Note: Alyna and I decided to both respond to the childhood obesity epidemic but through our unique lenses. I focus on how your nature or genetic predisposition leaves you vulnerable to obesogenic environments. Alyna, on the other hand, focuses on how your nurture as a child influences your weight. We would recommend reading Alyna’s blog post first, for I describe the interplay between nature and nurture at the end. I stand by my opinion that obesity is more influenced by nature but want to acknowledge this confluence.
Introduction: What Are the Benefits of Fat?
Many children and adolescents may struggle to answer the above question. In 2013 to 2016, 45.2% of adolescent girls aged 16 to 19 and 30.1% of adolescent boys aimed to lose weight. However, some adolescents devoted to weight loss are not clinically obese. I want to ensure that my discussion of the global obesity crisis does not encourage unnecessary and unhealthy weight loss. Poor nutrition can weaken your immune system, stunt your growth, and have other tragic consequences, many of which relate to reproductive health.
Females with body fat in areas used for reproductive purposes (such as the breasts, hips, and buttocks) tend to produce healthier offspring. Birthing a child requires a lot of energy and calories, so the ability to metabolize stored fat is important, and lacking a buffer would be dangerous. A significant selective pressure hominids faced was food shortages, which only those with fat buffers could overcome. Due to these evolutionary advantages, natural selection favored hominid females with appropriate levels of body fat.
Possibly, Venus figurines crafted by early humans worked alongside natural selection by expressing favoritism of corpulent and thus more fertile women. In these figurines, artists did not develop women’s facial features but accentuated their fatty reproductive areas.
However, there is a critical difference between carrying healthy fat, being clinically overweight (with a BMI of 25 kg/m2 or above), being clinically obese (with a BMI of 30 kg/m2 or above), and being morbidly obese (with a BMI of 40 kg/m2 or above).
The Modern Obesity Crisis
Over two decades ago, the World Health Organization (WHO) officially stated that the growing obesity crisis was a “global epidemic.” Recently, the percentage of U.S. adults considered morbidly obese more than tripled; from 1976 to 1980, 1.4% of this demographic was morbidly obese while 5.1% was from 2003 to 2004. Other high-income countries, such as the UK, have experienced similar statistical shifts. According to estimations, the prevalence of morbid obesity will increase by 130% between 2010 and 2030.
Where?
Despite this discussion of westernized countries, the prevalence of being overweight and obese has increased most rapidly in Africa and Asia. In 2015, the most obese children resided in China and India, and, in that same year, the most obese adults resided in not just the U.S. but also China. Although the growth of the obesity epidemic seems to have decelerated in developed countries recently, the problem is escalating quickly in developing ones, and no country has been able to restore a lower prevalence of obesity. In fact, the prevalence of obesity is estimated to rise by 33% by 2030.
Why?
The cause of obesity, when boiled down, is intaking more energy than you are expending over an extended period of time to create chronic positive energy balance. Energy intake is how many calories you acquire from fat, carbohydrates, protein, and alcohol. It is mainly influenced by the nature of the food and drink you consume, including its energy density and quantity. The following equation summarizes energy expenditure: your basal metabolic rate, or how much energy you expend when at rest + how much energy you need for digestion and the absorption of nutrients + how much energy you exert through physical activity. Despite these many contributing factors, it is mainly influenced by the nature of your physical activity, including what type of exercise you perform as well as how intense and frequent it is.
Acknowledging the Influence of Nurture on Obesity
Today’s environment is often said to be obesogenic, or conducive to weight gain, and is thus often blamed for the obesity epidemic. To be obesogenic, our environment 1) increases energy intake and 2) decreases energy expenditure.
1) How Does Our Modern Environment Increase Energy Intake?
As mentioned, our hominid ancestors constantly struggled to obtain sufficient food, especially large amounts of three important ingredients: fat, sugar, and salt. In response, they grew to crave these substances. Who knew your addiction to fatty cheeses, sugary candies, or salty bacon was originally a useful biological adaptation? However, in most parts of the world today, this selective pressure has been alleviated because we have easier access to fat, sugar, and salt.
Globally, adults in developed countries have significantly increased their energy intake since the 1970s, for they can easily acquire many inexpensive foods that are rich in calories and fat. Recently, foods with more appealing tastes and higher energy densities have become more inexpensive and accessible as the production, processing, storage, and preparation of food have evolved. Technology that helps produce, transport, and market food has spread across the globe. Thus, large supermarkets that make inexpensive processed foods full of fat, sugar, and salt more accessible have replaced traditional food markets.
