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10/10/2022
Is Obesity Genetic or Environmental? From Adoption and Twin Studies to the Recent Rise in Obesity
Summary
１. Children whose parents have obesity are known to be more likely to develop obesity themselves. However, it is difficult to determine the extent to which genetic factors and the family environment contribute to this association.
２. In adoption studies by Stunkard et al, the adoptees’ weight class was associated with the BMI of their biological parents rather than that of their adoptive parents. Twin studies also found strong similarities in BMI even among identical twins who had been raised apart.
These findings suggest that genetic factors play an important role in individual differences in BMI and that the shared family environment alone cannot fully explain similarities in BMI.
Discussion
These studies primarily examined the influence of genetic and environmental factors on individual differences in BMI. Building on these findings, the discussion considers the recent global rise in obesity from both genetic and environmental perspectives.
３. Because it is unlikely that the the human genetic makeup has changed substantially within just a few decades, genetics alone cannot readily explain the worldwide rise in obesity since the 1970s.
４. Rather than viewing obesity as a simple question of genetics or environment, interactions between genetic predisposition and environmental factors are also considered important.
５. Changes in the food environment since the 1970s involve more than changes in energy intake. In addition to changes in food processing and refining, eating patterns—including meal frequency, the intervals between meals, and the timing of food intake—have also changed.
６. According to my hypothesis, changes in food digestibility and eating patterns may make intestinal starvation more likely to occur, potentially leading to an upward shift in the body-weight set point through adaptive physiological responses.
７. The “shared family environment” examined in adoption and twin studies alone cannot capture the full range of environmental changes that may be involved in the recent rise in obesity. To better understand this rise, it is important to consider which environmental factors interact with genetic predisposition and influence body-weight regulation.
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Contents
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- Genetic and environmental influences: evidence from adoption and twin studies
- Discussion
(1) Can genetics alone explain the recent rise in obesity?
(2)The gene–environment interaction perspective
(3) How should we think about changes in the environment?
To what extent do genetics and the living environment contribute to obesity?
Obesity often runs in families, and children whose parents have obesity are known to be more likely to develop obesity themselves [1,2].
However, family members share not only genetic factors but also environmental factors, such as diet and lifestyle. It is therefore difficult to distinguish the extent to which genetic factors and the “shared family environment” each contribute to individual differences in BMI (body mass index).
（Source: Magnific）
In this article, I will first examine the influences of genetic and environmental factors on individual differences in BMI based on findings from adoption and twin studies. I will then explore the recent global rise in obesity from both genetic and environmental perspectives in light of these findings.
1. Genetic and environmental influences: evidence from adoption and twin studies
Adoption and twin studies have long been used to investigate the relative influences of genetics and environment on obesity. By separating and comparing biological relatedness with the family environment in which individuals were raised, researchers can estimate the contribution of each.
For example, if the BMI of adoptees more closely resembles that of their biological parents than that of their adoptive parents, this would suggest a greater influence of genetic factors. Conversely, if their BMI more closely resembles that of their adoptive parents, this could indicate a greater influence of the shared family environment [3].
Albert J. Stunkard and his colleagues conducted pioneering research in this field.
Adoption study: Stunkard et al. (1986)
Stunkard et al. investigated the influence of genetic factors and the family environment on obesity in 540 adult adoptees in Denmark. The adoptees were classified into four weight classes—thin, median weight, overweight, and obese—and the relationship between their weight class and the BMI of their biological and adoptive parents was examined.
The results showed a strong association between the adoptees’ weight class and the BMI of their biological parents, whereas no association was found with the BMI of their adoptive parents [4].
(Author: pch.vector/ Source: Magnific)
Moreover, the association between adoptees and their biological parents was not limited to those with obesity but was observed across a broad range of body sizes, from very thin to obese.
The researchers concluded that genetic factors play an important role in determining body size in adulthood, whereas the shared family environment alone showed no clear influence [4].
Adoption study (biological siblings): Sørensen et al. (1989)
Sørensen, Stunkard, and colleagues later used the same Danish adoption data to compare the BMI of adoptees who had been separated from their biological parents early in life with that of their biological siblings who had been raised by their biological parents.
The results showed that the higher the adoptees’ weight group, the higher the BMI of their biological siblings who had been raised separately. This trend was more pronounced among full siblings who shared both biological parents. Among half-siblings who shared only one biological parent, BMI also increased steadily across the adoptees’ weight groups, but the increase was weaker [5].
The study suggested that the association in BMI was stronger among full siblings, who share more of their genetic background, than among half-siblings. In addition, comparing siblings from the same generation helps reduce the potential influence of generational differences in environment, which can be a concern in parent–offspring studies [5].
Twin study: Stunkard et al. (1990)
Stunkard et al. investigated the influence of genetic and environmental factors on BMI in Swedish twins. The study included identical and fraternal twins who had been raised apart, as well as identical and fraternal twins who had been raised together [6].
