Obesity: a contagious disease among peers?

3 June 2024

With nearly 15% of children suffering from obesity or overweight, China has become the most affected country by obesity in the world. A growing number of research suggests that obesity can be transmitted between couples, siblings and close friends. As important members of the social network, classmates may also contribute to the spread of this disease. However, this phenomenon has rarely been studied. To address this topic, a recent study examined and compared data from Chinese children from 26 different schools. According to this work, several anthropometric parameters and lifestyle habits such as diet and physical activity are associated with those of classmates.

Over the past 40 years, obesity has become a major health issue among children and adolescents worldwide (Caprio et al., 2020 ; Lister et al., 2023). While many lifestyle risk factors have been identified – diet, lack of physical activity – the role of social influence is gaining interest. In particular, numerous studies demonstrate the importance of social ties on an individual’s weight and behaviours (Christakis et al., 2007 ; Datar et al., 2018).

Childhood is an important stage during which individuals develop health behaviours and establish their social relationships as they enter school. At this stage, classmates are important actors who may influence the behaviors of their peers. In this context, a cross-sectional study (Liu et al, 2024) conducted in China investigated whether obesity and obesity-related behaviours spread between classmates.

Similarities between peers in obesity characteristics

A total of 3,670 students in 4th, 5th or 6th grade enrolled in 26 schools took part in the survey. The collection of several types of data was carried out from May to June 2017 and included:

  • Anthropometric data: height, weight, BMI and percentage of body fat ;
  • Physical activity levels: using the PAQ-C questionnaire;
  • Dietary intakes: self-reported by parents using a food frequency questionnaire. Food groups included vegetables, fruit, fast food, snacks and sweetened beverages.
  • Family characteristics: place of residence, monthly household income and parental education levels;
  • Geospatial characteristics: number of restaurants, supermarkets, physical activity zones within a 1km radius of each child’s school.

The mean age of the children surveyed was 10.8 years. Average BMI was 18.9 and body fat percentage 20.8%.

According to the results, there were similarities between the BMI, body fat percentage, physical activity level and dietary intake in 4th and 6th grade children and their classmates. A gender effect was also observed for BMI and body fat percentage. Indeed, these anthropometric parameters were more strongly associated with same-sex classmates within the same class. This association between BMI and social ties is consistent with previous studies conducted within the same school (Asirvatham et al., 2014; Lim & Meer, 2018; Loh & Li, 2013; Nie et al., 2015).

Health behaviors depend on the strength of bonds between peers

To explain these observations, several studies suggest that close friends tend to adopt similar health behaviours (Ali & Dwyer, 2010; Balsa et al., 2011; Purwono & Rodkin, 2014; Daw et al., 2015; Montgomery et al., 2020). Several mechanisms derived from behavioral sciences and potentially involved in the contagion of obesity, have been identified:

  • Behavioral modeling;
  • Social norms;
  • Social control;
  • Social support;
  • Social comparison (Cunningham et al., 2012; Hammond, 2010; Smith et al., 2020).

However, research on the subject remains limited and requires further investigation.
Previous studies have shown that children’s weight status is more strongly associated with same-sex peers when these are defined as close friends (Renna et al., 2008; Salvy et al., 2012; Chung et al., 2017). Associations between social ties and health behaviours may thus suggest that behavioral homophily depends on:

  • The place where the behavior occurs;
  • The strength the bonds (Bahr et al., 2009).

Network interventions as preventive strategies for childhood obesity

This study is one of the first to explore the potential “contagion” of obesity in schools by examining children’s weight status and health behaviours.

As the results suggest, modeling healthy behaviours in the classroom can encourage children to adopt health-promoting habits. In particular, several studies have demonstrated the effectiveness of network interventions in promoting behaviour change (Hene et al., 2022; Jancey et al., 2023; Polman et al., 2023; Valente, 2012). Future work is thus invited to consider the inclusion of classmates in the design of obesity interventions.