Medicinal Herbs - What Defines Quality?
Medicinal herbs are sourced from nature and hence, unlike conventional chemical drugs, vary from batch to batch. This can be readily understood by comparison with another plant product — wine. In technical terms, wine is the fermented juice of the fruit of Vitis vinifera. However, factors such as the grape variety, climatic conditions, soil type, time of harvest and fermentation conditions can all determine whether a batch of wine will be either poor or good quality (which is subsequently reflected in the price of the wine).
In the case of wine, factors such as the texture, colour, aroma and taste determine if the product is of good quality or otherwise. For medicinal plants, the situation is much more complex. Most of the chemical components of herbs which are important for therapeutic activity are secondary metabolites.
By definition, secondary metabolites are not thought to be important for the growth and survival of the plant, although they are sometimes produced in higher levels in response to infection, insect attack and adverse growing conditions. One consequence of this is that even though they may give an impression of quality, the appearance and colour of a herb are not necessarily indicators of its therapeutic quality. Indeed, plants grown under adverse conditions may sometimes have a poor appearance but higher levels of secondary metabolites. One corollary of this is that while herb batches which have a good appearance and a pleasant taste might be suitable for a herbal tea, they may not be optimum for use as medicines. An example is chamomile. Chamazulene is considered to be an important active component, and varieties of chamomile have been bred which have higher levels of this compound.
Since chamazulene imparts a bitter taste to the flowers, these high medicinal grade varieties are not suitable for a herbal tea taken for pleasure. How can the medicinal quality of a herb be determined? One approach adopted by the various pharmacopoeias and used by manufacturers is to set minimum levels of marker chemical compounds for a herbal raw material. These are seen to give an indication of activity, and hence quality. This approach is fraught with difficulties. Even where the marker compound is known to contribute to the therapeutic activity of the herb (and this is not always the case), herbalists stress that the chemical complexity of the plant confers the sum total of its activity. But until we better understand how individual herbs work in their chemical totality, it is a good starting point.
The uncertainties can be lessened by choosing phytochemical classes of marker compounds (flavonoids, essential oil, oligomeric procyanidins etc) rather than just individual chemical components. In addition, testing for different marker compounds (or groups of marker compounds) in the one plant can lead to a better assessment of activity. As one researcher recently commented: “If a batch of a herb contains low levels of one marker compound, and then by a completely different test also shows low levels of another class of marker compound, we can begin to form the conclusion that it is of poor quality.”
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