（Source: Magnific)
The results showed that BMI was strongly similar even among identical twins who had been raised apart. The analysis estimated that heritability—the proportion of variation in BMI attributable to genetic factors－was 74% in men and 69% in women.
In contrast, no clear additional effect of being raised in the same family environment was observed [6].
The researchers concluded that genetic factors have a strong influence on BMI, whereas the influence of the family environment shared by twins appears to be relatively small [6].
Review of twin, family, and adoption studies: Maes et al. (1997)
Maes et al. reviewed previous twin, family, and adoption studies and analyzed data from more than 25,000 twin pairs and 50,000 biological and adoptive family members. They found that the weighted mean correlation for BMI was highest among identical twins (0.74), followed by fraternal twins (0.32), siblings (0.25), and biological parents and their children (0.19), while the correlation was very low among genetically unrelated members of adoptive relatives (0.06) [7]. It should be noted that this review was not limited to twins or family members who lived apart.
These findings suggest that genetic factors play an important role in individual differences in BMI, while the shared family environment alone cannot fully explain the observed similarities in BMI [7].
2. Discussion
These adoption and twin studies primarily examined the influence of genetic and environmental factors on individual differences in BMI; they were not designed to explain the recent rise in obesity itself. How, then, should we interpret these findings in relation to the worldwide increase in obesity since the 1970s?
(1) Can genetics alone explain the recent rise in obesity?
Adoption and twin studies suggest that genetic factors play an important role in individual differences in BMI.
However, the prevalence of obesity began to rise rapidly in many populations from the 1970s onward, and it is unlikely that the the human genetic makeup changed substantially within just a few decades [8,9]. Genetic factors may influence an individual’s susceptibility to obesity, but genetics alone cannot readily explain the worldwide rise in obesity.
In other words, the factors that explain differences in BMI among individuals need to be distinguished from the factors that have contributed to the rise in obesity across the population as a whole.
(2) The gene–environment interaction perspective
An important point to keep in mind is that the environmental influence found to be minor in adoption and twin studies was primarily that of the environment shared by family members or twins.
In contrast, since the 1970s, our diets and lifestyles have changed substantially across society as a whole. Greater availability of high-calorie foods and more opportunities to eat out have been accompanied by declines in physical activity associated with the widespread use of cars, televisions, computers, and other technologies. Such environmental changes cannot be ignored when considering the factors behind the rise in obesity within the population.
(Author: pvproductions/ Source: Magnific)
Today, obesity is generally viewed not as a simple question of genetics or environment, but as the result of interactions between genetic predisposition and environmental factors [8,9].
Even among people living in similar environments, there are individual differences in susceptibility to weight gain. Changes in the environment may also make genetic predispositions that were previously less apparent become more pronounced.
(3) How should we think about changes in the environment?
The recent rise in obesity has often been discussed in terms of increased energy intake and decreased energy expenditure. However, changes in the food environment cannot be understood solely in terms of energy intake.
As food processing and refining have become more widespread, softer and more easily digestible foods have become increasingly common.
In particular, studies have reported that people who consume a higher proportion of ultra-processed foods tend to have higher BMIs and a greater risk of obesity [10].
Eating patterns have also changed, including meal frequency, the intervals between meals, and the timing of food intake.
Studies have reported associations between obesity and behaviors such as skipping breakfast and consuming a higher proportion of daily energy intake at dinner [11,12].
According to the hypothesis I propose in this blog, these changes in the food environment and eating patterns may make intestinal starvation more likely to occur, potentially leading to an upward shift in the body-weight set point through adaptive physiological responses. There may also be individual differences in susceptibility to intestinal starvation and in the physiological responses it triggers, and these differences may partly reflect variation in genetic predisposition—for example, physiological characteristics related to digestion and absorption.
In other words, the “shared family environment” examined in adoption and twin studies alone cannot account for the full range of environmental changes that may be involved in the recent rise in obesity.
To better understand this rise, it is important to consider which environmental factors—including food digestibility and eating patterns—interact with genetic predisposition and influence body-weight regulation.
<References>
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[3] Jason Fung. The Obesity Code. Greystone Books, 2016, Pages. 22-3.
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[6] Stunkard AJ et al. The body-mass index of twins who have been reared apart. N Engl J Med. 1990;322(21):1483–1487.
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[10] Elizabeth L et al. Ultra-Processed Foods and Health Outcomes: A Narrative Review. Nutrients. 2020 Jun 30;12(7):1955.
[11] Sakurai M et al. Skipping breakfast and 5-year changes in body mass index and waist circumference in Japanese men and women. Obes Sci Pract. 2017 Apr 3;3(2):162-170.
[12] Simona Bo et al. Consuming more of daily caloric intake at dinner predisposes to obesity. A 6-year population-based prospective cohort study. PLoS One. 2014 Sep 24;9(9):e108467.