Obesity is one of the major problems we face in our modern world, obesity is a growing problem that has impacted almost every nation now both developed and developing. This would not be a major issue if not for the fact that obesity can lead to elevated risks for the development of several serious health conditions which include: insulin resistance, type 2 diabetes mellitus, hypertension and cardiovascular disease, fatty liver disease, and some types of cancer(1). It should be to no surprise then that there have been a lot of research into why obesity has just in the last 50 or so years become such a major problem and many signs point towards the reason being a genetic factor that was not an issue until our modern environment interacted with the genes causing a mismatch of old prehistoric genetics and a modern environment coming into conflict(2). In this blog it will be shown that mismatch happened because some people have genes that predispose them to having difficulty burning calories because the genes are a byproduct of selection on some other trait that is advantageous(1). More specifically it will be shown that the ability to thermoregulate in extreme heat or cold afforded powerful survival advantages and this ability is controlled by the brown adipose tissue (BAT) which also has the secondary function of controlling the ability to burn excess fat and calories and that this is why some people are more susceptible to obesity than others and how our environment impacts this(2).
As Humans left Africa and went more northward they were put under different selection pressures than the ones in more southern lands due to climate differences. Northern lands like Europe or Asia where colder than ones closer to the equator and as such having the a better ability to regulate ones body heat gave those living in these colder environments a selective advantage(2). This is achieved by having a superior BAT thermogenic function because BAT controls the ability to produce body heat and this would be selected for because the lack of BAT would lead to exceedingly high mortality rates in infants born in colder climates, as would have been the case for the ancestors of Europeans and east Asians who flourished in ice age Europe and Siberia(2). Infants possess abundant amounts of BAT at the time of birth, which accounts for 1.4% of total body weight. BAT plays a critical role in thermoregulation infants and young children, impaired BAT development in infants has been implicated in the high mortality rates of preterm infants. Thus, genes that augment BAT function would be highly advantageous in cold climates(2). The byproduct of all this BAT selection is that this came with equally efficient fat burning and energy wastage capability because as stated beforehand BAT has a secondary function of controlling the ability to burn excess fat and calories.
Human migration out of Africa with dates provided (5)
Brown adipose tissue Tissue up close (6)
Comparatively in the more warm climates close to the equator like Africa, Oceania or central and south America the warmer climate would put selective pressure on genes for heat adaptation, which would greatly have improved survival chances in the hot climates(2). But as a byproduct of heat adaption the functionality of BAT would go down and thus the ability of burning excess fat and calories would be lessened. There is evidence for this as seen in the higher rates of obesity in ethnic groups who ancestor come from hot climates and lower rates of obesity in those whos ancestor were from colder climates. For example in the United States of America people of European and east Asian descent are less likely to be obese than those of African, Hispanic and Native American decent(3 & 2). This can be seen in a study published in the journal "Endocrinology" that found 26% of Americans of European and East Asian descent are obese compared to 38 % of US blacks, Hispanics, and Pima Indians(2). And in another study by the CDC They found that 12.7% Asian and 37.9% of white Americans where obese whilst 46.8% black and 47% Hispanic Americans were obese (CDC Graph below)(3).
This is further reinforced when we study white Europeans further and find that there is a great disparity in Obesity rates dependent on location. A great example is the Scandinavian countries, whose population reflect hundreds of generations of genetic adaptation to extreme cold, have much lower rates of obesity than the rest of Europe despite having similar lifestyles and caloric intake(2).
Now this explains the genetic part of the genetic-environment but there is still the environmental factor to go over. It can clearly be seen that the environmental cause of this mismatch is a result of our modern lifestyles, that mix poor quality food with less physical activity than decades before(1). A key issue is why people have excess intake of energy and the answer to that is individuals not only eat food to get the necessary energy but also for some critical nutrient. When food is of high quality, it may be that by eating enough food to satisfy one’s daily energy demands, the demands for the nutrient are always met(1). But with the advent of low quality processed foods individuals have to eat more food in order to get the necessary nutrients, leading to them eating more food than their daily energy needs would demand. Combine this with a more sedentary society that uses less energy that if individuals continued to eat food to meet their energy demands, then they would reduce their intake, but this might mean their intake of the nutrients would be below requirements, and they would be nutrient deficient. Thus they would eat more food for the nutrients whilst being too sedentary to burn off the extra calories that turn to fat(1). An example of this will be in the Graph below.
