Understanding the relationship between depression and trajectories of physical multimorbidity accrual: longitudinal analysis of UK Biobank data

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There are increasing numbers of people who have multimorbidity which means living with multiple physical and/or mental health conditions. Increasing multimorbidity has a number of causes. People are living longer and are more likely to survive life-threatening illness than in the past (for example, anyone who survives a heart attack will then have to live with chronic heart disease). Living longer and surviving life-threatening illness are of course good things, but multimorbidity poses challenges to health services and research which often focuses on single conditions. We need to understand how people develop multiple conditions better, and this is the focus of this research. The combination of physical conditions with depression is very common. About one in five people with a physical condition have depression, and this combination is associated with worse physical and mental health outcomes. It is therefore important to understand how physical conditions and depression are related. Existing research has often just looked at people at one point in time. This means that it isn't possible to know if depression causes physical disease, or if physical disease causes depression, or both. Existing research also does not always account for things which might be related to both physical disease and depression such as heavy drinking or lack of exercise. The aim of this research is to examine the relationship between physical disease and depression using data for half a million middle-aged people followed up for ~10 years as part of the UK Biobank study. There are three elements to the planned work:First, we will develop a number of different ways of measuring the patterns of physical conditions that people get in middle and older age. There are three different ways of doing this: (1) Simply counting how many physical conditions people develop over 10 years; (2) Identifying the 'shape' or trajectory of how they develop new physical conditions (eg developing none vs slowly developing new conditions vs rapidly developing them); and (3) Identifying if there are groups of physical conditions that develop together (for example, different conditions caused by smoking, but there may be unexpected groupings as well). We will also explore different ways of measuring depression in the data, because we don't know whether the severity of depression affects the relationship with physical conditions. Second, we will examine whether depression before the start of the study predicts how people develop physical conditions over the next 10 years. We will do this using a number of methods, based on the different ways of measuring patterns of physical conditions described above. We will carefully account for the effect of other individual characteristics which might be linked to both depression and patterns of physical condition such as gender, smoking, alcohol, obesity, and how rich or poor a person is. Third, we will examine whether different patterns of physical condition predict whether or not people develop new or recurrent depression. Again, we will carefully account for the effect of other individual characteristics which might be linked to both depression and patterns of physical conditions. The study will add significantly to our understanding of the patterns of physical conditions which people develop in middle age, and the links between physical conditions and depression. The same approach can be used in the future to examine how physical conditions are linked to other mental health problems like anxiety or schizophrenia. It will also make possible additional studies that examine if there are common genetic (inherited) causes of both depression and physical conditions, or if there are common mechanisms affecting different diseases like inflammation. Finally, better understanding of patterns of disease will help identify people who may need more help from health services to improve their quality of life and future health.