Chapter 34 Botanical Medicine—Understanding Herbal Preparations
The clinical application of botanical medicine involves the use of various herbal preparations. That being the case, it is imperative that practitioners and physicians using herbal medicines understand the differences among the various forms. Commercial herbal preparations are whole fresh and dried herbs, teas, tinctures, fluid extracts, and tablets or capsules. In addition, juices, purified compounds, and specific fractions of plants (e.g., volatile oil preparations) can also be considered herbal medicines. Each form has advantages and disadvantages and varying ways of expressing its medicinal strength. Any form may be an effective medicine as long as it can deliver an effective dosage of active compounds. The following section reviews the major extraction forms, discusses the benefits of standardized botanical extracts, and emphasizes the importance of delivering a clinically effective level of active compounds regardless of the form of the herbal preparation.
An extract is a concentrated form of an herb obtained by mixing the crude herb with an appropriate solvent (such as alcohol or water, or both). A major advance in the herb industry has been the improvement in extraction and concentration processes.
When an herbal tea bag is steeped in hot water, it is actually a type of herbal extract known as an infusion. The water is serving as a solvent in removing some of the medicinal properties from the herb. Teas are often better sources of bioavailable compounds than powdered herbs, but are relatively weak in action compared with tinctures, fluid extracts, and solid extracts. These forms are commonly used by the lay public and herbal practitioners for medicinal effects.
Tinctures are typically made using an alcohol and water mixture as the solvent. The herb is soaked in the solvent for a specified amount of time, depending on the herb. This soaking is usually from several hours to days; however, some herbs may be soaked for much longer periods of time. The solution is then pressed out, yielding the tincture.
Fluid extracts are similar to, but more concentrated than, tinctures. Although they are most often made from hydroalcoholic mixtures, other solvents may be used (e.g., vinegar, glycerin, propylene glycol). Commercial fluid extracts are usually made by distilling off some of the alcohol, typically by using methods that do not require elevated temperatures, such as vacuum distillation and counter-current filtration. However, some small manufacturers produce fluid extracts in a similar manner to tinctures via percolation at room temperature.
A solid extract is produced by further concentration of the extract by the mechanisms described earlier for fluid extracts, as well as by other techniques such as thin-layer evaporation. The solvent is completely removed, leaving a viscous extract (soft solid extract) or a dry solid extract depending on the plant, portion of the plant, solvent, and drying process (if any) used. The dry solid extract, if not already in powdered form, can be ground into coarse granules or a fine powder. A solid extract can also be diluted with alcohol and water to form a fluid extract or tincture.
The potencies or strengths of herbal extracts are generally expressed in two ways. If they contain known active principles, their strengths are commonly expressed in terms of the content of these active principles. Otherwise, the strength is expressed in terms of their concentration. For example, tinctures are typically made at a 1:5 to 1:10 concentration. This means one part of the herb (in grams) is soaked in five parts solvent (in milliliters of volume), meaning there is five times the amount of solvent (alcohol/water) in a tincture as there is herbal material. Fluid extracts are typically 1:1.
A 4:1 concentration means that one part of the extract is equivalent to, or derived from, four parts of the crude herb. This is the typical concentration of a solid extract. A quantity of 1 g of a 4:1 extract is concentrated from 4 g of crude herb.
Typically, 1 g of a 4:1 solid extract is equivalent to 4 mL of a fluid extract and 20 to 40 mL of a tincture. Some solid extracts are concentrated as high as 100:1, meaning it would take nearly 100 g of crude herb, or 100 mL of a fluid extract, or 1 L of a tincture, to provide an equal amount of herbal material in 1 g of a 100:1 extract.
Before the 1980s, the quality of the extract produced was often difficult to determine because many of the active principles of the herbs were unknown. However, advances in extraction processes, coupled with improved analytic methods, reduced this problem of quality control.1,2 The concentration method of expressing the strength of an extract does not accurately measure potency because there may be great variation among manufacturing techniques and raw materials. By using a high-quality herb (i.e., rich in active compounds), it is possible to have a more potent dried herb, tincture, or fluid extract compared with the solid extract that was made from a lower quality herb. Standardization of herbal extracts for key active constituents has been suggested to be the solution to this problem.3
Standardized extracts (also referred to as guaranteed potency extracts) refer to an extract guaranteed to contain a “standardized” level of active compounds or key chemical marker. Stating the content of active compounds or key chemical marker rather than the concentration ratio allows for more accurate dosages to be made. However, the complex composition of an herbal medicine makes it unwise to ignore the other constituents.
The best scenario for determining the quality of an herb is the level of active components or key biological markers, along with taking into consideration the complete chemical profile. Regardless of the herb’s form, it should be analyzed to ensure that it contains these components at an acceptable standardized level. More accurate dosages can then be given. This form of standardization has emerged as the preferred method, but it is not without its shortcomings. Chief among them is that assigning too much importance to individual components rather than the entire chemical profile leads to the assumption that controlling for the level of marker compounds is equivalent to controlling the entire pharmacologic effect of the herbal medicine. However, stating the content of active constituents versus drug concentration ratio does allow for more accurate dosages to be based on active constituents and provides the greatest degree of consistency and assurance of quality, especially if attention is also given to the other constituents.
Although referred to in terms of active constituents, it must be kept in mind that these are still whole extracts and not isolated constituents. For example, a Uva ursi extract standardized for its arbutin content, say 10%, still contains all of the synergistic factors that enhance the active ingredient’s function.
A tremendous range of sophistication exists in processing herbs, from crude herb to highly concentrated standardized extracts. Nonetheless, there are some common stages. Some of the processes in the production of herbal products and the machines that perform these functions are discussed in the following section.3
When plants are collected from their natural habitat, they are said to be “wild-crafted.” When they are grown using commercial farming techniques, they are said to be “cultivated.” Collection of plants from cultivated sources ensures that the plant collected is the one that is desired. When an herb is wild-crafted, there is a greater chance of picking the wrong herb and of variation in potency. However, some herbal practitioners believe that wild-crafted herbs are inherently superior because growth in their natural habitat produces an herb with constituents and properties more consistent with traditional use. The use of analytic techniques can be employed to guarantee that the plant collected is the one desired and that its concentrations of medicinal constituents are within an acceptable range.
The mode of harvesting varies from hand labor to sophisticated equipment. The mode is not as important as the time of year. A plant should be harvested when the part of the plant being used contains the highest possible level of active compounds. Again, this is ensured by using analytic techniques.
After harvesting, most herbs have a moisture content of 60% to 80% and cannot be stored without drying. Otherwise, important compounds could break down or microorganisms could contaminate the material, or both. As many of the desired compounds are heat labile, the majority of herbs require relatively mild conditions for drying. Commercially, most plants are dried within a temperature range of 100°F to 140°F. During drying, the plant constituents must not be damaged or suffer losses that would prevent them from conforming to accepted standards. With proper drying, the herb’s moisture content is reduced to less than 14%.