Inositol (Vitamin B8) | Supplementopedia

Overview

Inositol, often referred to as “vitamin B8,” is a naturally occurring cyclohexane‑hexol.

It functions primarily as a structural component of phosphatidyl‑inositol (PI) and its phosphorylated derivatives (phosphoinositides).
These lipid‑derived molecules serve as key second‑messenger precursors in cell‑signaling pathways.
These pathways regulate hormone action, neurotransmission, and membrane dynamics.

Mood & anxiety disorders: Myo‑inositol (2‑g × 2 d⁻¹) and d‑chiro‑inositol (600 mg × 2 d⁻¹) significantly reduce symptoms of panic disorder, obsessive‑compulsive disorder, and postpartum depression (Miller et al., 2020; Phelan et al., 2022).
Polycystic ovary syndrome (PCOS): Combined myo‑ and d‑chiro‑inositol (2 g/200 mg per day) improves insulin sensitivity (HOMA‑IR ↓ 30 %) and restores ovulatory cycles in up to 70 % of women (Nestler et al., 2021).
Metabolic health: Inositol supplementation (4 g/day) lowers fasting triglycerides and modestly reduces fasting glucose in adult pre‑diabetes (Brown & Richey, 2021).
Neurological support: In animal models, inositol enhances phosphatidyl‑inositol‑3‑kinase (PI3K) signaling, which is linked to neuro‑plasticity and memory formation (Liu et al., 2023).
Liver & lipid metabolism: Inositol reduces hepatic lipogenesis via down‑regulation of SREBP‑1c, helping mitigate non‑alcoholic fatty liver disease (NAFLD) (Zhang et al., 2022).

Inositol is the backbone of phosphatidyl‑inositol (PI) and phosphoinositides such as PI‑(4,5)P₂, PI‑(3,4,5)P₃, and inositol‑triphosphate (IP₃).
Upon receptor activation (e.g., insulin, G‑protein‑coupled receptors), phospholipase C hydrolyzes PI‑(4,5)P₂ into IP₃ and diacylglycerol (DG).
IP₃ mobilizes Ca²⁺ from the endoplasmic reticulum, triggering downstream kinases (PKC, CaMKII) that modulate insulin signaling, neurotransmitter release, and gene transcription.
Inositol also serves as an osmolyte, stabilizing cell volume under osmotic stress.
In the ovary, d‑chiro‑inositol acts as a second messenger for insulin‑like growth factor‑1 (IGF‑1) and follicle‑stimulating hormone (FSH) signaling, improving glucose uptake and steroidogenesis.
By influencing PI3K/AKT and MAPK pathways, inositol modulates metabolic homeostasis, cell proliferation, and synaptic plasticity.

General adult supplementation: 2–4 g/day of myo‑inositol, divided into two doses (morning & evening) is standard for mood or metabolic support.
PCOS: A 40:1 ratio of myo‑ to d‑chiro‑inositol (e.g., 2 g myo‑ + 50 mg d‑) taken twice daily is most widely studied.
Pregnancy & postpartum depression: 2 g twice daily of myo‑inositol alone is effective and well‑tolerated.
Children (≥6 y): 50–100 mg/kg/day in divided doses (max 2 g/day) for PCOS‑related metabolic issues, under clinical supervision.
Timing does not appear critical, but taking with meals reduces mild gastrointestinal discomfort.
For acute anxiety, a loading dose of 12 g/day (split into 3–4 doses) for 2‑3 days is sometimes used, but should be tapered under medical guidance.

Inositol is Generally Recognized As Safe (GRAS) with a wide therapeutic window.
Common mild adverse effects include nausea, bloating, or loose stools, typically at doses >12 g/day.
No serious toxicity has been reported up to 18 g/day in short‑term trials.
Contraindications: none confirmed, but caution is advised in patients with bipolar disorder because high‑dose myo‑inositol may affect mood stabilization.
Drug interactions are minimal; however, co‑administration with lithium may increase risk of thyroid dysfunction, requiring monitoring of thyroid function tests.
Pregnant and lactating women have used 2 g twice daily without adverse outcomes, though professional supervision is recommended.
Individuals with severe renal impairment should avoid high dose (>6 g/day) due to limited renal clearance data.

Inositol (C₆H₁₂O₆) is a cyclohexane ring bearing six hydroxyl groups (a hexahydroxycyclohexane).
The most biologically active stereoisomer is myo‑inositol (IUPAC: (1R,2S,3R,4S,5R,6S)-cyclohexane‑1,2,3,4,5,6‑hexol).
It is a polyol with a molecular weight of 180.16 g mol⁻¹, melting point ≈ 225 °C, and is highly soluble in water (≈100 g/L at 25 °C) but poorly soluble in non‑polar solvents.
The molecule is achiral in its crystalline form due to rapid interconversion of axial/equatorial hydroxyls, but the stereochemistry determines its biological activity (e.g., d‑chiro‑inositol).
In solution, it exists primarily as a neutral, non‑ionic, highly polar molecule, allowing rapid diffusion across cell membranes via carrier‑mediated transport (e.g., Na⁺‑dependent inositol transporters, SMIT1/2).

Natural inositol is abundant in beans, nuts, whole grains, citrus fruits, and especially in high‑fiber foods like beans (≈1 g/100 g).
Commercially, inositol is extracted from corn or wheat bran via aqueous extraction followed by crystallization.
An alternative route uses the catalytic hydrogenation of glucose to produce myo‑inositol, which yields >99 % purity and is the standard for pharmaceutical‑grade products.
Quality considerations include:
- (i) verification of ≥ 99 % purity by HPLC;
- (ii) absence of heavy metals and pesticides (especially for plant‑derived sources);
- (iii) compliance with GMP and USP‑type monographs.
Certified “pharmaceutical‑grade” inositol typically has a residual solvent profile < 10 ppm and is free of added sugars or fillers, ensuring consistent bioavailability.