While it seems like it might be a relatively easy question to answer, the number of calories a person burns each day can vary widely based on several factors:
Genes: The Centers for Disease Control and Prevention (CDC) states, "Most likely, genes regulate how our bodies capture, store, and release energy from food." Your parents' and their parents' genes can be a determining factor on how your body uses and burns calories.
Body Composition / Muscle Mass: Because muscle tissue uses more energy than fat tissue, individuals with larger, more muscular bodies, burn more calories.
Gender: In general, men have more muscle mass than women and will burn more calories.
Weight: The more weight a person is carrying on their body, the more calories they will require. As the weight is reduced, so are the calories needed by the body.
Age: According to the National Institutes of Health, as we age, lean muscle mass tends to deteriorate, especially if an individual is less active. Muscle loss can slow down the rate at which your body burns calories.
Illness/Trauma/Disease: Fever, burns, hyperthyroidism, Cushing's disease, and polycystic ovary syndrome are examples of other factors that can cause a shift in how the body uses calories.
Drugs: Seizure medications, antidepressants, corticosteroids, and nicotine can slow or increase the rate at which the body uses energy.
How Energy is Used
Basal Metabolic Rate (BMR) - is the minimum amount of calories the body needs to maintain regular functioning (breathing, moving blood around the body, building and repairing tissues, etc.). The BMR accounts for the greatest amount of calories burned daily--approximately 60 to 70 percent of total energy use. Depending on the person, this can vary 25 to 30 percent between individuals. BMR declines about two percent every 10 years after the age of 30.
Thermic Effect of Food (TEF) - this is the energy the body uses to digest, absorb and process food. This accounts for approximately five to 10 percent of daily energy use. The TEF value is higher for carbohydrate and protein-rich foods, because these take more energy to convert than fat does.
Physical Activity - Activity and exercise can use up an additional 20 to 40 percent of energy. People who tend to fidget throughout the day can burn anywhere from 100 to 800 extra calories a day.
How to Determine Your Calories
Calories can be determined with calorimetry (indirect or direct) or by estimations. Direct calorimetry is very expensive and is rarely used. This method figures calorie needs by measuring heat that emanates from the body, typically with the use of an isolated chamber. Indirect calorimetry, on the other hand, measures the amount of oxygen used by an individual since there is a relationship between the body's use of energy and oxygen. Indirect calorimetry can be performed with a number of instruments including a mounted cart or backpack.
The easiest, and cheapest, method for determining calories is by using an estimate. The method most widely used in hospitals and by registered dietitians is the Harris-Benedict Equation, which takes various factors into consideration including weight, height, age, physical activity and illness.
Technology also provides many tools and apps like calorie calculators, heart rate monitors, and wearable fitness trackers. However, to get the most accurate information about your personal calorie needs, talk to your doctor or dietitian.
Mandy Seay is a bilingual registered and licensed dietitian who holds both a bachelor's degree in nutrition and in journalism. After gaining 30 pounds while living abroad, Mandy worked to lose the weight and regain her health. It was here that she discovered her passion for nutrition and went on to pursue a career as a dietitian. Mandy currently works as a nutrition consultant and freelance writer in Austin, Texas, where she specializes in diabetes, weight management and general and preventive nutrition. She recently published her first book, Your Best Health, a personalized program to losing weight and gaining a healthy lifestyle. Please visit Mandy's website at Nutritionistics.com.