Intermittent fasting works by cycling between periods of eating and not eating, and the most common approach is the 16:8 method: you eat within an eight-hour window and fast for the remaining 16 hours. There’s no single “correct” way to do it, but there are specific practices that make fasting safer, more sustainable, and more effective. Here’s how to set it up properly from the start.
Choose a Fasting Schedule
The two most popular protocols are the daily time-restricted approach and the weekly 5:2 method. Each works differently, and the best one for you depends on your lifestyle and how much structure you want.
The 16:8 method means you eat during an eight-hour window each day, such as noon to 8 p.m., and fast for the other 16 hours. This is the most beginner-friendly option because you’re essentially skipping one meal and sleeping through a large portion of the fast. Some people narrow their window further to six hours as they get more comfortable.
The 5:2 method lets you eat normally five days a week. On the other two days (non-consecutive, like Monday and Thursday), you eat just one meal of 500 to 600 calories. This can feel easier socially since most of your week is unrestricted, but those two low-calorie days can be challenging at first.
Alternate-day fasting (ADF) alternates between normal eating days and fasting or very-low-calorie days. A large network meta-analysis of randomized controlled trials found ADF produced the most weight loss among intermittent fasting methods, averaging about 5 kg (11 pounds) more than control diets. However, it’s also the hardest to sustain long-term.
OMAD (one meal a day) compresses all your eating into roughly one hour. This is an advanced approach and not where most people should start. The jump from your normal eating pattern to OMAD is steep enough that hunger, irritability, and energy crashes often derail it within the first week.
What You Can Have During the Fast
Water, black coffee, and plain tea are the standard go-to drinks during your fasting window. They contain essentially zero calories and won’t trigger a meaningful metabolic response. Sparkling water is fine too.
Artificial sweeteners are more complicated than most fasting guides suggest. Research from the University of Illinois found that swallowing sucralose (the sweetener in Splenda) caused significantly higher insulin levels in people with obesity compared to drinking plain water. Even in people at a healthy weight, sucralose altered insulin dynamics. The researcher noted that different artificial sweeteners have very different chemical structures, so these findings may be specific to sucralose rather than applying to all zero-calorie sweeteners. If you want to play it safe, skip the diet soda and sweetened drinks during your fasting hours.
Anything with calories breaks the fast. That includes milk or cream in your coffee, bone broth, juice, and “small snacks.” Even 50 calories can shift your body out of the fasting state.
Ease Into It Gradually
Starting with a full 16-hour fast on day one is a common mistake. If you normally eat from 7 a.m. to 10 p.m., that’s a 15-hour eating window. Cutting it to eight hours overnight is a shock to your system.
A more practical approach is to shorten your eating window by one to two hours each week. Start with a 12-hour fast (for example, stop eating at 8 p.m. and have breakfast at 8 a.m.), then move to 14 hours the following week, then 16 hours the week after. This gives your hunger hormones time to adjust. Most people find the first three to five days of any new window are the hardest, and then hunger signals start to shift.
What to Eat When You Break Your Fast
The quality of your meals during the eating window matters as much as the timing. A large meta-analysis published in Nutrition Reviews found that the effectiveness of intermittent fasting for weight loss depends primarily on how much you reduce your overall calorie intake, not on the meal timing itself. In other words, fasting creates a structure that makes it easier to eat less, but if you compensate by overeating during your window, you won’t see results.
Break your fast with something that won’t spike your blood sugar dramatically. A meal built around protein, healthy fats, and fiber-rich vegetables is ideal. Think eggs with avocado and greens, or grilled chicken over a salad with olive oil. Protein is especially important: it helps preserve muscle mass during a calorie deficit and keeps you fuller longer through the rest of your eating window.
Avoid breaking a fast with highly processed carbohydrates, sugary foods, or large amounts of fruit juice. After hours without food, your body is more sensitive to blood sugar swings, and a sugar-heavy first meal can leave you feeling sluggish and hungry again within an hour or two.
