Intermittent fasting, particularly the 16:8 method, is a popular strategy for managing weight. Many people wonder how many calories the body burns during the 16-hour fasting window. The number of calories burned is highly specific to the individual, as the body does not suddenly burn extra calories just because food intake has stopped. The calories used during this period are the same ones the body uses every hour to maintain basic life functions. The goal of the 16-hour fast is not to increase the total burn, but to shift the primary fuel source the body uses.
Calorie Burn is Driven by Your Basal Metabolic Rate
The foundation of calorie expenditure, whether fasting or feeding, is the Basal Metabolic Rate (BMR). BMR represents the minimum amount of energy the body requires to keep vital organs functioning while at complete rest. This includes the energy needed for breathing, circulating blood, maintaining body temperature, and cellular repair. For most people, BMR accounts for the largest portion—around 60% to 75%—of their total daily energy expenditure.
The body’s energy needs for BMR are relatively constant and do not drastically change in the short 16-hour window of fasting. The calories burned during the fast are essentially 16 hours’ worth of BMR, plus any calories expended through physical activity. A person with a 24-hour BMR of 1,800 calories, for example, burns about 75 calories every hour, meaning they would burn approximately 1,200 calories during the 16-hour period.
It is a misconception that fasting significantly elevates metabolism or calorie burn above this baseline. While some studies suggest a temporary, slight increase in metabolic rate during short-term fasting, any change is modest. The primary effect of the fast is on the source of the fuel, not the rate at which it is consumed.
Metabolic Shifts During 16 Hours of Fasting
The significance of the 16-hour fast lies in the metabolic changes that occur as the body transitions from using consumed food to using stored energy. After a meal, the body is in a “fed state” with high insulin, primarily using glucose from carbohydrates for fuel. Once meal glucose is used, the body enters the “post-absorptive” phase and begins drawing on glycogen—stored glucose—in the liver and muscles.
Typically, the liver’s glycogen stores become depleted after about 10 to 12 hours without food. This depletion signals a metabolic shift, marking the start of the true fasting state. Low insulin and higher glucagon levels signal the body to begin breaking down stored body fat through lipolysis.
During lipolysis, fat cells release free fatty acids, which the body uses for fuel. The liver converts some fatty acids into ketone bodies, which serve as an alternative energy source for the brain and other organs. The 16-hour window is often long enough to initiate this switch, mobilizing stored fat to meet BMR energy needs.
Estimating Your Calorie Burn During the Fasting Window
Since calories burned during the 16-hour fast are based on BMR, estimating this number requires calculating an individual’s specific BMR first. BMR is influenced by factors including age, sex, height, and weight. People with more lean muscle mass generally have a higher BMR because muscle tissue requires more energy to maintain than fat tissue.
To estimate a 24-hour BMR, common formulas like the Mifflin-St Jeor equation are widely used for approximation. Once the 24-hour BMR is calculated, the hourly calorie burn rate is determined by dividing the BMR by 24. Multiplying this hourly rate by 16 provides the estimated calories burned during the 16-hour fasting period.
For example, a person with an estimated 24-hour BMR of 1,500 calories burns approximately 62.5 calories per hour (1,500 / 24). Over the 16-hour fasting window, this results in an estimated burn of 1,000 calories (62.5 x 16). These formulas offer an estimate, and the actual rate of calorie burn can vary by as much as 10%.
How Fasting Affects Overall Daily Energy Expenditure
While BMR remains the dominant factor in calories burned during the fast, the 16-hour window affects the Total Daily Energy Expenditure (TDEE). TDEE is the total calories burned daily, which includes BMR, physical activity, and the Thermic Effect of Food (TEF). TEF is the energy required to digest, absorb, and metabolize the food consumed, typically accounting for about 10% of a person’s total calorie intake.
By abstaining from food for 16 hours, the body eliminates the TEF during that period, which slightly reduces the overall TDEE for the day. However, the main effect of intermittent fasting on body weight is achieved by compressing the eating window. This often leads to an unintentional reduction in total calories consumed during the 8-hour feeding period.
This reduction in calorie intake is the primary mechanism that facilitates weight loss, not a heightened metabolic rate during the fast. For short-term fasting, the body’s BMR remains stable or may even slightly increase due to hormonal changes, preventing a significant metabolic slowdown. This is different from chronic, long-term calorie restriction, which can lead to adaptive thermogenesis—a metabolic slowdown where the body attempts to conserve energy. The 16-hour fast is generally considered too short to trigger this significant long-term metabolic adaptation.