Written by Lily Moorhouse
You may have heard metabolism described as ‘energy in vs energy out’, and whilst this isn’t completely untrue, metabolism is far more complicated than that…
What is metabolism?
Metabolism is the combination of all the chemical reactions within your body, which are essential for life. The processes involved in metabolism can be split into two categories; catabolism and anabolism. Catabolism is the process of conserving energy, with your body breaking down food molecules into simple molecules which releases energy for anabolism. Anabolism is the utilisation of energy to create the molecules that your body needs for growth and repair [1]. We also need energy for movement, alongside many other processes within our body.
Put simply, metabolism is the combination of all the reactions in the body which turn food into energy, and the reactions which use that energy. Metabolic rate is commonly interchanged with metabolism, however it has a distinct meaning, it is defined as the amount of energy your body requires to maintain metabolic processes within the body.
What makes up our metabolic rate?
There are four main ways in which our body uses energy. These add together to make your metabolic rate, which changes day to day based on your activity levels and natural fluctuations in energy needs. The metabolic rate is the energy you burn, and therefore how much energy your body requires you to eat to maintain these processes.
Basal Metabolic Rate (BMR)
The largest component of metabolic rate, which accounts for about 65-75% of our total metabolic rate. The energy required for BMR includes all basic bodily functions such as breathing, blood circulation, and breakdown and regeneration of cells. If you want to know more about the components of energy expenditure, you can read our blog about energy requirements and eating disorder recovery.
Physical Activity
Whilst you may think of movement as the main use of energy, it actually only makes up about 20% of your daily expenditure - 15% through non-exercise activity thermogenesis (NEAT) including things like standing up from your bed, moving your arm to pick something up, or walking to get somewhere [1]. Exercise activity thermogenesis (EAT) makes up about 5% of energy expenditure, although this varies depending on activity levels [2].
Thermic Effect of Food (TEF)
Approximately 10% of your energy is used to digest food - some of the energy stored from eating food is used by your body to digest your next meal! [2]
How do I know what my metabolic rate is?
Metabolic rates differ from person to person and change over time. BMR is influenced by many factors such as age, weight, sex, height, body composition and genetic factors. The energy required for physical activity and digestion however, vary a little more based on our activity levels and what and how much we've eaten, and how efficient our digestive system is at breaking down food into nutrients.
At TCN, we believe that unless you are joining scientific research or there is a need to have a deep understanding of your metabolic rate (for example high level athletes), then we don't really need to know the exact number. You can find equations online which estimate energy requirements, however they won't ever be accurate because of missing data such as genetics and body composition. And because our energy requirements change over time, you would need to be calculating this frequently over your lifetime.
The only way to more accurately determine metabolic rate is by using scientific equipment.
How is metabolic rate impacted by an eating disorder?
When you eat less than your body requires, your metabolism slows down as the body adapts to having less energy to use. A survival response is to reduce metabolic processes (such as heart rate, digestion, temperature regulation, hair growth) to conserve energy. Energy conservation means that you have less energy, reduced need for sleep, feel colder, experience headaches and dizziness and become more sensitive to light and noise [4]. We know that when people restrict their energy intake at a significant level, metabolic processes can reduce by up to 30%-50%.
However, when eating starts again, metabolism speeds up, and energy requirements can be higher than average. This means when you start eating an increased amount, your body increases energy requirements. This is because your body uses energy to repair any damage that has happened during periods of restriction [4]. You can read more about this in our blog about hypermetabolism.
If you need support in your recovery, we offer 1:1 support at the TCN clinic to help you overcome food rules and find food freedom. You can find out about more about how we can support you here.
Lily Moorhouse
TCN Team
Talia Cecchele Nutrition is a team of registered dietitians that specialise in eating disorder recovery and disordered eating. We aim to bring balance back to nutrition, help you to break free from food rules and find food freedom. We offer virtual consultations and group programs so whether you are based in London, the United Kingdom or around the world we would love to support you. To enquire about a private consultation please fill out a contact form.
REFERENCES:
[1] Ayesha Judge, Michael S. Dodd; Metabolism. Essays Biochem 8 October 2020; 64 (4): 607–647. doi: https://doi-org.nottingham.idm.oclc.org/10.1042/EBC20190041
[2] Todorovic, V.E., Micklewright, A. and British Dietetic Association. Parenteral And Enteral Nutrition Group (2018). A pocket guide to clinical nutrition. Birmingham: Parenteral And Enteral Nutrition Group Of The British Dietetic Association.
[3] Kalm, L.M. and Semba, R.D. (2005). They Starved So That Others Be Better Fed: Remembering Ancel Keys and the Minnesota Experiment. The Journal of Nutrition, [online] 135(6), pp.1347–1352. doi:https://doi.org/10.1093/jn/135.6.1347.
[4] The Effects of Starvation. (n.d.). Available at: https://insideoutinstitute.org.au/assets/the%20effects%20of%20starvation.pdf
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