Intermittent fasting works by cycling between periods of eating and not eating on a set schedule. The most popular approach, called 16:8, has you fast for 16 hours and eat all your meals within an 8-hour window. But there are several formats to choose from, and the best one depends on your lifestyle, your schedule, and how your body responds. Here’s how to get started.
Choose a Fasting Schedule
Every intermittent fasting method splits your day or week into fasting periods and eating periods. The difference is how long each lasts.
- 16:8 (time-restricted eating): Fast for 16 hours, eat within an 8-hour window. Most people skip breakfast and eat from roughly noon to 8 p.m. This is the easiest starting point for beginners because a large chunk of the fasting window happens while you sleep.
- 18:6: Fast for 18 hours, eat within 6 hours. A natural next step once 16:8 feels comfortable.
- 20:4: Fast for 20 hours, eat within 4 hours. This usually means one large meal and one smaller meal or snack.
- OMAD (one meal a day): A 23-hour fast with a single 1-hour eating window. This is an advanced approach and not necessary for most people.
- 5:2: Eat normally five days per week. On the other two days (non-consecutive), limit yourself to 500 to 600 calories total. This works well if you prefer not to fast every single day.
If you’ve never fasted before, start with 16:8 for at least two to three weeks before considering a longer fast. Your hunger hormones need time to adjust, and jumping straight to 20:4 or OMAD often leads to overeating during the eating window or quitting altogether.
What You Can Have During the Fast
During your fasting window, stick to zero-calorie or near-zero-calorie drinks. Water, black coffee, and plain tea are all fine. Diet soda and other zero-calorie caffeinated beverages are also generally permitted in fasting research without disrupting results. Adding cream, sugar, honey, or milk to your coffee introduces enough calories to shift your body out of a fasted state.
There’s no universally agreed-upon calorie number that “breaks” a fast, but the practical rule is simple: if it triggers digestion, it counts as eating. A splash of lemon juice in water is negligible. A tablespoon of cream is not.
What Happens in Your Body While Fasting
The first several hours of a fast are unremarkable. Your body burns through its most recent meal and then begins tapping stored glycogen in your liver and muscles. Once glycogen runs low, typically 12 to 16 hours in, your liver starts converting fatty acids into ketones for fuel. This is when fat burning ramps up meaningfully.
Insulin levels drop as you fast. A meta-analysis of people with impaired blood sugar and cholesterol found that intermittent fasting lowered fasting insulin by an average of 13.25 mU/L and improved insulin resistance scores. Lower insulin makes it easier for your body to access stored fat and helps cells respond more efficiently to insulin when you do eat again.
Longer fasts also activate a cellular cleanup process where your cells break down and recycle damaged components. Animal research shows this process ramps up noticeably around 24 hours of fasting and peaks around 48 hours. For people following 16:8, you’re likely getting a mild version of this benefit. You don’t need to fast for two days to see results, but it’s worth understanding that longer fasts do trigger deeper cellular repair.
How to Break Your Fast
What you eat first matters more than most people realize, especially if you’ve been fasting for 16 hours or longer. Eating refined carbohydrates on an empty stomach causes a sharp blood sugar spike followed by a crash that leaves you hungry again quickly.
A better approach: start your meal with fiber and protein, then move to fats and carbohydrates. High-fiber, low-calorie foods with high water content, like vegetables, soups, or a small portion of beans and nuts, slow digestion and blunt glucose spikes. Plant-based proteins or lean proteins increase satiety so you’re less likely to overeat. Save starchy or sugary foods for the end of the meal, after fiber and protein have already begun digesting. This sequencing keeps blood sugar steadier and reduces the bloating or sluggishness some people experience when they break a fast with a heavy meal.
Realistic Weight Loss Expectations
Across 27 clinical trials involving nearly 950 participants, intermittent fasting consistently produced weight loss ranging from 0.8% to 13% of baseline body weight. In the largest head-to-head comparison, 244 obese adults lost an average of 5 kg (about 11 pounds) over 52 weeks with intermittent fasting, compared to 6.65 kg (about 14.7 pounds) with traditional calorie-restricted diets. That difference was not statistically significant. All 12 studies that directly compared intermittent fasting to calorie restriction found equivalent results.
The takeaway: intermittent fasting doesn’t produce faster weight loss than simply eating less. What it does offer is a framework some people find easier to maintain. Instead of tracking every calorie at every meal, you follow a time-based rule. For people who struggle with portion control or constant snacking, that structure can make a real difference in adherence.
One encouraging detail: about 79% of weight lost through intermittent fasting comes specifically from fat, rather than muscle or water. That’s a strong ratio, and it held up in level I clinical evidence.
Considerations for Women
A common concern is that fasting disrupts menstrual cycles and reproductive hormones. This worry traces largely to a single rodent study that used extremely young rats, equivalent in age to a 9-year-old human. Since intermittent fasting isn’t recommended for children, that study doesn’t tell us much about adult women. Human trials have not found that intermittent fasting disrupts estrogen or menstrual regularity in adult premenopausal women.
There is one useful timing detail. Research on women with elevated androgen levels (a feature of PCOS) found that benefits to hormone markers were strongest when eating was limited to earlier in the day. Premenopausal women may see better hormonal results by finishing their eating window by around 4 p.m., rather than eating late into the evening. If you’re a woman starting intermittent fasting and you notice changes to your cycle, shortening your fasting window or shifting your eating window earlier are reasonable first adjustments.
Who Should Avoid Intermittent Fasting
Intermittent fasting is not appropriate for everyone. People under 18 should not fast, as calorie restriction can interfere with growth and development. Anyone with a history of disordered eating, including anorexia, bulimia, or binge eating disorder, should avoid it because the rigid eating and fasting schedule can reinforce harmful patterns. Pregnant and breastfeeding women need consistent calorie and nutrient intake and should not fast.
People with diabetes, particularly those on insulin or medications that lower blood sugar, need medical guidance before fasting because the combination of medication and prolonged fasting can cause dangerously low blood sugar. Athletes and highly active individuals may find it difficult to get enough fuel within a compressed eating window, which can impair performance and recovery.
Practical Tips for the First Two Weeks
The adjustment period is real. Most people feel noticeably hungry during their usual breakfast time for the first three to five days. This fades as your hunger hormones recalibrate. Drinking water or black coffee during those early morning hours helps.
Keep your eating window consistent. If you eat from noon to 8 p.m. on weekdays but shift to 10 a.m. to 6 p.m. on weekends, that’s fine. Constantly changing your window by several hours, or skipping fasting days entirely, prevents your body from settling into a rhythm. Consistency matters more than perfection.
Don’t compensate for the fast by eating more. Intermittent fasting works partly because people naturally eat a bit less when their eating window is shorter. If you eat the same total calories you were eating before, just compressed into fewer hours, you likely won’t see meaningful changes on the scale. Focus on whole foods, adequate protein, and reasonable portions during your eating window. The fasting schedule handles the rest.