The Nutrient Transition
Many parts of the world have undergone the nutrient transition, meaning locals have begun to consume highly processed foods replete with saturated fats, sugar, and calories while lacking in fiber. Additionally, the World Trade Organization has facilitated food trade between nations. As a result, the diets of low- and middle-income countries have become more westernized and thus heavy in refined carbohydrates, added sugars, fats, and animal products but light in legumes, vegetables, and grains. Countries affected by this westernization have experienced rapid rises in the rates of obesity and metabolic diseases.
The nutrient transition was primarily caused by the increase in sellers of foods, especially fast food. According to a UK study, between 1980 and 2000, there was a roughly 80% increase in the number of food distributors. Surrounded by more distributors of takeaway food in the places where you live, work, and commute, you eat more takeaway food and thus raise your BMI as well as your risk of obesity.
Food Quantity
Additionally, portion sizes have increased. According to experimental studies, both adults and children consume more food and non-alcoholic beverages when served larger portions. A 200% increase in portion sizes has caused a 35% increase in consumption.
The abundance of food advertisements today, especially ones promoting less healthy foods, is yet another part of the problem. According to a U.S. study, almost half of the commercials on children’s programs are food advertisements; an average hour of such programs includes 11 food advertisements. Likewise, the unhealthy but tempting snack options at supermarket checkouts or in vending machines can cause you to intake more calories.
The Root of the Problem
Thus, we have increased our caloric intake while decreasing our intake of micronutrients, meaning vitamins and minerals. In contrast, earlier in time, we consumed locally grown foods that had undergone little processing. They were still rich in nutrients and fiber but lower in sugars, sweeteners, salt, and preservatives.
Overall, the root of the global obesity epidemic is our inability to shed our ancestors’ adaptation by silencing our cravings for now plentiful substances that, when consumed in abundance, have adverse health effects.
The Influence of Prenatal Food Environments on Obesity
Evidence shows that your food environment before birth can affect your energy intake and thus risk of obesity. If you are under- or overnourished in the womb, your neural circuits that work with leptin, a hormone connected to your level of fullness, and that affect your appetite are permanently changed.
Through animals that serve as models for humans, scientists have observed that infants of mothers who are malnourished during pregnancy have less leptin at birth. Thus, they have larger appetites, possibly to help their bodies grow to an appropriate size. Such infants can be at a greater risk of obesity if born into an obesogenic environment. The hypothalami of infants who are overnourished in their mothers’ uteri and who are large when they are born are less responsive to leptin, so they are not as sensitive to feelings of fullness and may also become obese.
How Can We Limit Our Energy Intake?
Research is being conducted on how to best decrease energy intake. Proposed strategies include adding health information to food labels and preventing fast food restaurants from being close to schools and workplaces. In the U.S., menus have begun to include calorie counts and nutrition information; some regions have begun to manage the use of ingredients such as trans fats; food advertisements have been limited; and a tax on sugar-sweetened beverages has been introduced. The UK plans to reduce childhood obesity by clearing supermarket checkouts of foods with a lot of fat, sugar, and salt as well as by prohibiting the advertisement of such foods on TV before 9 pm.
2) How Does Our Modern Environment Decrease Energy Expenditure?
Your environment affects how much physical activity you get and thus how much energy you expend, and physical activity levels have decreased recently. In Finland, the amount of physical activity adults performed when working and commuting decreased. However, the amount of physical activity they performed during leisure time increased.
1972 | 2002 | |||
| Percentage of Men | Percentage of Women | Percentage of Men | Percentage of Women | |
| Occupational Physical Activity | 60 | 47 | 38 | 25 |
| Commuting Physical Activity | 30 | 34 | 10 | 22 |
| Leisure Time Physical Activity | 66 | 49 | 77 | 76 |
A similar trend is present in the U.S. What percentage of U.S. jobs in private industry do you think involves physical activity that is at least moderately intense? In the early 1960s, the answer to this question was nearly 50% while currently, it is below 20%. Thus, on average, people are estimated to burn over 100 calories less through their occupation per day than they did in the past. During most of the day, those who are the appropriate age are usually working. Therefore, the fact that U.S. residents, as with Finland’s residents and those of other countries, are performing slightly more physical activity during leisure time will likely not compensate for their decreased occupational physical activity.
I want to remind you that the obesity crisis is not limited to high-income countries; rather, it is a global epidemic. In 2016, the lowest percentage of men to engage in physical activity, 12.3%, was attributed to Oceania. Men from East and Southeast Asia as well as sub-Saharan Africa were close behind at 17.6 and 17.9% respectively.
