Basal Metabolic Rate, known as BMR, is the rate at which the body expends energy to maintain essential life functions while at complete rest. The majority of these essential functions continue mostly unnoticed—things like the heart beating, the lungs inhaling and exhaling air, the kidneys filtering waste, new cell creation, maintaining proper body temperature, and more. A BMR calculator measures the sum of kilocalories required to maintain these essential body functions. On average, this sum is approximately 60-70% of a person's total daily energy expenditure.
Because BMR measures these activities while at complete rest, a true calculation of BMR can only be done after a 12-hour fast and a good night's rest and in the absence of any physical activity or emotional excitement, as these factors can temporarily influence metabolic rate.
Many factors affect BMR. Most notable are age, body shape, body composition, and hormones. Children and adolescents generally have a very high BMR because their bodies are in a constant state of growth, which requires a constant supply of food in order to gain energy. Remember that cheeseburger, fries, and milkshake that ‘magically disappears’ when a teenager eats it? This is why. As we age, our BMR slows as our body shifts from an anabolic (building) state to a catabolic (breaking-down) state.
Individuals with higher lean body mass—meaning more muscle—have a higher BMR than those without, as muscle requires significant energy to be maintained. Those with high body fat have a much lower BMR because fatty tissue requires very little energy to be maintained. For this reason, men tend to have higher BMR than women based on average body composition.
The hormone thyroxine, produced in the thyroid, directly impacts BMR. Individuals with normal to elevated thyroxine levels (as found in hyperthyroidism) experience higher BMR than those with reduced thyroxine levels (as found in hypothyroidism).
Other factors affecting Basal Metabolic Rate include the total surface area of the body (higher surface area requires higher BMR to maintain body temperature), the presence of a fever or other physical stressors (which can raise BMR), fasting or starvation (both reduce BMR), smoking and caffeine (both increase BMR), sleep (reduces BMR), and emotional stress (increases BMR).
While you can calculate an estimated BMR using a BMR calculator (which utilizes the following formulas or accepted variations), calculations are only estimates. Your actual BMR is affected by many factors, as stated above.
Resting Metabolic Rate, or RMR, is the rate at which the body expends energy to maintain essential life functions while at rest, but can include the act of digestion or recent physical activity. RMR measurements will be slightly higher than BMR measurements.
Total Metabolic Rate, or TMR, is the cumulative energy required to maintain essential life functions as well as all physical activity, everything from grocery shopping to a strenuous workout. Even slight increases in muscle activity can significantly raise the metabolic rate during exercise and several hours after.
Because so many factors can affect your BMR—age, height, weight, body composition, hormones, stress levels, nutrition, caffeine intake, and more—it is difficult to set acceptable guidelines or standards for BMR measurements. On average, BMR decreases almost linearly with age due to changes in hormones and loss of lean muscle mass as strenuous physical activity tends to decline with age.
In order to offset or "fight back" against this decline in BMR, you must maintain healthy lean muscle mass. In other words, muscle is the key. While cardio exercise is good for your heart, it will not raise your BMR. Instead, incorporate regular strength training into your routine to build and maintain muscle mass.
Online BMR calculators provide a fairly accurate measure of BMR. While these BMR calculators do not include factors such as stress level, caffeine intake, nutrition, or hormone levels, they utilize measurable factors and basic BMR calculations to properly measure actual BMR. If you require an exact measurement, many doctor's offices, nutrition professionals, and even gyms can run tests using a special calorimetry device.
While it is not necessary to know your Basal Metabolic Rate to lose weight, it may be helpful. And knowing your TMR (total metabolic rate) is an even more helpful tool, as TMR measures the total kilocalories that your body burns on a daily basis. Remember that weight loss is not as simple as creating a calorie deficit, as many believe. Other factors, like hormones, can significantly affect your weight loss (or weight gain) goals.