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  • Maddy Belaustegui

Have you ever wondered if your metabolism is fast or slow? You can actually have it tested.

Have you ever heard from others that they have a fast or slow metabolism? Maybe you’ve heard it about yourself or said it about other people. But are metabolisms really that different and if they are different how do you know how fast your metabolism is? Here’s everything you need to know, including how you can have your metabolism measured

  1. What is metabolism? Metabolism refers to chemical reactions within the body that maintain life which includes breaking down molecules and building new molecules. When people say metabolism they are usually referring to the digestion and use of food as fuel for the body. When someone eats food, the body breaks down the food into its components which vary depending on the type of food eaten. These molecules are then used within the body to fuel activity like the movement of muscles or build new parts of the body like growing hair or nails. 

  2. Can metabolism be fast or slow? To understand this a person must first understand total daily energy expenditure (TDEE). TDEE is the total amount of energy a person needs throughout a day. This includes a person’s basal metabolic rate (BMR), which is the energy needed to simply continue living and functioning. This also includes nonexercise activity thermogenesis (NEAT) which is the energy needed for daily activities other than exercise such as walking around the house, getting up from a chair, or getting dressed. Lastly the thermic effect of food (TEF), energy required to break down the food eaten for additional energy, and exercise (EAT) are included as well. Each of these different components of TDEE account for different amounts of energy used throughout the day. BMR makes up about 80% of our daily energy expenditure, NEAT is about another 15%, TEF uses about another 10%, and the last 5% goes towards exercise.  Two people can have vastly different TDEEs. Oftentimes a fast metabolism is used to describe someone who has a low weight but eats a high volume of food and a slow metabolism is the opposite, someone with high weight who eats less food. There are two main possible reasons for the differences in these people. The difference between someone with a fast metabolism and someone with a slow metabolism may be a difference in TDEE. The person with “fast metabolism” may have more muscle mass which increases their BMR, may have high NEAT that is hard to notice, or any other number of circumstances that increase their TDEE. The second possible reason would be that this person may choose less energy dense food giving the impression that they eat a large amount in terms of calories because they eat a large amount in terms of volume. It’s also possible that this person may have a high TDEE while consuming high volume/ low energy foods at the same time. The second person who is perceived to have a “slow metabolism” may be in the opposite position. They might have a low TDEE while eating energy dense foods even if the volume is low. The high TDEE can be the result of a mixture of job type, genetics, muscle mass, and more. Energy dense foods can often have many more calories in a small volume than people think. Some energy dense foods include most fried foods, most fast food options, and ultra processed foods. It’s possible to feel like someone eats very little because the food looks small when in reality the food is densely packed. Energy dense food can be great for some and misleading for others.  These differences can lead us to believe that our body has chosen a specific metabolism speed and stuck to it. However, as discussed there are many more factors to one's metabolism making it appear fast or slow. While not all of these factors can be altered, there are many facts that can change. Visiting a dietitian can help you understand your TDEE and how to make adjustments to your specific diet to modify what people often think of as “fast or slow metabolism”. 

  3. How to test your metabolism.  Imagine the body as a furnace which requires something to burn in order to keep the flame going. If the body is the furnace then food is the fuel it needs. Similar to the way a fire produces smoke, the body produces gas, CO2, when it burns food for energy.  Indirect calorimetry measures these gasses, specifically oxygen and carbon dioxide. Here's how it works: A person breathes into a special machine. This machine then calculates the amount of oxygen you inhale and the amount of carbon dioxide you exhale. Why oxygen and carbon dioxide? Because the amount of oxygen your body uses and the amount of carbon dioxide it produces are directly related to how many calories you are burning. So, in simpler terms, indirect calorimetry is like a "gas analysis" for your body's calorie-burning process. It's a way to find out our TDEE, how many calories your body needs in a day.  Having your metabolism tested with an indirect calorimeter can be really useful for things like weight management, nutrition planning, or figuring out your energy needs if you're an athlete or have certain medical conditions. Indirect calorimetry is available at Desert Dietitians and the appointment only takes 30 minutes. You will need to come to your appointment without exercising that day and having fasted for 4 hours. The test only takes 10 minutes wearing a mask like this one. If you’d like to make an appointment simply visit today or call us at 702-485-7235 

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