The Root of the Problem
One of the main reasons for decreased physical activity is new technologies. These have caused industrial processes to rely on automatic equipment as opposed to manual labor. Additionally, recent years have seen the invention and increased popularity of TV, computers, video games, and other activities involving screens. For all these reasons, we are spending more time doing leisure activities focused on sitting or reclining (which increase the likelihood of childhood and adolescent obesity, in particular) rather than on energy expenditure.
Also in recent years, human populations have spread from central urban areas to areas that serve a single purpose and tend to rely on cars. In other words, urban areas have spread into land that was initially the countryside. This movement, called the urban sprawl, has accelerated recently, likely because the global population has grown. The areas to which humans are migrating are obesogenic, for they cause residents to engage in less physical activity by relying on motorized transportation to places such as work, school, and shops. Studies conducted across multiple countries display that if low- and middle-income countries are more urbanized and economically developed, residents are likely to engage in less physical activity.
How Can We Maximize Our Energy Expenditure?
Numerous tactics have been proposed to combat the frightening decrease in physical activity from 50 years ago. They include making common destinations, such as parks and shops, more accessible to popularize active forms of transportation; constructing new infrastructure to support these modes of transportation as well public transportation; and practicing land-use mix, or making different uses of land feasible in a given environment.
The Influence of Nature on Obesity
After completing my research on nurture’s effect on obesity, I understood that features of today’s world have caused a general increase in the prevalence of obesity. Yet one question remained: why do only some suffer from obesity and others stay at a moderate body weight quite easily even if both groups reside in similar environments? According to “The obesity epidemic – Nature via nurture: A narrative review of high-income countries,” “[g]enetic factors are hypothesised to explain a large proportion of the variation in susceptibility to obesity.”
Twin Studies
As support that your genes influence your BMI, identical twins (who split from a single zygote and thus share all their genes) tend to have more similar BMIs than fraternal twins (who are born from two separate zygotes and thus share only 50% of their genes on average).
Twin studies that support this conclusion have produced statistics measuring BMI heritability, or how much of a causal relationship exists between genetic variation and BMI variation. Keep in mind that 0% indicates a nonexistent relationship while 100% indicates that genetic variation is the sole cause of BMI variation. Scientists analyzed 31 similar adult twin studies to estimate that BMI heritability in adults is from 47 to 90%. They did the same to 45 twin studies on child participants to conclude the following: 1) BMI heritability is consistently high during childhood and adolescence at 45 to 85% and 2) BMI heritability increases from mid-childhood, when it is at around 42%, to the beginning of adulthood, when it is at around 75%.
So, Which Genes Are at Fault?
Scientists have struggled to pinpoint the specific genes that influence BMI variation. In 2007, a study found that the presence of FTO variants is linked to adults and children being obese. On average, adults with one high-risk allele of the FTO gene are 1.2 kg heavier than those with two low-risk alleles while those with two high-risk alleles are 3 kg heavier. Approximately 50% of the population contains at least one high-risk allele of the FTO gene.
Subsequently, scientists discovered almost 1,000 single-nucleotide polymorphisms (SNPs) tied to BMI. In other words, let’s say there is a significant difference in BMI between Individual A and Individual B. Looking at their genomes, you would see almost 1,000 differences in single nucleotides, and these differences would contribute to their differing BMIs. However, FTO variants and polymorphisms cause only 6% of BMI variation. Scientists believe that 1) rare genes, 2) many common genes with minimal effects, and 3) the interactions between multiple genes as well as between genes and the environment account for the rest of the influence genetic variation has on BMI variation.
Changing “Or” to “And”: How Nature and Nurture Work Hand in Hand
Overall, levels of being overweight and obese have significantly increased recently, a fact that reflects the increasingly obesogenic nature of our modern environment. There are also significantly more people suffering from the most severe form of obesity. This increase at the very end of the weight spectrum has skewed BMI distribution, and it reflects the fact that those genetically prone to obesity are especially sensitive to today’s obesogenic environment. “The obesity epidemic – Nature via nurture” puts it best: “as is typically the case in debates of genetic versus environmental contribution to any phenotype, when it comes to the obesity epidemic, it is not nature or nurture; rather, it is nature via nurture.” However, I believe that nature plays a more significant role in obesity by making you more vulnerable to an obesogenic environment.