(1) Figure legend - "In scenario A, the food is of good quality, and the lifestyle is active. Food is eaten to provide for
the energy demands of the active lifestyle ( yellow bar), and the needs for the critical nutrient are exceeded by
the intake of the good-quality diet ( green bar). In scenario B, there has been reduction in energy demands by
conversion to an inactive lifestyle. In this situation, if individuals eat to satisfy their energy needs, they will have
a shortfall in their nutrient intake relative to demands. If the individual in this situation were to eat to satisfy
demand for the limiting nutrient rather than energy, then excess energy intake (orange bar) would need to be
burned off (scenario C). A similar situation could occur if the lifestyle remained active but the quality of the diet
changed (a higher ratio of energy to nutrient; scenario D). If individuals responded to this situation by eating to
satisfy their nutrient demand, they would again have an excess intake that would need to be burned off (
orange bar) (scenario E)."
As seen if someone over consumes energy because of the requirement for a particular nutrient, then the ability to burn off the excess energy depends on levels of BAT. Individuals with large BAT depots burn off the excess and remain lean, whereas those with lower levels of BAT are be unable to burn off the excess consumption and become obese(1). Because of this obesity is a maladaptive consequence of variation in adaptive selection on BAT capacity driven by its primary function for thermogenesis(1). The environmental trigger is the change in the energy to nutrients ratio in modern food that stimulates overconsumption of energy.
There is great danger in Obesity is that it may become Dysevolutioned or have already become Dysevolutioned which by that I mean that we focus more on treating the symptoms of obesity with patient blaming and focusing on individual treatments of Obesity with weight loss programs and books and the such instead of attacking obesity at the source, our environment so much of our food is nutrient bare and very unhealthy there should be regulations on how unhealthy foods can be that way we try to cure obesity for all not just treat it symptoms.
So in conclusion, the Obesity issue in modern society is caused by a mismatch interaction of genes that primarily regulate the ability to produce body heat with brown adipose tissue (BAT) but also have a secondary function of regulating the ability to burn fat and calories. Those who have greater levels of BAT can burn fat and calories easier than those with low levels of BAT. This BAT level gene interacts with an environment that has poor quality food that leads to more consumption of food in order to acquire the necessary nutrients needed to function but has the side effect of the consumption of more energy than is needed for the day is that the excess energy is stored in the form of fat causing people to become fat. Those with high levels of BAT have a easier time burning this excess fat off than those with low levels of BAT and that is why Obesity has become such a prevalent disease in only the last 50 or so years. All this makes Obesity a perfect example of the mismatch hypothesis as discussed in my Evolution class on 4/1/2021.
Sources
(1) John R. Speakman. Evolutionary Perspectives on the Obesity Epidemic: Adaptive, Maladaptive, and Neutral Viewpoints (abdn.ac.uk) The Annual Review of Nutrition. 2013.
(2) Sellayah D, Cagampang FR, Cox RD. On the evolutionary origins of obesity: a new hypothesis - PubMed (nih.gov). Endocrinology. 2014
(3) Hales CM, Carroll MD, Fryar CD, Ogden CL. Prevalence of Obesity Among Adults and Youth: United States, 2015-2016 - PubMed (nih.gov), NCHS.
(4) Early Human Migration - World History Encyclopedia (ancient.eu) (migration out of Africa map)
(5) https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhuHjxO2HikQb20jwPkOC8xO3E3BRqt641dKW5SChT1SrRZujX2Z9TWHtZaDd7UpYo8m-qmQKIxKvptKazBPiWRj_munCvTt3T7GyNiO_bk_M0Ruy6hq4Wk6htFkeRm32APRMiVFmVyRMpP/s558/The-brown-adipose-tissue-Source-pathologymcdukeedu-research-PTH225html_W640.jpg ( BAT imagine)
