Energy expenditure and the accuracy of predictive equations in myotonic dystrophy type 1.

Loneliness has gained increasing recognition as a major societal and public health challenge in recent years (Goldman et al., ). Previous research has demonstrated associations between loneliness and a range of adverse health outcomes, most notably depression (Abdellaoui et al., ; Goosby, Bellatorre, Walsemann, & Cheadle, ), severe infections (Elovainio et al., ), cardiovascular disease (Abdellaoui et al., ; Hakulinen et al., ), and all-cause mortality (Elovainio et al., ; Wang et al., ; Yu et al., ). However, the mechanisms underlying these associations remain incompletely understood. Adverse health behaviors represent a potential mechanism through which loneliness might affect health (Christiansen, Larsen, & Lasgaard, ). The evolutionary theory of loneliness offers a useful theoretical framework, proposing that individuals who feel lonely may display hypervigilance to social threats (Hawkley & Cacioppo, ; Qualter et al., ). Over time, this sustained alertness may impair executive functioning and self-regulation, thereby heightening susceptibility to impulsive coping strategies and maladaptive health behaviors (Hawkley & Cacioppo, ). Consistent with this perspective, cohort studies in older adults and genetic evidence demonstrate that loneliness is associated with physical inactivity (Hawkley, Thisted, & Cacioppo, ; Pengpid, Peltzer, & Anantanasuwong, ; Shankar, McMunn, Banks, & Steptoe, ), smoking (Shankar et al., ; Wootton et al., ), and underweight (Pengpid et al., ). Among adults experiencing loneliness, such adverse health behaviors might contribute to multiple negative health outcomes, including an increased risk of depression (Liang et al., ), severe infections (Elovainio et al., ), cardiovascular disease (Hakulinen et al., ; Liang et al., ), obesity (Liang et al., ), and premature mortality (Elovainio et al., ). However, recent cohort studies of older adults that adjusted for a broad range of confounders have found no evidence for links between loneliness and subsequent physical activity (Hong et al., ; Kobayashi & Steptoe, ; Schrempft, Jackowska, Hamer, & Steptoe, ), alcohol use (Hong et al., ; Kobayashi & Steptoe, ), smoking (Hong et al., ; Kobayashi & Steptoe, ), and overweight (Hong et al., ; Kobayashi & Steptoe, ). These findings suggest earlier findings may partly reflect bias, such as residual confounding. Moreover, research on loneliness-health behavior linkages predominantly focused on older adults (Hawkley & Capitanio, ; Kobayashi & Steptoe, ), failing to systematically examine these behavioral precursors of disease and disability across the first half of life. Adolescence is a crucial developmental period when future adult behaviors are dynamically shaped, and in which loneliness is particularly prevalent (Luhmann & Hawkley, ; von Soest, Luhmann, & Gerstorf, ). However, findings from older adult populations may not directly generalize to earlier life stages, as risky health behaviors such as alcohol use, physical exercise, and smoking in adolescence and emerging adulthood are often socially embedded and shaped by peer norms, social network processes, and identity formation (Montgomery et al., ), potentially reducing the relative importance of loneliness as an independent driver of these behaviors. Across early life stages, prior research has linked loneliness and lack of social connections to poorer self-rated health and cardiometabolic risk factors such as high blood pressure, elevated body mass index (BMI), cholesterol, and inflammation (Goosby et al., ; Rangul et al., ; Yang et al., ). A large-scale Danish study further demonstrated associations between loneliness and health behaviors among young people (Jensen et al., ); yet its cross-sectional design precluded it from establishing the temporal ordering and directionality of these associations. By contrast, a recent US longitudinal cohort study applying a causal inference framework found no support for causal associations between adolescent loneliness and multiple health behaviors in adulthood, including smoking, alcohol use, physical activity, and overweight/obesity (Kim, Wilkinson, Holt-Lunstad, & VanderWeele, ). Such causal inference approaches involve adjustment for preexisting levels of the exposure, the outcome, and a wide range of confounders to minimize the risk of reverse causality and residual confounding (Mulder, Usami, & Hamaker, ; VanderWeele, , ), consequently providing more robust insights into potential causal linkages. However, systematic investigations into links between loneliness and adverse health behaviors across the first half of life – from adolescence through emerging adulthood and young adulthood – remain scarce.