METABOLISM OF DIETARY PHENOLIC ACIDS

摘要

Phenolic acids are secondary plant metabolites widely distributed in foods. They have attracted much attention in the past few years, because they occur ubiquitously in plants and play protective roles. Phenolic acids can be classified into two groups; hydroxybenzoic acids and hydroxycinnamic acids. Gallic acid and protocatechuic acid are two major hydroxybenzoic acids, while hydroxycinnamic acids include caffeic acid, ferulic acid, coumaric acid and sinapic acid (Figure 1). Different from hydroxycinnamic acids, hydroxybenzoic acids generally occur at very low levels in some red fruits, black radish and onions, representing approximately 10 ppm on a fresh weight basis. However, some studies report protocatechuic acid as a major active compound in some Chinese herbal medicines while gallic acid is abundant in tea leaves, accounting for 4:5 g/kg fresh tea leaves. Because of their limited occurrence, hydroxybenzoic acids have not been studied extensively, especially with respect to their metabolism. Caffeic acid and ferulic acid which are major hydroxycinnamic acids, occur abundantly in fruits and cereal grains, respectively, mainly in the bound form. Most hydroxycinnamic acids can be glycosylated or esterified with quinic acid, shikimic acid and tartaric acid. Chlorogenic acid, which is found in many fruits at high concentrations and especially in coffee, is an ester formed between caffeic acid and quinic acid. A single cup of coffee may contain 70-350 mg chlorogenic acid. Caftaric acid is a derivative of caffeic acid with tartaric acid, and is abundant in grapes, spinach, lettuce and wine. Ferulic acid is present in the free form in tomato and beer, mainly esterified with arabinose residues in the plant wall arabinoxylans and as feruloylquinic acid. Several dimers of ferulic acid are also found in cereals, p-Coumaric acid is a phenolic coumarin derivative, and is found in various fruits.