Making Sense of Metabolism


By O2X Nutrition  Specialist Andrea Givens, MS, RD, CSSD

Are calories from wearable devices accurate? Does muscle tissue burn more than fat? Why the hype around METCONs?

Metabolism is a complex topic, however, this article is not a lesson on Kreb’s cycle. You don’t have to become an expert in biochemistry to understand metabolism basics that will help inform your daily habits for health and performance.

Metabolism encompasses all the processes in your body that turn the food you eat into usable energy. Proteins, carbohydrates, fats in food supply energy, or calories, are the units used for energy. Everything you do requires energy, including basic physiologic functions such as your heart pumping blood, nerve transmission, physical movement, and even digesting food. Yes, it takes energy to make energy, which is called the thermic effect of food. More on that later.

The metabolic cost or how much energy is required daily to be a living, breathing human is called resting metabolic rate (RMR). You may have RMR testing offered in your area at a local sports or wellness clinic, where it measures gas exchange by having you breathe into a device. Alternatively, you can obtain RMR from a body composition assessment such as an InBody which estimates it based on your lean body mass.

RMR can be estimated through predictive equations using online calculators that ask for basic demographics including sex, age, height, and weight. We simplify this equation for you below, so no need to navigate away from this article to Google a RMR calculator. Since RMR is the amount of energy needed just to be alive, you should never eat less than your RMR for a sustained length of time. If you are chronically under eating, your body will conserve energy, or lower your metabolic rate, to ensure it has enough energy for vital organ function. This leaves minimal energy for everything else, resulting in fatigue, irritability, poor sleep, bad mood, and impaired performance. 

So how much should you be eating? Once RMR is determined, it is multiplied by an activity factor which gives you a total daily energy expenditure (TDEE). TDEE accounts for RMR plus exercise, activities of daily living, and the thermic effect of food. Energy balance (calories in – calories out) determines body weight. Eat less than your TDEE to lose weight, and more to gain weight. See the simplified version below from the O2X Human Performance for Tactical Athletes book:

It only takes a slight reduction, about 20%, of the calories you consume daily to gradually lose 1-2 lbs per week. This would be 500 calories a day for a 2400-calorie diet. Reducing 500 calories may be as simple as switching from sugar-sweetened beverages to calorie-free drinks like sparkling water, or drinking less alcohol. To gain 0.5-1.0 lbs per week, this requires strength training (see O2X App for training programs) as well as eating in a 500+ calorie surplus daily. Body recomposition, or losing fat while building muscle can be possible by following a well-programmed strength training plan, moving 8k+ steps a day, and being intentional about your nutrition: eat enough but not too much, ensure colorful vegetables and fruits on your plate as much as possible, get ~30-40 grams of protein per meal, and limit added sugar and alcohol. Remember the thermic effect of food? Protein has the highest TEF of all the macronutrients, so not only is it satiating, you burn calories by consuming it. Even though eating protein burns calories, this accounts for around only 10% of TDEE. 

Energy balance can also be affected by burning more calories daily (through movement, not steak). Moving more through increasing daily step count and exercise is obvious, but you can increase your RMR by building more muscle. Muscle tissue has a higher metabolic cost than fat, it takes a lot of energy to build and run your muscle machinery: make enzymes, mitochondria, and all other parts of muscle cells. Muscle is so energy expensive, the saying “if you don’t use it, you lose it” is true. If muscles are not being used, your body is going to cut the cost from the budget. Your workout routine should include both strength training to build and maintain muscle, as well as a conditioning program that includes training all energy systems—explosive, high-intensity anaerobic, and low-intensity aerobic. These workouts, appropriately nicknamed “metcon” for metabolic conditioning, are not just for burning calories. They will improve your metabolic efficiency, which positively impacts your health and all functional areas of your life.

What about calories from wearable devices? Consumer devices aren’t held to the same standards as medical devices, so the accuracy of estimated energy expenditure can range widely because each company uses its own predictive equation algorithm to estimate calories based on heart rate. While the exact number is not something to live and die by, the data can be useful to monitor trends, and help guide your performance plate construction—higher energy expenditure days are fueled by a larger portion of your plate made up of carbohydrate-rich foods. 

For wearable device accuracy, make sure your demographic information in the app settings is correct. The software uses algorithms that include weight, age, and gender for the calorie equations, so take the time to input your information. Next, the wearable should be worn correctly. For wrist-worn devices that measure heart rate via photoplethysmography, they should be worn above the wrist bone. Be mindful that tattoos may impact accuracy because the ink can block light from the sensor technology that takes the heart rate measurement. 

About O2X Nutrition Specialist Andrea Givens:

Andrea Givens, MS, RD, CSSD, is an accomplished O2X Nutrition Specialist with extensive experience as a Registered Dietitian and a Board-Certified Specialist in Sports Dietetics. She completed her education at San Diego State University and gained expertise in clinical dietetics, working with veterans at the VA. Transitioning into sports nutrition, Andrea has worked with a wide range of athletes, including collegiate athletes, professional baseball players, and Olympic qualifying teams. With concurrent master’s degrees in Nutrition and Exercise Physiology, she conducted research on optimizing performance for military personnel. Currently, Andrea provides nutrition expertise to O2X tactical athletes nationwide and conducts research in the Warfighter Performance Department of the Naval Health Research Center. She is actively involved in professional organizations and is an avid outdoor athlete with a passion for challenging pursuits.

About O2X Human Performance:

O2X Human Performance provides comprehensive, science-backed programs to hundreds of public safety departments, federal agencies, and the military. O2X works with clients to elevate culture, improve mental and physical well-being, support healthy lifestyles, and reduce healthcare costs associated with injuries and illnesses. Driven by results and cutting-edge research, O2X programs are designed and delivered by a team of Special Operations veterans, high-level athletes, and hundreds of leading experts in their respective fields of human performance.