Vitamin B8 (Inositol)

Inositol is a carbocyclic polyol, a six-membered carbon ring bearing a hydroxyl group on each carbon, and it exists as several stereoisomers of which myo-inositol is by far the most common in living organisms ^[1]. Historically it was labeled vitamin B8 and counted among the B-complex, but this classification was dropped once it became clear that the human body makes its own inositol, chiefly in the kidneys, from glucose ^[1]. For that reason it is regarded as a conditionally useful nutrient rather than an essential vitamin ^[1].

Within cells inositol is incorporated into phosphatidylinositol and related membrane phospholipids, which are the source of a family of signaling molecules known as inositol phosphates and phosphoinositides ^[1]. These second messengers help relay signals from hormones and growth factors across the cell membrane, participate in the release of intracellular calcium, and contribute to the intracellular action of insulin ^[1][3]. Myo-inositol and its epimer D-chiro-inositol appear to play distinct and complementary roles, and the balance between the two is thought to matter for tissues such as the ovary ^[3].

Inositol is widely distributed in the diet, occurring in fruits such as citrus and melon, in beans, grains, and nuts, and in the phospholipids of many foods ^[1]. In plants much of it is stored as phytic acid, a phosphate-rich form that is not readily absorbed by humans and that can bind dietary minerals, whereas inositol from phospholipids is more available ^[1].

Interest in inositol has centered on polycystic ovary syndrome, a common hormonal and metabolic disorder in which insulin resistance is frequent ^[2][3]. Because myo-inositol and D-chiro-inositol influence insulin signaling and ovarian steroid production, supplements of one or both have been trialed to improve metabolic measures, ovulation, and related symptoms, often in a ratio reflecting their natural proportions in the body ^[3]. Systematic review of these trials, including the analysis informing recent international guidelines, found the evidence limited and inconclusive, with possible benefits for some measures but uncertainty overall, though myo-inositol was generally well tolerated ^[2]. Inositol has also been examined in other metabolic and psychiatric conditions and in pregnancy, with mixed results ^[1][2].

Inositol is sold as a dietary supplement rather than a licensed medicine in most countries, and it is available as myo-inositol, D-chiro-inositol, and combinations of the two, as well as the related compound inositol nicotinate ^[1]. It has a long record of low toxicity, and reported side effects at high intake are mainly mild digestive symptoms ^[1][2].

Inositol is well tolerated even at the high doses used in studies, with the main side effects being mild GI upset, nausea, or loose stools at several grams. It is considered low-risk and is even studied in pregnancy for PCOS-related outcomes. Most people handle it comfortably.