BMR: Know Your Number and Get Results
Trying to stay on a weight loss program ends up being a daily battle with a mathematical equation. Since the key to losing weight is to be in a calorie deficit, meaning burning more calories overall than what you take in, most people will be constantly crushing numbers all day with their exercise regimen and food intake. But how exactly do you know how many calories you are really burning? Unfortunately, the cardio machines at the gym are not accurate with the calorie burn they state. Some machines will have the option to put in your weight for a better estimate, but even then it is not 100% accurate. The same goes for many fitness watches.
If you are one of these calorie counters or interested in learning the best way to lose weight for your body, it is important to know your basal metabolic rate, or BMR. This value is basically an estimate of the amount of calories your body requires to function for 24 hours if all you did was rest. Yes, our bodies need calories just for organ function, breathing, and to keep our hearts beating. If you go below this number you will risk losing muscle mass and possibly have lower organ function over time. Almost no one really lays down all day without moving so trying to stay at a calorie intake right around your BMR with a regular lifestyle of working, running errands, and taking care of a household and family, would be considered a calorie deficit and cause weight loss.
To measure BMR, the most accurate method is in a lab setting where measurements of carbon dioxide and oxygen are taken after a certain amount of time spent fasting, usually 12 hours. Since this is not an ideal way for everyone to get tested, a formula can be used that has been shown to be quite accurate as well. This formula is called the Mifflin-St. Jeor equation and is shown below.
Men: (10 x bodyweight in kg) + (6.25 x height in cm) – (5 x age in years) + 5
Women: (10 x bodyweight in kg) + (6.25 x height in cm) – (5 x age in years) – 161
This formula will equate to an estimate of what your basic daily calorie needs should be. Weight and height are a major determinant of BMR because the bigger and taller one’s body is the more fuel they will require for larger organs and muscles. Age plays a role since metabolism slows significantly, especially after the age of 30. This is mostly due to the loss of muscle mass as we age, which we can counteract by simply engaging in strength training on a regular basis. Body composition is different for men and women, which is why women usually require between 5% and 10% fewer calories than men. Women tend to carry more fat and less muscle mass than men, which accounts for slower rates of burning.
While this formula is a more accurate measure of BMR than others, it is still not 100%. Without knowing the exact amount of calories you burn with a given activity, the BMR value can vary. Devices that can measure your precise heart rate and breathing during activities may give a more accurate measurement of BMR. However, using the estimate given by the Mifflin-St. Jeor equation, you can still get results with weight loss (or gain) staying within the range of the calculated BMR.
The Role of Muscle
One reason BMR values highly differ amongst people with a similar body weight, height, and age is the variation in muscle tissue. Two 30 year-old females weighing 130 pounds at 5’5″ height can have the same BMR using the equation shown above. If one of those females is highly athletic with a low body fat percentage, meaning a decent amount of muscle, and the other sedentary with double the body fat and half the muscle mass, then the rate of daily calorie burn can vary significantly. The higher the body fat and the lower the muscle mass the slower the metabolism will be. This is why strength training is emphasized for those who want to lose weight since muscle burns many more calories than fat, even at rest.
Essentially “calories in versus calories out” is the concept to keep in mind when it comes to losing or gaining weight. Therefore it is important to know what your BMR is and how it can vary as we age and fluctuate in bodyweight. Any physical activity incorporated throughout the day will create that calorie deficit if you are sticking to calorie intake around your BMR value. Of course, you will have days where your calorie intake goes way over due to a social outing or indulgence in a favorite high-calorie meal, but you should not worry about these days. It is best to look at your calorie intake over the course of a week or two and if there is an overall deficit, then losing weight will be successful.
- Skeletal Muscle Metabolism is a Major Determinant of Resting Energy Expenditure, Journal, Jun 04, 2018 ,