Proper fasting comes down to choosing the right schedule for your goals, staying hydrated, and breaking your fast carefully. Most people do well with a daily time-restricted approach, such as eating within an eight-hour window and fasting for 16 hours. But the details matter: what you drink during the fast, how long you go, and what you eat afterward all affect whether fasting helps or harms you.
Choosing a Fasting Method
The two most popular approaches are daily time-restricted eating and the 5:2 method. In daily time-restricted eating (often called 16:8), you eat all your meals within a six- to eight-hour window and fast the remaining 16 to 18 hours. Most people skip breakfast, eat their first meal around noon, and finish dinner by 8 p.m., though you can shift the window to whatever fits your schedule.
The 5:2 approach works differently. You eat normally five days a week and limit yourself to one 500 to 600 calorie meal on the other two days. The two low-calorie days shouldn’t be consecutive.
Some people practice OMAD (one meal a day), which compresses all eating into a single meal. This is more extreme and harder to sustain, but it extends the fasting window to roughly 23 hours. If you’re new to fasting, starting with 16:8 for a few weeks before trying longer windows lets your body adapt gradually.
Longer fasts of 24, 36, 48, or 72 hours are not necessarily better. According to Johns Hopkins Medicine, going too long without eating can actually encourage your body to store more fat in response to perceived starvation.
What Happens in Your Body During a Fast
For the first several hours after your last meal, your body runs on glucose from the food you recently ate and from glycogen, the stored form of sugar in your liver and muscles. This is essentially business as usual.
Around 18 hours into a fast, your liver’s glycogen stores are largely depleted. Your body begins breaking down protein and fat for energy, and this triggers a gradual shift into ketosis, where fat becomes your primary fuel source. The transition doesn’t happen like flipping a switch. It builds over hours, and most people enter meaningful ketosis somewhere between 18 and 36 hours.
Fasting also triggers a significant rise in human growth hormone, which helps preserve lean tissue and supports fat burning. Research published in Frontiers in Endocrinology found that during a 24-hour water-only fast, growth hormone levels increased roughly 5-fold in men and 14-fold in women. People with lower baseline levels saw the most dramatic increases.
Autophagy, the process where your cells break down and recycle damaged components, is one of the most talked-about benefits of fasting. Animal studies suggest it ramps up between 24 and 48 hours of fasting, but as the Cleveland Clinic notes, there isn’t enough human research yet to pinpoint the ideal timing. Shorter fasts likely trigger some degree of cellular cleanup, but the full effect requires longer periods without food.
What You Can Drink While Fasting
Water is the safest bet and should be your primary drink throughout any fast. Plain black coffee and unsweetened tea are generally considered acceptable because they contain negligible calories and don’t trigger a meaningful insulin response. Adding cream, sugar, honey, or flavored syrups changes the equation and effectively breaks your fast.
Artificial sweeteners are a gray area. Some evidence suggests certain sweeteners can provoke a small insulin response even without calories, though the research is mixed. If your goal is maximum metabolic benefit, plain water, black coffee, and plain tea are the cleanest choices. Sparkling water and mineral water are also fine.
Bone broth, which contains protein and a small number of calories, technically breaks a fast. Some people use it during longer fasts to maintain electrolytes, accepting the trade-off. If you’re fasting primarily for weight loss rather than autophagy, a cup of bone broth is unlikely to derail your progress.
How to Break Your Fast Safely
The longer your fast, the more carefully you need to reintroduce food. For a standard 16:8 fast, you can eat a normal meal without much worry, though starting with something moderate rather than a massive plate of heavy food will feel better.
For fasts lasting 24 hours or longer, ease back in. Foods that are high in fat, sugar, or even fiber can overwhelm your digestive system after a period without food, leading to bloating, cramping, and discomfort. Greasy meals, baked goods, sodas, and even raw vegetables, nuts, and seeds can be surprisingly hard to digest right away.
Good options for breaking a longer fast include:
- Cooked vegetables and simple soups or broths
- Small portions of protein like eggs or fish
- Easily digested carbohydrates like rice or cooked sweet potato
A Mediterranean-style diet, built around vegetables, olive oil, whole grains, and lean protein, is often recommended for regular eating windows during a fasting regimen. Avoiding refined carbohydrates and added sugars during your eating periods supports weight loss and keeps blood sugar more stable heading into your next fast.
For fasts of several days (which should only be done under medical supervision), reintroduction needs to happen gradually over multiple days. In clinical settings, researchers have reintroduced food starting at 800 calories per day and increasing to 1,600 over four days. Eating too aggressively after a prolonged fast carries a real risk of refeeding syndrome, a dangerous shift in electrolytes that typically occurs within the first five days of refeeding and can affect your heart and muscles.
Protecting Muscle Mass
One of the biggest concerns about fasting is losing muscle along with fat. During shorter fasts, the spike in growth hormone helps protect lean tissue, and your body preferentially burns fat once it enters ketosis. The risk of meaningful muscle loss during a 16- to 24-hour fast is low for most people.
Longer fasts present a different picture. Once glycogen is gone and fat metabolism is well underway, your body still breaks down some protein for processes that can’t run on fat alone. Animal research shows that fasting can cause roughly 11% loss of hindlimb muscle mass, along with measurable decreases in muscle force. Standard caloric restriction has also been linked to muscle wasting, particularly in older adults.
To minimize muscle loss, keep your fasting windows moderate (under 24 hours for most people), eat adequate protein during your eating windows, and maintain some form of resistance exercise. Strength training sends a powerful signal to your body that muscle tissue is in active use and shouldn’t be broken down for fuel.
Who Should Avoid Fasting
Fasting isn’t appropriate for everyone. People with diabetes face real risks from extended periods without food, as blood sugar can drop dangerously low, especially if you’re on insulin or medications that lower glucose. If you take blood pressure or heart medications, fasting can disrupt your sodium and potassium balance.
People who are already at a low body weight risk losing too much, which can weaken bones, suppress the immune system, and sap energy. Anyone with a history of eating disorders should approach fasting with extreme caution, as the restriction can trigger or worsen disordered patterns. Pregnant or breastfeeding women need consistent nutrition and should not fast.
If you take medications that need to be taken with food to prevent nausea or stomach irritation, a fasting schedule may interfere with your treatment. In these cases, adjusting your eating window to accommodate your medication timing, or choosing a less restrictive approach like the 5:2 method, may work better than a daily extended fast.
Practical Tips for Your First Fast
Start with a 14- or 16-hour overnight fast and see how you feel before extending it. Most of those hours are spent sleeping, which makes it easier than it sounds. If you normally finish dinner at 7 p.m. and skip breakfast until 9 or 11 a.m., you’ve already completed a 14- to 16-hour fast.
Hunger tends to come in waves rather than building steadily. The first few days are the hardest. After about a week of consistent time-restricted eating, most people find their appetite adjusts and the fasting window feels natural. Staying busy during the morning hours helps, and drinking water or black coffee can blunt hunger pangs.
Pay attention to how you feel during your eating window too. If you’re overeating or bingeing when you break your fast, the window may be too restrictive. Fasting should reduce your total calorie intake slightly, not lead to a cycle of deprivation and overconsumption. If you find yourself consistently lightheaded, irritable, or unable to concentrate, shorten the fast or add a small protein-rich snack to your routine.