We are always interested in research which can show what conditions may cause chronic inflammation

Australian Center for Precision Health, University of South Australia Cancer Research Institute,
Adelaide, Australia, 2
South Australian Health and Medical Research Institute, Adelaide, Australia and 3
Population, Policy and Practice, UCL Institute of Child Health, London, UK
*Corresponding author. Australian Center for Precision Health, University of South Australia Cancer Research Institute,
GPO Box 2471, Adelaide, SA 5001, Australia. E-mail: Elina.Hypponen@unisa.edu.au
Received 14 December 2021; Editorial decision 29 March 2022; Accepted 8 April 2022

Abstract
Background: Low vitamin D status is often associated with systemic low-grade inflam-
mation as reflected by elevated C-reactive protein (CRP) levels. We investigated the cau-
sality and direction of the association between vitamin D status and CRP using linear and non-linear Mendelian randomization (MR) analyses.

Methods: MR analyses were conducted using data from 294 970 unrelated participants
of White-British ancestry from the UK Biobank. Serum 25-hydroxyvitamin D [25(OH)D] and CRP concentrations were instrumented using 35 and 46 genome-wide significant variants, respectively.

Results: In non-linear MR analysis, genetically predicted serum 25(OH)D had an L-shaped
association with serum CRP, where CRP levels decreased sharply with increasing
25(OH)D concentration for participants within the deficiency range (<25 nmol/L) and lev-
elled off at – 50 nmol/L of 25(OH)D (Pnon-linear ¼ 1.49E-4).

Analyses using several pleiotropy-robust methods provided consistent results in stratified MR analyses, con-
firming the inverse association between 25(OH)D and CRP in the deficiency range
(P ¼ 1.10E-05) but not with higher concentrations. Neither linear or non-linear MR analysis supported a causal effect of serum CRP level on 25(OH)D concentration (Plinear ¼ 0.32 and Pnon-linear ¼ 0.76).
Conclusion: The observed association between 25(OH)D and CRP is likely to be caused by vitamin D deficiency.

Correction of low vitamin D status may reduce chronic inflammation.
Key words: Non-linear Mendelian randomization, vitamin D, serum 25-hydroxyvitamin D concentration, C-reactive
protein, chronic inflammation

The full study can be read below