1 · Inositol · second messenger
On the insulin pathway PI3K makes PIP3, not IP3, so taking inositol does not directly supply a second messenger to insulin signalling.Inositol is a six-carbon cyclic polyol, not a vitamin (the body makes its own), but it is the backbone of membrane phospholipids and of a family of signalling molecules.First, a correction that is often gotten wrong — there is no IP3 on the insulin pathway:
Insulin binds its receptor → receptor tyrosine kinase phosphorylates IRS-1 → activates PI3K → PI3K adds one more phosphate to membrane PIP2, producing PIP3 → PDK1 / Akt → translocation, glycogen synthesis.PI3K's product is PIP3, not IP3. IP3 and DAG come from a *different* route: phospholipase C (PLC) cleaves PIP2 in half, giving IP3 and DAG. That route is driven mainly by Gq-coupled receptors (part of signalling in the ovary runs through it), not by insulin's main trunk.
Why the distinction matters: fuse the two routes and you get "supplementing inositol directly supplies insulin's second messenger." That step does not follow — insulin's glucose arm is a phosphorylation cascade, and it is not short of inositol as raw material. The real reasons inositol is discussed in are two different things, covered in the next steps.
The two stereoisomers do different jobs:
myo-inositol (MI): dominant in the ovary, tied to FSH signalling and glucose uptakeD-chiro-inositol (DCI): dominant in muscle/fat, weighted toward glycogen synthesis
The two interconvert via an epimerase.