The Science Behind Your Metabolic Rate
Metabolism is the process by which your body changes the food and liquid you take in into energy for use.
Resting metabolic rate or RMR is the minimum number of calories you need to perform basic functions, such as maintaining heart rate and body heat, breathing and keeping up brain activity. You will need this minimum amount of energy each day, even if you are not moving around. Two people with the same weight and height can have very different metabolic rates and energy needs.
How to Use Your RMR Results
Once you have your RMR results, you can easily calculate your Total Energy Expenditure based on your activity level.
Here’s how you can do it:
Without an accurate test, you could use your weight, height and age to estimate your RMR. However, everyone is different, and a small difference in your RMR can have a significant impact on your diet over time.
You can get an estimate of the number of calories you burn with exercise by using Fitnescity’s tool: Calories Burned By Exercise.
Factors That Affect Your Metabolism
The metabolic rate declines with age because of loss of skeletal mass in increased percentage of fat tissue.
Body Fat Percentage
Your RMR depends on the amount of fat you have. If your body fat percentage is high, your RMR might be lower than that of an individual with a lower body fat percentage.
The RMR is lower in women than in men, mostly because men, on average, have a greater muscle mass and a lower body-fat percentage than women.
A smaller body requires fewer calories to maintain the same physiological functions, whereas a larger body requires more calories.
Also, during weight loss your body may try to conserve energy in response to a lower calorie intake, this can also cause a reduction in RMR.
Taller people typically have greater body surface area and more lean body mass. As a result, they can have a higher RMR.
For every increase of 0.5 degrees C in internal temperature of the body, the RMR increases by about 7 percent. Therefore, a person with a fever of 42 degrees C (about 4 degrees C above normal) would have an increase of about 50 percent in RMR.
Starvation or serious abrupt calorie-reduction can dramatically reduce BMR by up to 30 percent. Likewise, restrictive, low-calorie weight-loss diets may cause BMR to drop by as much as 20 percent.
Physical exercise not only influences body weight by burning calories, it also helps raise RMR by building extra muscle. The greater the exercise intensity, the longer it takes the body to recover, which results in a longer and higher excess post-exercise oxygen consumption (EPOC).
Temperature outside the body affects the resting metabolic rate. Exposure to cold temperature causes an increase in the RMR, as the body tries to create the extra heat needed to maintain its internal temperature. A short exposure to hot temperature has little effect on the body’s metabolism because of compensatory increases in heat loss. However, prolonged exposure to heat can raise the RMR.
Some people are born with faster metabolisms, while others naturally have slower metabolisms.
Some hormones can increase or decrease metabolic rate. For instance, the thyroid hormones regulate a portion of metabolism. An individual who has hypothyroidism can have a low RMR.
Your Measured RMR Will Most Likely Be Different than Your Estimated RMR
You can estimate your RMR using the Mifflin-ST Jeor equation. However, the formula does not take into account all of the factors mentioned above, and as a result, it can under-estimate or over-estimate the RMR.
Men : 10 x weight (kg) + 6.25 x height (cm) – 5 x age + 5
Women : 10 x weight (kg) + 6.25 x height (cm) – 5 x age – 161
It is important to keep in mind that the equation only gives an estimate of the RMR and cannot entirely be relied upon, since each individual is different. For instance, you might find that your actual (measured) RMR is 200 calories less than your estimated one. The difference is enough to cause weight gain of several pounds over a few months.
The RMR test will also give you your Respiratory Exchange Ratio (RER). The RER is the ratio between the amount of carbon dioxide (CO2) produced in metabolism and oxygen (O2) used. This ratio indicates what your body is using for energy: carbohydrates or fat.
At rest, the RER is about 0.8. However, during exercise the number can exceed 1 as CO2 production by the working muscles becomes greater and more O2 is being used.
An RER of 0.70 means that your predominant energy source is fat while an RER of 0.85 suggests that you use a mix of fat and carbohydrates. If the value is 1.00 at rest, this means that your body relies mostly on carbohydrates for energy.
How to Boost Your Metabolism
If you eat enough protein, you will allow your body to build and repair its muscle tissue. As a result, this will help you maintain or increase your muscle mass.
If you increase your daily activity level and/or do strength training, you will not only burn more calories, but will also strengthen your muscle mass.
If you ear less sugar (and more protein and fat), you will boost your RMR. Unlike protein and fat, the amount of energy that it requires for the body to process sugar, is relatively small compared with fat and/or protein.