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What is Metabolism?

Disclaimer: This content is for informational purposes only, and should not be taken as professional medical advice.

Metabolism is a word commonly thrown around. You may have heard someone say “I have a fast metabolism” or maybe you have made assumptions about someone’s metabolism based on how they look. However, metabolism is a complex body function that is variable between people and is not known by the shape of a person’s body.


So, what is metabolism?


The body’s metabolism is the total sum of energy the body needs for all chemical reactions to support bodily functions. Metabolism includes two main types of reactions: anabolic and catabolic reactions. Anabolic reactions are reactions that build something, such as muscle repair from amino acids. Catabolic reactions are reactions that break down something, such as digesting food. The metabolic processes of the body are coordinated by the hormones produced by the thyroid gland.


Metabolism is generally thought to have 3 components:

  • Resting Metabolic Rate: the amount of energy a person’s body uses lying in a semi-wakened state all day long. This makes up the largest portion of someone’s overall energy intake.

  • Thermic Effect of Food: the amount of energy a person’s body uses to break down the chemical structure of each macronutrient in food, leaving it in its most absorbable form.

  • Physical Activity: the amount of energy a person’s body uses doing spontaneous or planned movement. This is the most variable part of overall energy needs




Let’s talk a little more about the resting metabolic rate.

As mentioned above, the resting (or basal) metabolic rate is the amount of energy our body needs at rest. Resting metabolic rate is largely unchangeable and is influenced by:

  • Age

  • Gender

  • Race/Ethnicity

  • Height

  • Fat-free mass (organ tissue and skeletal muscle tissue)

While fat-free mass is possible to change, there tends to be genetic predispositions for the amount of fat free mass for a person’s body to function optimally.


What is my body doing at rest that needs nutrition?

So many things!!


Our body and therefore our metabolism is at work 24 hours a day – everyday. Our body needs nutrition to fuel our brain thinking and processing; lungs expanding and contracting during breathing; heart expanding and contracting during a regular heartbeat; the digestion and absorption of food in the gastrointestinal tract; muscle repair; bone repair; production of antibodies to maintain immune function; and so much more. Our body does all these things at rest when in a well-nourished state.


What about if I’m not as nourished?

A well-nourished person is one who eats a physically satisfying amount of food that meets the body’s metabolism and has all food groups represented. This allows the body to maintain a balance of all body systems, called homeostasis.


When someone is not as nourished that homeostasis is thrown off balance and the body strives to bring it back into balance. Let’s take a closer look at how the body tries to bring things back into balance.


Low caloric intake and low carbohydrate intake lead the body to conserve fat and start to use protein from organ tissue and skeletal muscle tissue. This protein gets broken down into amino acids and enter a process to create new glucose (called gluconeogenesis) for the body to use. As we discussed earlier, fat free mass is one of the predictors of resting metabolic rate so when our fat free mass starts to reduce our resting metabolic rate may lower. The longer this continues the more protein and eventually fat will be used, but it’s not without damage.


Medical Complications of Being Undernourished

You see, protein is critical to the structural integrity of organ tissue and skeletal muscle tissue. When your body is doing everything it can to try to support its energy needs by breaking down protein, there isn’t enough metabolic energy left to perform all the building up (anabolic) reactions. This is the root of many of the medical complications that we see in an undernourished person. These may include:

  • Brain fog

  • Fatigue

  • Low resting heart rate (bradycardia)

  • Blood pressure shifts when moving from lying to standing (orthostatic vital signs)

  • Low breathing (respiratory) rate

  • Slower digestion

  • Constipation

  • Kidney damage

  • Liver damage

  • Among others!


It’s important to also remember that undernutrition is not body specific. We see individuals in all body sizes that are undernourished and can have any of these medical complications.


Can I heal my metabolism?

Absolutely! Consistent meals and snacks and incorporating foods across all food groups goes a long way in helping heal the metabolism. It will not heal overnight though! Healing the metabolism takes consistency over time.


If you find that you need more help in your eating disorder recovery journey, please contact me for individual nutrition counseling.

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