Do emerging adults know what their friends are doing and does it really matter? Methodologic challenges and associations of perceived and actual friend behaviors with emerging adults' disordered eating and muscle building behaviors.

Disordered eating and muscle building behaviors are common among emerging adults, and friends may be a particularly salient social influence. Epidemiologic research often includes questions about participants' perceptions of their friends. A less common approach, with greater logistical challenges, is to ask for friend nominations and then survey friends about their actual behaviors. The comparability of these different approaches is unknown. This study addresses the following research questions: 1) What is the feasibility of collecting data from emerging adults' friends in epidemiologic research? 2) Do perceptions of friends' weight- and shape-related behaviors align with friends' actual behaviors? and 3) Are perceptions or friends' actual behaviors more strongly and consistently associated with emerging adults' behaviors? Participants (N = 2383) in the EAT (Eating and Activity over Time)-2018 study in Minnesota, USA, were asked to nominate up to three friends and provide data about those friends' weight- and shape-related behaviors (i.e. perceptions); nominated friends were invited to complete an abbreviated survey and report on their own same behaviors (i.e. actual). Among the invited friends, 191 responded and were linked to the 152 EAT 2018 participants who nominated them. Descriptive statistics, Spearman's correlations, and logistic regression were used to address the research questions. The response rate for nominated friends was very low (9.9%), suggesting this approach may have low feasibility for epidemiologic studies of emerging adults. Emerging adults' perceptions of their nominated friends' weight and shape-related behaviors generally did not align well with the behaviors reported by those friends. Furthermore, analytic models found different associations between friends' behavior and EAT 2018 participant behaviors, depending on the measure of friends' behavior used (perceived or actual). Careful consideration of the pros and cons of each study design is essential to build an evidence base and support interventions regarding emerging adults' weight- and shape-related health.