A 3-day fast, often referred to as a prolonged fast, involves abstaining from all caloric intake for a continuous 72-hour period. The goal is to shift the body’s primary fuel source from glucose to stored body fat (ketosis), while also prompting cellular repair processes like autophagy. Maintaining the metabolic benefits of this extended fast depends entirely on consuming only zero-calorie liquids that do not trigger an insulin response. This ensures you remain safely hydrated and balanced throughout the 72 hours.
Hydration and Essential Electrolytes
Proper fluid intake is the most important factor for safety during a 72-hour fast, as the body loses significant water and minerals without food intake. Consuming at least two to three liters of plain, filtered, or unflavored sparkling water daily helps prevent dehydration, which can lead to headaches and fatigue. Water is necessary to support kidney function and the metabolic processes that intensify during a prolonged fast.
During a fast, lower insulin levels signal the kidneys to flush out sodium and water at a higher rate, quickly depleting the body’s store of essential minerals. This loss of electrolytes—specifically sodium, potassium, and magnesium—is the primary cause of symptoms like dizziness, muscle cramps, and “keto flu” experienced during extended fasting. Supplementing these minerals is necessary to maintain nerve and muscle function and regulate fluid balance.
Sodium is often the first mineral to become deficient; a simple solution is to add a quarter to a half teaspoon of sea salt or pink Himalayan salt to water throughout the day (1,000 to 2,000 milligrams). Potassium is also rapidly lost and can be supplemented using a potassium chloride salt substitute, aiming for 200 to 500 milligrams per day. Magnesium, which aids muscle relaxation, should be taken as a supplement like magnesium glycinate or citrate, with a daily goal of 300 to 400 milligrams.
It is important to source zero-calorie, unflavored electrolyte powders or create a homemade mixture to ensure the fast is not broken. These mineral supplements are a necessary safety measure to sustain the body’s electrical and fluid balance across three days. Start with a lower dosage and increase it slightly if symptoms of lightheadedness or muscle twitching occur to find the right balance.
Approved Calorie-Free Beverages
Beyond plain water and electrolytes, a limited selection of beverages can be consumed to provide a flavor change or a mild stimulant without disrupting the fasted state. These options must be strictly calorie-free and free of any additions that could provoke an insulin release.
Black coffee is a popular choice, providing caffeine that helps maintain alertness and energy during the fast. It must be consumed completely plain, without any dairy, creamers, or sugar, as these additions introduce calories and protein. Similarly, plain black or green tea is acceptable, offering a lower dose of caffeine and beneficial antioxidants.
Herbal teas, such as chamomile, peppermint, or ginger, are excellent non-caffeinated options for warmth and flavor diversification. These teas must be unsweetened and should not contain dried fruit or naturally sweet ingredients that might contribute trace amounts of sugar. Apple cider vinegar (ACV) mixed into water (one to two teaspoons) is also acceptable. ACV is non-caloric and may help with appetite management, but dilute it heavily to protect tooth enamel.
Items That Immediately Break the Fast
To maintain a strict 72-hour fast, entirely avoid any liquid containing calories or ingredients that cause an insulin spike. The core rule is that anything other than water or a zero-calorie, non-insulinogenic liquid will interrupt the metabolic state. This includes obvious caloric drinks like fruit juices, sodas, milk, and smoothies, or coffee/tea containing sugar, honey, or cream.
Avoid all artificial sweeteners, even those labeled zero-calorie, as some may trigger an insulin response or affect gut health, undermining the fast’s benefits. Ambiguous items like bone broth must also be excluded from a strict 3-day fast. Despite being low-calorie and recommended for shorter fasts, bone broth contains protein and fat, which is caloric and breaks the true fasted state. Finally, check flavored waters rigorously, as many contain citric acid, natural flavors, or trace amounts of sugar that negate the zero-calorie goal.