Intermittent fasting means cycling between periods of eating and not eating on a set schedule. The most common approach, the 16:8 method, has you eat all your meals within an 8-hour window and fast for the remaining 16 hours. But there are several variations, and the best one depends on your lifestyle, your health, and how your body responds. Here’s how to get started and what to expect.
Choose a Fasting Schedule
The four most popular methods each work differently, so pick one that fits how you already eat rather than forcing a dramatic change.
- 16:8 or 14:10 (daily): Eat only during an 8- or 10-hour window each day. A common version is eating between 11 a.m. and 7 p.m., essentially skipping breakfast. The 14:10 split (eating from 9 a.m. to 7 p.m.) is a gentler starting point. Fast most days of the week to see benefits.
- 5:2 (twice a week): Eat normally five days a week. On the other two days, cap your total intake at 500 calories, typically split into a 200-calorie meal and a 300-calorie meal. Space the two fasting days apart rather than doing them back to back.
- Alternate-day fasting: Every other day, limit yourself to about 500 calories (roughly 25% of your normal intake). Some people do a stricter version with zero calories on fasting days, but 500 is the more common and sustainable approach.
- Eat-Stop-Eat: Fast completely for a full 24 hours, once or twice a week. For example, finish dinner at 7 p.m. and don’t eat again until 7 p.m. the next day.
If you’ve never fasted before, the 14:10 method is the easiest entry point. You can shorten your eating window to 8 hours after a week or two once the routine feels natural.
What You Can Drink During a Fast
Water is your baseline. Plain black coffee and unsweetened tea are also fine and won’t break your fast. Green tea in particular may improve your body’s insulin sensitivity over time. Black coffee, in moderation, supports sugar metabolism and can help blunt hunger.
Avoid adding sugar, milk, or cream to your drinks. Even small amounts of calories trigger an insulin response that shifts your body out of a fasting state. Diet soda is technically calorie-free, but artificial sweeteners increase insulin resistance, which works against the metabolic benefits you’re fasting for. Bone broth contains calories and will technically break a fast, but it’s low enough in carbohydrates that some people use it strategically during longer fasts to replenish electrolytes without a major insulin spike.
Watch your caffeine intake. Too much coffee can cause dehydration, and when you’re dehydrated your body releases stress hormones that raise blood sugar levels. Two to three cups is a reasonable ceiling for most people.
How to Break Your Fast
What you eat first matters, especially after fasts longer than 16 hours. A large, carb-heavy meal will cause a sharp insulin spike and can leave you feeling sluggish or bloated. Instead, start with something small that combines protein, healthy fat, and fiber. Think eggs with avocado, a handful of nuts with some fruit, or a bowl of Greek yogurt.
Protein powder and branched-chain amino acid supplements trigger a strong insulin response that opposes the cellular cleanup processes fasting activates, so they’re not ideal as your first intake. Save protein shakes for later in your eating window if you use them. For your remaining meals, eat as you normally would. Intermittent fasting is a schedule, not a diet. That said, packing your eating window with processed food and sugar will undermine the metabolic improvements fasting provides.
Stay on Top of Electrolytes
One of the most common reasons people feel terrible during their first week of fasting is electrolyte depletion. When you’re not eating, your body flushes sodium, potassium, and magnesium faster than usual. This causes headaches, fatigue, dizziness, and muscle cramps that people sometimes mistake for hunger.
Adding a pinch of salt to your water during fasting hours can make a noticeable difference. You can also use calorie-free electrolyte supplements. Focus on getting enough potassium (bananas, potatoes, spinach) and magnesium (nuts, seeds, dark chocolate) during your eating window. If you’re doing longer fasts or exercising while fasted, electrolyte management becomes even more important, since people on blood pressure or heart medications are especially vulnerable to mineral imbalances during extended fasting periods.
Exercise While Fasting
You can work out during your fasting window, but set realistic expectations. A 2017 analysis of five studies found that fasted exercise does not lead to greater fat loss or changes in body composition compared to fed exercise. Your body does burn more fat for fuel during a fasted workout, but that increased fat oxidation doesn’t translate into lower body fat over time. Long-term calorie balance is what drives fat loss, not whether you ate before your workout.
There are some trade-offs. Fasted training can mean reduced performance and longer recovery times, partly because your body breaks down protein looking for fuel before it turns to fat stores. On the other hand, several studies show fasted cardio improves endurance by increasing your VO2 max, which is your body’s ability to deliver oxygen to working muscles. If you’re a runner or cyclist, occasional fasted sessions could improve your aerobic capacity.
For strength training, you’ll likely get better results eating beforehand or scheduling your lifts during your eating window. For light cardio or moderate-intensity sessions, fasted training is generally fine.
Adaptations for Women
Intermittent fasting can disrupt estrogen and progesterone levels in pre-menopausal women. The hormone that regulates your menstrual cycle is highly sensitive to environmental stressors, and fasting can signal to your body that food is scarce. In response, your body may suppress ovulation to prevent pregnancy during perceived famine conditions. The result can be skipped periods, mood changes, hot flashes, trouble sleeping, low sex drive, acne, or hair loss.
This doesn’t mean women can’t fast, but timing matters. The best windows to try fasting are a day or two after your period starts and the week following. Avoid fasting during the two weeks before your period is due, since that’s when you’re most likely ovulating and your hormones are most vulnerable to disruption. The week before your period is when your body handles stress the worst, so skip fasting then too.
Starting with a shorter fasting window (14:10 instead of 16:8) and fasting fewer days per week gives your body time to adjust without major hormonal fallout.
Who Should Avoid Fasting
Intermittent fasting isn’t safe for everyone. People with diabetes face real risks from skipping meals, since blood sugar can drop dangerously low. If you take medications for blood pressure or heart disease, fasting increases your risk of sodium and potassium imbalances. Anyone who needs to take medication with food to prevent nausea or stomach irritation will have trouble fitting that into a restricted eating window.
If you’re already at a low body weight, fasting can push you into territory that weakens your bones, suppresses your immune system, and tanks your energy levels. Pregnant or breastfeeding women, children, and anyone with a history of eating disorders should also avoid structured fasting.
What to Expect in the First Two Weeks
The first three to five days are the hardest. Hunger tends to come in waves, peaking at the times you normally eat, then fading. Your body produces the hunger hormone ghrelin on a schedule based on your eating habits, so it takes roughly a week for those signals to shift to match your new pattern. During this adjustment period, expect some irritability, difficulty concentrating, and mild headaches.
By the end of the second week, most people report that fasting feels routine. The hunger waves become shorter and less intense. Energy levels stabilize, and many people notice improved mental clarity during fasting hours. If you’re still struggling significantly after two weeks, consider trying a wider eating window or fewer fasting days rather than quitting entirely. A 14:10 schedule done consistently will produce more benefit over time than a 16:8 schedule you abandon after a month.