A group of adipocytes makes up adipose tissue. Obvious visual indicators of a large concentration of adipose tissue can be found by the subcutaneous fat present under the epidermis, be it on the chest, arms, lower back, belly, thighs, and so on. Visceral fat on the other hand, is much harder to detect, and while it serves a purpose as a cushion for internal organs, too much of it can lead to health complications.
The effects obesity (screw epidemic) pandemic are well documented, and every day new obesity cases are being broadcasted all over the media. Taking the extreme examples out of the picture, the symptoms for the majority of the populace are much more subtle than a massive gut and an inability to stand on two legs. The tightening of a pair of jeans, a dress that does not fit, a lower self-esteem, a harder time completing daily tasks, how many of these are you afflicted with?
How did it even come to this?
From an evolutionary standpoint, ancient humans had to live with a scarcity of food. Their bodies were forced to adapt to these changes by developing the ability to store and mobilize fat [1] [2]as a means of ‘reserve energy’ in harsher conditions as they migrated to new lands. Consequently, this gave homo-sapiens an evolutionary advantage over their simian/primate cousins in the form of heightened encephalization (brain:body mass), as well as better developed lower body in the form of stronger legs.
This phenomenon feeds back into itself, with the development of heightened intelligence being the catalyst for acquiring problem solving skills that ultimately remedied the environmental conditions that gave rise to it in the first place. As evidenced throughout history, humans as a species have flourished since then, making incredible advances in countless ways. However, as a consequence of these advancements, the focus shifted from survival to ‘#firstworldproblems’ , and as such human evolutionary development over the past few thousand years has been insignificant.
With a biological context for modern day homo sapiens’ presented, an understanding of this evolutionary mechanism is imperative if an individual wants to conquer the ubiquitous issue that is obesity. Needless to say; you’re stuck with the hand dealt.
The human body is an amazing machine though.
Complex carbohydrates, fibers, proteins, fats; the human body is more than capable of digesting and extracting nutrients from all the ingested food sources, then converting it into new tissue, recycling dead cells, executing metabolic processes, efficiently using extracted energy AND storing any excess in the process. Nothing goes to waste, only that which is to be excreted or dissipated as heat, and no other food sources are as efficient in providing energy to the human body as fat.
Fat, or scientifically known as triglycerides, is present in many food sources. When fat is consumed, it does not undergo digestion until the chyme reaches the gall bladder [3]. The gall bladder secretes bile to emulsify the triglyceride molecule, breaking it down into smaller micelles (smaller fat molecules), which is then further broken down into glycerol and fatty acids through lipase secreted from the pancreas. Since triglycerides far too big to be absorbed directly into the bloodstream, the small intestine absorbs the glycerol and fatty acids, then recombines them within its cells to reforge triglycerides. These triglycerides are then bundled in a phospholipid coating that makes them soluble in water, forming chylomicrons. The chylomicrons are then absorbed into the lymphatic system, then phased into the circulatory system [4].
Triglycerides are large molecules compared to other food groups, a lauric triglyceride molecule (fat from coconut oil) being C33H62O6 [5] and a basic starch molecule being (C6H10O5)n [6]. Through the process of lipolysis (lipo meaning fat, lysis meaning seperation) in the liver, a triglyceride molecule can be broken down into one glycerol molecule and three fatty acids, easily more than twice the amount of a normal sugar molecule (starch being a complex sugar). These fatty acids are converted to acetyl CoA through beta-oxidation (fatty acid oxidation) [7] and utilized in the Krebs cycle [8], which powers cellular functions throughout the body via the mitochondria present in cells.
When the bodily energy demands are met by way of heightened blood sugar, the excess acetyl CoA is returned to the liver/adipose cell and can be reverted to fatty acids [4], converted into bile salts, or triglycerides through de novo lipogenesis [7]. Both processes take up energy in the form of ATP, which the liver extracts from the extra acetyl CoA, reforging acetyl CoA molecules into their end-products. The bile can be transported to and re-used in the gall bladder to emulsify more fat. The rest of the acetyl CoA is reforged into triglycerides as a biological form of energy storage inside adipose cells [7], making them larger in the process. In some cases, the excess fat is also partially stored in the liver [9], eventually leading to fatty liver disease on the long run. But in the grand scheme of things, these processes only highlight the degree of efficiency of the human body managing its fuel reserves.
One interesting thing of note is that a study [10] (paywall) in 2008 has shown that the number of adipocytes in an adult human does not change, with the body renewing approximately 10% of all adipocytes annually. Implicitly, it changes the definition of fat-burning from: “destroying a fat cell to gain energy” to “extracting energy out of a fat cell to gain energy”. To put it simply, it also means that adipocytes don’t go away; they either shrink or are renewed.
In a nutshell:
Fat storage is a byproduct of human evolution, as fat is very energy dense. Most fat metabolizing processes are handled in the liver or adipose cells, where fat is broken down into fatty acids and glycerol. Fatty acids are converted to acetyl CoA which is used in bodily metabolic functions, and the excess is re-converted into fat and stored in adipose tissue. The amount of adipose tissue does not change, and so as more fat is stored, the bigger it becomes.
As the year draws to an end and people start re-evaluating the new-year’s resolutions they’ve made in 2018, especially the kind that had their eyes on a new fitness goal of some sort, it should be known that the journey to a healthier body is an uphill one. It’s no longer feasible to go to the gym and spend half the time staring at a phone, nor is it possible to chow down on that bag of doritos anymore and pray that it works itself off. The fact that the number of adipocytes are near-constant mean that a person is likely to be set further back for making bad choices, and more so in today’s hectic-sedentary hybrid lifestyle where the world we live in is permeated with temptation.
The next post focuses on carbohydrate and saccharide metabolism and how it ties into the issue at hand, so that a better understanding can be had of how food is metabolized, and why the body behaves the way it does. Hope you enjoyed the read, and for those who would love further reading, the links are just below. Have a nice day!
References:
[1] Evolutionary Perspectives on Fat Ingestion and Metabolism in Humans, May 2008.
[2] Body composition in Pan paniscus compared with Homo sapiens has implications for changes during human evolution, June 2015.
[3] How Fat Cells Work, Craig Freudenrich Ph.D., HowStuffWorks.com
[4] Lipid Metabolism, Anatomy and Physiology Textbook, Rice University
[5] Chemical Formula of Lauric Triglyceride, PubChem Open Chemistry Database
[6] Chemical Formula of Starch, Quora.com
[7] Lipid Metabolism
[8] Krebs Cycle Steps by steps Explaination, Microbiology Info.com
[9] Why Sugar is as bad as Alchohol, What I’ve learned, Youtube.
[10] Dynamics of Fat Cell Turnover in Humans, Nature, May 2008
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