Fasting means voluntarily going without food for a set period, and the simplest way to start is by extending the overnight gap you already take between dinner and breakfast. Most beginners do well with a 14- to 16-hour fast, eating all their meals within an 8- to 10-hour window during the day. From there, you can work up to longer fasts once you understand how your body responds.
Choose a Fasting Method
There are several well-known approaches, and the best one depends on your schedule and goals. Here are the most common:
- 16:8 or 14:10 (time-restricted eating): You eat within a set daily window. A 16:8 schedule might mean eating only between 11 a.m. and 7 p.m., while a gentler 14:10 version allows eating from 9 a.m. to 7 p.m. This is the easiest method for beginners because you’re simply skipping one meal, usually breakfast.
- 5:2 method: You eat normally five days a week and cap your intake at 500 calories on two non-consecutive days. You pick the days that work for you, as long as there’s a regular eating day between them.
- Alternate-day fasting: Every other day, you limit calories to about 500 (roughly 25% of a normal intake). On the off days, you eat normally.
- 24-hour fast (eat-stop-eat): You go a full 24 hours without food, typically once or twice a week. Most people go from breakfast to breakfast or lunch to lunch so the fast feels like it fits naturally into their routine.
If you’ve never fasted before, start with the 14:10 or 16:8 approach for at least a week or two before trying anything longer. Jumping straight into a 24-hour or multi-day fast often leads to unnecessary discomfort and makes it harder to stick with the practice.
What Happens in Your Body During a Fast
For the first few hours after your last meal, your body is still digesting and absorbing nutrients. This is the fed state, and your blood sugar and insulin levels are elevated.
Around 3 to 4 hours after eating, you enter an early fasting state. Blood sugar and insulin begin to decline, and your body starts tapping into stored glucose (glycogen) in the liver for energy. This phase lasts until roughly 18 hours into the fast.
Between 18 and 48 hours, your liver’s glycogen stores run out. Your body shifts to breaking down fat for fuel, producing compounds called ketone bodies in the process. This is the transition into ketosis, where fat becomes your primary energy source. It’s worth noting that shorter fasts of 12 to 18 hours often don’t reach full ketosis unless you’re also eating very few carbohydrates during your eating window. For most people doing a daily 16:8 fast, the metabolic benefits come more from the insulin drop and calorie reduction than from deep ketosis.
What You Can Drink While Fasting
Water is the obvious baseline, and you should drink plenty of it. Black coffee and unsweetened tea are generally considered safe during a fast because they contain essentially no calories.
If you want a hint of sweetness, some sweeteners are less likely to interfere with your fast than others. Stevia has shown no negative impact on insulin sensitivity in human trials, and in one study it actually reduced the blood sugar spike from a meal by 18%. Erythritol, even at doses of 20 to 55 grams for a 150-pound person, did not significantly raise blood sugar or insulin. Sucralose and aspartame also appear not to trigger a meaningful insulin response from a single dose.
Xylitol is the one to be cautious with. It produces a small insulin response, about 16% to 25% of what the same amount of sugar would cause. That’s probably not enough to completely derail a fast, but if you’re fasting specifically for metabolic benefits, it’s worth avoiding.
Anything with calories, including milk, cream, juice, or sweetened drinks, will break your fast.
How to Prepare Before You Start
A little planning in the days before your first fast makes a real difference. Headaches are one of the most common complaints, and they’re often caused by caffeine withdrawal rather than the fast itself. If you’re a regular coffee or tea drinker, gradually reduce your caffeine intake over three to four days before your fast so the sudden absence doesn’t trigger a headache.
Eat balanced, nutrient-dense meals in the days leading up to your fast. Focus on protein, healthy fats, and complex carbohydrates. These digest more slowly and help stabilize your blood sugar, which makes the first several hours of fasting much more comfortable. Avoid loading up on sugar or refined carbs in your last meal, as the resulting blood sugar crash will make hunger feel worse.
Pick a day when you’re reasonably busy but not under heavy physical or mental stress. Boredom is one of the biggest triggers for breaking a fast early, so a mildly active day with a normal routine tends to work better than a lazy weekend.
Managing Hunger and Side Effects
Hunger during a fast tends to come in waves rather than building steadily. A pang will hit, peak for 15 to 30 minutes, and then fade. Drinking water, sparkling water, or black coffee during these waves often helps them pass. Staying occupied is equally effective.
Headaches during fasting are common and typically feel like mild, dull pressure across the front of the head, sometimes extending to both sides. You might also notice tension in your neck and shoulders. These happen because your brain is adjusting to lower blood sugar, and your body releases stress hormones and histamine in response. Staying well-hydrated and tapering caffeine beforehand are the two most reliable ways to prevent them.
Other common side effects include irritability, difficulty concentrating, and feeling cold. These are normal in the first few fasts and tend to improve as your body adapts. If you feel dizzy, shaky, or genuinely unwell, eat something. There’s no benefit to pushing through symptoms that feel alarming.
Electrolytes for Longer Fasts
For fasts under 24 hours, most healthy people do fine with just water. But once you go beyond a full day, your body loses electrolytes through urine at a faster rate than usual, and you’re not replacing them through food. This is when supplementation becomes important.
The key targets during an extended fast are:
- Sodium: 1,500 to 2,300 mg per day. A pinch of salt in water a few times a day covers this.
- Potassium: 1,000 to 2,000 mg per day. Potassium supplements or a salt substitute containing potassium chloride can help.
- Magnesium: 300 to 400 mg per day. Magnesium supplements are widely available and also help prevent muscle cramps and sleep disruption.
Low electrolytes are behind many of the worst fasting symptoms: muscle cramps, heart palpitations, lightheadedness, and brain fog. Keeping up with sodium, potassium, and magnesium makes fasts beyond 24 hours dramatically more tolerable.
Exercise While Fasting
You can exercise during a fast, but your performance will take a hit, especially at higher intensities. In a controlled study where participants fasted for seven hours before cycling, fat burning increased by about 3 grams during the workout, but high-intensity performance dropped by nearly 4%. Participants also reported lower motivation, less energy, and less enjoyment of the exercise session.
Light to moderate activity, like walking, yoga, or easy cycling, works well in a fasted state and can actually help distract from hunger. Save intense workouts for your eating window, or schedule them shortly before you plan to break your fast so you can refuel afterward. If you exercise fasted and notice you’re significantly hungrier later, know that this is common. One study found people ate about 99 extra calories in the post-exercise meal after fasting, though their total daily intake was still 443 calories lower overall.
How to Break Your Fast
The longer your fast, the more carefully you should ease back into eating. For a 16-hour fast, this matters less. You can eat a normal meal, though it’s still wise not to gorge yourself. For fasts of 24 hours or more, your digestive system has been resting and needs a gentle restart.
Start with something small and easy to digest: a few bites of soft fruit, a small bowl of broth, some scrambled eggs, or a handful of cooked vegetables. Wait 30 to 60 minutes, then eat a fuller meal if you feel good. Foods that are high in fat, sugar, or fiber are the hardest on your system after a fast. A greasy meal, a large salad full of raw vegetables, or a pile of nuts can all cause bloating, cramping, and discomfort.
The most important rule when breaking a fast is to eat slowly. Your hunger signals will be strong, and it’s tempting to eat quickly and in large quantities. Give your body 20 minutes to register fullness before reaching for a second serving.