Fasting means voluntarily going without food for a set period, then eating during a designated window. The simplest way to start is with a 12-hour overnight fast (stop eating at 8 p.m., eat again at 8 a.m.) and gradually extend that window as your body adjusts. Beyond that basic framework, the method you choose, what you drink, and how you break your fast all matter for safety and results.
Choose a Fasting Method
Most people who fast for health or weight loss use one of a few well-established protocols. They differ mainly in how long you go without food and how often.
- 16:8 (time-restricted eating): Fast for 16 hours, eat within an 8-hour window. This is the most popular starting point. A typical schedule is eating between noon and 8 p.m.
- 18:6 or 20:4: Tighter versions of the same idea, with 6- or 4-hour eating windows. These are more demanding and better suited to people who’ve already adapted to 16:8.
- 5:2 diet: Eat normally five days a week. On two non-consecutive days, limit yourself to 500 to 600 calories.
- Alternate-day fasting: Alternate between 24-hour fasting periods and days of unrestricted eating.
- OMAD (one meal a day): A 23-hour fast with a single 1-hour eating window. This is an advanced approach and not recommended as a starting point.
If you’ve never fasted before, 16:8 is the easiest to stick with because most of the fasting happens while you sleep. You’re essentially skipping breakfast and not snacking after dinner.
What Happens in Your Body While You Fast
For the first 3 to 4 hours after eating, your body is digesting food and your blood sugar and insulin levels are elevated. This is the fed state, and your body is running on the calories you just consumed.
Between roughly 4 and 18 hours, blood sugar and insulin start to drop. Your body taps into glycogen, a stored form of sugar in your liver, to keep energy levels stable. This is when most people on a 16:8 schedule spend the bulk of their fast. You may feel mild hunger, but it typically passes in waves rather than building continuously.
After about 18 to 24 hours, liver glycogen runs low and your body shifts to burning fat for fuel. This produces ketone bodies, which your brain and muscles can use for energy. This metabolic state is called ketosis. Shorter fasts of 12 to 18 hours may not reach full ketosis unless you’re also eating very few carbohydrates.
Animal studies suggest that a cellular cleanup process called autophagy, where cells break down and recycle damaged components, ramps up between 24 and 48 hours of fasting. There isn’t enough human research yet to pin down exact timing, so don’t treat this as a guaranteed benefit of shorter fasts.
What You Can Drink
Water is always fine. Black coffee and unsweetened tea are also widely accepted during a fast because they contain essentially zero calories and don’t trigger a meaningful insulin response. Clear broth and sparkling water are other options.
Anything with sugar, cream, milk, or calories will break your fast by raising blood sugar and insulin. Diet sodas are technically calorie-free, but some people find that artificial sweeteners increase hunger and make fasting harder to sustain.
How to Start Gradually
Jumping straight into a 20-hour fast is a recipe for headaches, irritability, and quitting. A better approach is to build tolerance over one to two weeks.
Start with a 12-hour overnight fast. If you finish dinner at 8 p.m., don’t eat again until 8 a.m. Most people can do this without much discomfort since they’re asleep for most of it. After a week, extend by one hour on each end: stop eating at 7 p.m., start again at 9 a.m. Keep stretching the window until you reach 16 hours if that’s your goal.
During your eating window, eat balanced meals with protein, healthy fats, and fiber. Fasting doesn’t work well if your eating window becomes a free-for-all of processed food, because blood sugar swings will make your next fast feel significantly harder.
Considerations for Women
Fasting affects hormones, and pre-menopausal women may need to be more cautious with intensity and timing. Starting with a 12-hour fast and working up slowly to 16 hours is a safer entry point than jumping to longer protocols.
The menstrual cycle matters too. The week before your period, your body is more sensitive to stress, and fasting can amplify symptoms like fatigue, mood swings, and cravings. The best times to try fasting are a day or two after your period begins and during the week or so following it. Limiting fasting during the two weeks before your period is due can help avoid disruptions to your cycle.
What to Expect for Weight Loss
Across 27 clinical trials, intermittent fasting consistently produced weight loss ranging from 0.8% to 13% of baseline body weight. In studies lasting 2 to 12 weeks, BMI dropped by an average of 4.3%. For someone weighing 200 pounds, that’s roughly 1.5 to 26 pounds lost, with the wide range reflecting differences in protocol, duration, and individual factors like starting weight and diet quality.
Most of the weight lost through fasting is fat. One study calculated that 79% of weight loss came specifically from fat rather than muscle. That said, eating adequate protein during your eating window is important for preserving muscle mass, especially if you’re also exercising.
How to Break Your Fast
For fasts of 16 hours or less, most people can eat a normal meal without issues. But if you’ve fasted for 24 hours or longer, easing back in matters. Your digestive system slows down during extended fasts, and a large or heavy meal can cause bloating, nausea, or stomach cramps.
Good first foods after a longer fast include eggs, avocado, unsweetened yogurt or kefir, and bone broth. These are easy to digest and provide protein and healthy fats without spiking your blood sugar. Wait 30 to 60 minutes before eating a full meal, and keep the first portion small.
Who Should Not Fast
Fasting is not appropriate for everyone. People with kidney disease are at particular risk because the protein breakdown that occurs during fasting puts extra strain on the kidneys. People with diabetes who take insulin or blood sugar-lowering medications can experience dangerous drops in blood sugar during a fast. Pregnant or breastfeeding women, children, and anyone with a history of eating disorders should avoid fasting protocols.
Fasting beyond 72 hours carries serious risks including dangerous drops in blood pressure, severe low blood sugar, and malnutrition, and should not be attempted without medical supervision. Even for healthy individuals, extended water-only fasts of more than 24 hours deserve a conversation with a healthcare provider first.