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Your Metabolism Explained

Updated: Aug 26, 2023

Build your metabolism

Broken metabolism

Boost your metabolism

Improves your metabolism

You've heard all of these before I'm sure but what does 'your metabolism" even mean??

Your metabolism is composed of cells, cellular components and the reactions carried out by those cells that takes place in the body to create energy from the food and liquids we consume.

There are many metabolic pathways that occur in the body (if you want a full list, click here).

Here are the main ones:

  • Aerobic metabolic state is less of a metabolic process and more of a state that occurs when the body has enough oxygen to fuel metabolic reactions for energy production. Also called aerobic respiration, cell respiration, and oxidative metabolism. Activities that fuel this metabolic state include walking and other low intensity workouts or movements. The primary metabolic pathway in this state is glycolysis followed by the Krebs cycle and electron transport chain (explained below).

    • NADH and FADH2 are electron carrier molecules, meaning they are carrying hydrogen ions about the cell and play a role in energy production. When they aren't carrying hydrogens they are NAD+ and FAD+.

  • Anaerobic metabolic state again, is less of a specific metabolic process and more or a state that occurs when the body does not have enough oxygen to fuel aerobic metabolism. Generally, in aerobic respiration, the glycolysis pathway (explained below) takes off per usual in the muscles but then gets stopped right before the substrates enter the Krebs cycle due to the lack of oxygen. Pyruvate (the last substrate in glycolysis) gets converted into lactate and is sent to the liver to be processed into usable energy. This is the process that everyone refers to as "lactic acid build-up". (Check out the blog post I made all about Lactic acid build up for more info). Activities that fuel this metabolic state include HIIT training, weight lifting and high intensity workouts or movements.

  • Glycolysis: basic glucose metabolism which also includes the Krebs cycle (AKA TCA or Citric acid cycle) and the electron transport chain. Initially, this pathway uses energy to get going but then generates 36 ATP (that's a lot of energy), CO2, and water while using oxygen (hence aerobic).

    • Vitamins and minerals crucial for the functionality of this pathway: B6 (pyridoxine), B3 (niacin), B1 (thiamine), B5 (pantothenic acid) and B2 (riboflavin), manganese, magnesium, potassium, as well as coenzyme-Q.

    • There are specific pathways for the breakdown of fructose, galactose, mannose and sucrose (other forms of ingested sugar/carbs) but I'm not reviewing those here.

  • Gluconeogenesis: essentially this is the reverse pathway of glycolysis; it's an energy pathway that uses glycerol from fat, lactate from anaerobic metabolism, pyruvate (a product made from the breakdown of fats, carbs and proteins), and specific amino acids to make glucose when carbohydrates are not ingested and readily available.

    • Vitamins crucial for this pathway: B7 (biotin), B1, B6 and B3.

    • The hormones insulin and glucagon stimulate this pathway.

    • This is one of the primary pathways utilized when the body doesn't have enough fuel sources from incoming glucose/carbohydrates for energy production. The body will pull from non-carb sources to get energy. This is not ideal when the body begins to pull energy from amino acids which are proteins in the form of skeletal muscle.

  • Glycogenolysis: breaking down stored glycogen using it to make glucose to enter the glycolysis pathway. This is the primary pathway used when for energy when the body does not have carbs readily ingested and available. Glycogen is stored in the muscles and liver.

    • Liver glycogen is there to maintain blood glucose levels, while skeletal muscle glycogen is used during high-intensity exertion, and brain glycogen is an emergency energy source.

    • Vitamins crucial for this pathway: B6, B3, B1

    • Fun Facts: for every 1g of glycogen stored, 3g of water is also stored. Liver glycogen can be depleted during exercise in 30 minutes depending on the intensity of the workout.

  • Lipolysis/Beta-Oxidation: break down of fat molecules AKA triglycerides into fatty acids and glycerol. The fatty acids are then oxidized into Acetyl-CoA which can enter the Krebs cycle for energy production (AKA burning fat).

    • magnesium, B5, B3 and B2 are important in breaking down long chain fatty acids for form Acetyl-CoA for energy production.

  • Lipogenesis: (what people typically hate) the formation of fat that takes place in the liver and adipose tissue. If you ingest too many calories and the body does not need to use this energy in an immediate way or has the storage availability and capacity for it, the end product of other metabolic pathways, Acetyl-CoA, is converted into triglycerides.

    • Vitamins crucial for this pathway: B3, B5, B2

  • Adipogenesis: the maturation of fat cells.

  • Ketogenesis: production of ketone bodies from stored fatty acids in the body in the absence of glucose.

  • Ketosis: a metabolic state, not really a process, in which the body is using circulating fatty-acid derived ketones for energy instead of circulating glucose.

  • Cholesterol Biosynthesis: the formation of cholesterol (because yes, our bodies need cholesterol) from Acetyl-CoA.

  • Alcohol metabolism: the process of breaking down ethanol into acetaldehyde (an extremely toxic byproduct) further into acetate and further into Acetyl-CoA which can be used for energy production.

It's important to note that our bodies have metabolic pathways for the breakdown of nucleotides (DNA and RNA fragments), neurotransmitters, various peptide hormones like melatonin, insulin, and adrenaline, as well as various proteins and amino acids that need to be degraded and and excreted. OUR BODIES ARE SO COOL.

Summary of some of the above pathways:

Let's move on to the numbers...

As you can see, for all the break down and synthesis processes to function well, our metabolism requires a lot of energy, vitamins and minerals to work appropriately.

Let's look at some definitions of commonly used terms when referring to our metabolism or metabolic rate:

  • Total Daily Energy Expenditure (TDEE): the total amount of calories you burn on daily basis that can be determined via indirect calorimetry which "is the method by which measurements of respiratory gas exchange (oxygen consumption (V O2) and carbon dioxide production, (V CO2) ) are used to estimate the type and amount of substrate oxidized and the amount of energy produced by biological oxidation." (source). You can get this done where they also do DEXA scans.

  • Basal Metabolic Rate (BMR): the calories burned by the body to perform life sustaining internal processes. Basically, what you burn all day if you just laid in bed and didn't move. This makes up roughly 60-70% of your TDEE.

  • Resting Metabolic Rate (RMR): the same thing as BMR.

  • Resting Energy Expenditure (REE): the same things as BMR/RMR.

  • Non-exercise Activity Thermogenesis (NEAT): the calories burned doing basic movements like walking to the bathroom, typing, fidgeting, etc. This makes up roughly 10-15% of your TDEE.

  • Thermic Effect of Food (TEF): the calories burned during the breakdown/metabolic processes of food consumed. Protein has the highest thermic effect which theoretically means you're body burns more calories to consume protein than it does carbs and fats. This makes up roughly 10% of your TDEE.

  • Exercise Activity Thermogenesis (EAT): The calories burned performing intentional movement/workouts. This makes up only 5% of your TDEE!

  • Non-resting Energy Expenditure (NREE): comprises 30% of your TDEE.

One I commonly use in the clinical setting is Estimated Energy Requirements (EER) which is the amount of calories needed daily based on a persons BMR, activity level/factor (generally 1.2-1.3 unless highly active), anthropometric data and past medical history. I also commonly calculate protein, carbohydrates and fat needs which is what a nutrition coach does when they calculate someone's macros.

I know this was science heavy but I hope it was broken down well into simple terms that also gave you good insight into the metabolic processes of the body! This also shows how important it to properly fuel you body with balanced meals of protein, carbs and fats as well as micronutrients in order to keep all of these pathways functioning optimally!

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