What is Metabolism?

All living things require ongoing inputs of energy to continue living. This means that .

Metabolism is the process that controls this transformation. It includes all of the chemical reactions in your body’s cells that convert the energy in your food into energy that your body can burn as fuel to stay alive. Energy is measured in calories. [1] [2]

The process of metabolism balances two separate elements:

Anabolism

  • Builds up body tissues (in times of growth, healing or repair)
  • Energy stores as backup ‘fuel depots’

Catabolism

  • Breaks down body tissues and energy stores to get the necessary fuel to support body functions

[3]

When your metabolism balance is stable you will receive enough energy to support your body’s functions, including breathing, walking, and digesting (among many other functions) without having to store any extra energy as fat. [4]

How Does the Process of Metabolism Work?

There are three components that determine how many calories, or how much energy, your body needs every day:

Basal Metabolic Rate

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Your basal metabolic rate

Your basal metabolic rate is the amount of energy, or calories, needed by the body when it is resting. You aren’t consciously aware of the activity happening deep in your body, but even at rest your body needs to burn calories to breathe, keep your heart beating, circulate oxygen and nutrients throughout your body and grow new cells.

Basal metabolic rate is mainly determined by muscle mass. People with more muscle will burn more calories when they are at rest. As people age, they tend to lose muscle mass changing the proportion of muscle to fat.

Energy Used to Break Down Food

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The energy used to break down your food (dietary thermogenesis)

Energy is needed to break down food because food must be digested, absorbed, moved around the body and, if the energy is not used by the cells immediately, stored as fat.

Physical Activity Level

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Your levels of physical activity

Physical activity requires energy to burn as fuel. This activity can include anything such as exercising. Daily activity that isn’t exercise includes walking around, gardening, fidgeting, and grocery shopping. [1]

Changing your metabolism

In adults that are not very active, the basal metabolic rate accounts for 60% to 70% of total energy used, the food breakdown accounts for about 10%, and physical activity for the remaining 20% to 30%.

The only part of metabolism that is under direct control is physical activity. If activity levels are increased, this will increase the rate at which the basal metabolism works to burn energy.

So, with movement and physical activity, not only does the amount of energy burned increase directly as a result of the activity, but it also increases the rate at which your body burns energy while resting. [5]

One MET, or metabolic equivalent, is defined as the amount of oxygen consumed while at rest and it is used as a proxy for resting metabolic rate. Multiplying the MET by minutes of that activity can express how much energy is used during physical activity.

Activity in MET

In the US, the recommended minimal level of physical activity for health benefits is 500 MET-min/week.

Moderate Intensity
(Noticeable increase in breathing & heart rate)

  • Brisk walking ( 5 km/hr) ≈ 3.3 MET
    X 150 minutes = 500 MET-min/week
    (2.5 hours total, or about 20 minutes per day)
Vigorous Activity
(Large increase in breathing & heart rate)
  • Jogging (8 km/hr) ≈ 6 MET
    X 85 minutes = 500 MET-min/week
    (~1.4 hours total, or 20 minutes 4x per week)
More examples of METs for your favourite activities: here, here and here

Metabolic homeostasis

To stay alive and healthy, each cell and tissue must coordinate with all of the other cells and tissues in the body. When this coordination works you will be in a state of homeostasis, where there is balance among all the body systems. Homeostasis ensures that the body works properly. [2]

For example, humans require certain conditions, like the body’s core temperature, to stay within certain limits if they are to survive. When the body’s temperature drops below a threshold, the body needs to self-regulate and bring the temperature up in order to survive. This self-regulation is called metabolic homeostasis, which will cause your muscles to shiver, burning fuel but releasing heat and keeping the body’s temperature up. [4]

Metabolic syndrome

Metabolic dysfunction can be at the root of two key health indicators – inflammation and insulin resistance. Inflammation is the body’s response to insults or injuries. Insulin resistance lowers the sensitivity of your body to insulin, meaning you need larger amounts of insulin to lower your blood glucose levels compared to someone with high insulin sensitivity.

Metabolic syndrome is not a single disorder, rather it is a group of conditions that occur together, including: increased blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol and triglyceride levels. A diagnosis of metabolic syndrome requires that a patient has three of these conditions.

It is estimated that almost 20% – 30% of the middle-aged population (40s and 50s) are affected by this syndrome. Most people are unaware of their condition because it is ‘silent’ or impossible to notice without testing, except for the belly weight which is visible externally.
[7] [8]

Metabolic syndrome greatly increases your risks of experiencing cardiovascular disease (CVD), chronic kidney disease, fatty liver disease, and type 2 diabetes. These chronic illnesses account for 43% of all deaths in Canada.

Each of these diseases and conditions happens at a significantly higher rate in patients with metabolic syndrome. For example, the presence of diabetes is estimated to be 2.5 times higher in those with metabolic syndrome compared to those without (18.0% vs. 7.1%). The ten-year risk of a fatal CVD event is five times higher (4.1% vs. 0.8%) [8]

References

[1] https://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/metabolism/art-20046508

[2] https://my.clevelandclinic.org/health/body/21893-metabolism

[3] Wilson DF, Matschinsky FM. Metabolic Homeostasis in Life as We Know It: Its Origin and Thermodynamic Basis. Front Physiol. 2021 Apr 23;12:658997. doi: 10.3389/fphys.2021.658997. PMID: 33967829; PMCID: PMC8104125.

[4] Woods SC, Ramsay DS. Food intake, metabolism and homeostasis. Physiol Behav. 2011 Jul 25;104(1):4-7. doi: 10.1016/j.physbeh.2011.04.026. Epub 2011 Apr 28. PMID: 21530564; PMCID: PMC4422051.

[5] Leeners, B. et al. (2017) Ovarian hormones and obesity.Hum Reprod Update. 2017 May; 23(3): 300–321. doi: 10.1093/humupd/dmw045

[6] Chi, H. Immunometabolism at the intersection of metabolic signaling, cell fate, and systems immunology. Cell Mol Immunol 19, 299–302 (2022). https://doi.org/10.1038/s41423-022-00840-x

[7] https://www.mayoclinic.org/diseases-conditions/metabolic-syndrome/symptoms-causes/syc-20351916

[8] https://www.metabolicsyndromecanada.ca/

Original content, last updated February 3, 2025.
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