Hydroxycinnamic acids

The hydroxycinnamic acids, hydroxy derivatives of cinnamic acid, comprise a group of polyphenols that are formed in the secondary metabolism of plants. They often occur in the outer layers of plants and contribute to the stability of plant cell walls. Hydroxycinnamic acids and hydroxybenzoic acids are summarized under the term "phenolic acids".

Representative of the hydroxycinnamic acids:

• o-Cumaric acid (2-hydroxycinnamic acid)
• m-Cumaric acid (3-hydroxycinnamic acid)
• p-Cumaric acid (4-hydroxycinnamic acid)
• Caffeic acid (3,4-dihydroxycinnamic acid) - Coffee

Hydroxycinnamic acids consist of a C6-C3 skeleton and differ in the substituents. Biochemically, their synthesis takes place via the so-called Shikimiat pathway. Hydroxycinnamic acids can be converted into hydroxybezoic acids by degradation of the side chains (beta-oxidation). These can form simple phenols by decarboxylation.

Hydroxycinnamic acids and other hydroxycinnamic acid derivatives have, besides their supporting functions, important functional biological effects in plants. They act as cellular antioxidants (free radicals), have inhibitory effects against numerous fungi and bacteria and support the defence against pathogens. Some hydroxycinnamic acids also show antiviral effects. Furthermore, hydroxycinnamic acids have UV-protective effects. They are therefore also of pharmacological interest, e.g. as light protection agents.

Occurrence in food: The most common types of hydroxycinnamic acid and its derivatives present in food are: p-coumaric acid (4-hydroxycinnamic acid), caffeic acid (3,4-hydroxycinnamic acid), methyl esters of ferulic acid (3-methoxy-4-hydroxycinnamic acid) and sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid).

The hydroxy-cinnamic acid derivative that dominates in whole grain is ferulic acid (3-methoxy-4-hydroxy-cinnamic acid). Coupled to arabinose, it is present as a glycoside component of hemicelluloses. Certain vegetables (spinach, lettuce) are also rich in hydroxycinnamic acids. Hydroxycinnamic acids are found in wines as well as other phenolic acids.

Bioavailability (metabolism): Free hydroxycinnamic acids can be absorbed in both the small and large intestine. In contrast, esterified hydroxycinnamic acids are not absorbed in the small intestine. Humans have no enzymes in the intestine to break down esterified hydroxycinnamic acids. A metabolism of phenolic acid esters in the large intestine can only be achieved by enzymes (xylanases, esterases) of the microflora.

1. Burger A et al (1993) Hunnius Pharmaceutical Dictionary 7th Edition Walter de Gruyter Berlin-New York p. 1080