The word metabolism, as it pertains to our physiology, refers to all the ways our body utilizes energy (i.e. calories) to maintain life.
You likely have heard the term “metabolism” thrown around in society as it pertains to weight loss. Our diet and weight-loss obsessed culture has created a whole industry for companies to create meal plans, exercise plans, nutrition supplements and more based off this idea of metabolism. These are all marketing ploys to make you feel like you can speed up your metabolism and lose weight with just a change of your diet or a new exercise regime.
However, your metabolism isn’t that cut and dry. It’s not as simple as eating fewer calories than you use during exercise, despite what you may have heard or seen.
Losing weight is not as simple as eating fewer calories than you use during exercise.
The gold standard for measuring one’s metabolism would be to use a Metabolic Chamber. A metabolic chamber is a sealed room in which a subject would spend time in. The air that flows in and out of the sealed chamber is carefully monitored. The subject’s urine is also collected from their time in the metabolic chamber – this allows scientists to measure protein use. Between the air flow in, the air flow out, and the urine output, scientists can determine how much energy their subject in the metabolic chamber is using. You can read more about a metabolic chamber here. Metabolic chambers are not only massive equipment, they are also very costly. Despite them being the gold standard for measuring your metabolism, they aren’t very practical.
Other ways to estimate your metabolism are various calculations. You may have found a metabolism calculator online or have noticed one on your fitness watch. These equations usually ask you to input some basic information about yourself such as age, height, and weight. From some very basic information, they estimate your metabolic rate (how many calories your body uses on average). For some individuals, these calculations may be fairly accurate, but it is not uncommon for them to be significantly off.
One reason why these calculations can be significantly off is because they do not consider all aspects of someone’s metabolism. There are three main factors that contribute to our total energy expenditure:
Basal Metabolic Rate
Basal Metabolic Rate (BMR), a.k.a. Resting Energy Expenditure (REE), is the energy our body uses while at rest. Your body is constantly using energy even while you’re sleeping. Your heart beating, lungs inhaling and exhaling, veins constricting and dilating all require energy. In fact, the energy your body uses at rest accounts for 60-80% of the total energy your body uses!
For the most part, you don’t get much of a say in how much energy your BMR uses. This part of your metabolism is strongly influenced by genetics. However, studies show that some long-term behaviors can impact BMR. Most notably, yo-yo dieting (going on and off diets over a period of years) can slow your metabolism by negatively impacting your Basal Metabolic Rate.
If you are someone who has been on and off diets for years it may be helpful to think about BMR this way: it takes long-term behaviors to damage your BMR and it will take long-term behaviors to heal it. But healing is possible.
Next, we can consider the ways your body uses energy when you’re active. All activities above and beyond rest fall into this category. Activity factor can be further divided into the categories: sedentary activity and physical activity. Sedentary activities may be things like using your computer, reading a book, or sitting and chatting with friends. Physical activity includes not only the obvious things like going for a run or playing a sport, but also your activities of daily living. Cleaning your house, doing yard work, and folding laundry are all physical activities that contribute to the amount of energy your body uses.
Activity factor accounts for around 10-30% of the total energy your body uses. This can vary significantly depending on the individual. An athlete will burn more energy through physical activity than someone who may have mobility issues and are not able to be as active, for example. Activity factor can also vary from day to day. Imagine you go hiking one day and the next day you take a rest day, on the day of the hike you utilize a greater amount of energy being physically active than the next day when you decide to rest (sidenote: rest days are still necessary even though they may not cause as much energy expenditure!).
Thermic Effect of Food
Lastly, we must consider the energy that is used by digesting energy. Yes, the process of eating and digesting our food and converting it to usable energy requires energy! Thermic effect of food describes the process of our bodies heating up to metabolize the food we ingest and accounts for around 10% of the total energy our body uses (again, this varies from person to person). This facet of your metabolism will give us insight as to why diets do not work. Let’s look at the equation below:
Metabolism = Basal Metabolic Rate + Activity Factor + Thermic Effect of Food
If you go on a diet and reduce your food intake, the amount of energy your body needs to use to digest the reduced food intake is lowered. When Thermic Effect of Food is decreased, that results in your metabolism (i.e. the amount of energy your body is using) being decreased. When we eat less food, we use less energy.
On the flip side, when we eat more food, our Thermic Effect of Food increases and we use more energy to digest the increased amount of food. This is why you don’t gain erroneous amounts of weight after eating larger than usual amounts of foods on Thanksgiving Day – your metabolism increases to process the increased amount of food. There is a tipping point with this, however. Eventually, if you are consistently eating significantly more food than your body is using, over time you will gain weight. This is why some individuals requiring weight restoration as part of their recovery from an eating disorder will require a higher calorie diet for a period of time.
Stay tuned for Your Metabolism Explained: Part 2 to learn about more factors that influence your metabolism.