[Kevan Gelling (OP)]

From the Immune link to diabetes news item in 26th July's "The Science of Rubbish" podcast
The results suggest that inflammation in fatty tissue may play a role in obesity and the development of types 2 diabetes.

Is this why vitamin D3 deficiency is linked to obesity and diabetes?

[iko]

Hi DDave & Kevan,

One major link between vitamin D and immunity seems to be cathelicidin, one of the so called 'antimicrobial peptides' produced by white cells under control of the steroid hormone cholecalciferol (D3-vitamin!).  Cholecalciferol should not be classified as a vitamin anymore: over 95% is produced by our skin and not obtained from the diet, is not a cofactor for any enzyme in the biochemical machinery of our cells.  It is a real hormone, a steroid molecule binding to specific ligands and switching on over 200 genes in the nuclei of specialized cells.  One of these effects is the production of antimicrobial peptides inside macrophages, capable of controlling infections by viruses and intracellular bacteria (e.g.: mycobacteria).
There are many 'links' between vitamin D and immunity, even in this Forum.
I cut and paste this recent abstract, found crossing vitD3&immunity on PubMed:

The roles of cathelicidin LL-37 in immune defences and novel clinical applications.

Nijnik A, Hancock RE.
Centre for Microbial Diseases and Immunity Research, Department of Microbiology and Immunology, 2259 Lower Mall Research Station, University of British Columbia, Vancouver, Canada.

PURPOSE OF REVIEW: LL-37 is the only member of the cathelicidin family of host defence peptides expressed in humans. It is primarily produced by phagocytic leucocytes and epithelial cells, and mediates a wide range of biological responses: direct killing of microorganisms, chemotaxis and chemokine induction, regulation of inflammatory responses, as well as adjuvant, angiogenic and wound healing effects. In this review we will cover the recent advances in the understanding of LL-37 biology: its activities, the mechanisms of its induction and roles in immune defence. RECENT FINDINGS: Recent studies advanced our understanding of the mechanisms controlling LL-37 expression, demonstrating the key involvement of the vitamin D3 and the hypoxia response pathways, and the impacts of commensal and pathogenic microorganisms on its production. The synergistic and antagonistic interactions between LL-37 and other immune mediators have been further elucidated. Furthermore, studies in animal models and human patients further characterized the roles of cathelicidins in immunity, with roles in infectious and inflammatory conditions. The underlying properties of LL-37 have been exploited to create innate defence regulator peptides that represent a novel immunomodulatory approach to treating infections.
SUMMARY: The understanding of the biological properties and functions of LL-37 and other host defence peptides advances our knowledge of innate immunity, the interactions of the host with pathogens and the microflora, as well as the pathology of infectious and inflammatory diseases, creating many strategies and opportunities for therapeutic intervention.

Curr Opin Hematol 2009 Jan;16(1):41-7.