Although Vitamin D is well known for promoting bone and teeth health by regulating vital calcium levels, it does much more than that. It is now accepted that it plays an important role in immunity*.

The human gut is the first point of contact with microbes. Here resides much of our mucosal immunity (named GALT or Gut Associated Lymphoid Tissue). 

The gut is the Frontline of all frontlines in the body, as every mouthful of food comes with a heavy cargo of exterior microbes.

The Gut associated immunity is therefore in constant need to gauge its response to its surrounding environment: Sound the alarm when bad bacteria breach the barrier and dozing-off in tolerant mode when good (commensal) bacteria in the gut are at work for the host's benefit.

Expanding body of research suggest the pivotal role of Vitamin D plays in immunity*/**, and specially in gut immunity, through Vitamin D Nuclear Receptors (VDR) expressed in human cells. VDR's sense Vitamin D and trigger, amongst others, certain genes, that encode key proteins for gut regulation.

Such proteins, for example, help gut cells bind together better preventing "leakage" while others  called HDP (Host Defense Peptides or AntiMicrobial Peptides AMP) help control undesirable microbes.

* Vitamin D contributes to the normal function of the immune system.
** Vitamin D has a role in the process of cell division
Source of claims: Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children’s development and health.