If you’re lying on the couch watching television, do you still burn calories? The answer, of course, is yes. Even when you are not physically active, various systems in your body are still functioning to keep you alive.
The most limited number of calories needed to keep essential organs working is referred to as your Basal Metabolic Rate (BMR). This includes things like keeping your heart beating and your blood circulating, maintaining breathing to provide oxygen, and supporting body temperature.
About seventy percent of your actual daily caloric use is BMR. An additional ten percent is for thermogenesis, the digestion of food and elimination of waste products. The final twenty percent is attributable to various physical activities, like standing, walking, and so forth. Your caloric expenditure can also be affected by things like illness, ambient temperature, and stress. For example, if you start to shiver because you’re cold, this is your body’s way of warming up. Short muscle contractions release heat energy, which helps to raise your body temperature. This muscle movement uses calories.
BMR tends to decrease as you age. This is one of the reasons it seems to be harder to get rid of a few extra pounds as you get older. Reduction in energy expenditure and reduction in lean body mass tend to reduce BMR by about 5% per decade after you reach adulthood. Strength training and regular physical activity can limit this decrease.
Lean body mass requires more caloric expenditure and therefore promotes a higher BMR regardless of your age. Studies estimate that for every pound of lean body mass (which includes everything in your body except fat), you require 16 calories per day to maintain your body weight. If an average man has 15% body fat (therefore a lean body mass of 85%) and he weighs 180 pounds, he needs approximately 2,700 calories per day to maintain his weight.
This number is only an estimate. Every person has a different body configuration and there are many factors that affect total calories used. It has become quite popular to use BMR calculators to estimate calorie requirements. There are dozens of BMR calculators on the internet, and they all use similar models and will probably give you similar results. However, it’s important to remember that these are all estimates. The BMR for the man in the example given above would be about 1,900 calories, which is very close to seventy percent of his estimated daily calorie requirement.
The only way to know your BMR for sure is to begin tracking your calorie intake and see how you do. If you start losing weight, then you’re consuming fewer calories than your body is using each day. On the other hand, if your weight isn’t changing, you may need to further restrict your intake or do more physical activity if you're trying to lose weight.
BMR can give you an idea of caloric expenditure, but it's only a tool. Continued good health depends on basic principles like eating appropriate amounts of nutritious food and getting regular exercise.