D-Chiro-Inositol

Inositol is a carbocyclic sugar alcohol that can exist as several stereoisomers; the two of greatest biological importance are myo-inositol, by far the most common form, and D-chiro-inositol ^[1]. Inositol was once labeled vitamin B8, but because the body can make it from glucose it is not actually a vitamin ^[1]. D-chiro-inositol is not synthesized directly; instead it is produced from myo-inositol by a specific epimerase enzyme whose activity is driven by insulin ^[1].

Both isomers serve as the backbone of small messenger molecules called inositol phosphoglycans, which cells release in response to insulin and which help carry out insulin's effects on glucose metabolism ^[1]. Myo-inositol-based mediators are associated with glucose uptake, while D-chiro-inositol-based mediators are linked to glucose storage as glycogen, so the two play complementary roles ^[1]. Because the epimerase that converts myo-inositol to D-chiro-inositol depends on insulin, insulin resistance can disturb the ratio between the isomers within tissues ^[2].

D-chiro-inositol has been most studied in polycystic ovary syndrome, a common hormonal and metabolic disorder frequently accompanied by insulin resistance ^[4]. Supplementation with inositols can improve insulin sensitivity, help restore regular menstrual cycles, and reduce markers of excess androgen in women with PCOS ^[4]. Researchers have also described a D-chiro-inositol paradox in the ovary: while the isomer is helpful for whole-body insulin action, too much of it in the ovary appears to deplete local myo-inositol and can impair egg quality, which is why high-dose D-chiro-inositol on its own is viewed cautiously for fertility ^[2][3].

This has led to interest in combining the two isomers in a physiological ratio that roughly mirrors their proportion in healthy plasma, an approach meant to capture the benefits of myo-inositol while avoiding the drawbacks of D-chiro-inositol excess ^[1]. At the level of the ovary, the isomers pull in different directions on hormone production: D-chiro-inositol tends to promote androgen synthesis and dampen the aromatase enzyme that converts androgens to estrogen, whereas myo-inositol supports aromatase and follicle-stimulating hormone signaling ^[3].

D-chiro-inositol is sold as a dietary supplement rather than an approved drug, either alone or blended with myo-inositol, and it also occurs naturally in foods such as buckwheat and certain legumes ^[1]. It is generally regarded as well tolerated, and inositol therapy for PCOS has been described in reviews as effective and safe, although formulations and the ideal isomer balance continue to be debated ^[4].

Generally well tolerated; mild GI upset, nausea, or dizziness can occur. The notable caution is that high-dose D-chiro-inositol alone may impair oocyte quality in women with PCOS, so the balanced ratio with myo-inositol is preferred. Otherwise it has a good safety profile.