In part one of this series I have discussed that a 500 calorie per day deficit is necessary in order to lose one pound per week. In part two I showed how recording calories consumed in food, and burned in exercise can help achieve this. In this post I will cover the importance of resting metabolic rate (also known as basal metabolic rate).
Resting metabolic rate is the amount of energy that our bodies use at rest. This energy is used to make our hearts pump, to breathe, to metabolise the food we eat and all those other bodily functions that occur without our conscious thought. This is essentially the same as the basal metabolic rate. The reason that it is important is that it sets our ‘target’ calorie balance: i.e. Energy consumed – Energy burned = Resting Metabolic Rate – 500 (to loose one pound per week).
Unfortunately resting metabolic rate cannot easily be measured using scales or a tape measure, so most people who are trying to lose weight are not aware of what their value is. Resting metabolic rate can be estimated using various equations, if you know your height, weight, gender and age. The best known of these is called the Harris-Benedict equations. Again on-line calculators can be useful such as http://www.bmi-calculator.net/bmr-calculator/.
Unfortunately estimated resting metabolic rates may not be accurate. People may have a resting metabolic rate that is much lower than predicted, and in this case it can be difficult or impossible to lose weight despite consuming very few calories. People who have engaged in many diets in the past often have a low metabolic rate. When we diet and lose weight rapidly our body thinks it is starving, in order to survive it starts burning muscle, and dropping its metabolic rate. When we stop a diet, we tend to re-gain fat, but our resting metabolic rate stays low. This is why people who undertake many cycles of dieting often end up gaining weight, despite eating very little.
Resting metabolic rate can be measured accurately using a machine known as a calorimeter. These place an astronaut type helmet over a person’s head and measure the amount of oxygen and carbon dioxide they breathe out over a measured time period. Using this information, computers are able to calculate how many calories that person is burning at rest. This testing, called calorimetry, is the best way to determine resting metabolic rate, however calorimeters are not widely available.
At Oregon Weight Loss Surgery we obtain calorimetry in all our patients prior to surgery, and follow-up on any abnormal results. If you don’t have access to calorimetry I would suggest you start with using an equation like what I have linked to above, subtract 500 from it, and make this your daily calorie goal. If you do not lose weight at this calorie target (over a reasonable period such as two weeks), subtract another 500 from it and try again. An alternative approach if you weight is stable (i.e. you are neither gaining nor loosing weight) is to accurately track your calories consumed and burned. Your average daily calorie balance will be your metabolic rate, subtract 500 from it to loose one pound per week.
In this post I have covered the basics of resting metabolic rate, how it is estimated and how it is measured. This is a huge topic so next post I will discuss how we can increase our metabolic rate (or prevent it from dropping).