Intermittent fasting works by cycling between periods of eating and not eating on a regular schedule. Rather than changing what you eat, you change when you eat. Most people start with a 16-hour overnight fast and an 8-hour eating window, but several approaches exist depending on your lifestyle and goals.
The Main Fasting Methods
Three intermittent fasting protocols are the most widely practiced, and they differ mainly in how long and how often you fast.
Time-restricted eating is the most beginner-friendly approach. You eat all your meals within a set window each day, typically 8 hours, and fast for the remaining 16. A common version looks like eating between noon and 8 p.m., then fasting until noon the next day. Some people use a tighter window of 6 hours (eating from noon to 6 p.m.), while others start with a gentler 10-hour window and work their way down. The key is consistency: you pick the same window most days.
The 5:2 method takes a different approach. Five days a week you eat normally, and on two non-consecutive days you limit yourself to roughly 400 to 500 calories. Those low-calorie days aren’t full fasts, but they’re restrictive enough to trigger many of the same metabolic benefits. Harvard’s Nutrition Source notes this method is generally recommended only for healthy individuals without chronic diseases.
Alternate-day fasting is the most aggressive protocol. You alternate between unrestricted eating days and fasting days where you consume one small meal of about 25% of your normal calorie intake. So Monday might be unrestricted, Tuesday fasting, Wednesday unrestricted, and so on. A large BMJ network meta-analysis found that alternate-day fasting produced the greatest short-term weight loss of any intermittent fasting strategy, averaging about 3.4 kg (roughly 7.5 pounds) more than eating freely over trials lasting less than 24 weeks.
What Happens in Your Body During a Fast
When you stop eating, your body first burns through its available blood sugar and stored glycogen (the glucose reserve in your liver and muscles). Once those stores are depleted, typically after 12 to 16 hours without food, your body increasingly shifts to burning fat for energy. This is the metabolic transition that makes fasting effective for fat loss.
Fasting also appears to trigger a cellular cleanup process called autophagy, where your cells break down and recycle damaged components. Animal studies suggest this ramps up significantly between 24 and 48 hours of fasting, though researchers at Cleveland Clinic note there isn’t enough human data yet to pin down the exact timing in people. Most daily fasting protocols (like 16:8) likely produce modest autophagy compared to longer fasts.
What You Can Drink While Fasting
Staying hydrated is essential during your fasting window, and several beverages won’t interrupt the fast. Water, black coffee, unsweetened tea, and plain electrolyte supplements are all fine. A splash of apple cider vinegar in water is also considered acceptable.
Drinks that technically break a fast include bone broth (it contains protein and calories), bulletproof coffee (the added fat triggers digestion), and diet soda. While diet soda is calorie-free, the artificial sweeteners can provoke an insulin-like response in some people, which undermines part of the point of fasting. When in doubt, stick to water, plain coffee, or plain tea.
How to Break Your Fast
What you eat at the end of your fast matters more than most people realize. After hours without food, your body is primed to absorb nutrients quickly, and a large, carb-heavy meal can cause a sharp blood sugar spike followed by a crash that leaves you sluggish and hungry again within hours.
A better approach is to start with a smaller, balanced meal that includes healthy fats and protein. Research shows that consuming olive oil before or alongside carbohydrates significantly slows digestion, improves blood sugar control, and increases your body’s release of a satiety hormone called GLP-1. In practical terms, this means starting your eating window with something like eggs cooked in olive oil, avocado with vegetables, or a salad dressed with olive oil rather than a bowl of cereal or a sandwich. Save larger, more carb-heavy meals for later in your eating window when your digestive system has been active for a while.
How Much Weight You Can Expect to Lose
A 2024 systematic review published in The BMJ analyzed 93 randomized trials and found that intermittent fasting produces real but modest weight loss. In studies lasting less than 24 weeks, alternate-day fasting led to an average loss of about 3.4 kg (7.5 pounds) compared to unrestricted eating. Time-restricted eating (the 16:8 style most people use) produced smaller losses. In longer trials of 24 weeks or more, the most restrictive fasting strategies led to losses of roughly 2 to 3.6 kg (4.4 to 8 pounds) compared to eating freely.
