Ingredient review

Inositol

Description

What is it?

Inositol is a constituent of phospholipids in cellular membranes, a precursor of secondary messengers in metabolic pathways, a component of reproductive fluids, and essential for tissue growth. (32)(39) Previously considered an essential B vitamin (vitamin B8), it is now known that the human body can synthesize four grams of inositol per day, whereas approximately one gram per day can be obtained from the diet. (14) For this reason, inositol is now mainly referred to as an essential pseudovitamin. (24)

Not be confused with: D-pinitol (structurally related)

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Main uses

Endocrine disorders
Hormonal and reproductive disorders
Metabolic disorders
Neurological disorders

Formulations

Form
Comparison
Myo-inositol
Hormonal and reproductive:
↑ mature oocytes, embryo quality, & sex hormone-binding globulin (SHGB);
Hormonal and reproductive: ↓ LH/FSH ratio, total testosterone, HOMA index, and immature oocytes by MI compared with D-chiro-inositol supplements in polycystic ovarian syndrome patients (34)(40)
Endocrine:
More involved in regulating intestinal glucose absorption in rats (9)
More involved in glucose uptake from plasma via glucose receptor activation (16)
More involved in glucose utilization via its synthesis from glucose-6-phosphate, producing up to 4-5g of myoinositol per day (14)(16)
D-chiro-inositol
Endocrine:
More involved in glycogenesis than myoinositol with up to 50-100x more activity in rats (16)(23)
Can be synthesized from myoinositol via an epimerase enzyme, but in the presence of insulin resistance less can be synthesized due to impaired epimerase activity (4)(27)

Dosing & administration

Adverse effects

At doses greater than 12 g per day, gastrointestinal effects, such as nausea, flatulence, and diarrhea, may occur. However, the incidence of these events is considered mild and does not increase in severity as the dose increases. (8) Increasing doses (300 to 2400mg) of DCI has been shown to negatively impact oocyte quality in patients with PCOS. (25)

Pharmacokinetics

Absorption

  • Intestinal transportation of MI across the apical membrane is accomplished by the sodium/inositol symporter 2 (SMIT2) in rats. (1)
  • Softgel capsules with lower doses (0.6 g) provide equal bioavailability to higher doses of powder (2 g). (7)

Distribution

  • SMIT1/SMIT2 and the H+ myo-inositol transporter are responsible for inositol uptake throughout the body and brain. (14
  • Myoinositol can pass the blood-brain barrier for distribution to the brain and cerebrospinal fluid in rats. (36)(37)
  • Myoinositol is also distributed to seminal fluids, prostate, epididymis, and seminal vesicles, and is high in follicular fluid of healthy Px, but low in PCOS Px. (4)
  • D-chiro-inositol is highly distributed to fat, muscle, and the liver, but is low in the brain and heart in humans. (31)
  • D-chiro-inositol is high in follicular fluid of PCOS Px, but low in healthy Px. (4)

Metabolism

  • Myo-inositol is metabolized to D-glucuronic acid in the renal cortical tubules by myoinositol oxygenase. (14)
  • D-glucuronic acid is then converted by aldehyde reductase to L-gluconate. The metabolism of L-gluconate results in xylulose and ribulose, which can both be degraded through glycolysis. (14)

Excretion

  • If not reabsorbed by the SMT2 transporters on the apical membrane of the proximal convoluted tubules, MI and DCI inositol are both excreted in the urine. (14)(28)

References