Manage Electrolytes to Avoid Side Effects
Headaches, dizziness, and fatigue during fasting are usually not from hunger. They’re from low electrolytes, particularly sodium. When you fast, your kidneys excrete more sodium than usual, and if you’re also drinking a lot of water (which most fasting guides recommend), you’re diluting your remaining electrolyte stores further.
During fasting days or extended fasting windows, aim for roughly 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. For the 16:8 method, you can get most of this through your meals if you’re intentional about it. Salt your food adequately, eat potassium-rich foods like avocado and spinach, and consider a magnesium supplement if you’re prone to muscle cramps or poor sleep.
If you feel dizzy when standing up or develop a pounding headache, that’s a strong signal you’re low on sodium. A pinch of salt dissolved in water can resolve it within minutes.
Exercise Timing and Fasting
You can exercise while fasting, but the timing of your post-workout meal matters more than it would normally. When you train in a fed state, your body has circulating amino acids available for muscle repair, so the window for eating afterward is relatively flexible. When you train fasted, that window tightens considerably because your muscles are already in a depleted state and need fuel sooner to start the repair process.
The practical takeaway: if you work out during your fasting window, try to schedule your session so it ends close to when your eating window opens. Training at 11 a.m. when you plan to eat at noon works well. Training at 6 a.m. with no food until noon means your muscles go six-plus hours without the protein they need to recover, which can impair muscle retention over time.
If your only option is to train early in the morning during a long fast, lighter activities like walking, yoga, or low-intensity cycling are better choices than heavy resistance training. Save the hard lifting sessions for times when you can eat within an hour or so afterward.
What Happens in Your Body During a Fast
After about 12 hours without food, your body begins shifting from using glucose as its primary fuel to burning stored fat. Insulin levels drop, which allows fat cells to release their stored energy more readily.
Growth hormone levels can rise substantially during a fast. A study published in Frontiers in Endocrinology found that during a 24-hour fast, people with lower baseline growth hormone levels saw a median increase of over 1,200%. Growth hormone helps preserve lean muscle mass and supports fat metabolism, which is part of why fasting can reduce body fat without as much muscle loss as you might expect from simply eating less.
Autophagy, the process where your cells clean up damaged components and recycle them, is often cited as a major benefit of fasting. Animal studies suggest autophagy ramps up significantly between 24 and 48 hours of fasting. However, as Cleveland Clinic notes, there isn’t enough human research to pin down exactly when autophagy peaks in people. A standard 16:8 fast likely provides some autophagy benefit, but the dramatic cellular cleanup effects often discussed online are more relevant to longer fasts.
How Fasting Compares to Traditional Dieting
If you’re fasting primarily for weight loss, the evidence is clear: intermittent fasting produces results comparable to standard calorie restriction when the overall calorie deficit is similar. The meta-analysis in Nutrition Reviews found that time-restricted eating (like 16:8) led to an average weight loss of about 4.2 kg (9.3 pounds) and a waist circumference reduction of 3.5 cm compared to control diets. The 5:2 diet produced similar numbers.
Where standard calorie restriction with moderate to severe cuts outperformed fasting methods in absolute weight loss, the difference came down to a larger overall calorie deficit, not some metabolic advantage. This means intermittent fasting isn’t magic. It’s a tool that helps many people eat less without having to count calories at every meal. If that structure appeals to you, it’s a valid and well-supported approach. If counting calories and eating three meals a day feels more natural, you’ll get equivalent results.
Who Should Avoid Intermittent Fasting
Intermittent fasting is not appropriate for everyone. People with diabetes (type 1 or type 2) need to be particularly cautious because fasting can cause dangerous drops in blood sugar, especially if you’re on insulin or other glucose-lowering medications. Pregnant women, people with a history of eating disorders, those with chronic kidney or liver disease, and anyone with active peptic ulcers should avoid fasting protocols.
Children and teenagers are still growing and have higher caloric needs relative to their body size, making restrictive eating patterns a poor fit. If you’re underweight or recovering from illness or surgery, fasting can impair healing and recovery. People taking medications that need to be taken with food on a specific schedule may also find that fasting interferes with their treatment plan.