The question of how much weight someone loses after three days without food is common, but the answer requires a careful distinction between “weight loss” and “fat loss.” The number that appears on the scale after 72 hours is a combination of true caloric deficit and a temporary shift in the body’s water balance. The immediate, dramatic drop in weight seen in the first couple of days is overwhelmingly due to fluid loss, not the burning of stored body fat. This initial weight reduction is rapid but not permanent, making it important to understand the biological mechanisms at play.
The Source of Initial Weight Loss: Glycogen and Water
The most significant and immediate weight change during the first 24 to 48 hours comes from the body depleting its stored carbohydrates, known as glycogen. Glycogen is the body’s primary, readily available energy source, stored mainly in the liver and muscles. When food intake stops, the body first turns to these stores to maintain blood sugar levels and energy. Each gram of glycogen is chemically bound to a significant amount of water. Specifically, every gram of glycogen is associated with approximately three to four grams of water.
This physiological process accounts for the rapid drop on the scale, which can often be several pounds in the first two days. A person’s total glycogen reserves can hold a substantial amount of water, and once that reserve is depleted, the accompanying fluid is quickly shed. While this mechanism results in a lower number on the scale, it is a temporary phenomenon. This water weight will be quickly restored once carbohydrates are reintroduced into the diet and glycogen stores are replenished.
Sustained Loss: Shifting to Fat Burning
After the initial depletion of glycogen stores, typically between 18 and 36 hours, the body undergoes a significant metabolic transition. With its preferred fuel source exhausted, the body shifts to using stored fat for energy in a state called ketosis. The liver begins breaking down stored triglycerides, or body fat, into molecules called ketone bodies, which are then used as fuel by the brain and muscles. The amount of weight lost from actual fat during this 72-hour period is relatively modest. True fat loss is governed by the total caloric deficit created over the three days. Since one pound of body fat is roughly equivalent to a 3,500-calorie deficit, fat loss depends on the individual’s Basal Metabolic Rate (BMR) and activity level.
Calculating the Total: Individual Variables
The final number on the scale after a 72-hour fast is highly variable among individuals. The total weight loss is a combination of the substantial water and glycogen loss, plus the smaller amount of actual fat loss. This total loss typically falls within a range of five to twelve pounds, with the higher end of the range being almost entirely water. Several personal factors influence both the initial water loss and the sustained fat loss. An individual’s starting body weight and body composition, particularly the ratio of muscle to fat, will affect their total glycogen capacity and BMR. Activity level during the fast also plays a role in the total energy expenditure and subsequent fat loss.
Medical Safety and Re-feeding
Attempting a 72-hour fast carries important health and safety considerations, and it is not appropriate for everyone. Individuals with Type 1 diabetes, those who are pregnant or nursing, and anyone with a history of an eating disorder should not attempt this type of prolonged fasting. Consulting with a healthcare provider is prudent before undertaking a period without food, especially for those who are on specific medications. Maintaining electrolyte balance is a major concern during any extended fast, as the body loses sodium, potassium, and magnesium through increased fluid excretion. Replenishing these minerals, often through non-caloric electrolyte supplements, is important to mitigate side effects like headaches and dizziness.
A significant factor to understand is the phenomenon of weight regain, which occurs immediately after the fast ends. Once food and carbohydrates are reintroduced, the body rapidly restores its depleted glycogen reserves, and with them, the associated water returns. This process causes the scale weight to rise quickly, demonstrating that the initial dramatic weight loss was largely temporary fluid loss.