Intermittent fasting works by cycling between periods of eating and not eating on a regular schedule. Rather than changing what you eat, you change when you eat. The most common approach is the 16:8 method, where you fast for 16 hours and eat within an 8-hour window each day. Most people find that skipping breakfast and eating between noon and 8 PM is the easiest way to start, though you can shift the window to fit your schedule.
The Three Most Popular Methods
There’s no single “right” way to fast. The best method is the one you can maintain consistently. Here are the three most widely used approaches:
- 16:8 (daily time-restricted eating): You eat during a 6- to 8-hour window and fast the remaining 16 to 18 hours. This is the most beginner-friendly option because you’re essentially just skipping one meal and avoiding late-night snacking. A typical window runs from noon to 8 PM or 11 AM to 7 PM.
- 5:2 (modified fasting): You eat normally five days a week and limit yourself to one 500 to 600 calorie meal on the other two days. The two fasting days shouldn’t be consecutive. Many people choose Mondays and Thursdays.
- OMAD (one meal a day): You eat all your daily calories in a single meal, typically within a 1-hour window. This is the most aggressive daily approach and generally not recommended for beginners.
If you’ve never fasted before, start with 16:8. Give yourself at least two to three weeks before deciding whether it works for you, since the first week is the hardest and isn’t representative of the long-term experience.
What Happens in Your Body During a Fast
When you stop eating, your body goes through a predictable sequence. For the first several hours, it burns through glucose circulating in your blood. Once that’s used up, it taps into glycogen, the stored form of sugar kept in your liver (about 100 grams) and muscles (about 400 grams). Glycogen depletion begins roughly 12 to 24 hours after you stop eating carbohydrates, depending on how active you are and how much you had stored.
Once glycogen runs low, your body shifts to burning fat for fuel. The liver converts fatty acids into molecules called ketones, which your brain and muscles can use for energy. This transition, sometimes called “metabolic switching,” is a core reason intermittent fasting promotes fat loss beyond simple calorie reduction. With daily 16:8 fasting, you’re dipping into the early stages of this switch each day.
There’s also a cellular cleanup process called autophagy, where your cells break down and recycle damaged components. Animal studies suggest this ramps up significantly between 24 and 48 hours of fasting. The exact timing in humans isn’t well established yet, so shorter daily fasts likely trigger only modest levels of this effect.
What You Can Drink While Fasting
The goal during your fasting window is to avoid triggering a significant insulin response. Water, black coffee, and plain tea are all safe. Sparkling water and mineral water are fine too. Research on healthy men found that artificially sweetened drinks containing sucralose or aspartame did not cause any measurable rise in blood sugar or insulin over a two-hour period, so a diet soda technically won’t break your fast from a metabolic standpoint. That said, some people find that sweet-tasting drinks increase their hunger and make fasting harder.
What does break a fast: anything with calories. Cream in your coffee, a splash of juice, bone broth, and supplements containing sugar or fat all count. If it has calories, save it for your eating window.
Timing Your Eating Window
Your body’s metabolism is tied to your internal clock. Eating earlier in the day, aligned with daylight hours, appears to optimize how your body processes food. Research published in The Lancet Healthy Longevity found that time-restricted eating synchronized with circadian rhythms can improve blood pressure and blood sugar, both important markers for heart health.
In practical terms, this means a noon-to-8 PM window is good, but a 10 AM-to-6 PM window may be slightly better for metabolic health. The worst option is shifting your eating late into the night. If you work a standard daytime schedule, try to finish your last meal at least two to three hours before bed.
How to Handle the First Week
The first three to seven days are the adjustment period. Common side effects include hunger (obviously), headaches, irritability, difficulty concentrating, and low energy. This is sometimes called “fasting flu,” and it’s largely driven by two things: your body adjusting to a new fuel source and electrolyte shifts from changes in hydration.
Staying on top of your electrolytes makes a noticeable difference. During fasting periods, 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 sodium from adding a pinch of salt to your water, potassium from eating potassium-rich foods like avocado and spinach during your eating window, and magnesium from a simple supplement taken with your meal.
Most people find that hunger pangs come in waves rather than building continuously. If you ride out a wave for 20 to 30 minutes, it typically passes. Drinking water, black coffee, or tea helps. By the end of the second week, the majority of people report that the hunger fades significantly and fasting starts to feel natural.
Exercise and Fasting
Light to moderate cardio during a fasted state is generally fine and may even enhance fat burning, since your body is already primed to use fat for fuel. Walking, cycling, and easy jogging all work well in a fasted window.
Strength training is a different story. Research in The Journal of Nutrition found that while resistance exercise stimulates muscle protein building for up to 48 hours afterward, this effect is significantly blunted if you remain fasted. The net balance between muscle building and muscle breakdown stays negative without amino acids from food. There’s also an interactive effect: consuming protein after a workout produces a greater muscle-building response than either protein or exercise alone. If you lift weights, schedule your sessions during or just before your eating window so you can eat protein afterward. A meal containing 20 to 40 grams of protein within a couple hours of training is a practical target.
Who Should Be Cautious
Intermittent fasting isn’t appropriate for everyone. People with diabetes need to be especially careful, since skipping meals can cause dangerous swings in blood sugar, particularly if you’re on medication that lowers glucose. If you take blood pressure or heart medications, longer fasting periods can disrupt your sodium and potassium balance. Anyone who takes medications that need to be taken with food to avoid stomach irritation will have trouble fitting doses into a compressed eating window.
People who are already at a low body weight risk losing too much, which can weaken bones, suppress immune function, and drain energy levels. Intermittent fasting is also not well studied in older adults, so the benefits seen in younger populations may not translate. And anyone with a history of eating disorders should approach fasting schedules with caution, since the rigid structure around food restriction can trigger disordered patterns.
A Practical Starting Plan
If you’re ready to try it, here’s a simple week-one approach. Stop eating after dinner, around 8 PM. Skip breakfast the next morning. Have your first meal at noon. Eat normally between noon and 8 PM, focusing on whole foods, adequate protein, and plenty of vegetables. Repeat daily.
If 16 hours feels too aggressive right off the bat, start with a 12-hour fast (stop eating at 8 PM, eat at 8 AM) and push it back by an hour every few days until you reach 16 hours. There’s no prize for jumping straight to OMAD. Consistency over weeks and months matters far more than the length of any single fast. Track how you feel, not just the clock, and adjust your window based on your energy, sleep quality, and hunger patterns.