Phenolic Acids

Plant polyphenols are known for their positive effects on human health. Considerable amounts of flavonoids, a class of polyphenols having rather large molecules, are present in higher amounts in many fruits and other foods of plant origin. They are poorly absorbed from the intestine limiting their bioavalability and positive effects on human health. However, they also undergo considerable transformation by intestinal microflora and various products of degradation may be formed during the digestion, absorption, and metabolic processes in the gastrointestinal tract. Phenolic acids (PA) are a type of organic compounds much smaller and simpler in structure than flavonoids. Included in that class are substances containing a phenolic ring and an organic carboxylic acid function (C6-C1 skeleton).

Phenolic acids are formed by plants as secondary metabolites, meaning not essential for growth, development, or reproduction, and are widely spread throughout the plant kingdom. There are many different types of phenolic acids. Some of the common ones are gallic, chlorogenic, benzoic, coumaric, caffeic, ellagic and ferulic acids and their derivatives. Phenolic acids are among the major end products of flavonoids or other polyphenol metabolism by the intestinal microbes.

Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruits and vegetables, against oxidative damage diseases (coronary heart disease, stroke, and cancers). For instance, ferulic acid (4-hydroxy-3-methoxycinnamic acid) has been shown to reduce hypertension, lipid peroxidation, oxidative impairments, and enhance insulin secretion in rats, while p-methoxycinnamic acid has been found to stimulate insulin secretion from the pancreatic β-cells in rats. Caffeic acid has been shown to improve glucose utilization and reduce plasma glucose in diabetic rats, while chlorogenic acid attenuates hypertension and endothelial cell function in spontaneously hypertensive rats. Similarly, O-coumaric acid was found to be effective in improving the symptoms of metabolic syndrome and obesity. 3,4-dihydroxybenzoic acid (also known as protocatechuic acid) is another PA found in many edible and medicinal plants. It is a major metabolite of cyanidin-3-glucoside, an anthocyanin, metabolism and has been shown to have many health effects, including chemoprevention and improvement of antioxidant status; a major factor involved in many of the chronic diseases.

Presence of large amounts of monophenolic acids have been demonstrated previously in the colon of healthy humans. While the absorption of flavonoids and other large molecular weight polyphenols may be poor, PA can be absorbed into the circulation and may contribute to the health-promoting effects. Moreover, PA or their metabolic products may actually be the active compounds responsible for at least some of the health promoting effects associated with their parent compounds. However, the extent and importance of absorption of many of these PA is not known.

Parent flavonoids in the diet are often deglycosylated before absorption and are also conjugated by methylation, glucuronidation, sulfation, etc. during the absorption process. They are usually excreted through the urine, while extensively conjugated ones may also find their in the bile. While many studies have reported free or total PA, little is known about their conjugated counterparts. Since PA and other metabolites formed by the intestinal microflora and/or those produced in host tissues are excreted in the urine, it is imperative to identify and quantify the PA excreted in urine after ingestion of plant polyphenols. Therefore, measurement of just the free form may not accurately reflect the amount excreted, and studies investigating the health effects, bioactivity, or bioavailability of parent polyphenols should also consider their metabolites, like phenolic acids and their derivatives, not just the parent compound(s).

Overall, research on the effects of PAs for improving human health or their mechanism of action is in its infancy. Given that plant polyphenols are widely believed and extensively studied for their numerous health benefits, it is not difficult to surmise some positive effects for PAs as well. However, this by no means indicates a definite answer for any of the health maladies we face as individuals.