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Understanding how body mass index (BMI) and biological aging influence each other can guide strategies to reduce age\u2010 and obesity\u2010related health risks. We examined the bidirectional, longitudinal association between changes in BMI and biological aging, measured by frailty index (FI) and functional aging index (FAI), across late life.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>This longitudinal cohort study used data from the Swedish Twin Registry substudies, GENDER, OCTO\u2010Twin and SATSA, collected via in\u2010person assessments from 1986 to 2014 at 2\u2010 to 4\u2010year intervals. We analysed 6216\u20136512 evaluations from 1902 to 1976 Swedish twins. Dual change score models were applied to assess the bidirectional, longitudinal association between BMI and FI or FAI from ages 60.0\u201391.9. FI measured physiological aging, while FAI assessed functional aging through a composite score of functional abilities.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>At first measurement, mean age was 74\u2009\u00b1\u20098, and 41% were males. BMI\u2013FI relationship was bidirectional (<jats:italic>p<\/jats:italic> value\u2009\u2264\u20090.001): Higher BMI predicted a greater increase in FI over time (coupling effect [<jats:italic>\u03b3<\/jats:italic>]\u2009=\u20090.86, 95% confidence interval [CI]\u2009=\u20090.65\u20131.06, <jats:italic>p<\/jats:italic> value\u2009\u2264\u20090.001), and higher FI predicted steeper decline in BMI (<jats:italic>\u03b3<\/jats:italic>\u2009=\u2009\u22120.04, 95% CI\u2009=\u2009\u22120.05 to \u22120.03, <jats:italic>p<\/jats:italic> value\u2009\u2264\u20090.001). When including coupling from FI, BMI showed a nonlinear trajectory with a mean intercept of 26.32\u2009kg\/m<jats:sup>2<\/jats:sup> (95% CI\u2009=\u200925.76\u201326.88), declining more rapidly after age 75. When including BMI coupling, FI increased from a mean intercept of 7.91% (95% CI\u2009=\u20096.41\u20139.42), with steeper growth from ages 60\u201375. BMI\u2013FAI relationship was unidirectional (<jats:italic>p<\/jats:italic> value\u2009\u2264\u20090.001): Higher FAI predicted a steeper BMI decline (<jats:italic>\u03b3<\/jats:italic>\u2009=\u2009\u22120.02, 95% CI\u2009=\u2009\u22120.02 to \u22120.01, <jats:italic>p<\/jats:italic> value\u2009\u2264\u20090.001). By including FAI coupling, BMI had a mean intercept of 26.10\u2009kg\/m<jats:sup>2<\/jats:sup> (95% CI\u2009=\u200925.47\u201326.74), declining rapidly after age 75. FAI increased exponentially from a mean intercept of 36.49 (95% CI\u2009=\u200934.54\u201338.43).<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>Higher BMI predicted a steeper increase in FI, substantiating the hypothesis that obesity accelerates biological aging. Higher biological aging, measured as FI and FAI, drove a steeper BMI decline in late life, signalling that late\u2010life weight loss may result from accelerated aging. Higher BMI may accelerate aspects of the aging process, and the aging process, in turn, accelerates late\u2010life BMI decline, necessitating an integrated approach to manage both obesity and unintentional weight loss among older adults.<\/jats:p><\/jats:sec>","DOI":"10.1002\/jcsm.13824","type":"journal-article","created":{"date-parts":[[2025,5,8]],"date-time":"2025-05-08T07:40:15Z","timestamp":1746690015000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["A Longitudinal Study of the Bidirectional Temporal Dynamics Between Body Mass Index and Biological Aging"],"prefix":"10.1002","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4845-1180","authenticated-orcid":false,"given":"Peggy","family":"Ler","sequence":"first","affiliation":[{"name":"Department of Medical Epidemiology and Biostatistics Karolinska Institutet  Solna Sweden"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jonathan\u00a0K.\u00a0L.","family":"Mak","sequence":"additional","affiliation":[{"name":"Department of Medical Epidemiology and Biostatistics Karolinska Institutet  Solna Sweden"},{"name":"Department of Pharmacology and Pharmacy Li Ka Shing Faculty of Medicine The University of Hong Kong  Hong Kong SAR China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Chandra\u00a0A.","family":"Reynolds","sequence":"additional","affiliation":[{"name":"Institute for Behavioral Genetics and Department of Psychology and Neuroscience University of Colorado Boulder  Boulder Colorado USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Alexander","family":"Ploner","sequence":"additional","affiliation":[{"name":"Department of Medical Epidemiology and Biostatistics Karolinska Institutet  Solna Sweden"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Nancy\u00a0L.","family":"Pedersen","sequence":"additional","affiliation":[{"name":"Department of Medical Epidemiology and Biostatistics Karolinska Institutet  Solna Sweden"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Juulia","family":"Jylh\u00e4v\u00e4","sequence":"additional","affiliation":[{"name":"Department of Medical Epidemiology and Biostatistics Karolinska Institutet  Solna Sweden"},{"name":"Faculty of Medicine and Health Technology and Gerontology Research Center University of Tampere  Tampere Finland"},{"name":"Tampere Institute for Advanced Study  Tampere Finland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Anna\u00a0K.","family":"Dahl\u00a0Aslan","sequence":"additional","affiliation":[{"name":"School of Health Sciences University of Sk\u00f6vde  Sk\u00f6vde Sweden"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Deborah","family":"Finkel","sequence":"additional","affiliation":[{"name":"Center for Economic and Social Research University of Southern California  Los Angeles California USA"},{"name":"Institute for Gerontology J\u00f6nk\u00f6ping University  J\u00f6nk\u00f6ping Sweden"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ida\u00a0K.","family":"Karlsson","sequence":"additional","affiliation":[{"name":"Department of Medical Epidemiology and Biostatistics Karolinska Institutet  Solna Sweden"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"311","published-online":{"date-parts":[[2025,5,8]]},"reference":[{"key":"e_1_2_10_2_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0140-6736(23)02750-2"},{"key":"e_1_2_10_3_1","volume-title":"World Population Prospects 2022: Summary of Results","author":"United Nations Department of Economic and Social Affairs, Population Division","year":"2022"},{"key":"e_1_2_10_4_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.metabol.2018.09.005"},{"key":"e_1_2_10_5_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0140-6736(17)32129-3"},{"key":"e_1_2_10_6_1","doi-asserted-by":"publisher","DOI":"10.1111\/obr.12991"},{"key":"e_1_2_10_7_1","doi-asserted-by":"publisher","DOI":"10.1126\/scitranslmed.adh1175"},{"key":"e_1_2_10_8_1","doi-asserted-by":"publisher","DOI":"10.1093\/ageing\/afab039"},{"key":"e_1_2_10_9_1","doi-asserted-by":"publisher","DOI":"10.1007\/s11357-017-9960-3"},{"key":"e_1_2_10_10_1","doi-asserted-by":"publisher","DOI":"10.1007\/s10519-017-9851-5"},{"key":"e_1_2_10_11_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.bcp.2022.115012"},{"key":"e_1_2_10_12_1","doi-asserted-by":"publisher","DOI":"10.1093\/gerona\/glac197"},{"key":"e_1_2_10_13_1","doi-asserted-by":"publisher","DOI":"10.1007\/s11357-022-00567-7"},{"key":"e_1_2_10_14_1","doi-asserted-by":"publisher","DOI":"10.1093\/ajcn\/82.5.923"},{"key":"e_1_2_10_15_1","first-page":"718","article-title":"Unintentional Weight Loss in Older Adults","volume":"89","author":"Gaddey H. 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