BMR stands for “Basal Metabolic Rate”, which is the number of calories your body burns at rest to maintain life. While we like to think that our hardcore gym routine burns the most calories of the day, it actually doesn’t even come close. Instead, the BMR is responsible for 60-70% of the calories expended, through activities such as the beating of the heart, respiration, and body temperature maintenance. Here are some key factors that affect the BMR:
- Genetics. The one factor we can’t directly change. Some individuals have fast metabolisms, and some have slow metabolisms.
- Gender. Due to greater muscle mass and lower body fat percentage, men have a 10-15% faster BMR than women.
- Age. Because a younger person has a higher rate of cell division, once you are 20 years old, your BMR drops about 2% every 10 years.
- Weight. Due to increased body tissue volume, an obese individual actually has a higher metabolism than a thin person.
- Height. Tall thin people have a higher BMR than short people of equal weight. If both are on the same diet, the short person will gain much more fat.
- Body Fat %. A lower body fat % means a higher BMR.
- Diet. A strict diet or severe calorie restiction can reduce BMR by up to 30%. This is one of the reasons why people on a crash diet lose up to 20lbs of water weight, then plateau.
- Body temperature. For every 0.5 degree celsius increase in internal body temperature, the BMR increases approximately 7%. Physical activity significantly increases body temperature.
- External temperature. Prolonged exposure to extremely warm or very cold environments increases the BMR. People who live in these type of settings often have BMR’s that are 5-20% higher than those in other climates.
- Endocrine function. Thyroid glands that produce too much thyroxin can double the BMR, while BMR can drop by 30-40% in individuals with hypothyroidism, or inadequate thyroxin production.
- Exercise. In addition to increasing body temperature, exercise increases lean muscle mass, which burns more calories than fat – even when you’re not exercising.
The actual number of calories burnt by the BMR averages around 2000-2100 calories per day for women and 2700-2900 per day for men. The total day’s energy expenditure can dramatically increase this number, with very active athletes burning up to 6000-8000 calories per day.
So how do you determine what your personal BMR actually is? While there are advanced technologies, such as measuring heat output or expired gas exchange, there are also several different formulas. Here are three:
- Multiply. Take your body weight in lbs., and multiply by 15-16. This will give you an approximation of your BMR. If you want to lose weight, multiply by 12-13, and if you want to gain weight, multiply by 18-19. This method is very simple, but doesn’t account for body fat %, and will overestimate caloric needs for someone who is obese (30% body fat or more).
- Harris-Benedict formula. This formula uses height, weight, age and sex factors to determine BMR. It is more accurate than the multiplying factor, but also does not account for body fat %, and may also be prone to calorie overestimation for obese people. Remember, 1kg is 2.2lbs, and 1 inch is 2.54 cm.
- Men: BMR = 66 + (13.7 X wt in kg) + (5 X ht in cm) – (6.8 X age in years)
- Women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) – (4.7 X age in years)
- Katch-Mcardle formula. This formula is the most accurate, and accounts for body fat %. To find “lean mass in kg”, simply multiply your weight in kg by your body fat %.
- Men and Women: BMR = 370 + (21.6 X lean mass in kg).
Remember, the BMR does not take into account your activity levels. This is where activity multipliers can be used to determine your total daily energy expenditure.
- Sedentary = BMR X 1.2 (little or no exercise, desk job)
- Lightly active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
- Mod. active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
- Very active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
- Extr. active = BMR X 1.9 (hard daily exercise/sports & physical job or 2X day training, i.e marathon, contest etc.)
An alternative to using activity multipliers is to use a heart rate monitor or personal calorie measuring device like a bodybugg arm band (www.bodybugg.com), both of which can approximate calories burnt during daily activities.
The last step in using this information to lose body fat or gain weight, depending on your goals, is to adjust your caloric intake so that it falls above or below your total daily energy expenditure. Let’s say that you determine your BMR is 1800, and your additional energy expenditure is 700, for a total of 2500 calories. By decreasing your total caloric intake to 2000 calories per day, you will be at a 500 calorie per day deficit. Since a pound of fat is 3500 calories, this simple change can result in a loss of 1 pound of fat per week. A good place to start for caloric restriction is to consume about 15-20% less than the total daily energy expenditure.
Remember, if you consume too few calories, or decrease caloric consumption significantly, you can depress your metabolism, decrease thyroid hormone production, and lose lean muscle. A good guideline is to never consume more than 1000 calories per day less than your total energy expenditure. General health recommendations recommend that women never consume less than 1200 calories per day, and men never consume less than 1800 calories per day. Listen to your body! If you are constantly sluggish, fatigued, depressed or non-motivated, you may be overly restricting calories. But if you use your BMR to accurately adjust your energy intake levels, you can unlock the metabolic key to weight loss success!
Until next time, train smart,
M.S. PE, NSCA-CPT, CSCS