The Engine at Rest: Basal Metabolic Rate
Your Basal Metabolic Rate (BMR) represents the absolute minimum number of calories your body requires to keep you alive if you were to stay in bed all day and do absolutely nothing.
It is the energy required to fuel your brain, pump your heart, inflate your lungs, and maintain your core body temperature. For the vast majority of humans, BMR accounts for 60% to 75% of total daily energy expenditure.
The Mifflin-St Jeor Equation
Historically, dietitians used the Harris-Benedict equation (created in 1919) to calculate BMR. However, modern scientific literature overwhelmingly endorses the Mifflin-St Jeor Equation (published in 1990) as the most accurate predictive formula for contemporary lifestyles.
The Formula
The equation requires your gender, weight, height, and age. (Note: the mathematical constant differs slightly between men and women due to natural differences in muscle mass).
BMR = 10w + 6.25h - 5a + s
The Impact of Age and Muscle
Your BMR is not static. Notice that the formula subtracts points based on your age (). As you get older, your metabolism naturally slows down, largely due to age-related muscle loss (sarcopenia).
Conversely, muscle tissue is highly metabolically active. If you lift weights and build an extra 10 pounds of muscle, your BMR will permanently increase, meaning your body will burn more calories around the clock—even while you sleep.