The Behavioral Susceptibility Theory
In 2007, Professor Jane Wardle proposed the behavioral susceptibility theory (BST) of obesity (mapped in the diagram below). According to this theory, individuals who have larger appetites often overeat when their food environments give them the opportunities to do so by advertising tempting food and making it easily accessible. They are more stimulated by the sensory characteristics of food due to their genetics. Likewise, such individuals often overeat when their food environments provide them with larger portion sizes and more chances to eat. They are genetically predisposed to eat more when they are hungry and are less likely to stop eating when they are full. Overall, your genetics heighten your response to obesogenic environments and cause you to become obese.
Many common genetic variants linked to BMI are in or close to genes that control energy balance and especially appetite. For instance, studies display that the hypothalamus and pituitary gland (both of which regulate our hunger and feelings of fullness) as well as the hippocampus and limbic system become enhanced when you express BMI-related genes. Thus, according to these studies, genes that influence BMI also affect the way you feel, think, and learn about food, processes that involve your mind and nervous system as a whole.
Why Does Adulthood Come With Increased BMI Heritability?
As mentioned, twin studies show that there is a significant increase in BMI heritability when you transition from early childhood to the beginning of adulthood. Interestingly, during this transition, twins become less influenced by the family environment they initially shared.
The fact that your BMI and risk of obesity intensify after early childhood implies that your genetics actually influence your weight more when you are exposed to an obesogenic environment. Although this truth may seem counterintuitive, BST states that you do not completely express genes linked to obesity if you cannot eat as much as you want whenever you want, a freedom that comes with age. Overall, BMI heritability becomes more tied to age as you mature because older children can act in accordance with their genetic traits related to appetite. The idea that your response to your environment displays your genetic predispositions is called gene-environment correlation.
The Home Environment
In a large twin study, participants whose immediate communities or homes were salubrious displayed significantly less BMI heritability, 38%, than those in obesogenic environments, with a BMI heritability of 86%. The food, media, and physical activity present in environments influenced whether they were deemed healthy or obesogenic. Adults residing in walkable areas, which allowed them to perform more physical activity, displayed less BMI heritability.
The Role of Physical Activity
We have looked at large-scale obesogenic environments and smaller scale ones, but what is an environment of an even smaller scale? Your body. Through twin studies and additional research, scientists concluded that individuals who perform more physical activity combat their genetic predispositions to higher BMIs and weight gain. For instance, many data suggest that a lack of physical activity exacerbates the tendency of FTO variants to increase your BMI by around 30%.
The UK Biobank study shows that the degree to which your body is an obesogenic environment affects your genetic predisposition to a lower or higher BMI. Individuals susceptible to higher BMIs partly compensate for this trait by partaking in more physical activity and spending less time watching TV. These conclusions may support that regular physical activity helps you control your appetite by making you more sensitive to feelings of fullness.
Now that scientists better understand how intertwined nature and nurture are in influencing obesity, they can help our larger world answer this critical question:
How Can We Best Combat the Obesity Epidemic?
Policies should control food environments and allow for more physical activity to help those at risk of obesity due to their genetic predispositions and/or obesogenic environments. Additionally, clinicians should be trained to guide patients on how their lifestyles can affect their vulnerability to obesity. However, clinicians should also understand that factors beyond your control cause weight gain so that they can truly help patients with weight loss instead of simply instructing them to eat less and exercise more. They must realize that obesity is not simply a choice, and understanding that nature contributes more to obesity than nurture is a critical step in making this realization.
The Perspective of Paleonutritionists
Some anthropologists suggest that we make our diets more similar to that of our hominid ancestors. A field has emerged called paleonutrition studies, which claims that if we replicate our ancestors’ diets, we can combat the growing obesity crisis. Many works published about paleonutrition agree that we should avoid processed and refined food, fat, sugar, and salt.
However, other anthropologists view the approach of emulating our hominid ancestors’ diet as not helpful or even harmful. For instance, some biological anthropologists are skeptical about the fact that paleonutritionists suggest that there was one diet our hominid ancestors followed. In reality, hominids lived in many different environments and thus developed many different diets. Biological anthropologists also believe that paleonutritionists are romanticizing our ancestral history and spreading the extremely simplistic “noble savage idea”: we must return to the past, for hominids were expertly adapted to their environments and automatically knew how to make healthy dietary choices while modern times are in a much worse state. To some, paleonutritionists disregard that medical knowledge and technology have greatly advanced and benefitted modern humans.
Sources
- The Great Courses – Biological Anthropology: An Evolutionary Perspective
- Essentials of Biological Anthropology by Clark Spencer Larsen
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7222649/ (Note that this source focuses on high-income countries)
- Feature image: https://www.verywellhealth.com/childhood-obesity-overview-4014273