The authors are testing the idea the levels of vitamin D in blood cause differences in rheumatoid arthritis(RA). They compare relations between genetic variant

The authors are testing the idea the levels of vitamin D in blood cause differences in rheumatoid arthritis(RA). They compare relations between genetic variant of both (Vit D and RA) and show no or very weak causal association. Therefore there us no evidence that Vit D is a treatment for RA. More likely an effect of RA.