Fasting means voluntarily going without food for a set period, then eating during a designated window. The simplest way to start is the 16:8 method: you eat all your meals within an eight-hour window and consume nothing with calories for the remaining 16 hours. Most people do this by skipping breakfast, eating their first meal around noon, and finishing dinner by 8 p.m. But there are several other approaches, and the right one depends on your schedule, your goals, and how your body responds.
The Most Common Fasting Methods
The 16:8 method is popular because it fits naturally into most people’s routines. You’re already fasting while you sleep, so extending that window by a few hours on either side is the easiest entry point. Some people narrow this further to a six-hour eating window, sometimes called 18:6.
The 5:2 method takes a different approach. You eat normally five days a week and limit yourself to one 500 to 600 calorie meal on each of the other two days. Those two low-calorie days shouldn’t be back to back. This works well for people who find daily time restriction too rigid but can handle two lighter days per week.
OMAD, or one meal a day, is the most aggressive common protocol. You eat your entire day’s calories in a single sitting, typically within a one-hour window. This is harder to sustain and makes it difficult to get adequate nutrition in one meal, so it’s generally not a good starting point.
What Happens in Your Body When You Fast
For the first three to four hours after eating, your body is digesting and absorbing nutrients. After that, you enter an early fasting state where your body draws on glycogen, the stored sugar in your liver, for energy. This phase lasts until roughly 18 hours into the fast.
Around the 18-hour mark, your liver’s glycogen stores run out. Your body then shifts to breaking down fat for fuel, producing compounds called ketone bodies in the process. This metabolic shift into ketosis is the transition that many people are chasing with longer fasts. It’s also the point where your body starts relying on fat as its primary energy source rather than glucose.
Autophagy, the process where your cells break down and recycle damaged components, appears to ramp up somewhere between 24 and 48 hours of fasting based on animal studies. There isn’t enough human research yet to pinpoint exactly when this kicks in or peaks, so claims about autophagy starting at 16 or 18 hours are speculative.
What You Can Drink Without Breaking a Fast
The only way to guarantee you’re in a true fasted state is to consume zero calories. In practice, most experts agree that black coffee, plain tea, and water won’t meaningfully disrupt a fast. Adding cream, milk, sugar, or flavored syrups to your coffee will break it.
Zero-calorie sweeteners like stevia and sucralose are a gray area. Research on non-nutritive sweeteners generally shows no significant insulin response compared to plain water. A minority of studies have found that sweet taste alone can trigger a gut hormone response through taste receptors in the mouth, but the overall evidence is mixed and contradictory. If your goal is weight loss, a packet of stevia in your tea probably won’t derail your progress. If you’re fasting for metabolic or cellular benefits, plain water is the safest bet.
How to Start Your First Fast
Begin with the 16:8 method and ease into it over a week. If you normally eat breakfast at 7 a.m., push it to 9 a.m. for a few days, then to 11 a.m., then noon. Most of the discomfort people feel in early fasting comes from habit, not genuine hunger. Your body adjusts within a few days.
During your eating window, eat balanced meals. Fasting is not a license to binge. Focus on protein, vegetables, healthy fats, and complex carbohydrates. Getting enough protein matters because it helps preserve muscle. Research comparing intermittent fasting to standard calorie restriction shows similar retention of lean mass in most studies, but one larger trial of 116 adults found measurable losses in limb muscle mass after 12 weeks of time-restricted eating. Prioritizing protein at each meal helps counteract this.
Stay well hydrated. Drink water throughout your fasting window. If you’re fasting for longer than 24 hours, electrolytes become important. Aim for roughly 1,500 to 2,300 milligrams of sodium, 1,000 to 2,000 milligrams of potassium, and 300 to 400 milligrams of magnesium per day. You can get these through sugar-free electrolyte drinks or by adding a pinch of salt and a potassium-based salt substitute to your water.
What to Expect for Weight Loss
A systematic review of 40 studies found that intermittent fasting typically produces a loss of 7 to 11 pounds over 10 weeks. That’s roughly a pound a week, which is in line with what standard calorie restriction achieves. Fasting doesn’t burn fat faster than other methods at equal calorie deficits. Its advantage is behavioral: some people find it easier to skip meals entirely than to eat small portions all day.
The weight you lose in the first few days is mostly water and stored glycogen, not fat. Don’t be discouraged when the scale stalls after that initial drop. Real fat loss is slower and steadier. If you’re not losing weight after several weeks, you’re likely eating too many calories during your eating window, which is the most common pitfall.
Extended Fasts and Refeeding Safely
Fasts lasting 24 to 72 hours are sometimes practiced for deeper metabolic effects, but they carry more risk and require more preparation. For any fast beyond 24 hours, electrolyte supplementation is essential, not optional.
How you break a longer fast matters. After fasts shorter than about 15 days, refeeding syndrome (a dangerous shift in electrolytes when food is reintroduced) is a negligible risk for otherwise healthy people. But even after a 48- or 72-hour fast, jumping straight into a large, carbohydrate-heavy meal can cause bloating, nausea, and blood sugar spikes. Break longer fasts with a small, easily digestible meal: some broth, eggs, cooked vegetables, or a small portion of protein. Wait an hour, then eat a normal meal if you feel fine.
Who Should Avoid Fasting
Fasting is not safe for everyone. People with diabetes face real risks from blood sugar dropping too low during a fast, especially if they take insulin or medications that lower blood sugar. If you take blood pressure or heart medications, fasting can disrupt your sodium, potassium, and other mineral levels in ways that interact with those drugs. Anyone who needs to take medication with food to avoid stomach irritation will also have trouble with fasting schedules.
People who are already underweight risk losing too much weight, which can weaken bones, suppress the immune system, and drain energy levels. Fasting is also not appropriate for pregnant or breastfeeding women, children, or anyone with a history of eating disorders. The restriction inherent in fasting can reinforce disordered eating patterns in people who are vulnerable to them.