Our bodies undergo millions of chemical reactions throughout the day: building, breaking down and transporting molecules in an attempt to maintain balance, or homeostasis. Many of these reactions require energy. Most of that energy comes from our cells’ energy centers, called the mitochondria. The total of all of these chemical reactions and the energy requirement are referred to as metabolism. This energetic requirement over the course of a single day is referred to as our metabolic rate.
Mitochondria, or those energy centers, need oxygen to break down the foods we eat. That produces a form of energy called ATP. Carbon dioxide, water and heat also get broken down. A simple breakdown of this equation looks like this:
Food + Oxygen → Energy + Carbon Dioxide + Water + Heat
We can measure our body’s metabolic rate in two ways: how much heat our body produces or how much oxygen our body uses. Measuring heat production is complex and is represented as calories with a lowercase “c.” This familiar unit scientifically refers to the amount of energy required to raise 1 gram of water by 1 degree C. However, this is different than the Calorie with a capital “C” that we see associated with packaged food. One Calorie is actually equal to 1,000 calories (1 kCal). Measuring how much oxygen we use looks at how much we take in versus how much we breathe out. This far simpler measurement is presented as liters of oxygen consumed per minute.
Many factors can affect metabolic rate, including hormones, body temperature, our body size and composition, age, genetics and, most importantly, how active we are. Our metabolic rate skyrockets during exercise, and the effects of a single bout of exercise can last up to 48 hours!
However, even if we were to remain completely still for an entire day, the average adult would still burn around 1,200 to 1,600 kCals. This is due to the continuous activity of systems such as our brain, heart and muscles that don’t just turn off when the body is idling.
Measuring our resting metabolic rate (also called basal metabolic rate) can give doctors a glimpse at how our bodies are working. And, it helps them decide if they need to take a closer look.
Just like a well-tuned race car, the best ways to keep our engine running smoothly is giving it appropriate nutrition and taking frequent laps around the race track!
Ravi Kumar is a PhD candidate in exercise physiology at the University of Florida. He is an avid runner and sports enthusiast, and his research interests include skeletal muscle and mitochondrial biology and how these systems adapt to exercise, aging and chronic heart failure.