Daily energy expenditure, or total “calories burned” in a day, has three major components: basal or resting metabolic rate, the thermic effect of food (calories expended through digestion) and the energy cost of physical activity. Having a general understanding of your daily energy expenditure is a GREAT place to start to understand weight maintenance and loss. Once you understand how many calories your body expends just to maintain vital functions, you can then better understand weight loss in terms of how many calories you should subtract from your diet to lose weight, and/ or how much activity you should engage in to engage in a target calorie deficit (“burn calories”). This article will help you to understand those concepts and apply them to yourself with evidence- based equations provided.
Imagine you laid in bed all day. You didn’t get up to eat, pee, or even to change the channel from that rerun of Friends you have seen 84 times already. If you were to measure how many calories (energy) your body expended just to maintain this level of ultimate couch potato, you would get your basal metabolic rate (BMR), also known as resting energy expenditure (REE) or resting metabolic rate (BMR). For the sake of this article, we will continue to reference “BMR” as our standard.
The MOST accurate way to measure BMR is with is a portable indirect calorimeter, which is accurate to within 5% of your true BMR. HOWEVER, those are only used clinically, as they are costly and require trained personnel. Because those are so unavailable, we need reliable and accessible predictive equations instead.
A systematic review done by the (American Dietetic Association) compared four of the most commonly used equations to estimate BMR, including the Harris-Benedict, Mifflin-St Jeor, Owen, and WHO/FAO/UNU). When they compared these equations, the Mifflin-St Jeor equation was the most reliable equation, as it consistently predicted BMR within 10% of that measured with indirect calorimetry, and it also had the narrowest error range (meaning it was less likely that a measurement obtained was due to some kind of examiner/ participant error, etc).1
- NOTE: The populations studied were “healthy,” non- obese and obese populations. This study did not include those with a disease that might have impact on BMR, such as thyroid disease or diabetes mellitus, and were not taking medications known to affect BMR. Therefore, while this is the most reliable equation for a “healthy” person, keep in mind that the results of this study and the reliability of the equation cannot be applied to those with metabolic diseases, and those individuals can only have their BMR more accurately measured in laboratory, performed by a physician. (1)
This equation basically gives you a rough estimate of how many calories your body “burns” in a resting state, and accounts for about 70% of your total daily energy expenditure. Technically, taking your height, weight, age and gender into account, you would be able to maintain your weight while consuming this amount of calories each day, without any additional activity. It does not tell you how many calories to consume for weight loss purposes.
*insert calculator here: can we insert the actual calculator rather than the link to it?*
Most of us have seen this equation. Usually, when you use an online calculator to find your BMR, this is the equation you are actually plugging your information into. Like the equation listed above, this Harris-Benedict Equation is usually used to estimate BMR. The equations were published in 1919, and they have been commonly used for years. However, a study in 1990 found that this equation overestimated measured BMR by about 5% when compared to more precise indirect calorimetry. That could mean the difference of 100 extra calories for a calculated 2,000 calorie RMR.
Now that you have identified your BMR, you probably want to find out about how many calories you expend in an average day, rather than during an ultimate coach potato marathon. Our RMR calculator for the Mifflin St. Jeor equation has already crunched the numbers and done this for you! Again, this is an equation to help you understand how many calories you expend (“burn calories”) as you go about your day; it does not give you any information about weight loss. According to the equation, if you continue to eat the amount of calories given, you will maintain your current weight.
This is how we calculated your TEE: As a general rule of thumb, you can use your RMR calculation, and plug it into one of the following Harris- Benedict formulas corresponding with your activity level.
Now that you have an idea of how many calories need to eat in an average day just to maintain your weight, you probably want to use those numbers to help you achieve your weight loss goal.
We have all heard it: the “rule- of- thumb” for weight loss rules states that a total energy deficit of 500 calories per day, adding up to 3500 calories per week, is required to lose 1 pound of body weight. However, this calculation assumes that the 1 lb. loss is almost exclusively from fat.3,4 Researchers have found that muscle mass is also lost during weight loss, contributing to total pounds lost.5 Fat loss is not as simple as subtracting calories based off of a standard equation. Firstly, body composition and genetic makeup are two major contributing factors influencing rate and quality of weight loss. Additionally, not all calories are created equally, meaning that the quality of the calories also makes a difference in how much body fat will be lost. Finally, the methods used to achieve the calorie deficit are important. Basically, the 3500 calorie rule just doesn’t quite cut it. Next week, we will have an article on how to use this rule to best accomplish LONG- LASTING weight loss goals, dispelling common myths.
- Frankenfield, David et al. Comparison of Predictive Equations for Resting Metabolic Rate in Healthy Nonobese and Obese Adults: A Systematic Review. Journal of the American Dietetic Association. 2005.
- Roza A. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. The American Society for Clinical Nutrition, Inc. 1984.
- Ravussin E. et al. Determinants of 24-hour energy expenditure in man. Methods and results using a respiratory chamber. J Clin Invest. 1986.
- Zurlo F et al. Spontaneous physical activity and obesity: cross-sectional and longitudinal studies in Pima Indians. Am J Physiol. 1992.
- Levine J et al. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science 283. 1999.
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