| MadSci Network: Medicine |
I have good news and bad news. The good news is that work on compounds isolated from walnut trees has been going on since the last century, so a great deal is known about their potential medical value. The bad news is that the only biologically active compound unique to walnut trees is not a good medication: it kills plants, poisons other mammals, and is less efficacious as a therapeutic than other drugs.
Walnut trees contain a toxin called Juglone, which is a naphthaquinone found in all parts of plants of the walnut family, Jugandaceae (including walnuts, pecans, and hickories). Juglone has been identified as the toxin involved in black walnut toxicosis1, an equine ailment afflicting horses and ponies that ingest black walnut shavings. These animals get laminitis (an inflammation of the feet around the hooves) which makes them lame, as they refuse to lift their painful feet. Even topical application of juglone to the horses' feet caused local skin irritation. Juglone is strongly phytotoxic (it kills plants). While some gardeners find this useful as a weedkiller, other gardeners with Walnut trees in their yards fight to keep plants near the tree from dying off as the juglone leaches from the roots into the soil.
Juglone also has broad spectrum antimicrobial activity, killing many bacteria and fungi2. In fact, bark from the Persian walnut (Juglans regia L.) is used in some countries as a toothbrush and as a dye for coloring the lips for cosmetic purposes3. It also increased the pH of saliva, so brushing the teeth with this bark may improve oral hygiene. However, some hazardous effects on oral tissue from the topical cosmetic application of Persian walnut have also been demonstrated 4. Since other herbs and barks with better hygenic qualities exist5, the benefits of using Persian walnut are not outweighed by the risks.
The antibiotic, antimycotic, phytotoxic, and poisonous activities of juglone are probably all related. In fact, biochemical analyses have shown that juglone inhibits several essential enzymes which are involved in cellular pathways found in almost all organisms. One such inhibited enzyme, the K+ channel which is required for cellular depolarization and nerve signalling 6, may account for the mild sedative effect when juglone has been given to animals intravenously. In fact, the toxic nature of juglone makes it a candidate for chemotherapy7, except that continued usage causes cataracts.
Although Juglone has not been proven acutely toxic in humans, walnut trees pose an allergic threat to many people. Of all the allergenic trees, pollen extracts of Hickory elicit some of the highest allergic reactions, while Black Walnut pollen elicit more moderate reactions, that are still much worse than reactions to Maple and Elm pollens8. Similarly, walnut oil that has undergone less processing at lower temperatures tends to demonstrate qualitatively greater allergenicity and higher protein concentrations, making it a threat to patients with allergy, depending on the method of manufacture and processing of the oil9 .
To sum up, extracts from walnut trees, or any members of the Juglandaceae family, are useful for many things, but not for medications, since juglone is potentially toxic and raw extracts are potentially allergenic. Fortunately, there is no juglone present in the fruits of these trees, so walnuts and pecans are a healthy and delicious addition to foods, which makes them agriculturally valuable.
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