Myo-inositol vs D-chiro-inositol: what's the difference?
Last updated: August 2026 · Reviewed by the FactoWiki Editorial Team for clarity and source accuracy
They are two forms of the same molecule that do different jobs in the body. Myo-inositol dominates in the ovary and supports egg quality; D-chiro-inositol is more involved in insulin signalling in other tissues. The 40:1 myo to D-chiro ratio used in PCOS research reflects the natural ratio in plasma — and too much D-chiro appears to harm ovarian function.
Key takeaways
- 40:1 myo to D-chiro is the ratio used in trials, mirroring physiological plasma levels.
- High-dose D-chiro alone has been associated with worse egg quality — more is not better.
- Inositol has trial evidence in PCOS for ovulation and insulin sensitivity.
Two isomers, two roles
Inositol exists in several stereoisomeric forms. Myo-inositol is the most abundant and is the precursor for second messengers involved in FSH signalling in the ovary. D-chiro-inositol is made from myo-inositol by an enzyme called epimerase, and is more involved in glycogen synthesis and insulin signalling in liver, muscle and fat. Tissues differ enormously in their ratios — the ovary is myo-inositol dominant, which is the key point.
Why the ratio matters
Plasma normally contains myo and D-chiro inositol in roughly a 40:1 ratio, and this is the ratio most PCOS trials use. Research has found that high doses of D-chiro-inositol alone can reduce oocyte quality and worsen ovarian function — a genuine example of an ingredient where more of the seemingly active form makes outcomes worse. Products supplying D-chiro-inositol at high proportions are not better despite marketing that implies it.
What the evidence supports
In PCOS, myo-inositol at around 2 g twice daily has evidence for improved insulin sensitivity, more regular ovulation and improved menstrual regularity, with effect sizes sometimes compared to metformin but with far better tolerability. It has also been studied in gestational diabetes prevention and in IVF outcomes. Evidence quality is moderate — many trials are small — but it is among the better-supported supplements in women's health.
Key ingredients to understand
The ingredients most relevant to this question, with what the evidence actually supports:
- Inositol (Myo-Inositol) — Inositol is a vitamin-like compound with its best evidence in polycystic ovary syndrome (PCOS), where myo-inositol may improve insulin…
- Berberine — Berberine is a plant compound studied mainly for blood sugar, cholesterol and related metabolic markers. It has some of the strongest human…
- Vitamin D — Vitamin D is essential for bones and plays a role in immunity. Deficiency is common, and supplementing clearly helps if you are low — but large…
- Chromium (Picolinate) — Chromium is a trace mineral marketed for blood sugar and weight. Genuine deficiency is rare, and the evidence that supplementing helps blood…
Frequently asked questions
What dose should I take?
PCOS trials typically use 4 g of myo-inositol daily, usually split, often with 400 mcg folic acid. The 40:1 combination products supply roughly 4 g myo with 100 mg D-chiro.
How long before it works?
Menstrual and ovulatory changes generally take 3–6 months. Insulin markers may shift sooner.
Is it safe in pregnancy?
It has been studied in pregnancy for gestational diabetes prevention with reassuring results, but discuss it with your maternity team rather than self-prescribing.
Related on FactoWiki
- All health answers
- Ingredient evidence guides
- Inositol (Myo-Inositol) — ingredient guide
- Berberine — ingredient guide
- How we review
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