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What is Metabolism?

You often hear people talk about metabolism…’my metabolism is slow’, ‘my metabolism is fast’, or ‘my metabolism isn’t what it used to be!”. Have you ever wondered what exactly metabolism is? Or why people may differ from one another? This blog is going to explore exactly what metabolism means and future blogs will look at myths and facts surrounding metabolism.

Let’s start off by getting a good understanding of metabolism. Metabolism refers to all the chemical processes or reactions going on continuously inside your body that allow life and normal functioning or homeostasis. In other words, metabolism is the body’s ability to utilize essential biomolecules or macromolecules (protein, carbohydrates, fats, and nucleic acids (DNA or RNA). We obtain these macromolecules through food, however the macromolecules in food may not be in the form that our bodies can use, so we digest the food to break it down to its smallest components…. carbohydrates breakdown to glucose, proteins to amino acids, fats to fatty acids, and nucleic acids to nucleotides and then we rebuild them back up into usable forms for the body.

These chemical processes or reactions are often linked together in pathways which can be divided into two categories: catabolic pathways - the breakdown of organic matter to produce energy; and anabolic pathways - the construction of molecules from smaller units. This figure demonstrates metabolism. You can see there is a lot of catabolism or breaking down before we can build it back up into something our bodies can use!

Catabolic Pathways

Catabolic pathways involve the breakdown of complex molecules into simpler ones…you can think of it as getting back to the building blocks, and typically this pathway releases energy. Catabolism provides the energy our bodies need for physical activity, from cellular processes to body movements. Examples of catabolism are glycolysis - the breakdown of glucose to ATP; beta oxidation - the breakdown of fatty acids to Acetyl CoA which then heads to the Kreb’s cycle to produce ATP; and proteins being broken down into amino acids.

Anabolic Pathways

Anabolic pathways build complex molecules from simpler ones and typically need an input of energy. An example of anabolism is gluconeogenesis. This is when the liver and kidneys produce glucose from non-carbohydrate sources. Other examples of anabolism include the growth and mineralization of bone and increases in muscle mass.

Catabolism Fuels Anabolism

When we eat, our body breaks down nutrients or catabolism – this releases energy, which is stored in molecules of adenosine triphosphate (ATP) in the body. We then use the ATP for building up the simple molecules into usable complex molecules. Another way to think of this …. catabolism creates energy that anabolism consumes in order for us to be able to do such work as synthesizing hormones, enzymes, sugars, and other substances for cell growth, reproduction, and tissue repair.

Metabolic Rate

One of the most straightforward ways to understand your metabolism is to look at it as the accumulation of all the energy your body expends to function. We will refer to this is our total daily energy expenditure (TDEE) or the sum of resting energy expenditure and non-resting energy expenditure.

Resting Energy Expenditure (REE) is defined as the energy required by the body in a resting condition and it typically accounts for the largest portion of total energy needs. It is also called Basal Metabolic Rate (BMR) or Resting Metabolic Rate (RMR). People tend to use BMR and RMR interchangeably but there is a difference. BMR is the minimal energy expenditure needed for survival, or the number of calories we burn in order for our body to perform minimal survival functions, whereas our RMR is the energy required to maintain bodily functions at rest. In general, RMR may be a better indicator of daily energy needs than BMR.

Non-Resting Energy Expenditure (NREE) includes:

Thermic Effect of Food (TEF): when you consume food, your body must expend energy to digest the food that you consume. It is the amount of energy used for your food to be broken down into simple molecules that can be rebuilt into useable molecules for the body. Studies reveal that TEF typically accounts for 10% of an individual total daily energy expenditure.

Non-Exercise Activity Thermogenesis (NEAT): This is the number of calories expended during daily activities that are not categorized as structured exercise. NEAT includes activities such as housecleaning, movement from one room to another, or taking the stairs to the office.

Thermic Effect of Activity (Exercise): This varies from one person to another and even from one day to another for the same person as the length and intensity of the workout, and exercise frequency are factors that impact an individual’s weekly thermic effect of activity.

From this figure, we can see that BMR/RMR accounts for the largest percentage of energy expenditure and activity or exercise accounts for the smallest percentage (unless you are an elite athlete or have a highly demanding job). Can you change your metabolism? Simple answer is yes…if we move more, either through NEAT or TEA, then we may expend more calories during the day. The more complex answer is yes, no, maybe? Stay tuned for our next blog as we delve deeper into metabolism!

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