There is a huge body of evidence that phenolic compounds have effects on human health, and that is the topic of this chapter. Perhaps the oldest medical application of phenolic compounds is the use of phenol (1.1) as an antiseptic. Because of its negative side effects on living tissues, including blister formation, especially at higher concentrations, it is no longer used in this capacity. The modern antiseptic agents effective against the bacterium Staphylococcus aureus are, however, still compared to a 5% (w/v) solution of phenol. Phenol is still used as an oral anesthetic in throat lozenges, at a typical concentration of 1.4%.
Another very common use of phenolic compounds is in sunscreens. The presence of the aromatic ring results in the effective absorbance of the UV-B radiation (between 280 and 315 nm) from the sun and thus prevents sunburns. The most common active ingredient in many sunscreens is p-aminobenzoic acid (PABA; 7.1), which is actually not a phenolic compound. This compound has been widely used since the 1970's but is less popular nowadays due to the formation of skin rashes and acne. As a result many sunscreens are now PABA-free. Alternative active ingredients include salicylates such as octylsalicylate (7.2), cinnamates such as octyl methylcinnamate (7.3), benzophenone (7.4) and the related compound oxybenzone (7.5), and anthranilates, such as menthylanthranilate (7.6). Octyl methylcinnamate (7.3) is insoluble in water and is therefore commonly used in water-proof sunscreens. The most recently developed sunscreens use titanium dioxide and/or zinc oxide which reflect the light, rather than absorb it, and are considered more effective. They work best in relatively thick layers, which is less desirable from a cosmetic perspective. The more traditional sunscreens with phenolic compounds that absorb UV radiation are therefore still very common.
A concern of the widespread use of phenolic compounds is the estrogenic activity these compounds may display, which impacts the hormone balance and may result in breast cancer in women. In order to investigate this, Miller et al. (2001) used recombinant yeast in an estrogen assay to assess the activity of 73 phenolic additives in sunscreens, preservatives, perfumes, disinfectants, antioxidants and flavorings. Thirty-two compounds were shown to have activity in this assay. Twenty-two exhibited potencies relative to 17b-estradiol that ranged from 1/3,000 to 1/3,000,000. Forty-one compounds were inactive. The major criteria for estrogenic activity were the presence of an unimpeded phenolic OH group in a para-position and a molecular weight of 140-250 amu.
Kawamura et al. (2003) performed a similar study, also using a yeast-based assay to detect estrogenic activity, with UV-absorbing compounds in food plastics, as well as with benzophenone (7.4) derivatives used in sunscreens. They reported estrogenic activity higher than the known endocrine disrupting compound bisphenol A (7.7) for several benzophenone derivatives, including 2,4-dihydroxyphenone and 4-hydroxybenzophenone. Based on the specific estrogenic activity, they concluded that a hydroxyl group on the phenol ring of benzophenone has the biggest impact when it is present at the para-position, followed by the meta- and ortho-positions, which is consistent with the data reported by Miller et al. (2001).
Aside from medical applications, polyphenols, including the flavonoids and tannins, are an integral part of human and animal diets, because they represent one of the most numerous and ubiquitous groups of plant metabolites (Bravo, 1998). Although traditionally regarded as anti-nutrients, because of their bad taste, unappealing color, or cause of browning of tissues, polyphenols and other food phenolics are the subject of increasing interest because of their possible beneficial effects on health.
This chapter will highlight the positive effects of phenolic compounds on human health. This is by no means meant to be an exhaustive presentation of all the literature available on this topic. An entire book could be dedicated to it, and such books indeed exist. Several references for further reading are provided at the end of this chapter.
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