School education, physical performance in late midlife and allostatic load: a retrospective cohort study Åse M Hansen, 1,2,3 Lars L Andersen, 2 Carlos F Mendes de Leon, 4 Helle Bruunsgaard, 5 Rikke Lund 1,6 1 Department of Public Health, University of Copenhagen, Copenhagen, Denmark 2 National Research Centre for the Working Environment, Copenhagen, Denmark 3 Copenhagen Stress Research Center, Copenhagen, Denmark 4 Department of Epidemiology, Center for Social Epidemiology and Population Health, University of Michigan School of Public Health, Chicago, Michigan, USA 5 Department of Clinical Immunology, Center for Inflammation and Metabolism, and National University Hospital, Copenhagen, Denmark 6 Center for Healthy Ageing, University of Copenhagen, Copenhagen, Denmark Correspondence to Professor Åse Marie Hansen, Department of Public Health, University of Copenhagen, Østre Farimagsgade 5, Copenhagen K DK-1014, Denmark; asemarie.hansen@ sund.ku.dk Received 23 February 2015 Revised 25 November 2015 Accepted 22 December 2015 Published Online First 14 January 2016 To cite: Hansen ÅM, Andersen LL, Mendes de Leon CF, et al. J Epidemiol Community Health 2016;70:748–754. ABSTRACT Background The mechanisms underlying the social gradient in physical functioning are not fully understood. Cumulative physiological stress may be a pathway. The present study aimed to investigate the association between highest attained school education and physical performance in late midlife, and to determine to what extent cumulative physiological stress mediated these associations. Methods The study is based on data from the Copenhagen Aging and Midlife Biobank (CAMB; n=5467 participants, aged 48–62 years, 31.5% women). School education was measured as highest examination passed in primary or secondary school (3 categories). Cumulative stress was operationalised as allostatic load (AL), and measured as the number of biological parameters (out of 14) in which participants scored in the poorest quartile. Physical performance included dynamic muscle performance (chair rise ability, postural balance, sagittal flexibility) and muscle strength ( jump height, trunk extension and flexion, and handgrip strength). Results Among women, higher school education was associated with better performance in all physical performance tests. Among men, higher school education was associated with better performance only in chair rise and jump height. AL partially mediated the association between school education and physical performance, and accounted only for 2–30% of the total effect among women. Similar results were observed among men for chair rise and jump height. Conclusions These results might indicate that AL plays a minor role in the association between school education and late midlife dynamic muscle performance in both men and women, and in muscle strength among women. INTRODUCTION Functional problems in old age are not only related to current circumstances, but may be traced back to earlier life experiences. 1 Childhood socioeconomic circumstances have previously been shown to be an important predictor of later life health. 2 3 Socioeconomic position (SEP) throughout the life course has shown to have a strong impact on midlife physical performance measures such as chair rise. 4 Recent evidence from a systematic review and meta-analysis has shown that there are modest associations between childhood SEP and physical performance, and that not all associations are fully explained by the continuity of SEP from childhood to adulthood. 5 Early life SEP, operationalised as mother’s and father’s education, and midlife physical performance and the social environment in which a child grows up, has a strong association with physical performance in midlife. 5–7 A recent study aimed to investigate how different facets of SEP (educational attainment and income) affect the onset versus progression of health problems. 8 Also school education acquired early in life was predictive of the onset of a range of adverse health outcomes in midlife. 8 Poor adult SEP has been shown to be associated with lower grip strength in midlife, 9 and self- reported functional limitations (eg, running 100 m and climbing stairs) in midlife and in old age among men and women. 10 11 Social gradients in physical function tests referring to muscle strength in central parts of the body 10 have been less studied. 12 Physical performance in grip strength and walking speed in midlife are strong predictors of future disability, 13 14 morbidity 15 and survival. 16 Measuring levels of physical functions in popula- tions in midlife may provide a possibility to capture individuals who are at risk of onset of disability in later life, and for whom intervention might be effective. The mechanisms underlying the social gradient in physical functioning are not fully understood. Stress is an important contributor to physical pathological conditions in humans. Through poten- tial mediators, stress can lead to acute or chronic pathological, physical and mental conditions in individuals with a vulnerable genetic, constitutional and/or epigenetic background. 17 People with sus- tained economic hardship (a potential stressor) are more likely to have poorer physical, psychological and cognitive functioning. 18 The concept of allo- static load (AL) provides a theoretical framework for understanding the physiological pathways from socioeconomic disadvantage to later adverse health outcomes. 19 Cumulative socioeconomic disadvan- tage over the life course are strongly related to AL in adulthood. 20 AL was introduced as a multisystem view of the cumulative physiological toll that may be exacted on the body through the ageing of major peripheral physiological regulatory systems, including the autonomic nervous system, cardiovas- cular, metabolic and inflammatory processes. 21 The concept of AL was introduced as the course of a lifetime of efforts to adapt to life’s demands in the early 1990s by McEwen and Stellar. 22 In the pio- neering work on AL, the index was based on 10 physiological parameters and investigated as a risk factor of cardiovascular disease, cognitive and 748 Hansen ÅM, et al. 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