The Vitamins 187tissue uptake of inositol. There is some evidence that
impaired nerve conduction velocity in diabetic neu-
ropathy in both patients and experimental animals is
associated with low intracellular concentrations of
inositol, and inositol supplements may improve nerve
conduction velocity. However, high intracellular con-
centrations of inositol also impair nerve conduction
velocity, and supplements may have a deleterious
effect.
Taurine
Until about 1976 it was assumed that taurine was a
metabolic end-product, the only function of which
was the conjugation of bile acids. The occurrence of
changes in the electrical activity of the retina in chil-
dren maintained on long-term total parenteral nutri-
tion without added taurine has shown that it has
physiological functions, and has raised the question
of whether or not it should be regarded as a dietary
essential.
Ubiquinone (coenzyme Q, “vitamin Q”)
Ubiquinone is one of the electron carriers in mito-
chondria. Therefore, it has an essential function in all
energy-yielding metabolism and may also have a
general antioxidant role in membranes. Like vitamin
E, it can be anchored in membranes by the hydropho-
bic tail, with the reactive quinone group at the mem-
brane surface. Ubiquinone is readily synthesized in
the body, and there is no evidence that it is a dietary
essential, or that supplements serve any useful
purpose, although they may have non-specifi c anti-
oxidant actions and so spare vitamin E.
“Phytoceuticals”
In addition to the compounds with clearly defi ned
metabolic functions discussed above, various com-
pounds naturally present in foods, and especially in
foods of plant origin, have potentially benefi cial
effects, although they are not nutrients. Collectively,
they are known as phytoceuticals (substances of plant
origin with potential pharmaceutical action) or
nutraceuticals. The following compounds are exam-
ples of phytoceuticals:
● Many glucosinolates and glycosides either inhibit
the enzymes of phase I metabolism of foreign com-
pounds (the reactions that activate many potential
carcinogens) or induce the reactions leading to
conjugation and excretion of foreign compounds.
● Terpenes that are found in the volatile (essential) oils
of herbs and spices are potentially active as lipid-
soluble antioxidants, as are many of the carotenoids
that are not active as precursors of vitamin A.
● Compounds such as squalene, which are precursors
of cholesterol synthesis, may have a hypocholester-
olemic action, by reducing the activity of the rate-
limiting enzyme of cholesterol synthesis, hydroxy-
methylglutaryl-CoA reductase.
● Various water-soluble compounds, including poly-
phenols, anthocyanins, and fl avonoids, have anti-
oxidant action.
● Several plants (especially soyabeans) contain com-
pounds with estrogenic action (phytoestrogens)
that also have antiestrogenic action and appear to
be protective against the development of hormone-
dependent cancer of the breast and uterus.Reference
Horwitt MK. J Nutr 1956; 60 (Suppl 1): 1–43.Further reading
Bender DA. Nutritional Biochemistry of the Vitamins, 2nd edn.
Cambridge University Press, Cambridge, 2003.