A 24-hour fast involves abstaining from all caloric intake, forcing the body to rely entirely on internal energy stores. The total number of calories burned is highly individualized, depending on a person’s underlying metabolic rate and physical activity level. Understanding this expenditure requires establishing the body’s baseline energy needs and examining how the source of that energy changes when food is absent.
Establishing Your Baseline Calorie Burn
The majority of calories burned during a 24-hour fast are those required for the body’s fundamental, non-voluntary functions. This foundational energy requirement is quantified as your Basal Metabolic Rate (BMR) or Resting Metabolic Rate (RMR). BMR is the minimum number of calories needed to keep organs functioning while at complete rest. RMR is a slightly less restrictive measure, often used interchangeably, that accounts for energy expended during low-effort activities.
This baseline burn accounts for approximately 60% to 70% of the total calories you expend on any given day. Your unique BMR is determined by several factors, including your current body weight, height, age, and biological sex. Generally, people with greater body mass and a higher proportion of lean muscle tissue require more calories to sustain their functions at rest.
Since measuring BMR in a laboratory setting is impractical for most people, predictive equations have been developed to provide a close approximation. Formulas like the Mifflin-St Jeor or Harris-Benedict equations use personal metrics to estimate this foundational caloric expenditure. This calculated BMR serves as the starting point for determining the minimum number of calories burned during the 24-hour fast.
How Fuel Sources Shift During a 24-Hour Fast
While the rate of calorie burn is primarily set by your BMR, the source of that energy changes dramatically as the fast progresses. In the initial hours after your last meal, your body relies on the immediate supply of glucose circulating in the blood. Once this is used, the body transitions to its short-term energy reservoir: stored glucose, or glycogen, primarily located in the liver.
Stored liver glycogen is typically sufficient to maintain blood sugar levels for about 12 to 18 hours, depending on activity level and store size. As the 24-hour mark approaches, these glycogen stores become depleted, signaling a major metabolic shift. The body must then find a new way to fuel itself and maintain necessary blood glucose for the brain and other cells.
The body initiates a process called gluconeogenesis, where it begins creating new glucose from non-carbohydrate sources, such as amino acids from muscle or glycerol from fat. Concurrently, the body significantly increases the breakdown of stored body fat, a process known as fat oxidation. This release of fatty acids into the bloodstream is the key mechanism for fueling the body during the later stages of the 24-hour fast.
Fat oxidation also leads to the production of ketone bodies by the liver, which serve as an alternative, efficient energy source for the brain. By the 24-hour mark, glucose oxidation can decrease by about 50%, while fat oxidation increases by a similar amount. This demonstrates the near-complete metabolic transition to using stored body fat as the primary fuel source.
Estimating Total Caloric Expenditure
The total number of calories burned over a 24-hour fast is ultimately determined by your Total Daily Energy Expenditure (TDEE), which is your BMR plus the calories burned through all physical activity. For an average adult, this TDEE typically falls within a range of approximately 1,500 to 3,000 calories. This range accounts for the differences in body size, age, and activity level that influence an individual’s BMR.
To calculate your true daily burn, you must multiply your BMR by an activity factor that reflects your level of movement. Even when fasting, you are still expending energy through Non-Exercise Activity Thermogenesis (NEAT), which includes all non-structured movement like walking, standing, and fidgeting. If you maintain a normal level of activity during the fast, your total burn will be higher than your BMR alone.
The act of fasting itself does not cause the body to burn a significantly higher number of calories overall. Instead, fasting changes the composition of the calories burned. The body ceases to expend energy on the Thermic Effect of Food (TEF), which is the energy used for digestion. The final caloric number is an estimate of your TDEE minus the negligible TEF, with all energy now coming from stored reserves.