Nutrition 101: Understanding Metabolism

Welcome to the second of five blogs, as part of the series: Nutrition 101!

Make sure to check back at taylorwhisman.com every Saturday for new articles.  A full list of the topics covered can be found here.  Each article is published in a specific order, so make sure that you don’t miss out!  Subscribe (on the right if you’re on a computer and at the bottom if you’re on a smartphone) to get notified of new posts and to support the spread of easy-to-digest (pun intended) health information.

Now that we understand what nutrition is and how important it is, let’s get down the nitty gritty details!  This post in the series is all about metabolism.

Metabolism is a simple concept that many like to overcomplicate.  Metabolism is defined as “chemical processes occurring within the body to convert food to energy.”  Although the concept is simple, the multitude of chemical processes are rather complicated.  We won’t get into the chemical details here!  My goal with today’s blog post is to explain metabolism in a simple, easy-to-understand way!  If anything becomes confusing or unclear, reach out to me! 

Energy Balance

It is imperative to understand that your metabolism either converts food to energy or uses energy to make stuff in your body, like various tissues.  Hench, there are two parts to your metabolism – catabolism, which means “breaking down,” and anabolism, which means “building up.” 

To understand how your body gets energy from food or vice versa, it is necessary to understand the basics of thermodynamics.  In short, the laws of thermodynamics are used to explain the energy balance of the entire universe.  If you’re a part of this universe, that means it applies to you and your body… no one can escape the laws of thermodynamics!  

Energy balance is a vital concept in the nutrition world.  Our bodies, and the processes occurring within it, are in a constant state of exchanging energy to maintain balance.  Biochemical pathways get turned on and off simply because there is an energy imbalance.  This has direct implications for weight maintenance, weight loss, and weight gain.  It is up to our body to balance out the energy equilibrium by either storing what it doesn’t use or using stores when it doesn’t have enough.

Our bodies use energy in the form of a molecule called adenosine triphosphate (ATP).  ATP is often called the “energy currency” of the body.  Your body can harvest ATP from the calories in the food and beverages we consume.  More on this next week!

Energy balance is often referred to as the “calories in, calories out” concept.  After all, calories come from food, and calories are just another form of energy!  Calories in, calories out is not another diet or pyramid scheme.  This is an easy way to conceptualize thermodynamics within our own bodies!  Energy intake must match energy expenditure, or else there will be a change in mass.  Counting calories, or being aware of how many calories you consume, is not dieting.  It’s just keeping track of how much energy you are inputting.  In the next blog post in this series, I will explain calories in more depth and why it’s important to at least have a general idea how many calories you consume each day.

“Calories In”

A Calorie is in fact a unit of measure for energy.  The capital C in Calorie matters for scientific purposes, but from here on out, I will just use a lowercase c.  Any food or beverages you consume that have calories in it and can be referred to as “calories in.”  Factors influencing how many calories we intake is based on a lot that I don’t have the time or expertise to cover here.  However, know that it’s not always “hungry = eat, full = stop eating.”  Your environment, your psychological state, your culture, and your activity levels can affect the calories-in part of the energy balance equation.  

“Calories Out”

This is where we can really dive into the concept of metabolism and how our bodies use energy. 

Each day, your body uses a certain amount of energy, which can be estimated by a number called the total daily energy expenditure (TDEE).  This includes many factors and variables like your resting metabolic rate (RMR), your age, sex, weight, and physical activity.  

Figure. This pie chart visually represents the components of TDEE. RMR/BMR is in blue, TEA is in red, and TEF is in green. These are not exact values, but estimations. Thanks TDEEcalculator.net for the image!

Let’s look at what factors affect the TDEE a little bit closer, starting with the RMR.  The RMR, which is oftentimes referred to as the basal metabolic rate, is how many calories your body burns while completely at rest.  This includes things like breathing, circulating blood, organ processes, and basic neurological operations. 

One of my favorite things to learn about the RMR is that it’s dependent on fat mass and fat-free mass, so you can alter your RMR by body composition modifications.  As shown in the figure, your RMR accounts for 60-75% of your TDEE.  

The other two components of TDEE are the thermic effect of activity (TEA) and the thermic effect of feeding (TEF).  These account for 15-30% and 10% of your TDEE, respectively.  TEA consists of all the activities that you do, including exercise.  TEF consists of the energy used by your body to digest food.  Ever feel tired after a big meal?  That’s your TEF at work.  However, TEA is clearly more influential on your TDEE than your TEF. 

