A phenolic acid is a plant metabolite built from a phenol ring bearing a carboxylic acid group, a subgroup of the polyphenols widespread in plant foods. Common examples include caffeic acid and chlorogenic acid in coffee, ferulic acid in whole grains, and gallic acid in tea and berries. They contribute to the astringency, bitterness, and colour of foods and serve the plant as defences against pests and ultraviolet damage. In human nutrition they are studied as antioxidants: they scavenge free radicals in laboratory assays, and epidemiological studies link high intakes of phenolic-rich foods to lower cardiovascular and metabolic risk. Bioavailability is limited and complex, with gut microbes converting them into various metabolites, so the health story is still being written. Coffee is the dominant dietary source in most Western diets.