For most healthy adults, fasting for a full day is safe and comes with some measurable metabolic benefits. A 24-hour fast is one of the more common forms of intermittent fasting, and your body is well-equipped to handle it. That said, how you feel during and after the fast, and whether it’s a good idea for you specifically, depends on your health status, medications, and how you approach it.
What Happens in Your Body During a 24-Hour Fast
When you stop eating, your body shifts fuel sources in a fairly predictable sequence. For the first several hours, it burns through glycogen, the stored form of glucose in your liver and muscles. As those reserves deplete, your body increasingly turns to fat for energy. This transition is driven by falling insulin and leptin levels and rising levels of glucagon and growth hormone, which signal the liver to release glucose and ramp up fat burning.
Growth hormone rises significantly during a 24-hour fast. One study found it increased 5-fold in men and 14-fold in women over that window. People who start with lower baseline levels see the most dramatic spikes, with increases of over 1,000% in some cases. Growth hormone helps preserve muscle tissue while your body taps into fat stores, which is part of why short fasts don’t cause the muscle loss people sometimes worry about.
Your brain also gets some attention during a fast. As fat breaks down, your liver produces ketone bodies, which serve as an alternative fuel for the brain. One of these ketones can increase production of a protein called BDNF, which supports the growth and maintenance of brain cells and plays a role in learning and memory. Enhanced BDNF levels during intermittent fasting have been observed in humans, not just in animal studies.
Animal research suggests that autophagy, the process by which cells break down and recycle damaged components, may begin somewhere between 24 and 48 hours of fasting. There isn’t enough human data yet to pin down exactly when it kicks in, so a single day of fasting likely puts you at the early edge of that window rather than deep into it.
The Calorie Math Is Better Than You’d Expect
A common concern is that you’ll just overeat the next day and cancel out any calorie deficit. The data suggests otherwise. After a fast, people typically compensate by eating only about 60% of the calories they would have consumed during the fasting period. In one study, participants coming off a 36-hour fast increased their intake by just 20% above their normal eating. That means the net calorie deficit is real and substantial, even with some extra eating afterward. There’s individual variability here, but the idea that your body will force you to “make up” every missed calorie isn’t supported by the research.
Who Should Skip the Fast
A 24-hour fast isn’t appropriate for everyone. People who take medications that require food, particularly for blood sugar management, should not attempt it without medical guidance. The same goes for anyone with a history of eating disorders, since the restrict-and-eat cycle can reinforce unhealthy patterns. Pregnant or breastfeeding women have increased nutritional demands that make fasting a poor fit. Most clinical fasting programs also exclude people under 21.
If you take thyroid medication, some protocols allow continuing it at a reduced dose during a fast, but this is something to discuss with your prescriber rather than figure out on your own. People with low blood pressure, a history of fainting, or conditions that affect blood sugar regulation should also be cautious.
What to Drink During the Fast
Water is the obvious choice and should be your primary fluid throughout the day. Black coffee and plain tea are widely considered acceptable during a fast, though they aren’t completely neutral. Black coffee triggers a small insulin response, roughly a 15 to 20% increase, and can cause a minor bump in blood sugar of about 5 to 8 mg/dL. For most people pursuing a one-day fast, this is negligible and doesn’t meaningfully undermine the metabolic benefits. Adding cream, sugar, or fat to your coffee is a different story: that can raise insulin response by 40 to 60% and may blunt fat burning and autophagy.
The practical takeaway: black coffee and unsweetened tea are fine. Anything with calories in it is not fasting.
How to Break a 24-Hour Fast
What you eat when you start eating again matters more than most people realize. Diving into a large, heavy, or greasy meal is the fastest route to bloating, heartburn, and digestive discomfort. Your digestive system has been idle, and it needs a gentle restart.
Start with something small and easy to digest. Soups with lentils, beans, or rice are a solid choice because they provide carbohydrates for quick energy and protein for satiety without overwhelming your gut. Smoothies, fruit juice, or even a glass of milk can ease your body back into processing food. Dates are a traditional fast-breaker in many cultures for good reason: they’re a whole, unprocessed source of carbohydrates, fiber, vitamins, and minerals.
For your first real meal, lean toward lean proteins like fish or plant-based options rather than red meat. Keep fat moderate and avoid spicy foods until your second meal. Eat slowly and in smaller portions than you normally would. You can return to your regular eating pattern within a few hours, but that initial transition is where most people run into trouble if they’re not deliberate about it.
What You’ll Actually Feel
The hardest part of a 24-hour fast for most people is the stretch between hours 12 and 20. Hunger tends to come in waves rather than building steadily, and many people report that the worst wave passes and they feel surprisingly clear-headed afterward, likely related to the ketone production and hormonal shifts happening in the background.
Common side effects include irritability, difficulty concentrating (especially early on), mild headaches, and feeling cold. These are normal and temporary. If you feel dizzy, extremely weak, or notice heart palpitations, that’s a signal to eat something rather than push through. A one-day fast should feel uncomfortable at times but never dangerous.
Light activity like walking is fine during a fast. Intense exercise is generally a bad idea, since your glycogen stores are depleted and your performance and recovery will suffer. Save the hard workouts for fed days.