What Happens to Your Body After 40 Hours of Fasting?

By 40 hours without food, your body has moved well past the initial discomfort of skipping a meal and into a distinct metabolic state. Glycogen stores in the liver are largely depleted, fat is being broken down at a high rate, and ketone bodies have risen to levels that begin fueling your brain. But the shift in energy source is just one piece of a much broader physiological reorganization, touching everything from your heart rate and blood pressure to how your muscles handle protein, how your immune cells behave, and even how your gut bacteria rearrange themselves.

The Fuel Switch From Glucose to Fat

Your body stores a limited supply of glycogen, the quick-access form of glucose kept in your liver and muscles. Most of it is used up within the first 24 to 36 hours of a fast, depending on your activity level and how much you ate beforehand. Once glycogen runs low, the liver ramps up ketone production from fatty acids. These ketones, primarily beta-hydroxybutyrate (BHB), become a major energy currency for the brain, heart, and other organs that normally prefer glucose.

By 40 hours, BHB levels are climbing steeply. Clinical data from a hospital fasting protocol found that patients without underlying metabolic conditions hit BHB values above 2.7 mmol/L at around 44 hours into a fast, on average, with the rise following a characteristic S-shaped curve that accelerates in the second day.¹ That level is well into what researchers consider nutritional ketosis. You might notice a metallic or fruity taste in your mouth, or a faint acetone smell on your breath, both byproducts of ketone metabolism.

What Happens to Your Muscles

One of the common fears about extended fasting is that your body will start eating its own muscle for fuel. The reality is more nuanced. There is a brief spike in muscle protein breakdown during the first few days of a fast, but your body quickly pivots to protect its lean mass. A prospective trial in healthy men found that protein breakdown dropped by about 40% by the fifth day of fasting, while 3-methylhistidine, a marker of skeletal muscle breakdown, only rose transiently during the first four days before returning to baseline.² At 40 hours, you are right in the middle of this early phase where some muscle protein is being used, but the protein-sparing shift is already underway. The more ketones your body produces, the less it needs to break down amino acids for energy.

This protein-sparing mechanism is thought to be an evolved survival strategy. Once the body commits to burning fat, maintaining muscle is more advantageous than cannibalizing it, because muscle is what you need to hunt, forage, or otherwise find food. The signal to conserve protein appears to be driven in part by the same ketone elevation described above.

Hormonal Shifts

Several hormones change dramatically during a 40-hour fast, and they do not all move in the same direction.

Human growth hormone (HGH) is one of the most striking responders. Research on water-only fasting found that HGH levels increase substantially during fasting, alongside a drop in circulating markers associated with insulin resistance.³ A separate study in healthy adults found that 24 hours of fasting alone was enough to drive roughly a five-fold increase in growth hormone.⁴ By 40 hours, that surge has likely continued or plateaued at an elevated level. Elevated HGH during fasting helps mobilize fat stores and is part of the reason your body protects lean tissue while burning fat instead.

Thyroid hormones head in the opposite direction. Levels of T3, the most active thyroid hormone, drop measurably within just a couple of days of fasting. One study found that T3 fell from about 150 to 120 ng per 100 mL during short-term starvation.⁵ This decline is not a sign of thyroid disease. It is a deliberate energy-conservation move: your body lowers its metabolic thermostat to make its fat reserves last longer. You might feel slightly cooler than usual or notice that your hands and feet get cold more easily.

Insulin plummets, which is precisely what allows fat breakdown and ketone production to proceed. Meanwhile, ghrelin, the hunger hormone, shows surprising variability. The same study that measured HGH found no consistent average increase in ghrelin during 24-hour fasting, though individual responses varied widely. Some people saw large ghrelin spikes, while others barely budged.⁴ This helps explain why some people find extended fasting surprisingly easy on the hunger front, while others struggle throughout.

Your Metabolic Rate Slows, But Not by Much

A common worry is that fasting will “crash” your metabolism. In reality, the drop during a 40-hour fast is modest. The study that tracked 24-hour energy expenditure during fasting found an average decline of about 8%, which translates to burning roughly 130 to 170 fewer calories per day for most people.⁴ Interestingly, how much your metabolism dipped was linked to how much your ghrelin rose: people whose ghrelin increased by 200 pg/mL saw an extra 55 kcal/day drop in energy expenditure. So the same hormonal signal that makes you hungrier also helps your body conserve fuel, a tidy bit of biological engineering.

The T3 decline mentioned earlier contributes to this metabolic slowdown. But at 40 hours, the reduction is nowhere near the severe metabolic adaptation seen in prolonged caloric restriction over weeks or months. Once you eat again, these hormonal levels bounce back relatively quickly.