An important finding: when compared head-to-head with traditional calorie restriction (simply eating less every day), intermittent fasting showed no significant advantage in longer-term studies. The weight loss was similar. This suggests that fasting works primarily because it helps people eat fewer calories overall, not because of some unique metabolic magic. The real question is which approach you can stick with. For many people, skipping breakfast and eating in an 8-hour window feels easier than counting calories at every meal.
Exercise and Fasting
You can exercise while fasting, but the timing and type of workout matter. A 2017 meta-analysis of five studies found that working out in a fasted state does not lead to greater weight loss or changes in body composition compared to exercising after eating. So if you’re fasting purely for fat loss, there’s no advantage to scheduling your workout during the fasting window.
There is one potential benefit, though. Several studies have linked fasted cardio to improvements in VO2 max, your body’s ability to take in and deliver oxygen during exercise. Over time, this can improve endurance and allow you to train harder during high-intensity sessions. The trade-off is that fasted training can increase protein breakdown as your body hunts for fuel, which may impair recovery and reduce performance in the short term. If you lift weights or do intense training, it’s generally better to schedule those sessions during your eating window or shortly before your first meal so you can refuel afterward.
How Fasting Affects Women Differently
Women need to approach intermittent fasting with more caution than men. Fasting can significantly lower estrogen and progesterone levels by disrupting the hormonal signal (gonadotropin-releasing hormone, or GnRH) that regulates the menstrual cycle. This signal is highly sensitive to environmental stressors, and prolonged fasting can essentially tell your body that food is scarce and conditions aren’t safe for pregnancy. The result can be skipped periods, irregular cycles, and in some cases, difficulty conceiving.
Timing within your menstrual cycle also matters. The two weeks before your period are when your hormones are most vulnerable to fasting-related disruption, particularly the week immediately before your period when estrogen naturally drops and your body becomes more sensitive to the stress hormone cortisol. Many women find success with a modified approach: fasting during the first half of their cycle (the follicular phase, starting from your period) and easing off or shortening the fasting window during the second half. Starting with a gentler 12 to 14-hour fast rather than jumping to 16 hours gives you room to see how your body responds.
Who Should Avoid Fasting
Intermittent fasting isn’t appropriate for everyone. According to guidelines from Mass General Brigham, the following groups should avoid it entirely: anyone under 18, people who are pregnant or breastfeeding, and individuals with a history of eating disorders or disordered eating.
Several other groups should proceed only with medical guidance:
- Adults over 65, who may have higher protein and calorie needs
- People with diabetes, due to the risk of dangerous blood sugar drops
- Those with heart, kidney, or liver disease
- People with low blood pressure
- Anyone taking blood thinners, diuretics, blood pressure medications, or drugs that affect blood sugar, since fasting can change how these medications behave in your body
Getting Started: A Practical First Week
If you’ve never fasted before, jumping straight into a 16-hour fast often leads to headaches, irritability, and giving up by day three. A better approach is to ease in gradually. During your first week, try a 12-hour fast, which most people can achieve simply by not eating after dinner. If you finish dinner at 8 p.m., you eat breakfast at 8 a.m. This is barely different from what many people already do, and it lets your body adjust to a defined eating window.
During week two, push your first meal back by an hour or two. Eat from 10 a.m. to 8 p.m. (a 14-hour fast). By week three, you can try the full 16:8 schedule, eating from noon to 8 p.m. Throughout this process, pay attention to your energy levels, sleep quality, and mood. Some hunger during the fasting window is normal and typically fades after the first week. Persistent fatigue, dizziness, or difficulty concentrating are signs you’re pushing too hard or not eating enough during your eating window.
During your eating window, focus on nutrient-dense meals with adequate protein, healthy fats, and fiber. Fasting doesn’t give you license to eat anything you want during your window. The calorie reduction that drives weight loss only works if you don’t compensate by overeating during feeding hours.