That's a reasonable hypothesis. Another one is that it's a reverse causality: Several diseases may inhibit Vitamin D uptake. And don't forget: It can be multip

That's a reasonable hypothesis. Another one is that it's a reverse causality: Several diseases may inhibit Vitamin D uptake. And don't forget: It can be multiple causes combined. (I.e. it may be a mix of a confounder like being outside and reverse causality.)