Heart Rate, Blood Pressure, and the Vagal Shift

Fasting produces measurable changes in your cardiovascular system. Several studies have found that a day or two without food tends to lower resting heart rate and blood pressure. One trial measuring ambulatory vitals found that a fasting period reduced average arterial pressure from 81 to 78 mmHg and heart rate from 69 to 65 beats per minute, with an estimated 13% increase in stroke volume.⁶ A separate study confirmed that heart rate dropped during fasting, from about 83 to 77 beats per minute on average, alongside increased heart rate variability, a marker of healthy autonomic nervous system tone.⁷

The shift in autonomic tone is toward greater parasympathetic, or vagal, activity. In plain terms, your “rest and digest” branch of the nervous system becomes more dominant relative to the “fight or flight” branch. The researchers who measured ambulatory blood pressure interpreted this as consistent with improved short-term cardiovascular health.⁶ That said, if you stand up too fast during an extended fast, you may feel dizzy, because lower blood pressure combined with fluid shifts can make orthostatic hypotension more likely.

Electrolytes and Fluid Loss

One of the less-discussed but physiologically important effects of fasting is what happens to your electrolytes, particularly sodium and potassium. Classic research showed that sodium excretion increases sharply during the first days of a fast, declining gradually but persisting even through prolonged fasting, at levels exceeding what you would see from salt restriction alone.⁸ Potassium excretion follows a similar early spike before tapering to around 10 to 15 milliequivalents per day. A separate study confirmed progressive natriuresis and kaliuresis during three days of fasting, accompanied by a mean daily weight loss of about 800 grams, largely from water.⁹

This matters practically. Much of the weight you lose in the first two days of a fast is water, not fat, driven by the drop in insulin, which normally signals the kidneys to hold onto sodium. When insulin falls, sodium is released and water follows. If you are doing a water-only fast and not supplementing electrolytes, you can develop headaches, muscle cramps, or lightheadedness from sodium and potassium depletion. A curious finding from the older literature is that eating even a small amount of carbohydrate can abruptly halt sodium excretion and cause water retention, explaining why the scale sometimes bounces up sharply after breaking a fast even before meaningful fat regain has occurred.⁸

Inflammation Quiets Down

Fasting appears to suppress key inflammatory pathways, particularly the NLRP3 inflammasome, a molecular complex that triggers the release of inflammatory signals like IL-1β and IL-18. After 24 hours of fasting in healthy humans, researchers found that the inflammasome in monocytes became less responsive to activation, with reduced secretion of these inflammatory markers and lower activation of NF-κB, a master regulator of inflammation.¹⁰ In animal studies, this suppressive effect was even more pronounced at 48 hours and appeared to depend on a protein called SIRT3, which enhances the activity of antioxidant enzymes in mitochondria.¹¹

The ketones themselves seem to play a role here. BHB, the primary ketone body elevated at 40 hours, has been shown to directly inhibit the NLRP3 inflammasome in laboratory settings. So the same fuel switch that powers your brain during fasting also contributes to lowering the inflammatory background noise in your body. Whether this translates into meaningful long-term benefits for people with chronic inflammation is still an open question, but the short-term anti-inflammatory effect at 40 hours appears to be real.

Immune Cell Regeneration and Stem Cell Activation

Some of the most provocative fasting research involves the immune system. Work in both mice and early human data found that prolonged fasting reduces levels of IGF-1 (insulin-like growth factor 1) and the activity of a signaling pathway called PKA. This triggers long-term blood-forming stem cells to shift into a mode of self-renewal and stress resistance.¹² In mice, multiple cycles of prolonged fasting reversed the age-related tendency of the immune system to produce too many myeloid cells at the expense of other immune cell types, and it reduced the immunosuppression caused by chemotherapy.

Preliminary data in fasting human patients supported the protective effect on immune cells during chemo. The findings are preliminary and the human evidence is still thin, but the direction is clear: fasting appears to flip a regenerative switch in the immune system’s stem cell compartment, and the IGF-1 drop caused by fasting is central to that process.¹² At 40 hours, IGF-1 has begun to decline, though the full regenerative effect likely requires longer or repeated fasting bouts.

The Paradox of Temporary Insulin Resistance

Here is something that surprises most people: fasting, which is often promoted as a way to improve insulin sensitivity, actually produces temporary insulin resistance in your muscles. After a prolonged fast, skeletal muscle shifts almost entirely to burning fat, and in doing so, it becomes resistant to insulin’s signal to take up glucose. Research on 72-hour fasts found that this insulin resistance was profound, accompanied by the accumulation of both lipid and glycogen inside muscle fibers.¹³ A follow-up study confirmed this pattern with just three days of fasting and found that the degree of insulin resistance correlated with how efficiently your muscle fibers could burn fat.¹⁴

This is not pathological insulin resistance. It is a strategic reallocation: your muscles are saying, “We have all the fat we need right now, so save the glucose for the brain.” The brain cannot burn fat directly (it relies on either glucose or ketones), so by making muscles insulin resistant, the body ensures that whatever glucose is still circulating gets directed where it is needed most. This effect reverses once you eat again. But it does mean that if you check your blood sugar during a 40-hour fast, it might be slightly higher than expected, and your postprandial glucose response when you break the fast can be temporarily exaggerated.

