"I don't want to manage it, I want to get rid of it" : a narrative analysis of living with chronic plaque psoriasis, and an investigation into vitamin D as a treatment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutritional Science at Massey University, Albany, New Zealand

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As a chronic skin disease, plaque psoriasis can cause significant psychosocial, emotional and physical burden. Psoriasis sufferers perceive others as lacking understanding around what it is like to live with this condition, and there has been little research exploring the experience of psoriasis in depth. The burden of psoriasis can be compounded by the difficulty of treating it, and the inconveniences, side effects and risks of available treatments, suggesting the importance of finding a safe, effective and convenient treatment for psoriasis. Vitamin D and psoriasis have a long-standing relationship, with topical vitamin D analogues used to treat mild-tomoderate disease, and observational studies suggesting an association between higher systemic vitamin D (serum calcidiol) concentrations and less severe psoriasis. These findings suggest vitamin D3 supplements, which raise serum calcidiol concentrations, might improve psoriasis. In this thesis, two studies were conducted to address the limited in-depth understanding of the experience of psoriasis, and the need for a safe, effective treatment, respectively. The aims were 1) to gain a deeper understanding of the experience of living with psoriasis; and 2) to investigate whether oral vitamin D3 supplements can effectively treat psoriasis. For 1), data from semi-structured interviews with 10 men and women with psoriasis was analysed using narrative analysis. Narrative trajectories involving three predominant narrative forms shaped participants’ stories: restitution, where the focus was on overcoming psoriasis through trying to find an effective treatment or cure; chaos, where psoriasis was experienced as overwhelming and brought about a sense of hopelessness, and resignation, which was centred on begrudgingly accepting psoriasis in order to be able to get on with life. Participants had different narrative trajectories and shifted between forms over time, with the nature of experience linked with the relative stability and severity of a person’s psoriasis and their beliefs about their ability to manage it. For 2), a randomised, double-blinded, placebo-controlled trial was conducted with 101 participants ≥ 18 years allocated to 100,000 International Units (IU) vitamin D3/month (n = 67) for 12 months (200,000 IU at baseline), or an identical placebo (n = 34). Psoriasis severity (Psoriasis Area and Severity Index [PASI]) and serum calcidiol concentrations were assessed at 3-monthly intervals. The primary outcome was the difference in PASI between treatment and placebo over time, assessed using a linear mixed model. Psoriasis severity did not differ between groups at any time (group F(1, 106) = 0.59, p = 0.44, group*time F(4, 370) = 0.52, p = 0.72). Yet these findings are inconclusive, as serum calcidiol significantly increased from baseline in both the treatment and the placebo group, and a mild improvement in PASI score from baseline also occurred in each group. A non-predetermined secondary analysis was performed by assessing the strength of the relationship between serum calcidiol concentration and PASI score across the whole sample, and this showed a significant inverse relationship between the two variables, in that elevation of serum calcidiol concentration by increments from 25 nmol/L to 125 nmol/L was associated with very mild decreases in PASI (estimated range of decrease 0 – 2.6; p = 0.002). Therefore, despite being unable to determine a benefit of vitamin D3 supplements for psoriasis, these findings support the notion of a potential benefit of increasing serum calcidiol concentrations across the psoriatic population. In conclusion, this thesis offers insight into ways in which people can experience psoriasis over time: as a temporary and fixable condition that must be overcome, as an overpowering force and source of significant suffering, and as a permanent condition that is reluctantly accepted. As the findings emphasise the negative influence of the difficulties around managing and treating psoriasis on the experience of psoriasis, they provide further support for the need for an effective, safe and convenient treatment. While the findings were inconclusive in regards to whether oral vitamin D3 can help people to manage their psoriasis, the significant association between psoriasis severity and systemic vitamin D concentration supports continued research into this potential.
Psoriasis, Vitamin D, Therapeutic use, Research Subject Categories::INTERDISCIPLINARY RESEARCH AREAS::Domestic science and nutrition