When you stop eating, your body launches a predictable sequence of metabolic shifts to keep you alive on stored fuel. In the first few hours, it burns through sugar reserves. By 18 to 24 hours, it’s breaking down fat. Along the way, hormone levels change dramatically, your cells ramp up internal cleanup processes, and your kidneys start flushing sodium. Here’s what’s actually happening inside your body at each stage.
The First 18 Hours: Burning Through Glycogen
Your body’s first move is to tap into glycogen, a form of glucose stored in your liver and muscles. You carry roughly 100 grams of glycogen in your liver and several hundred more in muscle tissue, and this is your most accessible energy reserve. About 3 to 4 hours after your last meal, digestion wraps up and your body shifts into what’s called the early fasting state, drawing down those glycogen stores to maintain blood sugar.
By around 18 hours without food, liver glycogen is largely depleted. This is a turning point. Your body now needs to find fuel elsewhere, so it begins breaking down fat and, to a lesser extent, protein. One side effect of glycogen depletion is rapid water loss: every gram of glycogen is stored alongside 3 to 4 grams of water. As glycogen disappears, that water gets released and excreted. This is why people often lose several pounds in the first day or two of a fast, almost entirely from water rather than fat.
Hormonal Shifts Start Quickly
Fasting triggers a cascade of hormonal changes, and some of them are surprisingly large. Insulin, the hormone that tells your cells to absorb sugar from the blood, drops significantly. Comparing an overnight fast to a prolonged 72-hour fast, research published in the Journal of Applied Physiology found insulin levels fell by about 43%, while glucagon, the hormone that tells your liver to release stored energy, rose by about 35%. These shifts begin well before 72 hours; insulin starts declining within hours of your last meal.
Growth hormone is one of the most dramatic responders. During a 24-hour water-only fast, growth hormone levels can increase by 5-fold in men and up to 14-fold in women. People who start with lower baseline levels see the most striking jumps, with median increases around 1,225%. Growth hormone helps preserve muscle tissue and promotes fat breakdown, so this surge is essentially your body protecting lean mass while prioritizing fat as fuel.
Your Body Switches to Burning Fat
Once glycogen runs low, typically between 18 and 36 hours into a fast, your liver starts converting fatty acids into molecules called ketones. Ketones serve as an alternative fuel source that most of your tissues can use, including your brain. This metabolic state, called ketosis, is the hallmark of extended fasting. Your body doesn’t flip a single switch; the transition is gradual, with ketone production ramping up as glycogen stores empty out.
The brain normally relies almost exclusively on glucose, but it adapts remarkably well to ketones. One ketone in particular activates signaling pathways that increase production of a protein involved in forming new neural connections and maintaining existing ones. This is one reason some people report improved mental clarity during fasting, though the experience varies widely from person to person, and the early hours of a fast often feel worse before they feel better as blood sugar dips and the body adjusts.
Cellular Cleanup Ramps Up
Fasting activates autophagy, a process where your cells break down and recycle damaged components, old proteins, and dysfunctional structures. Think of it as your body’s internal maintenance system. When energy is abundant, cells focus on growth. When food is scarce, they shift into repair mode, dismantling what’s broken and reusing the raw materials.
The timing in humans isn’t perfectly nailed down. Animal studies suggest autophagy ramps up significantly between 24 and 48 hours of fasting, but measuring this in human tissues is far more difficult, and researchers haven’t established a precise peak window for people. What is clear is that this process is triggered by the same low-energy signals that drive the shift to fat burning: falling insulin, rising glucagon, and activation of cellular energy sensors.
Inflammation Drops Measurably
One of the more striking findings in fasting research involves the immune system. A study published in the journal Cell found that short-term fasting dramatically reduced the number of inflammatory immune cells (monocytes) circulating in the blood. These cells play a central role in chronic inflammation, the kind linked to heart disease, diabetes, and autoimmune conditions.
The mechanism works through the liver. When you stop eating, falling glucose and insulin levels trigger the liver to reduce production of a signaling molecule that normally pulls monocytes out of the bone marrow and into the bloodstream. Fewer monocytes in circulation means less baseline inflammatory activity. Fasting altered the expression of more than 2,700 genes in these immune cells, fundamentally shifting them toward a less inflammatory profile. People who practice intermittent or religious fasting tend to have lower baseline levels of several major inflammatory markers, including TNF-alpha, IL-6, and IL-1 beta.
Importantly, this anti-inflammatory effect didn’t compromise the immune system’s ability to respond to actual threats. When faced with an infection, fasted animals could still mobilize monocytes normally for emergency defense and tissue repair.
Your Kidneys Flush Sodium and Water
A less well-known effect of fasting is increased sodium loss through the kidneys, a phenomenon called fasting-induced natriuresis. The mechanism ties directly to falling insulin levels. Insulin normally stimulates your kidneys to reabsorb sodium. It does this partly by boosting the activity of a transporter in the kidney tubules that pulls sodium and glucose back into the body together, matched at a 1:1 ratio. When insulin drops during a fast, this transporter becomes less active, and sodium passes into the urine instead of being reclaimed.
Insulin also promotes sodium retention further down the kidney’s filtration system through channels in the collecting tubules. When insulin falls, those channels become less active too. The net result is that your kidneys release a substantial amount of sodium during the first day or two of fasting, pulling water along with it. Combined with the water released from glycogen breakdown, this explains the frequent urination, lightheadedness, and sometimes headaches people experience in the early stages of a fast. Electrolyte loss, not just hunger, is often what makes fasting feel physically uncomfortable.
What Changes After 48 Hours
Beyond two days, the metabolic picture stabilizes somewhat. Ketone production is in full swing, and your brain has largely adapted to using them as its primary fuel. Growth hormone remains elevated. Insulin stays low. Autophagy continues, though exact activity levels are hard to measure.
The body becomes increasingly efficient at sparing protein. Early in a fast, some muscle protein gets broken down to provide amino acids the liver can convert to glucose. But as ketone levels rise, the brain’s demand for glucose drops, and protein breakdown slows. This is a survival adaptation: your body shifts to protect muscle while maximizing fat use.
By 72 hours, the hormonal profile looks substantially different from the fed state. Insulin is roughly half of what it was after a normal overnight fast. Glucagon is up by about a third. The body is running almost entirely on fat-derived fuel, inflammatory activity is suppressed, and cellular repair processes are active. These are the changes that have made extended fasting a subject of serious scientific interest, though for most people, the practical benefits of fasting come from shorter windows of 16 to 36 hours rather than multi-day protocols.