The simplest way to start intermittent fasting is to stop eating after dinner and delay breakfast the next morning, giving yourself a 12-hour fasting window. From there, you gradually widen that window over days or weeks until you reach a schedule that fits your life. Most beginners land on a 16-hour fast with an 8-hour eating window, but several approaches work, and the best one is the one you can actually stick with.
Choose a Fasting Method
Intermittent fasting isn’t a single diet. It’s an umbrella term for several patterns that cycle between eating and not eating. The three most popular are time-restricted eating, the 5:2 diet, and alternate-day fasting.
Time-restricted eating (TRE) is where most beginners start. You pick a daily eating window, typically 8 to 10 hours, and fast for the remaining 14 to 16 hours. A common version: eat between noon and 8 p.m., fast from 8 p.m. to noon the next day. The fasting window can range anywhere from 12 to 21 hours depending on how aggressive you want to be, but 16:8 is the standard starting point.
The 5:2 diet means eating normally five days a week and sharply cutting calories on two days (consecutive or spread apart). On those two days, intake drops to roughly 500 to 600 calories. This works well for people who hate daily restrictions but can handle two tough days per week.
Alternate-day fasting alternates between regular eating days and fasting days. Some versions allow zero calories on fasting days, while modified versions allow 20% to 30% of your normal intake. This is the most demanding approach and not the best entry point for someone brand new to fasting.
For your first attempt, time-restricted eating with a 14- to 16-hour fast is the lowest-friction option. You’re already fasting while you sleep, so you’re really just skipping or delaying one meal.
Ease In Over Two Weeks
Jumping straight to a 16-hour fast often backfires. Big, sudden changes to eating habits are harder to maintain, and they produce more side effects. A better approach is to start with a 12-hour overnight fast and add 30 to 60 minutes to your fasting window every few days.
Here’s what that progression looks like in practice:
- Days 1 to 3: Stop eating by 8 p.m., eat breakfast at 8 a.m. That’s a 12-hour fast, which most people can handle immediately.
- Days 4 to 7: Push breakfast to 9 or 10 a.m. You’re now fasting 13 to 14 hours.
- Week 2: Shift your first meal to 11 a.m. or noon. You’ve reached the 15- to 16-hour mark.
This gradual ramp gives your body time to adjust. If you feel very hungry or unwell at any stage, stay at that window for a few extra days before extending further. There’s no penalty for going slowly.
What Happens in Your Body During a Fast
For the first several hours after your last meal, your body runs on glucose stored in your liver as glycogen. That fuel supply typically lasts about 12 hours. Once it’s depleted, your body flips what researchers call the “metabolic switch,” shifting from burning stored sugar to breaking down fat for energy.
This switch usually happens between 12 and 36 hours into a fast, depending on how full your glycogen stores were and how active you’ve been. For someone doing a daily 16:8 fast, the switch flips around hour 12 and stays on for roughly six hours, until you eat again at the end of your fasting window. That’s the core mechanism behind intermittent fasting’s fat-burning reputation.
During this fat-burning phase, your insulin levels drop and your body produces ketones as an alternative fuel source. Animal studies suggest a cellular cleanup process called autophagy (where your body recycles damaged cell parts) may begin between 24 and 48 hours of fasting, but there isn’t enough human research to pin down an exact timeline.
What You Can Drink While Fasting
Water, black coffee, and plain tea are safe during the fasting window. Diet sodas and other zero-calorie beverages are also generally permitted in clinical fasting studies without disrupting results. The key rule: avoid anything with calories, sugar, or cream, since those trigger an insulin response and effectively end the fast. Sparkling water and water with a squeeze of lemon are fine.
Staying well-hydrated matters more during fasting than during normal eating. Much of your daily water intake normally comes from food, so you’ll need to consciously drink more to compensate.
What to Eat When You Break Your Fast
What you eat during your eating window shapes your results as much as the fast itself. Across 12 studies that directly compared intermittent fasting to standard calorie restriction, both groups lost equivalent amounts of weight. The fasting schedule alone doesn’t override poor food choices.
When your eating window opens, prioritize lean protein (chicken, fish, Greek yogurt, beans), vegetables, and fruit. A practical target: get about 30% or fewer of your calories from carbohydrates, eat plenty of fiber, and include moderate amounts of healthy fat. Highly processed foods and sugary snacks spike your blood sugar rapidly, which can make the next fasting window feel much harder.
Meal planning makes a real difference here. Having food prepped and ready when your window opens prevents the common pattern of breaking a fast with whatever’s fastest and most convenient, which is usually the least nutritious option.
Expect These Side Effects Early On
The first week or two will likely involve some discomfort. The most commonly reported side effects are headaches, fatigue, mood swings, and dizziness. In one large survey of intermittent fasters, over half experienced headaches (mild in 36%, moderate in 18%, severe in about 8%), and nearly 70% reported some degree of lethargy. Mood swings affected more than half of participants.
Fasting headaches are usually caused by low blood sugar and tend to feel diffuse and dull rather than throbbing. They’re most common in the first week and typically fade as your body adapts to running on fat instead of a constant supply of glucose. Staying hydrated and making sure your eating-window meals include enough salt and electrolytes helps significantly.
If symptoms are severe or don’t improve after the first two weeks, try shortening your fasting window or switching to a less aggressive method like the 5:2 approach.
Exercise During Fasting
You can exercise while fasted, and there’s a measurable benefit to doing so. A meta-analysis of studies comparing fasted and fed exercise found that aerobic exercise in a fasted state produces significantly higher fat oxidation, meaning your body burns more fat per session than it would after eating.
That said, the difference matters most for moderate-intensity cardio like walking, jogging, or cycling. High-intensity or heavy resistance training may feel harder without fuel on board, especially in your first few weeks. A practical compromise: do lighter cardio during your fasting window and save intense workouts for after your first meal.
Realistic Weight Loss Expectations
Across 27 clinical trials involving nearly 1,000 participants, intermittent fasting produced weight loss ranging from 0.8% to 13% of baseline body weight. In studies lasting 2 to 12 weeks, BMI dropped by an average of 4.3%. Importantly, about 79% of the weight lost through intermittent fasting comes from fat rather than muscle, based on level-one evidence from a 2011 study by Harvie and colleagues.
The largest long-term comparison followed 244 adults with obesity over a full year. The intermittent fasting group lost an average of 5 kg (about 11 pounds), while the calorie-restriction group lost 6.65 kg (about 14.7 pounds). The difference wasn’t statistically significant, meaning both approaches worked about equally well. Combining intermittent fasting with deliberate calorie reduction during the eating window produced better results than fasting alone.
Who Should Avoid Intermittent Fasting
Intermittent fasting is not safe for everyone. It is not recommended for pregnant or breastfeeding women, frail older adults, people with immune deficiency conditions, or anyone with a history of or vulnerability to eating disorders. For people with diabetes, fasting carries real risk of dangerously low blood sugar, especially for those on insulin or medications that lower glucose. If any of these apply to you, this isn’t the right approach without direct medical supervision.