Parts of Metabolism

Metabolism is a word that is thrown around haphazardly and ignorantly. In this article, we will discuss what metabolism is, what it is made up of, how strong of an impact each facet has, as well as if it is possible to manipulate metabolism upwards or downwards.

What makes up metabolism?

To pull ourselves out of our ignorant state, we have to first understand what makes up metabolism. Metabolism is broken up into various components between basal metabolic rate (BMR), thermic effect of physical activity (TEPA), and thermic effect of food (TEF) [1]. Metabolism is broken down into these components, because these components encompass all related to energy use.
 

Basal Metabolic Rate

This term is sometimes interchangeably used with resting metabolic rate (RMR), and although they are slightly different, the difference is small (<10%)[2]. Basal metabolic rate is the energy needed to stay alive at a state of complete inactivity (lying down)[3]. This encompasses the energy your brain uses, your other organs use, your fat free mass (muscle, bones, etc.), as well as your fat mass – all when you are awake, but immobile and calm.

Thermic Effect of Physical Activity

The thermic effect of activity is the energy cost of sum physical activity coming from exercise activity thermogenesis and non-exercise activity thermogenesis (NEAT)[10]. This is due to the fact that we need to use energy to fuel movement.

Thermic Effect of Food 

The thermic effect of food is aptly described as the energy needed to break down various foods through digestion, their transport, and storage within the body [1][4]. Breaking foods down, like polysaccharides into absorbable monosaccharides, requires energy to power enzymatic reaction.

How much of each makes up metabolism?

Not only is it interesting and useful to know what makes up our metabolism, but to understand how we can change our metabolic capacity, we need to understand how much impact each of the three facets of metabolism have.

Basal Metabolic Rate

Basal metabolic rate is made up of various sections as it plays the biggest factor in total metabolism. Running between 50-70% of total metabolism, basal metabolic rate is broken up between organ energy use, fat free mass energy use, and fat mass energy use [1][5]. However, the majority components of basal metabolic rate, sitting at 60-70% of BMR, are the organs with the liver being most metabolically active [6][7][8].

Now, while lean mass makes up over 50% of body weight, it accounts for a little over 30% of basal metabolic rate [6]. On the other hand, fat mass accounts for the remaining 5-10% of basal metabolic activity [6]. This is the case, because muscle tissue uses 4.5 – 7.0 calories/lb/day while fat tissue uses 2.0 – 4.0 calories/lb/day [1][9].

So, in review:

BMR accounts for 50-70% of total metabolism.
Organs accounts for 60-70% of BMR.
Muscle accounts for 20-30% BMR.
Fat accounts for 5-10% BMR.

Values may be different based on if the measure takes into account smooth muscle and/or just skeletal muscle.

Thermic Effect of Physical Activity

Now that we have accounted for all metabolism in a completely rested state, we must also include physical activity, planned and unplanned. The thermic effect of physical activity accounts for 15-30% of total metabolic energy use [1][5]. Although it would be convenient to know how much of that 15-30% is partitioned between exercise and non-exercise thermogenesis, there is not enough data on the topic to appropriately segregate the two – hopefully in the future; one would think that non-exercise thermogenesis would be higher for most [10].

Thermic Effect of Food 

Now that we have accounted for metabolism at rest, as well as metabolism of movement, there is still the final, smallest component to total metabolism – thermic effect of food. Thermic effect of food accounts for approximately 10% of total metabolic energy use [1][3][5]. That effect increases and decreases depending on which macronutrient you consume; protein has a TEF of 20-30%, carbohydrates have a TEF of 5-10%, fat has a TEF of 0-3%, and if you are curious, alcohol has a TEF of 10-30%, as well [11]. However, since meals tend to be mixed, the accepted approximation is 10% [11].

Factors that increase or decrease metabolism?

Metabolism is relatively stable in day to day life, but it does change over large amounts of time and due to some acute factors. Here are some (not all) factors that have a major impact on metabolism:
 

Exercise

Seems obvious, but increasing or decreasing exercise (if you already exercise) certainly has an impact on metabolism in a direct sense from thermic effect of physical activity, but also basal metabolic rate increases if we increase our amount of fat free mass via strength exercise [1].

Consumption/Nutrition

Certainly the type of foods consumed can have a minor impact on metabolism in a direct sense by impacting the thermic effect of food, but the amount and type of food consumed can also increase or decrease the amount of fat mass and fat free mass. Being in a caloric deficit, and/or not consuming adequate amounts of protein has a detrimental impact on fat free mass, which is metabolically rich (accounting for 30% of basal metabolic activity), if adequate resistance exercise is not implemented to reduce or mitigate these effects; as much as a 25% drop in basal metabolic energy use can be seen within the first 2 weeks of energy restriction [12]. On the other side, an overconsumption in calories would technically create a slightly faster metabolism due to added fat free mass and fat mass; however, this increase would not offset the increased caloric intake.

