strenuous, then why do people still try it? Partly, it's the appeal of control—the notion of "restarting" the body and breaking with contemporary reliance on continuous snacking. There's also new science suggesting intermittent fasting, when done safely, might promote longevity and metabolic health but specialists caution there's a huge difference between intermittent fasting (such as 5:2 or 16:8 eating styles) and long-term starvation. The former can be safe and sustainable; the latter belongs in clinical trials or doctor's offices.Three days of fasting demonstrates just how adaptive the human body is. From tapping fat reserves to recycling junk cells, fasting biology is both interesting and intricate. But being interesting does not equate to being endorsed.For most people, safer fasting regimens—such as shorter intermittent fasts—are more researched and easier to stick to. Anyone thinking about extreme fasting should first see a healthcare professional. For one individual, it might seem like a "reset," but it might cause severe complications for another.What the 72-hour fast actually shows us is not so much about losing weight as it is about the precarious tightrope our bodies walk to sustain us. It's a message that health isn't about extremes, it's about sustainability, support, and regard for the limitations of human physiology.Disclaimer: The content of this article is for informational purposes only. Always consult a qualified healthcare professional before undertaking extensive dietary changes.