Fasting means voluntarily going without food for a set period, then eating during a specific window. The simplest way to start is the 16:8 method: stop eating after dinner, skip breakfast, and eat your first meal about 16 hours later. That gives your body an 8-hour eating window and a 16-hour rest period. From there, you can adjust the timing and duration to fit your goals.
The Most Common Fasting Schedules
There are several well-established fasting methods, and they differ mainly in how long you go without eating.
- 16:8 (time-restricted eating): Fast for 16 hours, eat within an 8-hour window. Most people do this by eating between noon and 8 p.m. This is the most popular starting point because it essentially just means skipping breakfast.
- 5:2: Eat normally five days a week, then limit yourself to 500 to 600 calories on two non-consecutive days. You’re never fully fasting, which makes this easier for people who struggle with hunger.
- OMAD (one meal a day): Fast for about 23 hours and eat one large meal in a 1-hour window. This is more advanced and not recommended as a starting point.
For most beginners, 16:8 is the easiest entry point. You spend roughly half the fasting period asleep, and the adjustment mainly comes down to tolerating a few hours of mild hunger in the morning.
What Happens in Your Body When You Fast
Your body moves through a predictable sequence once you stop eating. For the first three to four hours, you’re still digesting your last meal and running on that fuel. After that, blood sugar and insulin levels start declining, and your body begins converting its stored carbohydrate reserves (glycogen) into usable energy.
By around 18 hours without food, those glycogen stores are running low. Your body ramps up a process called lipolysis, where fat cells get broken down into smaller molecules your body can burn. This is the transition point most people are aiming for.
Between 18 hours and two days, your liver’s glycogen is fully depleted. Your body starts producing ketone bodies from fat, shifting into a metabolic state called ketosis, where fat becomes the primary fuel source. This is also the window where your cells begin clearing out damaged components and recycling them for energy, a maintenance process that may protect against age-related damage and chronic disease. The exact timing varies from person to person based on activity level, diet before the fast, and individual metabolism.
What You Can Drink Without Breaking a Fast
Water is the obvious choice. Black coffee is also widely consumed during fasting windows. Research shows that long-term coffee consumption doesn’t meaningfully worsen insulin resistance, though caffeine can cause a small, temporary bump in insulin levels. In practical terms, a cup or two of black coffee won’t derail the benefits of your fast.
Plain tea, sparkling water, and other zero-calorie drinks are generally fine. Anything with calories, including milk, cream, sugar, juice, or bone broth, will break your fast. Diet sodas are technically calorie-free, but artificial sweeteners may trigger an insulin response in some people, so opinions vary on those.
How to Start Without Feeling Terrible
The most common side effects when you begin fasting are headaches, fatigue, irritability, and constipation. These are temporary and usually fade within one to two weeks as your body adapts. Harvard Health recommends easing into fasting gradually, slowly reducing your eating window over several months rather than jumping straight into long fasts.
A few practical steps help:
- Stay hydrated. Drink plenty of water and calorie-free beverages throughout the fasting window. Dehydration is behind many of the headaches people blame on hunger.
- Start with a wider eating window. Try 12:12 (12 hours fasting, 12 hours eating) for a week or two before narrowing to 14:10, then 16:8.
- Expect hunger spikes. Your appetite hormones surge during fasting, especially early on. These waves of hunger typically last 20 to 30 minutes and pass. Staying busy helps.
- Watch for overeating after your fast. There’s a strong biological drive to overcompensate when you break your fast. Planning your first meal in advance helps you avoid eating everything in sight.
Electrolytes Matter for Longer Fasts
If you’re fasting for 24 hours or longer, your body loses electrolytes through normal processes that food would typically replenish. People who take blood pressure or heart medications are especially prone to mineral imbalances during extended fasts. For fasts beyond a day, aim for roughly 1,500 to 2,300 mg of sodium, 1,000 to 2,000 mg of potassium, and 300 to 400 mg of magnesium daily. You can get these through sugar-free electrolyte supplements dissolved in water or by adding a pinch of salt to what you drink.
Signs you’re low on electrolytes include muscle cramps, dizziness, heart palpitations, and brain fog. If these show up, add electrolytes before pushing through.
How to Break Your Fast
For standard intermittent fasts (16 to 24 hours), breaking your fast doesn’t require special planning. Eat a normal, balanced meal. The main thing to watch is portion control, since hunger hormones will push you toward overeating.
For extended fasts of several days or more, refeeding becomes genuinely important. Eating a large, carbohydrate-heavy meal after a prolonged fast can cause dangerous shifts in electrolytes, a condition called refeeding syndrome. Clinical guidelines recommend restarting food at roughly half your normal calorie intake and increasing gradually over four to seven days. Start with small, easily digestible meals. Foods like soup, cooked vegetables, eggs, or small portions of protein are easier on your system than a large plate of pasta or pizza.
Protecting Muscle During a Fast
One of the biggest concerns people have is losing muscle instead of fat. The research here is reassuring for shorter fasts. Randomized controlled trials generally show that intermittent fasting preserves lean mass at similar rates to normal eating. When compared to continuous calorie restriction (just eating less every day), some reviews suggest intermittent fasting preserves muscle equally well or slightly better.
That said, there are caveats. One study of 116 participants found that time-restricted eating over 12 weeks did reduce limb muscle mass. And research shows that compressing all your protein into a narrow eating window increases protein breakdown by about 13 grams per day compared to spreading meals across 12 hours. If keeping muscle is a priority, distribute your protein intake across at least two or three meals during your eating window, aiming for roughly 0.3 grams of protein per kilogram of body weight at each meal. Resistance training during a fasting program also makes a significant difference in holding onto muscle.
Who Should Not Fast
Fasting is not safe for everyone. Pregnant people should not restrict calories regardless of weight, as research links fasting during pregnancy with dehydration and fatigue, and the effects on the baby aren’t well enough understood to confirm safety. People with gestational diabetes face additional risks because fasting makes blood sugar harder to control.
Others who should avoid fasting or only fast under medical supervision include people who are underweight (losing more weight can compromise bone health, immune function, and energy levels), people with a history of eating disorders, children and adolescents, and anyone taking medications that must be taken with food to avoid nausea or stomach irritation. If you take blood pressure or heart medications, the electrolyte shifts during fasting can interact with your drugs in ways that require monitoring.