Illness

If you find yourself sick, metabolism picks up to attenuate the body’s need to recover as quickly as possible. The more serve the illness, the faster metabolism runs; surgery, for example, involves massive inflammation and protein turn over, and can increase metabolism from 25-30% to even 50-60% in extreme cases (burn unit cases, for example) [15][16].
 

SUMMARY

There we have it; metabolism is made up of 3 components starting with the most sizeable contribution coming from basal metabolic rate, then thermic effect of physical activity, and finally the thermic effect of food. As for how we can manipulate our metabolism, that comes down to changing body composition, mostly; this is accomplished by exercising and proper nutrition with the aim to put on more fat free mass. On a more acute level, things like illness can increase basal metabolic rate significantly, not to mention physical activity will increase metabolism in those moments of movement, as well.
 

Writer: Nicolas Verhoeven

                                                                                                        Citations                                                                                      

[1] Controversies in Metabolism. (n.d.). Retrieved from https://www.unm.edu/~lkravitz/Article%20folder/metabolismcontroversy.html

[2] Resting Energy Expenditure PROTOCOL. (2003, September). Retrieved from http://cdaar.tufts.edu/protocols/REE-protocol-edited.pdf

[3] Breaking Down Your Metabolism - McKinley Health Center - University of Illinois. (n.d.). Retrieved from http://www.mckinley.illinois.edu/handouts/metabolism.htm

[4] Reed, G. W. (1996). Measuring the thermic effect of food. American Journal of Clinical Nutrition, 63(2), 164-169. Retrieved from http://ajcn.nutrition.org/content/63/2/164.abstract

[5] Physical Activity: Energy expenditure. (2002). Retrieved from http://apjcn.nhri.org.tw/server/courses/12345phys-act/p2.php

[6] Javed, F., He, Q., Davidson, L. E., Thornton, J. C., Albu, J., Boxt, L., … Gallagher, D. (2010). Brain and high metabolic rate organ mass: contributions to resting energy expenditure beyond fat-free mass. American Journal of Clinical Nutrition, 91(4), 907-912. Retrieved from http://ajcn.nutrition.org/content/91/4/907.full

[7] Durnin, J. (1981, August). BASAL METABOLIC RATE IN MAN. Retrieved from http://www.fao.org/3/contents/3079f916-ceb8-591d-90da-02738d5b0739/M2845E00.HTM

[8] Berg, J. M. (2002). Biochemistry (5th edition). Retrieved from W H Freeman website: http://www.ncbi.nlm.nih.gov/books/NBK22436/


[9] Wang, Z., Ying, Z., Bosy-Westphal, A., Zhang, J., Heller, M., Later, W., … Müller, M. J. (2010). Evaluation of specific metabolic rates of major organs and tissues: Comparison between men and women. Am. J. Hum. Biol, 23(3), 333-338. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3139779/

[10] Levine, J. A. (2004). Non-Exercise Activity Thermogenesis (NEAT). Nutrition Reviews, 62, S82-S97. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/12468415

[11] Westerterp, K. R. (2004). Diet induced thermogenesis. Nutrition Metabolism (London), 1(5). Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC524030/

[12] Phinney, S. (1995). The Functional Effects of Carbohydrate and Energy Underconsumption. Not Eating Enough: Overcoming Underconsumption of Military Operational Rations. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK232461/

[13] Buckley, S. (1994). Metabolic response to critical illness and injury. AACN Clinical Issues Critical Care Nursing, 5(4), 443-449. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/7742135

[14] Jackson, N. C. (1999). The metabolic consequences of critical illness: acute effects on glutamine and protein metabolism. American Journal of Physiology - Endocrinology and Metabolism, 276(1), E163-170. Retrieved from http://ajpendo.physiology.org/content/276/1/E163.long

[15] Simsek, T., Uzelli Simsek, H., & Canturk, N. Z. (2014). Response to trauma and metabolic changes: posttraumatic metabolism. Turkish Journal of Surgery, 30(3), 153-159. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4379844/

[16] Gauglitz, G. G. (2014, December 12). Hypermetabolic response to severe burn injury: Recognition and treatment. Retrieved from http://www.uptodate.com/contents/hypermetabolic-response-to-severe-burn-injury-recognition-and-treatment


 

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