Isoflavones are polyphenolic compounds that are capable of exerting estrogen-like effects. For this reason, they are classified as phytoestrogens—compounds with estrogenic activity derived from plants (1). Legumes, particularly soybeans, are the richest sources of isoflavones in the human diet. In soybeans, isoflavones are present as glycosides (bound to a sugar molecule). Fermentation or digestion of soybeans or soy products results in the release of the sugar molecule from the isoflavone glycoside, leaving an isoflavone aglycone.
Resveratrol (3,4',5-trihydroxystilbene) belongs to a class of polyphenolic compounds called stilbenes (1). Some types of plants produce resveratrol and other stilbenes in response to stress, injury, fungal infection, and ultraviolet (UV) radiation (2). Resveratrol is a fat-soluble compound that occurs in a trans and a cis configuration (see figure 1). Both cis- and trans-resveratrol also occur as glucosides (bound to a glucose molecule). Resveratrol-3-O-beta-glucoside is also called piceid (3).
Throughout much of human evolution, it is likely that large amounts of plant foods were consumed (1). In addition to being rich in fiber and plant protein, the diets of our ancestors were also rich in phytosterols—plant-derived sterols that are similar in structure and function to cholesterol. There is increasing evidence that the reintroduction of plant foods providing phytosterols into the modern diet can improve serum lipid (cholesterol) profiles and reduce the risk of cardiovascular disease (2).
The mammalian lignans, enterodiol and enterolactone (see chemical structures), are formed by the action of intestinal bacteria on lignan precursors found in plants (1). Because enterodiol and enterolactone can mimic some of the effects of estrogens, their plant-derived precursors are classified as phytoestrogens. Lignan precursors that have been identified in the human diet include pinoresinol, lariciresinol, secoisolariciresinol, matairesinol, and others (see chemical structures).
Cruciferous vegetables, such as broccoli, cabbage and kale, are rich sources of sulfur-containing compounds called glucosinolates. Isothiocyanates are biologically active hydrolysis (breakdown) products of glucosinolates. Cruciferous vegetables contain a variety of different glucosinolates, each of which forms a different isothiocyanate when hydrolyzed (see Figure) (1). For example, broccoli is a good source of glucoraphanin, the glucosinolate precursor of sulforaphane (SFN), and sinigrin, the glucosinolate precursor of allyl isothiocyanate (AITC) (2).