LL-37.

LL-37 is cleaved from the C-terminus of the precursor protein hCAP18, and transcription of that precursor switches on when 1,25-dihydroxyvitamin D binds the vitamin D receptor at the CAMP promoter. This is one of the few human genes with a well mapped vitamin D response element, which is why vitamin D exposure and cathelicidin expression move together in cultured human cells. The relationship is transcriptional biochemistry, not an effect measured on a clinical endpoint.

Whichever form is taken, the signalling molecule is 1,25-dihydroxyvitamin D acting through its nuclear receptor, and the cathelicidin precursor gene is a direct target of that receptor in human myeloid and barrier cells. A 2026 review of the vitamin D to antimicrobial peptide axis returns to this induction step as the mechanistic core. The cited source is a narrative review, so it grounds the mechanism rather than any measured outcome.

Both 25-hydroxylation in the liver and 1-alpha-hydroxylation in the kidney run on magnesium-dependent enzymes. Low magnesium availability therefore limits how much active metabolite is generated from a given vitamin D intake, and the cathelicidin promoter only answers to the active metabolite. This is cofactor biochemistry, not a combination that has been tested for a cathelicidin readout in people.

Butyrate inhibits histone deacetylases, which loosens chromatin at several innate-defence promoters including the cathelicidin gene, and this induction has been characterised in human colon epithelial cell lines. The pathway is independent of the vitamin D response element, so the two inputs are described as converging on the same gene from different directions. Cell-culture gene expression is a marker, not a clinical outcome.

Colonising bacteria ferment fibre to short-chain fatty acids and interact with epithelial pattern-recognition receptors, both of which are recognised inputs to cathelicidin expression in gut lining cells. A 2026 randomised trial in infants sits behind this pairing, and LL-37 appears there inside a broader immune discussion rather than as the primary endpoint. Read it as a mentions-only link between a defined probiotic strain and innate peptide biology.

Lactoferrin sequesters iron and carries its own cationic antimicrobial fragment, while LL-37 disrupts anionic microbial membranes directly. In secretions such as milk, saliva and airway fluid the two occur together, which is the basis for describing them as complementary arms of the same barrier system. Co-occurrence in biology is not the same as a tested supplement combination.

Many human transcription factors are zinc-dependent, including regulators active at innate-immunity promoters, and zinc also supports tight-junction protein turnover in barrier tissue. That makes adequate zinc a background requirement for normal antimicrobial peptide expression rather than a direct inducer. No combination trial reporting an LL-37 readout supports this pairing.

Retinoic acid receptors and the vitamin D receptor share heterodimer partners and coregulators, so retinoid signalling can shift how strongly a vitamin D target gene responds. Reported effects on cathelicidin expression differ by cell type and direction, which keeps the confidence low. This is a laboratory gene-expression observation, not a human result.

LL-37 works by electrostatic attraction to negatively charged microbial membrane surfaces, and free calcium competes for those same anionic sites. In salt-sensitivity assays, adding divalent cations shifts the concentration needed for the same membrane effect. The finding is a property of the in vitro assay and says nothing about calcium intake in people.