The bioflavonoids are a variety of compounds (> 4000 are known) in all plants; most occur as glycosides (with a variety of sugars) and are hydrolysed to aglycones by digestive enzymes. They serve both to protect plants against attack and also as the pigments of many plants. They were at one time considered to be vitamins (vitamin P), although there is no evidence that they are dietary essentials, then during the 1970s were considered to be mutagens and carcinogens, because, as shown in Figure 7.21, they are polyphenols and can undergo redox cycling with the production of

Figure 7.21 The bioflavonoids.

oxygen radicals. Since the 1990s they have been considered as potentially protective antioxidants because they can form relatively stable radicals that persist long enough to undergo reaction to non-radical products.

The bioflavonoids are also inhibitors of phase I reactions and activators of phase II reactions. In addition, they can form inactive complexes with a number of carcinogens.

Epidemiological evidence suggests a beneficial effect of a high intake of bioflavonoids with respect to cardiovascular disease (accounting for perhaps 8% of the international variation in coronary heart disease). However, despite the actions noted above, there is no epidemiological evidence of a negative association with cancer. Intakes of bioflavonoids are estimated to be around 23 mg/day, of which almost half comes from tea; onions, apples and red wine are also good sources.


A number of compounds that are widely distributed in plants as glycosides and other conjugates have weak oestrogenic action, although they are not chemically steroids. As shown in Figure 7.22, what they have in common is that all have two hydroxyl groups that are in the same position relative to each other as the hydroxyl groups in oestradiol, so that they bind to oestradiol receptors (section 10.4). The amounts present in plants are increased in response to microbial and insect attack, suggesting that

Figure 7.22 Phyto-oestrogens — compounds that have two hydroxylgroups in the correct orientation to bind to the oestradiol receptor (see also section 10.4).

they function as antimicrobial or antifungal agents in plants. Legumes, and especially soya beans, are rich sources.

The phyto-oestrogens produce typical and reproducible oestrogen responses in animals, with an oestrogenic activity 1/500—1/1000 of that of oestradiol. They may be agonistic or antagonistic to oestradiol when both are present at the target tissue (isoflavones are mainly antagonistic, lignans are mainly agonistic). They compete with oestradiol for receptor binding (section 10.4), but the phyto-oestrogen—receptor complex does not undergo normal activation, so it has only a weak effect on the hormone response element of DNA.

Phyto-oestrogens also reduce circulating free oestradiol because they increase sex hormone-binding globulin synthesis in liver — more hormone is bound to globulin, and therefore less is available for uptake into target tissues. Vegetarians have higher levels of circulating sex hormone-binding globulin than do omnivores.

It is noteworthy that in Japan, where the traditional diet contains relatively large amounts of phyto-oestrogens from soya bean products, there is a low incidence of cancer of the breast and prostate, as well as an unexpectedly low incidence of osteoporosis (despite the lower peak bone density of Japanese women compared with European or American women; see section Although some prospective and case—control studies show a protective effect of soya bean consumption with respect to breast cancer, others do not.

There is a clear correlation between adiposity and breast cancer, which is almost certainly due to synthesis of oestradiol in adipose tissue; enterolactone and some of the other flavonoid phyto-oestrogens inhibit aromatase and will therefore reduce synthesis of oestradiol in adipose tissue.

isoprene limonene myrecene zingiberene Figure 7.23 Some potentially protective isoprenoids.

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