What if obesity were not only a matter of calorie intake?

Obesity has rapidly increased in the last decades. Such increase has been steeper in low and middle-income countries and, in high-income countries, among the poorest social classes. The main cause is the rapid and generalized diffusion of obesogenic environments, i.e. that boost obesity, which, from the global to the individual scale, promote a higher caloric intake and a lower expenditure of those calories, among other. While epidemiology and medicine have looked into the causes of obesity (rather from an individual point of view, using ad hoc cohorts), demographic studies considering the evolution of obesity within a population are scarce. However, this does constitute a population matter, in which demographic and social processes play a major role. As a consequence, those processes – that we can call ancillary – have been largely overlooked. 

Demographic and social processes at work

The first of those ancillary processes is the intergenerational transmission of obesity, which is, one the one hand, genetic (transmission of genes affecting the propensity to obesity), and on the other hand, cultural (transmission of behaviours and social determinants facilitating obesity). Second, homogamy – or the fact that individuals have a tendency to mate with similar individuals – can reinforce obesity transmission. This homogamy, in the case of obesity, is mainly indirect: it is the result of obesity not being evenly distributed across the social classes that define the pool of potential partners. The third process is differential fertility: in high-income countries, the social classes in which obesity is more prevalent are also those in which fertility is higher, such being the opposite of low-income countries. Finally, we should consider gene-environment interactions, or the fact that the same genes do not behave the same under two different social environments. Indeed, external influences can modify the manner in which genes operate, this process being more likely in populations that have experienced a strong mismatch between ancestral and present environments (as it is the case of some immigrant groups, or of countries that have experienced a rapid nutritional transition).

The effect of those processes on obesity prevalence

In two recently published articles (1), we study the effect of those four processes on obesity prevalence in a population, considering – as a first step – intergenerational transmission to be solely genetic. Using theoretical models, we draw four main conclusions. First, differential fertility is the process which could have a larger impact. Second, homogamy does not have a direct effect by itself, but can augment the effects the effects of differential fertility. This lack of effect of homogamy is contrary to results of other studies, due to our third conclusion: the manner in which genes are represented is crucial. Obesity is a largely polygenic trait (hundreds of genes contribute to it) and we consider it as such. However, it is usually represented in a simplified monogenic way, in which a single gene would be responsible for it (only a handful of human traits are monogenic: it is the case, for instance, of cystic fibrosis or colour blindness). Finally, we show that gene-environment interactions have non negligeable effects. In two other articles that we are writing, we complexify our models to include social classes and cultural transmission.

Thus, we aim most of all to draw attention to obesity as a demographic question, and not only medical or epidemiological. Certainly, it is a complex problem, in which biological, genetic, psychological, social or even economic elements coexist, but those elements should be integrated into a demographic framework that has not yet been fully considered by researchers.

Notes

Aldea, N., García-Aguirre, A., Beltrán-Sánchez, H., Daza, S., & Palloni, A. (2025). Generalized Stable Population and Agent-Based Models of phenotypic transmission in human populations, with an application to body size. Human Heredity, 1–26. https://doi.org/10.1159/000549053 ; Aldea, N., García-Aguirre, A., Beltrán-Sánchez, H., Daza, S., & Palloni, A. (2025). Genes, mating and fertility as determinants of global obesity trends. Mathematical Population Studies, 1–13. https://doi.org/10.1080/08898480.2025.2597019