Changes in Your Gut

Your gastrointestinal tract does not simply shut off during a fast. Without incoming food, the gut environment shifts, and your microbial population responds. A study of prolonged fasting in healthy men found major changes in gut microbiota composition, including roughly a six-fold expansion of bacteria from the Proteobacteria phylum and decreases of about 50% in Bacteroidetes and 34% in Firmicutes.¹⁵ A notable increase in the species Ruthenibacterium lactatiformans correlated strongly with fat metabolism markers, suggesting the gut bacteria themselves adjust to match the body’s shift toward fat burning.

Separate research on fasting in people with metabolic syndrome found that fasting enriched the gut’s capacity to produce propionate, a short-chain fatty acid linked to anti-inflammatory and appetite-regulating effects, along with genes involved in mucin degradation and diverse nutrient utilization.¹⁶ In other words, your gut flora do not simply starve when you do. They pivot to scavenging the mucus lining of the intestine and producing different metabolites. Whether these microbial shifts are beneficial or merely adaptive remains under investigation, but they illustrate how deeply a 40-hour fast affects even the ecosystems living inside you.

Cellular Cleanup and Autophagy

Autophagy, the process by which cells break down and recycle their own damaged components, is one of the most-discussed benefits of fasting. It is harder to measure in living humans than in lab dishes or animal models, so the evidence is less precise than what we have for metabolic or hormonal changes. An exploratory study in healthy young men who fasted intermittently for a month found increases in the expression of key autophagy-related genes, with ULK1 expression rising dramatically over the fasting period.¹⁷ These expression increases reversed within a week of stopping the fasting regimen, suggesting that autophagy upregulation requires ongoing fasting stimulus and does not become permanent.

At 40 hours, the conditions that promote autophagy are firmly in place: insulin is low, the nutrient-sensing pathways that normally suppress autophagy are dialed down, and cellular energy status is tilted in a way that favors recycling. But claiming a specific “autophagy peak” at a particular hour is not supported by current human data. The process is gradual, varies between tissues, and is difficult to quantify from a blood sample. What we can say is that the molecular signals driving autophagy are clearly active at this point in a fast.

How Men and Women May Respond Differently

Most fasting research has historically been done in men or in mixed groups without reporting sex-specific results, which leaves a meaningful knowledge gap. What evidence exists suggests that men and women do not respond identically to extended fasting. In men, rising levels of testosterone, growth hormone, and adrenaline during fasting tend to promote fat burning and may support muscle preservation. Women’s hormonal responses can vary depending on the phase of the menstrual cycle and individual hormonal profile.¹⁸

There is concern, though not yet definitive proof, that extended fasting can disrupt reproductive hormones in women more than in men. Some women report menstrual irregularities with repeated prolonged fasts, and the hypothalamic-pituitary-gonadal axis in women appears more sensitive to energy deficits. If you are a woman considering a 40-hour fast, this does not necessarily mean it is harmful, but it does mean the research is less settled and paying attention to how your cycle responds is worthwhile.

What Refeeding Looks Like

Breaking a 40-hour fast is not just the end of the fasting process; it is a physiological event in its own right. The sodium retention that kicks in with the first carbohydrate intake can cause a rapid water weight gain of a kilogram or more within 24 hours, which has nothing to do with fat regain. Your insulin level will spike more than usual when you eat, partly because of the temporary muscle insulin resistance discussed earlier, and partly because the pancreas tends to overcompensate after a period of suppression.

Gastric motility has slowed during the fast, so eating a large, heavy meal right away can cause bloating, cramping, or nausea. Starting with smaller, easily digestible food and eating slowly tends to be more comfortable. There is no clinical consensus on the ideal refeeding meal after a fast of this length, but most experienced fasters gravitate toward something moderate in both volume and glycemic impact rather than immediately sitting down to a feast.

The elevated growth hormone and the protein-sparing state do not switch off the instant food arrives. The hormonal environment remains favorable for muscle preservation and fat oxidation for some time after eating resumes, which is one of the reasons some people who fast periodically report that it feels different from simple caloric restriction in terms of body composition outcomes. That perception is consistent with the hormonal data, even if long-term controlled trials directly comparing 40-hour fasts to calorie-matched continuous restriction are scarce.

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