When doing intermittent fasting, you cycle between periods of eating and voluntary fasting, consuming only water and non-caloric beverages during the fasting window. The most popular approach is the 16:8 method: fast for 16 hours, eat within an 8-hour window. But the details of what happens in your body during those hours, what you can consume, and how to avoid common pitfalls make the difference between a sustainable practice and a miserable one.
How the Most Common Protocols Work
Intermittent fasting isn’t a single diet. It’s a category of eating schedules, and the one you pick should fit your daily routine. Here are the main options:
- 16:8 (time-restricted eating): Fast 16 hours, eat within 8. Common windows are 10 a.m. to 6 p.m., 11 a.m. to 7 p.m., or noon to 8 p.m. No calorie counting required.
- 18:6: A tighter version with a 6-hour eating window, typically noon to 6 p.m. or 1 p.m. to 7 p.m.
- 20:4: Fast 20 hours, eat within 4. This effectively limits you to one large meal and a snack.
- OMAD (one meal a day): A 23-hour fast with a single meal, often dinner.
- 5:2: Eat normally five days a week. On two non-consecutive days, limit intake to 500 to 600 calories.
- Alternate-day fasting: Every other day you eat 0 to 500 calories, alternating with normal eating days.
The 16:8 method is the most widely practiced because it’s the easiest to maintain. If your last meal is at 8 p.m. and you skip breakfast, you break your fast at noon the next day. Eight of those 16 fasting hours are spent sleeping.
What Happens in Your Body During a Fast
Your body stores energy as glycogen in the liver. When you stop eating, it burns through those glycogen reserves first. Once they’re depleted, your metabolism shifts to burning stored fat and producing molecules called ketones for fuel. This transition, sometimes called the “metabolic switch,” typically kicks in somewhere between 12 and 36 hours after your last meal. The exact timing depends on how full your glycogen stores were when you started and how physically active you are during the fast. Exercise accelerates the switch.
For most people doing a 16:8 schedule, the tail end of the fasting window is when fat burning begins to ramp up. With shorter fasts, you may only briefly enter this state before your next meal. Longer fasts (20:4 or OMAD) spend more time in it.
Your body also activates a cellular cleanup process called autophagy, where cells break down and recycle damaged components. Animal studies suggest this process ramps up significantly between 24 and 48 hours of fasting. There isn’t enough human research yet to pin down the exact timing, so claims that a 16-hour fast triggers meaningful autophagy are still speculative.
What You Can Drink While Fasting
During your fasting window, the goal is to consume nothing that triggers an insulin response. That means zero calories. What’s allowed:
- Water: Unlimited, and aim for at least 2 liters per day.
- Black coffee: No sugar, no cream, no flavored syrups.
- Plain tea: Black, green, or herbal varieties like chamomile or mint, unsweetened.
- Sugar-free electrolytes: Sodium, magnesium, and potassium supplements in pure form (no calories or sugar).
A splash of milk in your coffee or a diet soda with artificial sweeteners sits in a gray area. Purists avoid them. Practically, a small amount of milk (under 10 calories) is unlikely to derail the metabolic benefits for most people, but it does technically break the fast.
Does It Work Better Than Regular Dieting?
The honest answer: for weight loss specifically, not really. A 2022 trial published in the New England Journal of Medicine compared time-restricted eating with standard calorie restriction in people with obesity over 12 months. The fasting group lost an average of 8.0 kg (about 17.6 pounds), while the calorie-restriction group lost 6.3 kg (about 13.9 pounds). That 1.8 kg difference was not statistically significant, meaning the researchers couldn’t confirm it was due to the fasting schedule rather than chance. Body fat, waist circumference, blood pressure, and metabolic markers were also similar between the two groups.
What intermittent fasting does offer is a simpler framework. Instead of tracking every calorie, you watch the clock. For many people, having clear rules about when to eat is easier to follow long-term than counting and weighing food. The weight loss itself comes from eating fewer calories overall, which tends to happen naturally when your eating window shrinks. If you compensate by eating more during your window, the scale won’t budge.
Common Side Effects and How to Handle Them
The first week or two is the hardest. Your body is adjusting to a new fuel schedule, and several predictable side effects come with that transition.
Hunger is the most obvious one, and it tends to come in waves rather than building steadily. Most people find the hunger pangs peak around days 3 through 5 and then ease considerably as their body adapts. Drinking water or black coffee during these waves helps.
Headaches, lightheadedness, and irritability during early fasting days are often caused by electrolyte shifts rather than low blood sugar. When you fast, your kidneys excrete more sodium, and losing sodium pulls potassium and magnesium with it. A general target during low-carb or fasting states is 3,000 to 5,000 mg of sodium and 3,000 to 4,000 mg of potassium daily, plus 300 to 500 mg of magnesium. You can hit sodium targets by adding broth or bouillon during the fasting window (most have negligible calories) and salting your meals generously during the eating window. If you develop muscle cramps, that’s a reliable sign your magnesium is low.
Sleep disruption is less talked about but common in the first week, particularly if your eating window ends early in the evening and you’re used to snacking before bed. This usually resolves on its own.
How to Prepare and Ease In
Jumping straight into a 20-hour fast when you’ve been eating every 3 hours is a recipe for quitting. A more sustainable approach is to start with a 12:12 schedule (fast 12 hours, eat within 12) for a few days, then gradually push your fasting window to 14, then 16 hours over a week or two.
In the days before you start, reducing your intake of simple sugars and processed carbohydrates helps. These foods cause sharper blood sugar swings, which make the transition to fasting feel worse. Cutting back on alcohol also helps, since it disrupts the hormonal signals involved in hunger and blood sugar regulation.
During your eating window, focus on meals that include protein, healthy fats, and fiber-rich vegetables. These keep you full longer and smooth out your blood sugar before the next fast. Eating a large, carb-heavy meal right before your window closes can trigger a blood sugar crash a few hours later that makes the early fasting hours uncomfortable.
Who Should Not Fast
Intermittent fasting is not safe for everyone. It is not recommended for pregnant or breastfeeding women, frail older adults, people with immune deficiencies, or anyone with a history of eating disorders or at risk of developing one. The rigid eating-and-restriction cycle can reinforce disordered patterns around food.
People with diabetes face a specific risk: hypoglycemia. Fasting while taking insulin or certain oral diabetes medications can cause blood sugar to drop dangerously low. If you have diabetes and want to try intermittent fasting, your medication timing and dosages would need to be adjusted under medical supervision first.
Children and teenagers who are still growing, people taking medications that must be taken with food at specific intervals, and anyone who is underweight should also avoid fasting protocols.