Throughout history, extracts of wood barks have been used for the tanning of animal hides to make leather. It was not until 1957 when the functional principles of these preparations were formally defined as polyphenols—specifically those with high molecular weights and the ability to bind proteins (such as the collagen in animal skin). Known as tannins, these large and extraordinarily complex polyphenols occur in many plants, particularly in trees. The bark of the oak (Quercus spp.) is rich in a class of tannins known as ellagitannins, typified by abundant aromatic hydroxyl groups that, coincidentally, endow them with powerful antioxidant activity. But this is hardly the reason these compounds hold therapeutic promise.
Unless you’re a woodpecker, oak bark not a dietary staple. Further, the ornament of the American lawn seems an unlikely place to search for new medicines. So, why probe the oak tree for new therapies?
In two words, boats and wine. In the 13th century B.C., the Celts, who were skilled in building boats, arrived in Burgundy, France. They crafted smooth and large boat hulls, and in doing so, they bent wood using steam and heat. Their methodology was later applied to the construction of primitive barrels. During Napoleon’s time, the French planted oak forests to ensure adequacy for boat-building, but subversive innovations such as iron and steel forced creative thinking of alternative uses. Oak wood was well-suited for wine barrels and, fortuitously, favorably affected the texture and flavor. The pleasant notes of vanilla, toast and tea characteristic of oak-aged wine are primarily imparted by oak ellagitannins.
Oak-aged wines contain unique tannins that have been investigated in vitro and in vivo, showing antiatherogenic, antiinflammatory and antineoplastic promise . However, pharmacokinetic inquiry inspired new questions, as ellagitannins are not detected in human plasma following oral administration. The immense size and hydroxyl substitution of these compounds precludes effective absorption from the gut .
Recent studies have characterized a complex rendezvous of ellagitannins with microbes that naturally inhabit the colon . Contrary to our own genome, these bacteria express genes encoding enzymes needed to break down their elaborate structures into smaller compounds. Known as urolithins, these ellagitannin metabolites are easily absorbed and can reach relatively high (>1 mM) concentrations in plasma. New research is characterizing the therapeutic actions of urolithin A for mitophagy and skeletal muscle function in aging animals .
Oak contributes many different types of phenolics and polyphenols to wines, including lignin derivatives and chimeras such as flavano-ellagitannins. All of these compounds possess antioxidant activity and putative antineoplastic, antidiabetic and antiatherogenic properties.
With the complexity of wine further encumbered by the type of barrel in which it ages, I will leave you with a healthy respect for uncertainty and the assertion of Rilke from Letters to a Young Poet—“ Have patience with everything that remains unsolved in your heart….live in the question.”