When it comes right down to it, weight loss is all about calories. Knowing the number of calories your body burns to perform daily functions, which is referred to as the basal metabolic rate, or BMR, can help you determine the number of calories you need to lose weight. Consult your doctor before making any changes to your diet.
What is the BMR
BMR is the number of calories your body needs for basic body functions that keep you alive, such as your heartbeat, breathing and the regular upkeep of your body organs; it's essentially the number of calories you need if you did nothing but lie around all day. Surprisingly, BMR makes up the bulk of your calorie needs -- about 60 percent of your total energy expenditure, according to the University of Colorado at Denver -- with the calories you use to metabolize food and calories burned through activity and exercise making up the rest. Your muscles are your body's biggest BMR calorie burner. That's why men, who typically have more muscle on their frames, have higher BMRs than women. If you're trying to lose weight, knowing your BMR can help you better estimate your calorie needs for weight loss.
Determine Your BMR
A number of formulas are used to estimate BMR. According to a 2005 review study published in the Journal of the American Dietetic Association, the Mifflin-St. Jeor equation is the most accurate at estimating calorie needs in both non-obese and obese individuals.
The Mifflin-St. Jeor equation is different for men and women. You'll need your height, weight and age to do the calculations. Divide your weight in pounds by 2.2 to get kilograms. To calculate your height in centimeters, multiply the inches by 2.54.
For men: BMR = (9.99 x weight in kilograms) + (6.25 x height in centimeters) - 4.92 x age in years + 5.
For women: BMR = (9.99 x weight in kilograms) + (6.25 x height in centimeters) - (4.92 x age in years) - 161. For example, a 41-year-old female who weighs 162 pounds at 5-foot, 4-inches tall has a BMR of 1,388 calories.
While predictive equations provide a baseline for estimating calories, metabolic rates vary due to genetics and muscle mass.
Using BMR to Determine Your Calorie Goal
Eating fewer calories than your body needs to function will result in weight loss. To lose 1 pound a week, reduce your daily intake by 500 calories. While the BMR provides the number of calories your body uses for basic functions, the other factors that affect calorie needs, including activity and the thermic effect of food, also account for part of your total calorie needs.
To calculate the calories you need to maintain your current weight, multiply your BMR by an activity factor. If you do not exercise and have a desk job, your activity factor is 1.2. If you exercise one to three days a week, your activity factor is 1.375. If you exercise three to five days a week, your activity factor is 1.55. If you're involved in sports and exercise six to seven days a week, your activity factor is 1.725. If you're training for a marathon or have a physically demanding job, your activity factor is 1.9.
So, for example, a 41-year-old woman with a BMR of 1,388 calories who exercises three days a week needs 1,909 calories to maintain her weight: 1,388 x 1.375 = 1,909. Subtract 500 calories, and she needs 1,409 calories to lose 1 pound a week.
However, women should not eat fewer than 1,200 calories a day, and men should not eat fewer than 1,800 calories a day, according to the American College of Sports Medicine. If subtracting 500 calories puts you below those numbers, you'll need to cut fewer calories from your diet and exercise more to burn the rest.
Changing Your BMR for Weight Loss
Your BMR decreases as you lose weight, which means you need to eat even less as your weight goes down to continue to drop pounds. Loss of muscle may be partly responsible for some of the decrease in your BMR. You may be able to offset some of the changes in your BMR with strength-training exercises to build muscle. Work out your muscles two to three days a week using free weights, resistance bands or body resistance exercises such as squats, sit-ups and push-ups. If you're not sure where to begin, consult a fitness professional to help design an individualized exercise plan for you.