Intermittent fasting works by cycling between periods of eating and not eating on a predictable schedule. The most popular approach, called 16:8, means you eat within an eight-hour window and fast for the remaining 16 hours. It’s simpler than most diets because it focuses on when you eat rather than what you eat, though food quality still matters. Here’s how to get started and what to expect along the way.
The Most Common Fasting Schedules
There are two main approaches, and the right one depends on whether you’d rather fast a little every day or a lot on just a couple of days.
16:8 (daily time-restricted eating): You eat during a six- to eight-hour window and fast the rest of the day. Most people skip breakfast and eat from roughly noon to 8 p.m., but you can shift the window to fit your schedule. This is the easiest method for beginners because it essentially extends the overnight fast you’re already doing.
5:2 (weekly approach): You eat normally five days a week and limit yourself to one 500 to 600 calorie meal on the other two days. Those two days shouldn’t be back to back. For example, you might eat normally every day except Mondays and Thursdays, when you eat a single small meal.
Some people eventually work up to OMAD (one meal a day), but that’s an advanced schedule. Start with 16:8 or 5:2 and see how your body responds before considering anything more restrictive.
What Happens in Your Body During a Fast
For the first several hours after your last meal, your body runs on glucose from the food you ate. Once those stores run low, it begins tapping into fat for energy instead. This transition, sometimes called the metabolic switch, typically happens 12 to 36 hours after your last meal. The wide range depends on factors like how active you are, how large your last meal was, and your individual metabolism.
With a 16:8 schedule, you’re crossing that 12-hour threshold daily, which means your body gets regular practice switching fuel sources. Over time, this can make your metabolism more flexible and efficient at burning stored fat.
Fasting also triggers a cellular cleanup process called autophagy, where your cells break down and recycle damaged components. Animal studies suggest this ramps up between 24 and 48 hours of fasting, but there isn’t enough human research yet to pinpoint the exact timing. A standard 16:8 fast likely initiates early stages of this process, while longer fasts push it further.
How to Start: A Week-by-Week Approach
Jumping straight into a 16-hour fast can feel rough. A better strategy is to ease in gradually over two to three weeks.
- Week 1: Start with a 12-hour fast. If you finish dinner at 8 p.m., don’t eat again until 8 a.m. This is gentle enough that most people barely notice it.
- Week 2: Stretch the fast by two hours, adding an hour on each side. So if you were fasting from 8 p.m. to 8 a.m., now try 7 p.m. to 9 a.m.
- Week 3 and beyond: Continue extending until you reach a 16-hour overnight fast with an eight-hour eating window.
This ramp-up period lets your hunger hormones adjust. The first few days at each new interval may feel uncomfortable, but hunger during fasting hours typically fades within a week as your body adapts.
What You Can Drink While Fasting
Staying hydrated is essential, but the wrong drink will break your fast. Stick to zero-calorie, unsweetened options:
- Water (plain or sparkling)
- Black coffee (no cream, sugar, or flavored syrups)
- Unsweetened tea (green, black, herbal)
- Zero-calorie electrolyte drinks without artificial sweeteners
Avoid artificial sweeteners during your fast. Even though they contain no calories, some can trigger an insulin response that partially defeats the purpose. A splash of milk in your coffee or a diet soda technically interrupts the fasted state. If you need flavor, a squeeze of lemon in water is a safer bet.
What to Eat When You Break Your Fast
Your first meal matters more than you might think. After hours without food, your body is primed to absorb nutrients quickly, so a sugar-heavy meal can cause a sharp blood sugar spike followed by a crash that leaves you tired and hungry again.
Break your fast with a meal built around protein, fiber, and healthy fats. Protein and fiber slow digestion and prevent the kind of blood sugar rollercoaster that leads to overeating later. A practical plate looks like this: fill half with non-starchy vegetables (broccoli, leafy greens, peppers, cauliflower), add a palm-sized serving of lean protein (eggs, chicken, fish, legumes), and include a source of healthy fat (avocado, olive oil, nuts). If you’re adding carbohydrates, choose whole grains over refined options since they break down more slowly.
Fatty fish like salmon, herring, or tuna are especially good choices because their omega-3 fatty acids can help reduce insulin resistance over time. The goal isn’t perfection. It’s building meals that keep your energy stable through the eating window so the next fasting period feels manageable.
Adjustments for Women
Women’s hormones are more sensitive to fasting signals, so a more cautious approach often works better. The gradual ramp-up described above (starting at 12 hours and building to 16) is particularly important for women rather than jumping in at 16 hours on day one.
Your menstrual cycle also affects how well you tolerate fasting. The best times to fast are a day or two after your period begins and during the week or so after. Limit fasting during the two weeks before your period, and avoid it entirely the week right before it’s due. That’s when your body is most reactive to the stress of calorie restriction, and pushing through can disrupt your cycle, sleep, or mood.
If you notice your period becoming irregular, your sleep deteriorating, or your energy tanking, those are signals to shorten your fasting window or take more days off.
Managing Hunger and Side Effects
The first one to two weeks are the hardest. Common side effects include hunger (obviously), headaches, irritability, and difficulty concentrating. These are normal and almost always temporary.
Headaches during fasting are often caused by dehydration or low electrolytes rather than hunger itself. When you fast, your body excretes more sodium and water than usual, which can pull potassium and magnesium levels down with it. Drinking water with a pinch of salt, sipping on mineral water, or using an unflavored electrolyte supplement during your fast can prevent this. People exercising while fasting or living in hot climates will need to be more intentional about replacing minerals.
Hunger tends to come in waves rather than building continuously. If you can ride out a wave for 20 to 30 minutes, it usually passes. Black coffee and sparkling water are both effective at blunting hunger between meals. Staying busy during the last few hours of your fast helps more than willpower alone.
Who Should Be Cautious
Intermittent fasting isn’t appropriate for everyone. People with diabetes face real risks from skipping meals, since it can cause dangerous drops in blood sugar, especially if they’re on medication. Those taking blood pressure or heart medications may be more prone to imbalances in sodium, potassium, and other minerals during extended fasts. If you take any medication that needs to be taken with food to avoid nausea or stomach irritation, a restricted eating window can make that difficult to manage.
People who are already at a low body weight should be cautious. Losing additional weight through fasting can weaken bones, suppress the immune system, and drain energy levels. Fasting is also not recommended for anyone pregnant, breastfeeding, or with a history of eating disorders.
Realistic Expectations for Weight Loss
Intermittent fasting can help you lose weight, but it works primarily because it helps you eat fewer total calories, not because of some metabolic magic. A randomized one-year trial comparing 16:8 time-restricted eating to standard calorie reduction found no additional benefit from the restricted eating window when total calories were the same. Both groups lost similar amounts of weight.
That said, many people find it easier to eat less when they have a defined window. Instead of grazing from morning to midnight, an eight-hour window naturally eliminates late-night snacking and often cuts out one meal’s worth of calories without requiring you to count anything. For people who struggle with traditional calorie counting, this structure can be a more sustainable alternative. The best fasting schedule is one you can maintain consistently over months, not the most extreme one you can white-knuckle through for a week.