Confused yet with all of these acronyms yet? Just keep in mind that your RMR is the most influential on your TDEE.  Meaning, instead of working out to “burn off” the extra calories, it’s best to just not eat them in the first place.  Another great option, and I know I’m biased here, is to build more muscle and reduce body fat.  Your RMR decreases proportionally to the amount of fat mass that you have on your body.

Putting it all together

How exactly does this information help us with nutrition?  Well, as stated above, the only way to shift our body mass up or down is to alter the energy balance!  It really does boil down to calories in, calories out.  You can use a combined effect of manipulating your nutrition and physical activity to make your specific energy balance work the way that you want!  You can’t always control how fast or slow your body changes, but one of the things that you can control is proper nutrition protocols and strategic activity.  

For instance, if I’m trying to lose weight, I would need to decrease my calories in.  I can also increase my calories out to enhance the effect!  For weight gain, it would be the opposite in each case.  In the case of maintaining weight, you would just maintain your current energy intake and daily activity regimen.  Starting to make sense?    

Before I conclude, there are a few things that I’d like to mention.

  1. You cannot out-exercise a bad diet.  This is due to the components of TDEE.  The RMR is influenced by body composition, which is influenced by diet.  The TEA contributes much less to the TDEE than the RMR; therefore, if you gain fat mass from increased calories in, you will not be able to burn enough calories to compensate for this.  Exercise simply isn’t that efficient at burning calories!  Additionally, apps and watches that tell you how many calories you burn suffer from large errors. 
  2. If you’re in a caloric deficit and you aren’t losing weight, you are not in starvation mode.  If you’re not familiar with this, this is a common term thrown around for why individuals can’t lose weight when on a reduced-calorie diet.  This is explained by the metabolism slowing down so much that when calories are reduced, weight is just maintained or gained and is impossible to lose.  If you’re the average person with no real medical conditions, then this is not happening to you.  You’re not losing weight because you’re missing calories somewhere.  Weekend calories still count.  Try being a bit more strategic in your approach. 
  3. If you’re eating a lot and still can’t gain weight, you’re not eating enough.  I find that many people think their metabolism is out of control and they simply can’t eat enough to keep up with it.  This is the exact opposite of the issue in number 2.  You are in fact not eating enough in this case.  Eating a bunch of junk food is not a muscle-building diet!  Try tracking your calorie intake more diligently.  

Next week, we will discuss calorie intake in more depth, along with macronutrients and how they are important.  Stay tuned!

What did you think of the article?  Let me know in the comments, or if you have any questions about this blog or how to take charge of your health and nutrition, you can email me at twhismancnc@gmail.com.


Goldberg, G. R., Prentice, A. M., Davies, H. L., & Murgatroyd, P. R. Overnight and basal metabolic rates in men and women. European Journal of Clinical Nutrition, 42(2), 137–144.
Kornberg, H. (2018). Metabolism. Retrieved from Encyclopedia Britanniahttps://www.britannica.com/science/adenosine-triphosphate
LibreTexts Project. (2018). 1st Law of Thermodynamics. Retrieved from https://chem.libretexts.org/Textbook Maps/Physical and Theoretical Chemistry Textbook Maps/Supplemental Modules (Physical and Theoretical Chemistry)/Thermodynamics/The Four Laws of Thermodynamics/First Law of Thermodynamics
Lumen Boundless Chemistry (2018). The Laws of Thermodynamics. Retrieved from https://courses.lumenlearning.com/boundless-chemistry/chapter/the-laws-of-thermodynamics/
Poehlman, E. T. (1989, Oct.). A review: exercise and its influence on resting energy metabolism in man. Medicine & Science in Sports & Exercise, 21(5), 515–525.
Ravn, A. M., Gregersen, N. T., Christensen, R., Rasmussen, L. G., Hels, O., Belza, A., Astrup, A. (2013). Thermic effect of a meal and appetite in adults: an individual participant data meta-analysis of meal-test trials. Food & Nutrition Research, 57, 10.3402/fnr.v57i0.19676. http://doi.org/10.3402/fnr.v57i0.19676
Szalay, J. (2015, Nov.). What Are Calories? Retrieved from Live Science https://www.livescience.com/52802-what-is-a-calorie.html 
University of Kansas Department of Chemistry. (2018). The Chemistry Behind Eating. Retrieved from https://chem.ku.edu/sites/chem.ku.edu/files/docs/CHEM190/hormones_eating.pdf
U. S. Department of Health and Human Services. (2000). Healthy People 2010. Retrieved from http://www.cdc.gov/nchs/healthy_people/hp2010.htm


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