Whether you are looking to lose weight, maintain you current weight or gain weight (i.e. muscle mass) it is important to understand Basal Metabolic Rate (BMR). BMR is a term widely used in the areas of weight loss and nutrition. Metabolism is a term often used interchangeably with BMR when, in fact, the two words have totally different meanings.
Metabolism: The processes that the body needs to perform in order to function properly. These processes are either anabolic (tissue building) or catabolic (breaking down of tissue). Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles (skeletal and smooth).
Basal Metabolic Rate: The amount of energy expressed in calories that a person needs to keep the body functioning at rest. Basal metabolic rate (BMR) affects the rate that a person burns calories and ultimately whether you maintain, gain, or lose weight. Your basal metabolic rate accounts for about 60 to 75% of the calories you burn every day. It is influenced by several factors.
To keep it simple, metabolism consists of the processes our bodies need to perform to stay alive and Basal Metabolic Rate (BMR)is the number of calories your body burns to perform these basic functions. For example, envision a car parked in the driveway idling. You are the car, the engine (and all the processes happening) represents your metabolism, and the fuel it burns to make that happen is your basal metabolic rate.
What is Basal Metabolic Rate (BMR)?
BMR is the minimum caloric requirement needed to sustain life in an individual at complete rest. It is responsible for burning up to 70% of the total calories you expend each day. Because the human body adapts to many things on an ongoing basis, this figure varies. Calories are burned by bodily processes such as respiration, the pumping of blood around the body and maintenance of body temperature. Keep in mind the body will burn more calories due to the activities of daily living (i.e. walking, talking, eating/digestion), not to mention any physical activity or exercise that you may include.
BMR is determined by a combination of several genetic and environmental factors:
- Genetics (some people are born with faster metabolisms and some with slower metabolisms)
- Gender (men have greater muscle mass and a lower body fat percentage, resulting in a higher basal metabolic rate than women)
- Age (BMR decreases with age, dropping 2% per decade after the age of 20 years)
- Weight (the heavier the weight, the higher the BMR)
- Body Surface Area (tall, thin people have higher BMR compared to a shorter person with the same weight)
- Body Fat Percentage (the lower your body fat percentage, the higher your BMR)
- Diet (restrictive low-calorie weight loss diets may cause your BMR to drop as much as 20%)
- Body Temperature (for every increase of 0.5 ºC in internal temperature of the body, BMR increases by 7%)
- External temperature (exposure to cold temperature causes an increase in the BMR, as will prolonged exposure to heat)
- Glands (thyroxin, the primary hormone that regulates BMR, speeds up the metabolic activity of the body. The more thyroxin produced, the higher the BMR)
- Exercise (physical exercise helps raise BMR by building extra lean tissue)
Basal Metabolic Rate and Weight Loss
BMR is the largest factor in determining overall metabolic rate and how many calories you need to maintain, lose or gain weight. A common mistake made by many people trying to increase their BMR (for weight loss) is they reduce their caloric intake drastically. They choose a highly restrictive diet and decrease their calories by a significant amount. This does the exact opposite of what you would expect. Instead of increasing your BMR, your body goes into starvation mode (thinks that there isn’t enough food) and your metabolism slows down. In turn, this decreases your BMR by upwards of 30%.
Calculating Basal Metabolic Rate
The Harris-Benedict equation has been the standard for decades and is still the most widely used for estimating BMR. The Harris-Benedict equation for BMR is outlined below:
- For Men: (13.75 x w) + (5 x h) – (6.76 x a) + 66
- For Women: (9.56 x w) + (1.85 x h) – (4.68 x a) + 655