Common fasting protocols

Protocol	Eating window	Fast duration	Difficulty
12:12	12 hours	12 hours	Beginner - align with sleep
16:8	8 hours	16 hours	Moderate - most popular IF protocol
18:6	6 hours	18 hours	Moderate–high
20:4 (Warrior Diet)	4 hours	20 hours	High - one large meal per day
OMAD	1 hour	23 hours	Very high - one meal per day
5:2 Diet	Normal 5 days	Calorie restriction 2 days/week	Moderate
24-hour fast	Normal eating between fasts	24 hours	High - once or twice per week
36-hour fast	Normal eating before and after	36 hours	Very high - experienced only

What happens in your body

During a fast, your metabolism shifts through distinct phases:

0–4 hours (fed state): Digestion continues; blood glucose and insulin are elevated; the body burns glucose from the recent meal.
4–16 hours (post-absorptive): Digestion is complete; blood glucose drops; insulin falls; the body begins drawing on liver glycogen for fuel.
16–24 hours (gluconeogenesis): Liver glycogen is depleted; the body ramps up gluconeogenesis (making glucose from non-carbohydrate sources). Fat breakdown increases. Mild ketone production begins.
24–48 hours (early ketosis): Ketone production increases significantly; the brain begins using ketones as an alternative fuel. Autophagy (cellular clean-up) processes are elevated.

Electrolytes during fasting

During fasts longer than 24 hours, maintaining electrolytes is important. Low insulin causes the kidneys to excrete more sodium, which can secondarily deplete potassium and magnesium. Symptoms of electrolyte imbalance include headache, fatigue, muscle cramps, and heart palpitations. A pinch of salt in water, a magnesium supplement, or a dilute broth can help prevent these symptoms without breaking the metabolic state of the fast.

How to break a fast

After a long fast, re-introduce food gradually. Start with small, easily digestible foods: bone broth, a small piece of fruit, fermented foods, or soft cooked vegetables. Avoid a large, high-carbohydrate meal immediately after an extended fast, as this can cause refeeding syndrome symptoms (blood glucose and insulin spike) and significant discomfort.

Exercise during fasting

Moderate exercise (walking, light cardio, yoga) is generally safe and well-tolerated in a fasted state. Some research suggests fasted cardio may increase fat oxidation. However, high-intensity training, heavy resistance exercise, and prolonged endurance sessions are better performed in a fed state for both performance and recovery. Listen to your body - dizziness or weakness during fasted exercise is a signal to eat or reduce intensity.

Coffee and tea during a fast

Black coffee and plain tea (without milk, sugar, or cream) typically do not break a metabolic fast. They contain negligible calories and some evidence suggests they may enhance the metabolic effects of fasting. However, strict interpretations of fasting (such as water-only fasts) exclude all beverages except water. Bulletproof coffee (with added fat) does break a fast from a caloric standpoint, even though it minimally affects insulin levels.