Intermittent fasting works by cycling between periods of eating and not eating, rather than changing what you eat. The most popular approach, the 16/8 method, has you fast for 16 hours and eat within an 8-hour window each day. But there are several variations, and the best one depends on your schedule, your goals, and how your body responds. Here’s how to get started and what to expect along the way.
Choose a Fasting Method
Most intermittent fasting protocols fall into two categories: daily time-restricted eating or weekly fasting days. The daily methods limit when you eat each day, while the weekly methods include full or near-full fasting days spread across the week.
16/8 (most common): Fast for 16 hours, eat within an 8-hour window. Typical windows are 10 a.m. to 6 p.m., 11 a.m. to 7 p.m., or noon to 8 p.m. This is the easiest starting point because it essentially means skipping breakfast and not snacking after dinner.
18/6: A tighter version with 18 hours of fasting and a 6-hour eating window. Common windows run from noon to 6 p.m. or 1 p.m. to 7 p.m.
20/4: Twenty hours of fasting with a 4-hour eating window. This compresses your meals into a short period, typically in the late afternoon or early evening.
OMAD (one meal a day): You eat a single large meal, fasting for roughly 23 hours. Most people choose dinner.
5:2: Eat normally five days a week. On two non-consecutive days, limit intake to 500 to 600 calories.
Alternate-day fasting: Alternate between normal eating days and fasting days where you consume zero to 500 calories.
If you’re new to fasting, start with 16/8 for at least two to three weeks before considering a shorter eating window. Many people find 16/8 sustainable long term and never need to go further.
What Happens in Your Body During a Fast
When you stop eating, your body first burns through its stored glucose (glycogen) in the liver. Once those reserves run low, it shifts to burning fat for fuel, a process sometimes called the “metabolic switch.” This switch typically happens between 12 and 36 hours after your last meal, depending on how much glycogen you had stored and how physically active you are during the fast. Exercise speeds it up.
Once fat burning ramps up, your liver converts fatty acids into molecules called ketones, which your brain and muscles can use for energy. This is the same basic mechanism behind ketogenic diets, but intermittent fasting triggers it through timing rather than carb restriction. The shift to fat-derived fuel also appears to help preserve muscle, since the body preferentially burns stored fat rather than breaking down muscle protein for energy.
Fasting also triggers a cellular cleanup process called autophagy, where your cells break down and recycle damaged components. Animal studies suggest this kicks in somewhere between 24 and 48 hours of fasting, though the exact timing in humans isn’t well established. You don’t need to fast that long to get benefits, but it’s worth knowing that the body’s repair processes ramp up the longer you go without food.
How to Structure Your Eating Window
What you eat during your window matters. Fasting creates a calorie deficit naturally for most people, but filling your window with highly processed food can undermine both weight loss and how you feel during fasts. Focus on meals that include protein, healthy fats, and fiber-rich carbohydrates. These keep you full longer and make the next fasting period easier to get through.
Aim for at least two full meals within your eating window, plus a snack if needed. Trying to cram a full day’s nutrition into one rushed meal often leads to digestive discomfort and poor nutrient absorption. If you’re doing OMAD, eat slowly and make sure the meal is large and nutrient-dense enough to meet your daily needs.
There’s some evidence that earlier eating windows may offer a slight metabolic advantage. One small study comparing an 8 a.m. to 5 p.m. window versus a noon to 9 p.m. window found that both improved blood sugar responses, but the early window produced lower fasting glucose levels. The study was limited (15 male participants over one week), so the effect isn’t definitive. That said, eating earlier in the day aligns with your body’s natural circadian rhythm, when insulin sensitivity tends to be highest. If your schedule allows it, an earlier window is worth trying.
What You Can Have During the Fast
Water, black coffee, and plain tea won’t break your fast. These are fine and even helpful, since caffeine can blunt hunger. Anything with calories, including milk in your coffee, juice, or diet drinks with certain sweeteners, can trigger an insulin response and technically interrupt the fasted state.
Staying hydrated is essential. You’re not getting the water that normally comes from food, so drink more than usual. If you experience headaches, dizziness, or muscle cramps during fasts, electrolytes are likely the issue. General targets during fasting are 1,500 to 2,300 mg of sodium, 1,000 to 2,000 mg of potassium, and 300 to 400 mg of magnesium per day. A pinch of salt in your water or a sugar-free electrolyte mix can solve this quickly.
What to Expect in the First Two Weeks
The first three to five days are the hardest. Hunger tends to spike around the times you’d normally eat, especially in the morning if you’re skipping breakfast. This is largely a hormonal habit. Ghrelin, your hunger hormone, is released on a schedule based on your usual meal pattern. Within a week or two, those hunger signals shift to match your new eating window.
Other common early side effects include irritability, low energy, difficulty concentrating, and mild headaches. These typically resolve within the first week as your body adapts to burning fat more efficiently. If they persist beyond two weeks, your eating window meals may not contain enough calories or nutrients.
Most people notice that fasting becomes significantly easier after the 10- to 14-day mark. Hunger during the fasting window drops noticeably, energy levels stabilize, and many people report sharper mental clarity during the fasted state, likely related to the mild increase in ketone production.
Weight Loss and Body Composition
Intermittent fasting works for weight loss primarily because it reduces how much you eat. A shorter eating window naturally limits calorie intake for most people, without requiring calorie counting. The metabolic switch to fat burning provides an additional advantage, but the calorie deficit does the heavy lifting.
When it comes to preserving muscle, the picture is nuanced. Randomized trials comparing intermittent fasting to standard calorie restriction generally show similar or even slightly better preservation of lean mass with fasting. However, the extended fasting window does increase the period during which your body breaks down muscle protein, and the compressed eating window can make it harder to consume enough protein to fully stimulate muscle repair. If maintaining or building muscle is a priority, spread your protein intake across two or three meals within your window, aiming for 20 to 40 grams of protein per meal. Resistance training during a fasting regimen also helps signal your body to hold on to muscle.
Tips for Making It Stick
Start your fast after dinner. If you finish eating at 8 p.m. and sleep from 11 p.m. to 7 a.m., you’ve already fasted 11 hours before you even start your day. That leaves just 5 more hours before a noon eating window opens.
Stay busy during the last few hours of your fast. Hunger comes in waves that last 20 to 30 minutes, not hours. If you distract yourself through one wave, the next one often doesn’t arrive for a while. Black coffee or sparkling water can help bridge the gap.
Be flexible with your window on social occasions. Shifting your window by an hour or two for a dinner out won’t erase your progress. Rigid adherence that leads to quitting is worse than occasional adjustments that keep you consistent over months.
Don’t combine intermittent fasting with severe calorie restriction. The point is to compress your eating, not to starve. Eating too little during your window leads to muscle loss, nutrient deficiencies, and the kind of low energy that makes fasting unsustainable.
Who Should Avoid Intermittent Fasting
Intermittent fasting is not recommended for pregnant or breastfeeding women, frail older adults, people with weakened immune systems, or anyone with a history of or risk for eating disorders. For people with diabetes, fasting increases the likelihood of dangerously low blood sugar, especially for those on insulin or medications that lower glucose. If you have diabetes or another chronic condition, talk with your doctor before starting, since medication timing and dosages may need adjustment.