Overview of BMR
Basal Metabolic Rate (BMR) is the rate the body uses energy to sustain basic body functions. Included in that description is respiration, heartbeat, renal function, and blood circulation (paraphrased from “Advanced Nutrition and Human Metabolism 5th edition” by Gropper, Smith, and Groff). You may wonder why BMR is such a pervasive term in the nutrition and health community. Mainly the reasoning for BMR’s significance is that it is the amount of energy below which immediate health can become an issue, we will discuss this later; first let us differentiate BMR and RMR. Many people confuse BMR with RMR (Resting Metabolic Rate) and they are similar. BMR is usually taken after a 12 hour fast after a full sleep cycle when the body’s energy expenditure is at its slowest, RMR does not require a 12 hour fast, and only requires that the body be at a resting state (no exercise) for a 2 to 4 hour period before hand. RMR is usually about 5 to 10 percent higher than BMR. Because RMR does not require much in the way of preparation, it’s normally what is taken, and BMR is assessed as about 10% lower.
BMR is categorized as usually between 55% and 75% of your daily energy needs, not including exercise over and above what is considered normal daily activity (for some this is walking, for others this could be climbing rock walls all day) . The reasoning for the amount of variation is simply environmental and genetic effects that govern people. Why one person could have a BMR of 75% of total energy needs and another have 55% is mainly “luck of the draw”, but some of it is due to overall health. Some people genetically have more mitochondria in their cells, or larger arteries capable of delivering more oxygen rich blood to the cells that need them. Through training and exercise, we can increase the amount of mitochondria in cells (mitochondria combine calorie based fuel and oxygen or non-oxygen catalysts to create energy), and strengthen the heart muscle to deliver more oxygen per beat, and increase the amount of dissolved oxygen in the blood, these all lead to more efficient (and lower) total daily energy expenditure and to some degree counteract the increase in calories burned for gaining muscle mass.
The issue of why BMR is important to nutrition and by extension weight loss is because many people feel that BMR is the cut off point for calorie deficit where someone who is under the “line” of obesity can be. In other words, they stress that the body needs at least this many calories to perform its normal activity without forcing the body to slow those autonomic processes thus reducing calorie burn, levels of good health, and physical stamina. There is some merit to this case, but it is usually (in the author’s opinion) a widely abused statistic to use. Personally, as a trainer and a student in human metabolism, I feel that BMR should be a very rough guide, something to take note of, and be watchful for, but not something to hang your hat on. BMR is NOT, let me repeat, BMR is NOT your maintenance calories, so eating your BMR does not mean you’re receiving the amount of macronutrients you need to sustain weight.
Some Quick notes:
1) Because of their extremely high available energy stores, those who are obese or morbidly obese can usually afford to create a deficit that takes them below their BMR. Remember, it’s BMR + daily activity + exercise activity that created your Total Daily Energy Expenditure (TDEE); and it’s calories consumed plus energy from fat stores and to a far smaller degree energy from amino acid breakdown that is your total energy delivered; therefore the more available adipose (body) fat available, the more energy available to the body. There are some caveats to this equation though, the body can only release so much fat at once, regardless of total fat volumes, which is somewhat dictated by fat surface area; also keep in mind that the body does not run on a 24 hour clock. Food you ate this morning may or may not still be available to you this night depending on what kind of food, how active you are, and a host of other metabolic functions.
2) People who have been in a state of semi-starvation (starvation mode) will have a drastically reduced TDEE and consequently their BMR will lower as well. This is something that can affect how much weight is lost. Because energy required is lower than normal, the deficit you created will actually be smaller than you think, eliciting smaller fat losses, higher muscle cannibalization, and reduced systemic function and immune system. This is an insidious process, people don’t “feel” like they’re in starvation mode because the body adjusts hormone levels to reduce hunger when it thinks it’s in a period of famine, but there are obvious signs to a trained professional. Dull or dry hair, stunted nail growth, fatigue, reduced strength levels, sometimes high ammonia levels in the urine, yellow tint to the skin, decreased defenses to common viruses such as colds, higher minor infection rates, slower healing rates for cuts and bruises. All of these can be symptoms of the famine response.
3) BMR is usually taken via a formula this is an estimate though, true TDEE and BMR are done in a lab or hospital or other facility that has oxygen measurement equipment. If you truly want to know your BMR and/or TDEE, ask your primary care physician where the closest facility is for an indirect calorimetry test. There is one more accurate test called a direct calorimetry test, but it uses very specialized equipment and is generally very expensive and time-consuming.
In summary BMR is a tool, like many others, that you should have in your arsenal to help your nutrition and health. Similar to your weight, your Body Fat percent, your VO2 Max, and your waist to hip ratio, it’s another number to evaluate. It should neither be analyzed in a vacuum, nor should it be ignored. It should simply be part of your overall plan to develop a healthier lifestyle and nutrition program.