Plant foods are a rich source of polyphenols, the most well known and extensively researched of these being the flavonoids. Polyphenols are of interest to nutritional scientists because they possess phenolic rings within their structure. This ring structure confers chemical and physical properties that may be beneficial to human health. For example, many polyphenols are known to be effective reducing agents and can therefore donate electrons in order to be able to quench free radical chain reactions. This antioxidant action may reduce inflammation and decrease cellular damage (figure 1). There are many non-flavonoid polyphenols present in foods that are commonly eaten that are less well known and characterised. Of particular interest are the hydroxycinnamic acids, a group of phenolic acids which may play an important role in human health and nutrition.
Figure 1. The antioxidant function of flavonoids. R indicates radical, and ∙ symbolises free electron. Flavonoids donate a free electron (and lose a proton) and become oxidised themselves. The free radical is reduced and the chain reactin broken.
The major hydroxycinnamic acid in foods is caffeic acid. Caffeic acid is present in foods mainly as an ester bound to quinic acid, forming a compound called chlorogenic acid (5‑caffeoylquinic acid). The structure of caffeic acid and chlorogenic acid is given in figure 2. Chlorogenic acid is present in a number of commonly eaten food such as coffee and some fruits. Coffee drinkers may ingest 0.5 to 1 g of chlorogenic acid per day, whereas the intake of non‑coffee drinkers may be < 100 mg per day. Research with health illeostomy patients suggests that both caffeic acid and chlorogenic acid are both well absorbed from the small intestine1, as both are detected in urine following ingestion. This is important because in order for these compounds to illicit biological responses, they must be present in the plasma.
Figure 2. The molecular structure of caffeic acid and chlorogenic acid.
Like flavonoids, evidence suggests that hydroxycinnamic acids act as antioxidants, and so they may inhibit LDL oxidation and protect from cardiovascular disease. For example, research published in the American Journal of Clinical Nutrition in 2007, fed healthy subjects a 200 mL cup of filtered coffee after an overnight fast. The subjects consuming the coffee had a reduced susceptibility of LDL particles in the blood to become oxidised. The researchers concluded that this was probably on account of phenolic acids (such as caffeic acid) within the coffee being incorporated into the LDL particles, thus protecting them from oxidation. Caffeic acid is generally the most abundant phenolic acid present in fruit. It may represent between 75 % and 100 % of the hydroxycinnamic acid in apples, cherries, plums, kiwis and blueberries, mainly in the outer parts of the ripe fruit.
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