BACKGROUND:The oak tree has been admired for its stately stature and used in many different ways. About 600 different species occur, all in the genus Quercus. This term comes from two Celtic words meaning "fine tree". Druids viewed the oak tree as sacred.
The bark of oak trees has a long history of medicinal use, especially that of the English oak, Quercus robur. The bark is dried and then ground into a powder. This is used to make a tea or decoction (a remedy made by boiling the plant material). The decoction can be applied to the skin, and was sometimes used to make suppositories. The tea was used as a gargle for mouth ulcers or consumed for acute diarrhoea.
Oak bark contains between 10 and 20 per cent of tannins, which are astringents. These compounds constrict and shrink tissues and are used in traditional remedies to dry up wounds and skin inflammation. Many different types of plants and chemicals act as astringents, including strong tea or dry wine. They can be recognised by the dry, scratchy sensation they leave in the mouth.
EVIDENCE FROM STUDIES
Tannins have been studied extensively in lab settings. They react with proteins to make them insoluble, which creates a layer of material on the tissue that acts as a barrier. Tannins also cause membranes to become less porous, which reduces the amount of mucous or fluid coming through the tissue.
In spite of its long tradition of use, very little clinical research has been conducted on oak bark. A study of tree barks used for wound healing in Russia identified oak bark as having the most activity against staphylococcal infections. This has led to interest in oak bark being used against MRSA, the antibiotic-resistant infections that are becoming more common.
An ointment containing oak bark extract, benzoic acid and salicylic acid has been tested on MRSA-infected wounds in animals. The number of bacteria in the wounds was significantly reduced, but they were not eliminated. Another study examined the effect of the same commercial ointment on foot ulcers in diabetic patients. The wounds received conventional treatment in addition to either the oak bark ointment or silver sulphadiazine cream. Those treated with oak bark healed significantly faster.
However, while silver sulphadiazine cream has been used to prevent wound infections, studies have found that it can slow wound healing. Therefore, it was not the best cream to compare with oak bark. In addition, all three ingredients in this ointment have reputations for healing wounds. Therefore, these studies cannot determine which of the components, or which combination, caused the observed effects.
PROBLEMATIC ASPECTS
Oak bark is not recommended for extensive or more serious wounds. Care should be taken if the skin is broken.
Use around the mouth or genitalia has led to rashes which have spread to other parts of the body. Internal use can lead to gastrointestinal problems. Long-term use of oak bark either externally or internally has led to liver and kidney problems.
RECOMMENDATIONS
The long tradition of use, and established activity of tannins, suggests that oak bark may provide relief for certain types of eczema or minor wounds and burns. However, hardly any research has examined the preparation in humans, and what is available has serious limitations.
The evidence for internal use is even more limited. Short-term use of oak bark tea for diarrhoea may not be problematic, but more effective treatments are available. Long-term use of oak bark orally or on the skin should be avoided.
Dónal O’Mathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University
