The alkylresorcinols are a group of 1,3-dihydroxybenzene derivatives found in cereal grains. Wheat and rye are good sources of the alkylresorcinols, whereas barley contains much lower concentrations. At position 5 of the benzene ring, alkylresorcinols possess an odd numbered alkyl chain between 15 and 27 carbons long, and this feature gives the molecules amphiphilic properties (water and lipid solubility). The most common chain length for alkyl side chain is C19 and C21 in wheat and rye. The alkyrecornicols possess antioxidant activity and are bioavailable in humans. Antioxidants have health effects in humans, and so the alkylresorcinols may explain some of the health benefits of consuming cereal grains. However, refining of the cereal grain to remove the bran layer strips the cereal grain of most of its alkylresorcinol content. High intakes of alkylresorcinols are only possible therefore if whole grains are consumed. It has been suggested therefore that alkylresorcinols are good markers for whole grain intake in humans.
Alkylresorcinols are thought to be metabolised in a similar way to tocopherols. This metabolism involved an initial step using a cytochrome p450 mediated ω-oxidation of the alkyl side chain and this creates hydroxylated alkylresorcinols, and these are then oxidised to form carboxylated alkylresorcinols. A β-oxidation step then shortens the side chain producing hydrophilic phenolic metabolites. The metabolites formed from the breakdown of alkylresorcinols are still largely unknown, but 2,5-dihydroxybenzoic acid (3,5-DHBA) and 3-(3,5-dihydroxyphenol)propanoic acid (3,5-DHPPA), have been identified and are thought to be major alkylresorcinol metabolites in humans. Detection of these metabolites in the urine is thought to provide a reasonable reflection of the long term ingestion of wheat and rye, which may be considered fairly accurate representations of whole grain intakes in humans. In this regard the urinary concentrations of 3,5-DHBA and 3,5-DHPPA have been assessed.
For example, in one study1 researchers used both mice and humans subjects in order to further understand the role of alkylresorcinols in human nutrition. Using analytical chemistry the researchers identified three phenolic acid derivatives of the alkylresorcinols along with seven of their phase 2 conjugates. Of these 5-(3,5-dihydroxyphenyl)pentanoic acid (3,5-DHPPTA) and 2-(3,5-dihydroxybenzamido) acetic acid (3,5-DHBA glycine) were novel. Subjects then consumed whole grains for four days which included a daily intake of roughly 61 mg of alkylresorcinols. When the researchers analysed the urine of the subjects for 32 hours following ingestion of whole grain diet, they found that the novel metabolites 3,5-DHPPTA and 3,5-DHBA glycine as well as the established metabolites 3,5-DHBA and 3,5-DHPPA, where all useful markers of whole grain intakes. Use of such markers in epidemiological research to assess long term whole grain intake is therefore justified.
Wheat and rye are the main source of alkylresorcinols in the human diet, and regular intakes of these cereal grains may supply high amounts of these important antioxidant nutrients. Rye contains around 100 mg per 100 g of alkylresorcinols and wheat contains around 50 mg per 100 grams of alkylresorcinols. Triticale is another cereal grain that is rich in alkylresorcinols with concentrations of around 50 mg per 100 grams. Barley contains around 2 to 5 mg per 100 grams of alkylresorcinols. Because the alkylresorcinols are concentrated in the outer parts of cereal grains in the bran, refining removes most of the alkylresorcinol content. Those who eat refined grains therefore have low intakes of alkylresorcinols. Oats contain few alkylresorcinols, but do contain other antioxidants such as tocopherols, tocotrienols, phenolic acids, phytic acid, avenanthramides and flavonoids. Rice too can possess high levels of antioxidants, with dark varieties for example being good sources of anthocyanins.
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