Think you need to run a marathon just to burn off breakfast, lunch and dinner? Think again. The human body requires a significant amount of energy (i.e. calories) just to function regularly. Each day, your body must breathe, blink, circulate blood, control body temperature, grow new cells, support brain and nerve activity and contract muscles. Staying alive is hard work, people! The amount of energy (in the form of calories) that the body needs to function while resting for 24 hours is known as the basal metabolic rate, or BMR. This number of calories reflects how much energy your body requires to support vital body functions if, hypothetically, you were resting in bed for an entire day. In fact, your BMR is the single largest component (upwards of 60 percent) of your total energy burned each day.
While you can’t magically change your BMR right away, knowing your personal number, how it’s calculated, and which factors most influence your metabolism, can help you use this data point to create a smarter strategy for weight loss (or maintenance).
BMR: Your Basic Burn
To most accurately calculate BMR, an expert takes measurements of carbon dioxide and oxygen analysis after a subject has fasted for 12 hours and has had eight hours of sleep. However, a rough estimation of this data is possible using the Mifflin-St. Jeor equation, a formula introduced in 1990. Since it’s proven to be more accurate than previous BMR formulas, the Mifflin-St. Jeor equation is now considered the standard when it comes to calculating BMR.
Mifflin St. Jeor Equation
For men: BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) + 5
For women: BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) – 161
“You’ll want to use a BMR as a rough estimate to set your basic needs,” says Dr. Jennifer Sacheck, Ph.D, an associate professor of nutrition at Tufts University and co-author of Thinner This Year. She notes that this won’t vary too much for a male or female of the same age and body weight. Why the emphasis on weight, height, age and gender?
Weight and height: “The more mass you have, the more fuel you need to sustain larger organs,” notes Dr. Sacheck, explaining why heavier and taller individuals have a higher BMR. When you lose weight, your BMR decreases and you require fewer calories per day. In contrast, when you gain dense, heavier muscle, your BMR will increase.
Age: According to Dr. Sacheck, metabolic rate decreases as you age because muscle mass declines by five to 10 percent each decade after the age of 30. Luckily, it’s not a certain fate for the over-30 crowd. “We can mitigate that when we’re engaged in strength training,” says Dr. Sacheck. She recommends circuit training that incorporates full-body resistance exercises (think lunges, squats, core work on a balance ball). “Strength training individual muscle groups in isolation won’t be as effective in strengthening your body for daily movement that always incorporates a mix of muscle groups,” she says.
Gender: Since body composition (ratios of lean muscle, bone and fat) differ between men and women, research shows a woman’s BMR is typically around five to 10 percent lower than a man’s.
Calculate Your BMR
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Keep in mind, unless you have sophisticated tools to analyze your breathing or you’re closely monitoring your heart rate, you can’t calculate exactly how many calories you’re burning with exercise and digestion alone. Plus, Dr. Sacheck notes that stress levels and illness can also slightly or moderately change your BMR. Nevertheless, a formula-based estimate is a good place to start if you want to keep your diet in check.
TDEE: Energy, Explained
Once you know your BMR, you can make a more realistic guess of your total daily energy expenditure, or TDEE. This reflects the entire amount of calories, or energy, your body burns during a given day when you’re sleeping, ingesting and digesting food, working and exercising. To truly reflect the energy you’re burning, TDEE takes into account two additional aspects.
1. Thermic Effect of Activity (TEA): This is the amount of calories burned while exercising. The more intensely your muscles are working — sprinting during intervals or flexing while lifting weights — the more calories you’ll burn. And if you’ve completed a higher intensity workout, your body will have to work even harder to replenish its oxygen stores, resulting in an afterburn effect known as EPOC.
2. Thermic Effect of Feeding (TEF): When you digest food and absorb its nutrients, your body uses energy in the form of calories. “It’s only roughly three to five percent of your daily calorie needs,” Dr. Sacheck says, noting that proteins and fiber have the highest thermic effect, meaning they require the most amount of calories to digest per calorie consumed.
So how do we put a number on our TDEE? Taking into account your activity level and BMR, the calculators below can give an approximation of how much fuel your body requires. Note: If weight loss is your goal, you’ll want to create a calorie deficit. Aim to consume 90% of your TDEE.
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Knowing your BMR is important no matter if your goal is to lose weight, gain muscle, run harder or even taper from a training plan. It’s the first step to getting an idea of how much fuel you need to keep your engine roaring all day long. The next step is determining which healthy meals match up with your TDEE and leave you satisfied and energized. But it doesn’t stop there! When your body fluctuates or you change your exercise routine, revisit the BMR calculator to know if you should be eating more or less. When in doubt, consult with your doctor or nutritionist to make sure you’re on the right track.
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Originally published April 17, 2014. Updated September 2017.