Metabolism

What is metabolism? The word metabolism is thrown around all the time, but what does it really mean? What is your metabolism? Is there a spot in your body that is your metabolism? Is your metabolism a body system that consists of certain organs like the endocrine system or the digestive system?

According to Mirriam-Webster’s Dictionary, metabolism is the chemical process by which a plant or animal uses food, water, etc., to grow and heal and to make energy. Dictionary.com puts it a little simpler saying that metabolism is the chemical processes that occur within a living organism in order to maintain life. If that still doesn’t make sense, metabolism is basically a process of taking food, making energy and your body using that energy. So, no, there is no specific area in your body that houses your metabolism, your whole body is involved as your whole body is comprised of cells where this metabolic process happens!

There are many calculations us dietitians use to determine someone’s energy needs and they all have various factors in them to individualize the needs for that one specific person. The most basic calculation is the basal metabolic rate (BMR) otherwise known as resting energy expenditure (REE). This equation determines how much energy a body will use just for basic survival, energy to keep your heart beating, your lungs breathing, and your organs functioning; the energy you’d need to maintain life if you’re in a coma, basically.

In order to find your daily caloric needs you’ll need to multiply your BMR by an activity factor and a stress factor. For example—Kathy is an Olympic athlete and exercises rigorously for 3 hours/day—she’ll need a high activity factor to compensate for the calories she’s burning while exercising. However, Robert is an accountant and sits at his desk all day then watches TV when he gets home from work, he doesn’t go to the gym and doesn’t like much activity outside—Robert will need a lower activity factor as he is more sedentary. Stress factor can include emotional stress as well as physical stress (injury) but in practice we usually focus on physical stress/injuries. It makes perfect sense that the more injured you are, the more energy your body will need to heal you—burns = high stress, fell over at the bar drunk and have a little bruise on your elbow = low stress.

Have you ever heard someone say that they have a “high metabolism” or a “slow metabolism”? Metabolism is known to slow as we age, but there are other factors that can slow down or speed up someone’s metabolism. When energy intake is less than a person’s BMR, metabolism will slow in order to reserve energy in the body, makes sense right? If your flashlight is running out of battery it’s light is much dimmer than it would be with a new battery. If someone is in starvation mode and has lost a significant amount of weight, then starts eating again, their body may go into a hypermetabolic state where their metabolism increases quickly to account for the caloric increase—this is NOT a good way to try to increase your metabolism. In addition, still somewhat controversial, most research points to resistance exercise as a way to increase your metabolism. How does this work? Same way that aging slows the metabolism—lean body mass requires a lot of energy to function, the more lean body mass you have, the higher your metabolic rate (in a perfect situation). There are other contributing factors, like body fat percentage, fat free mass, etc. but it makes sense that as we age, we lose lean body mass and therefore our metabolism slows. As we build lean body mass through resistance training our metabolism increases.

Another thing to take into consideration is the Set Point Theory. The Set Point Theory states that each person’s body has a pre-determined set weight that it wants to be at no matter what eating habits, exercise habits, lifestyle choices, etc. someone may engage in. So screw everything you just read, your body is going to be where it wants to be.

I believe there is some validity in the set point theory, I’ve read research and watched a documentary on a man who runs ½ marathons weekly and still has abdominal adiposity and a BMI classified as overweight. Do I believe that external factors can influence our weight/metabolism—much research has pointed to yes. The best thing I can say is just trust your body, listen to your hunger/fullness cues and let your body guide you, not the other way around, your metabolism will fall where it needs to be. If you’re wanting to lose weight, do it in an informed and healthy way—talk with a healthcare provider (a registered dietitian or a PCP) and be kind to your body. The only way I believe someone can be a centenarian is if they honor their body, treat it well, and follow its guidance.