Preprint · Jan 2026
The Interaction of Multiple Social Determinants on the Burden of Multimorbidity
By McCormack R
1.0 Background Multimorbidity is commonly defined as the coexistence of two or more chronic conditions in the same individual (World Health Organization, 2016). It was a term first introduced by van den Akker in 1996 (van den Akker, 1996). Prior to this, terms such as ‘comorbidity’ or ‘multiple chronic conditions’ were used, but these were inadequate for appreciating the interplay and complexity that underpin having multiple co-existing conditions of equal weight. Boyd and Fortin clarified in their 2010 revision of the definition that multimorbidity was defined as ‘the co-existence of two or more chronic conditions, where one is not necessarily more central than the others’ (Boyd & Fortin, 2010). The revision of the definition of multiple chronic conditions was integral in placing the patient at the forefront of treatment. This facilitated a shift towards a more holistic healthcare approach, one that appreciated multiple conditions in the context of the whole system rather than focusing solely on a single disease. Through this lens, treatment, especially in primary care, could be shaped by the patient’s experience and priorities rather than in response to one condition. The distinction between multimorbidity and related terms is also important for research, as studies can be compared and synthesised when structured around a common framework. Secondarily, without clarity in definitions, there can be biased sampling or inappropriate population selection in clinical trials, both of which undermine the repeatability and reproducibility of the research. Beyond the need for conceptual clarity, there is a growing imperative to address multimorbidity given its profound clinical, social, and economic impact. Clinical guidelines are often crafted around single diseases because they are based on evidence from randomised controlled trials, which exclude older people and those with multiple conditions (Wallace, 2015). When a patient then presents with multimorbidity, healthcare systems and professionals are inadequately prepared to manage these increasingly complex conditions. This inadequacy is demonstrated in inappropriate polypharmacy, poorer outcomes, and fragmented care. Furthermore, the healthcare system is strained by the immense costs of these conditions; health expenditures increase with the number of chronic conditions, adding a substantial financial burden to an already crippled system (Soley-Bori, 2021). In tandem with this, multimorbidity doesn’t just affect an ageing population; over 20% of those aged 20-64 currently have multimorbidity. Projections estimate that from 2019 to 2049, there will be an 84% increase in the total number afflicted (Head, 2024). When considering multimorbidity, it is imperative to acknowledge that it exists within the context of the conditions in which we grow, live, work, and age, as well as the systems put in place to address these illnesses; these conditions are often called the social determinants of health (WHO, 2025). These non-biological factors are the drivers of differences in health outcomes. Therefore, any plan to tackle health inequality must centre on social health. This idea is reflected in Michael Marmot’s 2010 Review “Fair Society, Healthy Lives”, where he argues that change in social policy is necessary for change in health outcomes. These strategies concentrate on areas such as fairer employment, healthy living standards, and sustainable communities (Marmot, 2010). Addressing social determinants of health is integral to challenging health disparities between the healthiest and unhealthiest in society, and as Álvarez-Gálvez explores in his 2023 review, individuals in poorer social conditions are disproportionately more affected by more severe and complex multimorbidity (Álvarez-Gálvez, 2023). Therefore, to change the patterns and prevalence of multimorbidity comes from addressing the social determinants which shape it. No social condition happens in isolation. Each social determinant has a unique role but is also entangled in relationships with all other co-occurring determinants. When we explore these relationships, we can speculate that health outcomes will be better or worse as a result of this interaction. Some research has already examined this additive effect; Ahnquist concluded that although both low social capital and low economic status are independently associated with poor health outcomes, when combined, these outcomes are exacerbated (Ahnquist, 2012). This synergistic effect has also been demonstrated by Sun et al. (2009) where an interaction of poverty and individual social capital in urban China led to increased likelihood of poor self-rated health (Sun, 2009) Yet there is still a gap in the literature; this review aims to understand the possible protective and exacerbating effects these social determinants of health may have on one another and their specific roles in multimorbidity. Further, this review aims to understand the magnitude of the interactions and, potentially, construct a hierarchical ranking of the determinants by their impact on multimorbidity, which could help order those with the greatest modifiability and therefore offer the most health gains. In this way, changes made to policy would be most impactful and most achievable. 2.0 Review Question Objective: How does the interaction of two or more social determinants of health contribute to the burden of multimorbidity? Population: Adult population/working age population Exposure: 2 or more social determinants of health Comparator: 1 or no social determinants of health Outcomes: Multimorbidity