Resting Metabolic Rate

How many calories do we really need daily?
Often we hear clients say “I have a slow metabolism, I just look at food and gain weight.” Is it really that we have a slow metabolism or is it that we are consuming more (calories) than our body needs and expending (exercising) less? How do you really know how many calories you need daily?

Metabolism is how the body processes food into heat or “usable energy.” The rate at which an individual converts food to energy is called the metabolic rate. A test, called a Resting Metabolic Rate (RMR), is available to measure how many calories a person burns during the course of a day at rest (meaning, doing no activity at all). The RMR represents the largest component of calorie expenditure (~70%). This gives a baseline of the amount of calories you need to consume in a day just for all your organs to function (ie. breathing, heart beating, etc.).

How is RMR measured?

We measure Resting Metabolic Rate through a method called Indirect Calorimetry. First, for the most accurate results, we make sure the person being tested has fasted for about 11 hours prior to testing (that means no food or caffeine) and has not performed any exercise the morning of the test. We want the test subject to be in a rested and relaxed state. Next, we obtain baseline information to include height, weight and date of birth. After that, a mask which has a long tube on it and is attached to an analyzer will be placed over the individual’s nose and mouth. The person will lie down in a quiet room for about 10 minutes and breathe into the mask and tube. The analyzer measures the rate of oxygen and carbon dioxide as the person breathes in and out and from this, calculates the number of calories burned. The result would be given in kcals/day (i.e. a 135 pound woman who exercises regularly might have a Resting Metabolic Rate of 1650 kcals/day). This number differs greatly from person to person dependent on height, weight, age, sex, and muscle mass and is therefore more precise than using a generic formula. To watch an RMR test being done at LWI, click here.

How are the results from a RMR used by an exercise physiologist

From an exercise standpoint, the results from an RMR test can change based on a person’s muscle mass. Basically, as your muscle mass increases, the amount of calories you burn at rest increases, therefore, your RMR increases. So, to ‘boost’ your metabolism, it is recommended that you include strength training to your exercise routine to assist with gaining muscle and preventing loss as we age, since muscle loss is the main contributor to a ‘slowed’ metabolism.

How are the results from a RMR used by a Registered Dietitian?

From a nutrition standpoint, results of an RMR test can be used to determine your daily calorie needs and develop a personal meal plan depending on your weight goals. The results of the test are multiplied by an activity factor if you are not actually ‘resting’ all day (ie. lying down), and additional calories will be included for exercise. These 3 components together give your total calorie/energy needs each day if you wanted to maintain your weight. If your desire is to lose weight then a certain number of calories are subtracted and if your desire is to gain weight then a certain number of calories is added, both depending on your goal.

Since every person has a unique metabolism based on their physical make up and physical behavior, knowing your RMR can really help ‘fine tune’ your daily meal plan and exercise routine so that you achieve your weight goals whether it is to lose, maintain or gain.