A 24-hour fast can trigger several beneficial metabolic changes, including lower insulin levels, increased fat burning, and a significant rise in human growth hormone. But it also comes with real trade-offs, and the benefits depend heavily on your starting health, how often you do it, and whether you eat well during non-fasting periods.
What Happens in Your Body During a 24-Hour Fast
When you stop eating, your body shifts fuel sources over the next several hours. For the first 6 to 8 hours, it burns through stored glucose (glycogen) in your liver. After that, it increasingly relies on fat for energy, a state that deepens as the fast continues. By the 18- to 24-hour mark, fat burning is well underway, and your body is producing more ketones, which your brain and muscles can use as fuel.
One of the most dramatic hormonal shifts involves human growth hormone. A study published in Frontiers in Endocrinology found that during a 24-hour fast, people who started with low baseline levels of this hormone saw a median increase of 1,225%, with some individuals experiencing increases as high as 20,000%. Those who already had higher levels saw a more modest rise of around 50%. Growth hormone helps preserve lean tissue during fasting and supports fat metabolism, so this surge is one reason fasting appeals to people interested in body composition.
Insulin levels also drop substantially. With no incoming food, your pancreas produces far less insulin, which allows your body to access stored fat more efficiently. This temporary drop is the core mechanism behind many of fasting’s proposed benefits. However, the picture on insulin sensitivity is more nuanced than you might expect. Research in the American Journal of Physiology found that prolonged fasting improved how the liver responds to insulin but actually decreased whole-body insulin sensitivity in the short term. In other words, a single fast creates a mixed metabolic picture, not a straightforward improvement.
Autophagy: The Cellular Cleanup Effect
Autophagy is the process where your cells break down and recycle damaged components. It’s essentially cellular housekeeping, and it’s one of the most frequently cited reasons people try extended fasts. Animal studies suggest autophagy ramps up somewhere between 24 and 48 hours of fasting. The Cleveland Clinic notes that not enough research has been done to pinpoint exactly when autophagy peaks in humans. So while a 24-hour fast may put you at the beginning of this window, it’s not clear how much cellular cleanup actually occurs in that time frame versus a longer fast.
Heart Health and Cholesterol
There’s some evidence that intermittent fasting, including periodic 24-hour fasts, may improve cardiovascular markers. The Mayo Clinic notes that some studies have found fasting can lower LDL cholesterol, the type most associated with heart disease risk. The effects on blood pressure and triglycerides are less consistent across studies, and most of the cardiovascular research has looked at fasting as a repeated practice over weeks or months rather than a single 24-hour session. A one-time fast is unlikely to move the needle on heart health. Regular fasting as part of a broader healthy eating pattern is where any meaningful benefit would come from.
The Muscle Question
If you exercise regularly, you’re probably wondering whether a 24-hour fast breaks down muscle. The short answer: it does increase muscle protein breakdown compared to eating normally, but the effect from a single day is relatively small. A review in Frontiers in Nutrition found that fasting periods of 16 hours or longer lead to greater muscle protein breakdown over a 24-hour cycle compared to eating three to five meals throughout the day. The researchers also noted that whole-day fasting would have a greater impact on muscle mass than shorter fasting windows like 16:8 time-restricted eating, largely because of the longer period spent in an energy-deficit state.
That said, the surge in growth hormone during fasting provides some protective effect against muscle loss, and most people won’t see noticeable muscle wasting from an occasional 24-hour fast. The concern becomes more relevant if you’re fasting frequently, training hard, or not eating enough protein during your feeding periods. If preserving muscle is a priority, timing a protein-rich meal right when you break your fast helps counteract the breakdown that occurred.
Brain Function and Mood
You may have heard that fasting boosts a protein called BDNF, which supports the growth and survival of brain cells. Animal research has been promising on this front, but human data tells a much less clear story. A systematic review in the journal Medicina examined multiple human studies on fasting and BDNF levels. Six studies found no significant change, five found an increase, and five found a decrease. Studies specifically using fasting durations close to 24 hours showed no significant changes in BDNF. So the idea that a 24-hour fast sharpens your brain through BDNF is, at this point, not well supported in humans.
What many people do report is a subjective sense of mental clarity during a fast, particularly after the initial hunger passes. This likely relates to the shift toward ketone metabolism and elevated adrenaline levels rather than any lasting structural change in the brain.
Common Side Effects
A 24-hour fast is long enough to produce noticeable side effects, especially if you’re not used to it. Headaches, irritability, fatigue, difficulty concentrating, and constipation are all common. Harvard Health Publishing notes that there’s also a strong biological push to overeat after a fasting period because appetite hormones and the brain’s hunger center go into overdrive once food becomes available again. This rebound hunger can easily cancel out the calorie deficit from the fast if you’re not mindful about what and how much you eat afterward.
There’s also a metabolic consideration worth knowing about. Calorie-restricted approaches, including intermittent fasting, can slow your metabolism and increase appetite over time, the same adaptations that make any calorie-reduction strategy harder to sustain long term. Fasting doesn’t appear to bypass this effect compared to simply eating less every day.
Who Should Avoid 24-Hour Fasts
Certain groups face real risks from a full day without food. People with diabetes, particularly type 1 diabetes, face the danger of dangerously low blood sugar. Johns Hopkins Medicine specifically notes that no studies have tested intermittent fasting in people with type 1 diabetes, and the risk of hypoglycemia during fasting is a serious concern.
Other groups who should be cautious or avoid 24-hour fasts entirely include:
- People taking blood pressure or heart medications, who may develop imbalances in sodium, potassium, and other minerals during extended fasting
- People who take medications with food to prevent nausea or stomach irritation
- Anyone with a history of eating disorders, since strict fasting windows can trigger or worsen restrictive or obsessive food behaviors
- People who are already underweight, as further weight loss can affect bone density, immune function, and energy
- Older adults, for whom there is limited evidence on safety or benefit
How to Approach It Practically
If you’re healthy and want to try a 24-hour fast, the most common approach is eating dinner one evening and not eating again until dinner the following day. This means you sleep through a good portion of the fast and only experience one full waking day without food. Staying well hydrated with water, plain tea, or black coffee helps manage headaches and fatigue.
There’s no established medical consensus on exactly how often you can safely do a 24-hour fast. Most practitioners who recommend it suggest once or twice a week at most, with normal, balanced eating on the other days. Starting with shorter fasts of 16 to 18 hours and gradually extending to 24 hours gives your body time to adapt and helps you gauge how you respond before committing to a full day.
The bottom line is that a 24-hour fast produces real metabolic shifts, some clearly beneficial in the short term, others more ambiguous. It’s a tool, not a cure-all, and its value depends entirely on the context of your overall diet, activity level, and health status.