Your basal metabolic rate (BMR) is the amount of calories your body burns each day just by functioning – for example, through basic things like sustaining internal body temperature, pumping blood, breathing. So even if you did nothing by lie on the couch all day, your body would still be burning calories. Understanding this figure is key to determining the amount of calories you need to consume and burn in order lose weight.
So what determines your BMR? There are a number of different factors that will influence how many calories your body burns at rest, so each person’s result will be different. Some of the things that impact your BMR include:
- Genetics. Some people are just born with faster metabolisms than others- this is just another part of your genetic makeup.
- Gender. In general, men tend to have a lower body fat percentage than women, as their bodies are designed to build up muscle more easily. As a result, men will often have a higher BMR than women.
- Age. As your get older, your BMR will decrease. After the age of 20, it generally drops about 2% per decade.
- Weight. A heavier body requires more calories to function, so it will have a higher BMR than someone who weighs less.
- Height-weight ratio. A larger body surface area will have a higher BMR.
- Body fat percentage. People with less body fat will have a higher BMR.
- Diet. If you regularly follow very restrictive, low-calorie weight loss diets, you can actually cause your BMR to drop by as much as 20%. This happens when your body goes into starvation mode.
- Internal temperature. A body temperature increase boosts your BMR, so when you have a fever, your body will use more calories.
- External temperature. A cold climate increases your BMR, because the body needs to use more energy to warm you up to a normal functioning temperature.
- Thyroid function. Thyroxin, which is produced by the thyroid gland, plays a key role in regulating your BMR. If you have an overactive or underactive thyroid, this will impact how much energy your body burns at rest.
- Exercise. Regular physical activity builds up extra lean tissue that helps to raise your BMR.
Estimating your BMR
There are a number of scientific formulas that can be used to estimate your BMR, taking into consideration many of the factors above. Of course, no equation can calculate your BMR with absolute accuracy, so keep in mind that these results are approximate.
Some of the most popular BMR estimation formulas include:
The Harris-Benedict formula
Based on total body weight, considering height, weight, age and gender.
- Men: BMR = 66 + (13.7 X weight in kg) + (5 X height in cm) – (6.8 X age in years)
- Women: BMR = 655 + (9.6 X weight in kg) + (1.8 X height in cm) – (4.7 X age in years)
This equation is more accurate, as it is based on lean body weight- but this means you need to know your lean body mass (LBM).
- Men and women = BMR= 370 + (21.6 X LBM in kg)
Using your BMR in your weight loss plan
Once you’ve estimated your BMR, you can use this information to more effectively plan your calorie intake to help you reach your weight loss goals. Your BMR tells you how many calories you need per day to stay active and healthy. In other words, this is your daily caloric maintenance level. So, to keep your weight stable, you should remain close to this level.
To lose a few kilos, you need to create a calorie deficit. So, your daily intake of calories should be less than your BMR plus the extra calories burned through exercise. You can either add your exercise calories manually, or use the following formula to approximate based on your usual level of activity:
- Sedentary (little or no exercise, desk job) = BMR X 1.2
- Lightly active (exercise/sports 1-3 days a week) = BMR X 1.375
- Moderately active (exercise/sports 3-5 days a week)= BMR X 1.55
- Highly active (exercise/sports 6-7 days a week)=BMR X 1.72
- Extremely active (exercise/sports and physical job or training twice a day)= BMR X 1.9