Fasting is the voluntary abstinence from food and drink for a set period, often undertaken for health reasons. This eating pattern focuses on when you consume calories, cycling between periods of eating and not eating. This allows the body to transition from using glucose for fuel to burning stored body fat. This guide details various fasting protocols, preparation requirements, and the process for safely reintroducing food.
Defining Popular Fasting Schedules
The most common methods of fasting are based on Time-Restricted Eating (TRE), which limits the window for caloric intake each day. The 16/8 method is the most popular form of TRE, where an individual fasts for 16 continuous hours and consumes all meals within the remaining 8-hour window. This schedule easily incorporates the hours spent sleeping.
A slightly shorter version, the 14/10 method, involves a 14-hour fasting period and a 10-hour eating window, often serving as a gentler start for beginners. The primary goal of these daily schedules is to create a consistent routine that allows the body to enter a fasted state and begin using stored energy. These schedules are often considered the simplest to integrate into a busy life because a significant portion of the fast occurs overnight.
Periodic fasting methods involve restricting calories on specific, non-consecutive days of the week. The 5:2 method is a well-known example, requiring normal eating for five days a week, but restricting caloric intake to 500–600 calories on the other two days. These reduced-calorie days are spaced out, such as a Wednesday and a Saturday, to prevent back-to-back fasting.
Alternate-Day Fasting (ADF) involves alternating between a day of normal eating and a fasting day, which is sometimes a complete fast or a day of severe caloric restriction. The “fasting day” in ADF typically means consuming no more than 25% of one’s usual caloric needs, often limited to a single small meal. These longer, periodic fasts require more planning than daily TRE methods and are often seen as a more intensive approach.
Essential Preparation and Hydration Guidelines
Before beginning any fasting schedule, establish a clear plan for both your eating and non-eating periods, setting specific start and end times for consistency. Mental preparation is also important, as the initial days often involve feelings of hunger, irritability, and decreased energy levels. Starting with a shorter fasting window, like 12 or 14 hours, allows the body to gradually adapt to the new pattern.
Maintaining adequate hydration is a fundamental component of any fasting protocol, as the body loses more water and electrolytes when insulin levels drop. Water should be the primary fluid source, consumed consistently throughout the day to prevent dehydration. Zero-calorie beverages, such as black coffee and plain tea, are generally acceptable during the fasting window because they do not significantly trigger an insulin response.
Electrolyte supplementation becomes important, particularly during fasts extending beyond 24 hours or for individuals who sweat often. Fasting can lead to a significant loss of sodium, potassium, and magnesium, which are necessary for nerve function, muscle contraction, and fluid balance. Electrolyte imbalances can cause headaches, dizziness, and fatigue, commonly referred to as “keto flu” symptoms.
Sodium is often the most important electrolyte to replace, with some experts suggesting a loss of 2–5 grams per day while fasting. Choosing a zero-calorie electrolyte supplement that is free of added sugars and artificial sweeteners is important to avoid breaking the fast. Individuals can also add a small amount of sea salt to their water to help replace lost sodium.
How to Properly End a Fast
The process of reintroducing food, or refeeding, must be approached with caution, especially after fasts lasting 24 hours or longer. The digestive system temporarily reduces the production of enzymes during the fasted state, making it sensitive to a sudden influx of food. Starting with small portions of easily digestible foods prevents digestive distress, such as bloating and nausea.
The first meal should ideally be liquid-based and nutrient-dense, such as bone broth, which is gentle on the stomach and helps replenish fluids and minerals. Cooked, non-starchy vegetables and small amounts of healthy fats, like avocado or olive oil, are excellent next choices. Fermented foods, such as plain yogurt or sauerkraut, can also be introduced in small amounts to support the gut microbiome.
Avoid heavy meals, raw vegetables, excessive sugar, and processed carbohydrates immediately after a fast, as these can cause blood sugar spikes and digestive discomfort. For longer fasts, the refeeding process should be gradual, slowly increasing meal size and complexity over 2 to 3 days. Rapid refeeding after very prolonged fasts carries the risk of refeeding syndrome, a dangerous shift in fluids and electrolytes.
Who Should Not Attempt Fasting
Fasting is not appropriate for every individual and should be avoided by certain populations for safety reasons. Pregnant or breastfeeding women should not attempt fasting, as their nutritional requirements are significantly elevated. Children and adolescents who are still in periods of rapid growth and development should also avoid restricting their caloric intake.
Individuals with a history of disordered eating or eating disorders should steer clear of fasting, as the practice can trigger unhealthy behaviors and worsen their condition. Those with Type 1 diabetes or who take medications that affect blood sugar, such as insulin, face the risk of dangerous hypoglycemia (low blood sugar). People with certain chronic health conditions, including low blood pressure, kidney disease, or liver disease, must consult a healthcare provider before making any changes to their eating pattern.