Fasting is the voluntary abstinence from certain or all food, drink, or activity for a specified period. This ancient practice is observed for diverse reasons, ranging from spiritual discipline and mental clarity to metabolic health improvement and weight management. Modern approaches to fasting vary widely, focusing on limiting the time frame for eating, restricting total caloric intake, or excluding specific substances or behaviors entirely.
Time-Restricted Eating Schedules
Time-restricted eating (TRE) focuses on establishing a consistent daily cycle of fasting and eating without strictly counting calories. The most popular method is the 16/8 protocol, where an individual fasts for 16 consecutive hours and consumes all calories within an 8-hour window. This protocol is manageable because the fasting period includes sleep, often requiring only the skipping of breakfast or a late dinner.
Alternate-Day Fasting (ADF) involves alternating between a day of normal eating and a “fasting” day. Modified ADF permits the consumption of a small number of calories (500 to 600, or about 25% of normal intake) on the fasting day. This pattern creates a weekly caloric deficit while offering the psychological ease of eating freely every other day.
The 5:2 method is a similar weekly schedule allowing five days of normal eating and two non-consecutive days of significantly reduced caloric intake. The restriction on these two days is similar to modified ADF, recommending 500 calories for women and 600 calories for men. These time-based methods shift the body’s metabolism to burn stored fat after exhausting the glucose from the last meal.
Protocols for Complete Caloric Restriction
Fasts focused on complete caloric restriction are maintained for a continuous, defined period. Water fasting is the most common, involving the consumption of only plain water, black coffee, or herbal tea, ensuring zero caloric intake. Most medically unsupervised water fasts are recommended to last between 24 and 72 hours, as the body transitions into a deeper state of metabolic switching known as ketosis.
Extended water fasts lasting 72 hours or more are sometimes undertaken to promote cellular cleanup, a process called autophagy. These should only be performed under medical supervision due to safety concerns. The lack of calories forces the body to rely entirely on stored energy reserves, and electrolyte imbalance and dehydration become significant risks during these longer periods.
Juice fasting involves consuming only fresh fruit and vegetable juices. Since these juices contain natural sugars, vitamins, and minerals, they provide a limited number of calories, often delaying the deep metabolic shift triggered by a water fast. This caloric content makes juice fasts easier to sustain for longer periods, but the high sugar content of some fruit juices can lead to blood sugar spikes. The most extreme form is dry fasting, which involves abstaining from both food and all liquids, including water. This protocol is highly discouraged for durations beyond 12 to 16 hours due to the rapid onset of severe dehydration and risks to kidney function.
Targeted Dietary Exclusion Fasts
Targeted exclusion fasts remove specific substances or food groups from the diet, rather than restricting meal timing or total calories. These practices are often adopted for spiritual reasons, identifying food sensitivities, or resetting eating habits. A common example is abstaining from refined sugar and processed foods, which removes high-calorie, nutrient-poor items while allowing whole, unprocessed meals.
The Daniel Fast is a prominent religious exclusion fast that typically lasts 21 days and restricts all meat, dairy, alcohol, caffeine, and added sugars. Participants are limited to a whole-food, plant-based diet consisting of fruits, vegetables, whole grains, nuts, seeds, and water. Unlike time-restricted models, participants may still eat three meals a day, but the strict elimination of entire food categories shifts the focus to simpler, more mindful eating. This approach can be a helpful tool for individuals seeking to eliminate known compounds.
Behavioral and Technology Fasts
Fasting extends beyond food and drink to include the voluntary restriction of certain behaviors or technologies that consume time and mental energy. The goal is to create mental space, reduce cognitive load, and foster a greater sense of presence. A technology fast, for instance, involves abstaining from specific digital devices or platforms for a designated time.
Many individuals choose to fast from social media applications, news consumption, or all screen time during evenings or weekends. This practice aims to reduce the constant influx of information and associated anxiety or distraction. Other behavioral fasts can involve abstaining from shopping, specific forms of entertainment, or even speaking for a set period. Removing a habitual activity redirects the time and attention to other pursuits, often resulting in improved concentration and better sleep patterns.