A fasting diet works by cycling between periods of eating and not eating, rather than restricting specific foods. The most popular approach, the 16:8 method, has you eat within an eight-hour window and fast for the remaining 16 hours each day. But there are several variations, and the best one depends on your schedule, your goals, and how your body responds. Here’s how to get started.
Choose a Fasting Schedule
Fasting diets come in a few well-tested formats. Each one restricts when you eat rather than what you eat, though food quality still matters for results.
- 16:8 method: Fast for 16 hours, eat during an 8-hour window. A common setup is eating between 11 a.m. and 7 p.m., effectively skipping breakfast. This is the most popular starting point because it fits naturally around a lunch-and-dinner schedule.
- 14:10 method: A gentler version with a 10-hour eating window, such as 9 a.m. to 7 p.m. Good for beginners or people who find 16 hours too aggressive at first.
- 5:2 method: Eat normally five days a week, then cap your intake at 500 calories on two non-consecutive days. You pick which days work for you (say, Tuesday and Thursday), as long as there’s a regular eating day between them.
- Alternate-day fasting: Every other day, you limit calories to about 500, which is roughly 25% of a normal intake. On the other days, you eat as you normally would.
- 24-hour fasts: You fast completely from one meal to the same meal the next day, breakfast to breakfast or lunch to lunch. Most people do this only once or twice a week.
If you’ve never fasted before, the 14:10 or 16:8 method is the easiest entry point. You can always shift to a longer fast later once your body adjusts.
What Happens in Your Body During a Fast
For the first 12 hours or so, your body runs through its stored glycogen, which is the quick-access sugar reserve in your liver and muscles. Once that supply runs low, your metabolism shifts toward burning fat for fuel. This transition typically kicks in between 12 and 16 hours of fasting, which is exactly why the 16:8 window is so popular: it pushes you just past the point where fat burning ramps up.
If you fast longer, other processes come online. Between 12 and 24 hours, your cells begin a recycling process where they break down damaged components and repurpose the parts. This cellular cleanup intensifies significantly around the 24-hour mark and deepens between 24 and 48 hours. You don’t need to fast that long to see benefits from a fasting diet, but it explains why some people occasionally do longer fasts.
Fasting also shifts your hunger hormones. Levels of ghrelin (the hormone that triggers hunger) and leptin (the hormone that signals fullness) both decrease over time during sustained fasting. This is why many people report that their appetite actually quiets down after the first few days of sticking to a schedule, even though the early days can feel rough.
What You Can Drink While Fasting
During your fasting window, stick to zero-calorie beverages. Water, plain or sparkling, is the baseline. Black coffee and plain tea are fine for most people, but adding sugar, milk, or cream introduces calories that break the fast. Some people sip diluted apple cider vinegar (a teaspoon or two mixed into water) to curb cravings, which won’t meaningfully affect your fasted state.
Technically, anything with calories ends the fast. That said, small amounts of fat, like a splash of MCT oil or coconut oil in coffee, will break a strict fast but won’t kick you out of fat-burning mode. Bone broth falls into the same gray area: it contains calories, but the electrolytes can help if you’re feeling drained during longer fasts. Whether these small exceptions matter depends on your goals. If you’re fasting primarily for weight management, keeping your fasting window completely calorie-free gives you the cleanest results.
How to Structure Your Eating Window
What you eat during your feeding window matters just as much as when you eat. Fasting isn’t a license to binge. If you pack your eating hours with processed food, you’ll undermine the metabolic benefits and likely feel terrible.
When you break your fast, start with something easy to digest. A large, heavy meal on an empty stomach can cause bloating, nausea, or an energy crash. A moderate portion of protein and vegetables works well as a first meal. Save your larger, more calorie-dense meals for later in the eating window when your digestive system has had time to wake up.
Across your eating window, aim for balanced meals that include protein, healthy fats, fiber-rich vegetables, and complex carbohydrates. Protein is especially important because it helps preserve muscle mass during periods of calorie restriction. If you’re only eating two meals a day, each one needs to carry more nutritional weight than the three smaller meals you might be used to.
Exercise and Fasting
You can work out while fasting, but timing and intensity matter. Light to moderate exercise, like walking, yoga, or easy cycling, is generally well tolerated in a fasted state. High-intensity resistance training is a different story. Performance tends to be better if you haven’t eaten within about two hours before lifting, but also haven’t been fasting for an extremely long time. Training toward the end of your fasting window or early in your eating window hits that sweet spot for most people.
What you do after the workout is more important than what you do before it. Eating protein and carbohydrates after exercise supports muscle recovery and replenishes your energy stores. If you train fasted in the morning and your eating window doesn’t open until noon, your recovery will be delayed. Some people shift their eating window to accommodate their workout schedule, eating from, say, noon to 8 p.m. and training at 11 a.m. so their first meal doubles as post-workout fuel.
Side Effects in the First Week
The adjustment period is real. During your first several days of fasting, you may experience headaches, fatigue, dizziness, irritability, or difficulty concentrating. These symptoms are largely driven by the metabolic transition your body is making and by changes in hydration and electrolyte balance. Drinking enough water and getting adequate salt during your eating window helps significantly.
Constipation is another common early complaint, since you’re eating fewer meals and potentially less fiber overall. Prioritizing vegetables and whole grains during your eating window keeps things moving. Some people also notice changes in their menstrual cycle, particularly with longer or more aggressive fasting protocols. If this happens, scaling back to a shorter fast (like 14:10) often resolves it.
Most of these side effects fade within one to two weeks as your body adapts to the new schedule.
Who Should Avoid Fasting
Fasting is not appropriate for everyone. People with a history of eating disorders should avoid it, as the rigid eating-and-restriction cycle can trigger or worsen disordered patterns. Pregnant or breastfeeding women need consistent caloric intake and shouldn’t fast. People at high risk of bone loss and falls are also advised against it, since fasting can affect nutrient absorption and energy levels in ways that increase fall risk.
If you take medication that requires food at specific times, particularly for blood sugar management, fasting can complicate your dosing schedule and lead to dangerous drops in blood sugar. Adjusting a fasting protocol around medication needs medical input.
Tips for Sticking With It
The biggest predictor of success with a fasting diet is consistency over the first two to three weeks. Your hunger hormones recalibrate during this period, and most people find the fasting window genuinely easier to tolerate after the initial adjustment. A few practical strategies help:
- Start with a shorter fast. Begin at 12 or 14 hours and add an hour every few days until you reach your target window. Jumping straight into a 20-hour fast often leads to bingeing and quitting.
- Keep busy during fasting hours. Hunger tends to come in waves that last 20 to 30 minutes. Staying occupied makes them pass faster.
- Anchor your window to your social life. If you eat dinner with family, build your eating window around that. A protocol you can maintain without isolating yourself is one you’ll actually follow.
- Stay hydrated. Thirst masquerades as hunger more often than most people realize. A glass of water or cup of black coffee can quiet a craving almost immediately.
Fasting doesn’t need to be an all-or-nothing commitment. Many people fast on weekdays and eat normally on weekends, or fast four days a week instead of seven. The flexibility is part of what makes it sustainable long term.