Fasting, at its simplest, means choosing a window of time when you don’t eat. The most common approach is intermittent fasting, where you cycle between eating and not eating on a daily or weekly schedule. There’s no single “right” way to do it, and the best method depends on your lifestyle, your goals, and how your body responds. Here’s what you need to know to get started.
The Main Fasting Methods
Most people who fast use one of four popular schedules. They all work by limiting when you eat rather than strictly controlling what you eat.
- 16:8 (time-restricted eating): You fast for 16 hours and eat within an 8-hour window each day. For example, you eat between noon and 8 PM and skip breakfast. This is the most popular starting point because it mostly just means delaying your first meal.
- 5:2: You eat normally five days a week and limit yourself to 500 to 600 calories on two non-consecutive days. The “fasting” days aren’t true fasts, but the calorie restriction is low enough to trigger some of the same metabolic benefits.
- Alternate-day fasting (ADF): You alternate between normal eating days and fasting days, where you consume either nothing or up to 500 calories. This is more aggressive than 5:2 and harder to sustain socially.
- One meal a day (OMAD): You eat a single meal within roughly a one-hour window, fasting for the remaining 23 hours. This is the most extreme daily option and not where most people should start.
How to Start If You’ve Never Fasted
The 16:8 method is the easiest entry point. Most people already fast while they sleep, so you’re essentially extending that overnight fast by a few hours. If you normally eat dinner at 7 PM, just push your first meal the next day to 11 AM. That’s it.
For the first week, don’t worry about perfection. Start with a 12-hour fast (say, 8 PM to 8 AM) and push it back by an hour every few days until you reach 16 hours. Hunger during the morning tends to come in waves rather than building steadily, and most people find the urge passes within 20 to 30 minutes. Staying busy during those first morning hours helps more than willpower does.
If daily fasting feels too restrictive, try the 5:2 method instead. Pick two days that aren’t back-to-back, like Monday and Thursday, and eat lightly on those days. Having only two “hard” days per week makes it easier to plan around social meals and workouts.
What Happens in Your Body While You Fast
When you stop eating, your body first burns through its stored sugar (glycogen) for energy. As those reserves dwindle, your liver starts converting fat into molecules called ketones, which your brain and muscles can use as fuel. This shift from burning sugar to burning fat is often called “metabolic switching,” and it’s the core mechanism behind fasting’s effects on body composition.
Fasting also ramps up a cellular cleanup process where your cells break down and recycle damaged components. Animal studies show this process accelerates after about 24 hours of food restriction and peaks around 48 hours. In humans, the exact timeline is harder to pin down, but extended fasts beyond 24 hours appear to meaningfully increase this recycling activity. For most people doing a daily 16:8 fast, the primary benefit comes from the metabolic switch toward fat burning rather than deep cellular cleanup.
What You Can Drink Without Breaking Your Fast
Water, black coffee, and plain tea won’t break your fast. These have zero or near-zero calories and don’t trigger a significant metabolic response. Sparkling water and herbal teas are also fine.
The gray area starts when you add things to your drinks. A splash of milk or cream in your coffee technically breaks the fast because it contains calories, but in small amounts it won’t knock you out of fat-burning mode. The same goes for adding butter, coconut oil, or MCT oil to coffee. These add calories (and therefore end a strict fast) but keep your body burning fat rather than sugar. If your primary goal is weight loss, a small amount of fat in your morning coffee is unlikely to derail your progress. If you’re fasting for the cellular cleanup benefits, stick to zero-calorie drinks.
Anything with sugar, including fruit juice, sweetened coffee drinks, and sodas, definitively breaks a fast.
What to Expect for Weight Loss
Fasting works for weight loss, but it’s not magic. A systematic review of 27 clinical trials found that intermittent fasting produced weight loss ranging from 0.8% to 13.0% of starting body weight, depending on how long people stuck with it and which method they used. Studies lasting a full year showed participants losing around 5 kg (about 11 pounds) on average with intermittent fasting.
Here’s the important part: when researchers directly compared intermittent fasting to traditional calorie-counting diets, the results were essentially the same. In the largest head-to-head trial of 244 obese adults, those who fasted lost about 5 kg over a year while calorie counters lost about 6.7 kg, a difference that wasn’t statistically significant. Fasting doesn’t burn more fat through some special mechanism. It works because it naturally limits how much you eat. If you compensate by overeating during your feeding window, you won’t lose weight.
Common Side Effects
Expect some discomfort in the first one to two weeks. Headaches are common, especially if you normally eat breakfast or drink sugary beverages in the morning. These typically fade as your body adapts. Fatigue and irritability can show up in the first few days, and some people experience constipation from the change in eating patterns.
Fasting raises cortisol, your body’s primary stress hormone. Cortisol increases almost immediately after fasting begins, and longer fasts of several days can dramatically elevate levels and shift the natural cortisol peak from morning to afternoon. For most people doing a 16:8 fast, this increase is modest and manageable. But if you’re already dealing with high stress, poor sleep, or anxiety, adding fasting on top of that can make things worse. Pay attention to how you feel, not just how you look.
Dizziness is another common complaint, particularly if you exercise while fasted or don’t drink enough water. Staying well-hydrated and adding a pinch of salt to your water can help maintain electrolyte balance.
Special Considerations for Women
Women’s hormonal systems appear to be more sensitive to fasting than men’s. Animal research on alternate-day fasting found that it disrupted the reproductive cycle, lowered key reproductive hormones, and increased the risk of irregular cycles. Time-restricted eating (like 16:8) showed much milder effects, with little impact on reproductive hormones in some studies.
If you notice changes to your menstrual cycle after starting a fasting routine, that’s a signal to scale back. Many women do well with a shorter fasting window of 12 to 14 hours rather than a full 16. Starting conservatively and increasing gradually gives your body time to adapt without disrupting your hormonal balance.
How to Break Your Fast
For daily fasts of 16 to 20 hours, breaking your fast doesn’t require much ceremony. A normal, balanced meal works fine. That said, diving straight into a large, heavy meal can cause bloating and digestive discomfort, especially when you’re new to fasting. Starting with something easy to digest, like eggs, yogurt, a piece of fruit, or a small salad, gives your digestive system a gentle restart before a larger meal.
For longer fasts of 24 hours or more, ease back in more carefully. Eat a small meal first, wait 30 to 60 minutes, then eat more if you’re still hungry. Soups, cooked vegetables, and lean proteins are good first foods after an extended fast. Avoid processed or sugary foods as your first meal, since your body will be more sensitive to blood sugar swings after not eating for a full day or longer.
Who Should Avoid Fasting
Fasting isn’t appropriate for everyone. According to the Mayo Clinic, it’s not recommended for people with a history of eating disorders, anyone who is pregnant or breastfeeding, or individuals at high risk of bone loss and falls. People with diabetes need to be especially careful, since fasting directly affects blood sugar regulation and can interfere with medication timing. If any of these apply to you, talk to your doctor before trying a fasting protocol.