Supplementary Material for: Weight Status and BMI-Related Traits in Adolescent Friendship Groups and Role of Sociodemographic Factors: The European IDEFICS/I.Family Cohort

<b><i>Background:</i></b> During adolescence, health behaviors and weight status are increasingly influenced by friendship and peer networks. This paper examines resemblances in weight-related characteristics and how they differ by sociodemographic factors. <b><i>Methods:</i></b> Over 3,000 friendships were reported by 1,603 adolescents, aged 11–16 years, who participated in the school-based I.Family study in 6 European countries. Each “source child” named 1–10 friends for whom standardized weight-related traits were available in the same survey. The mean value of the friends’ traits weighted by time spent together was calculated, and related to the source child’s trait. Country, age and sex of the source child, parental education, and immigrant background were considered for confounding and moderation. <b><i>Results:</i></b> Source children’s <i>z</i>-scores of body fat percent and BMI were positively associated with their friends’ characteristics, in particular if they had highly educated parents. Positive associations were also found regarding the frequency of fast-food consumption, impulsivity, screen time, preference for sugar-sweetened foods, and hours spent in sports clubs, in increasing order of effect size. Additionally, correlations were observed between friends’ cognitive and school functioning and being bullied. No associations were seen for a preference for high-fat foods, weight concerns, and health-related quality of life. Finally, parental education and immigrant background were associated between friends in all countries except Sweden, where no associations were observed. <b><i>Conclusion:</i></b> Adolescent friends shared a number of weight-related characteristics. For weight measures per se, positive associations with friends’ characteristics were only observed in adolescents with high parental education. Associations regarding energy-balance behaviors and indicators of school-related well-being did not differ by parental education. Parental education and immigrant background correlated positively in friends in most countries showing that social aggregation is already occurring in adolescence. The wide spectrum of friendship associations in weight-related traits and behaviors suggests that health promotion initiatives in adolescents should be directed towards peer groups in both school-related and leisure-time environments. <b><i>ISRCTN Registry:</i></b> Pan-European IDEFICS/I.Family children cohort (ID ISRCTN62310987; https://doi.org/10.1186/ISRCTN62310987).

**背景：**
在青春期阶段，健康行为与体重状态愈发受到友谊与同伴社交网络的影响。本研究旨在探讨与体重相关特征的相似性，及其随社会人口学因素变化的差异。

**方法：**
来自6个欧洲国家、参与基于学校的I.Family研究的1603名11~16岁青少年，共报告了超过3000段友谊关系。每位“源受试者”可提名1至10名好友，且上述好友的标准化体重相关特征均可从同一调查中获取。本研究以共处时长为权重，计算好友特征的平均值，并将其与源受试者自身的特征进行关联分析。同时纳入源受试者所在国家、年龄、性别、父母受教育程度以及移民背景，以控制混杂效应并检验调节作用。

**结果：**
源受试者的体脂百分比z分数与体质量指数（Body Mass Index, BMI）z分数与其好友的特征呈显著正相关，该关联在父母受教育程度较高的青少年群体中尤为明显。按效应量递增排序后，快餐食用频率、冲动性、屏幕使用时长、偏好含糖食品以及在体育俱乐部的锻炼时长，均呈现出相似的正相关关联。此外，好友间的认知功能与学校表现、受欺凌情况也存在相关性。未观察到受试者对高脂食品的偏好、体重焦虑以及健康相关生活质量存在此类关联。最后，除瑞典外，所有国家的好友间父母受教育程度与移民背景均存在正相关，而瑞典未观察到此类关联。

**结论：**
青春期好友间存在多项与体重相关的特征共性。就体重指标本身而言，仅在父母受教育程度较高的青少年群体中，可观察到其与好友特征的正相关关联。而与能量平衡行为及学校相关幸福感指标的关联，则未因父母受教育程度而呈现差异。多数国家的好友间父母受教育程度与移民背景均呈正相关，这表明社会聚集现象在青春期阶段已然显现。体重相关特征与行为的友谊关联覆盖广泛，这提示青少年健康促进举措应同时针对学校与休闲场景中的同伴群体。

**ISRCTN注册信息：**
泛欧洲IDEFICS/I.Family儿童队列（登记号ISRCTN62310987；https://doi.org/10.1186/ISRCTN62310987）。