Silicon in horsetail and comfrey

Horsetail is famous among herbalists as a source of silicon, present in the plant in various forms including the water soluble silicic acid. The plant contains between 5% and 8% silicic acid (Duke). Experiments by Piekos and Paslawaska demonstrated that 55 mg of silicon in soluble form may be extracted in 200 ml of water in which 2.0 grams of Equisetum arvense plant is boiled for 3 hours, for a total yield of 2.75% soluble silicic acid by weight of the original horsetail.

Silicon has no officially recommended dietary intake, but estimates of the requirement for humans range from 5-20 mg per day (Groff et al). Silicon was found to be essential in the 1970s for normal development of the connective tissues, mucopolysacchardies, cartilage, elastin, and bone (Carlisle). It is an important rate-limiting enzyme cofactor in the formation of the collagen matrix of bone, and its presence facilitates bone repair and the uptake of other minerals into bone. Estimated requirements are based on average dietary intakes rather than on observed effects in humans, which are difficult to measure. Most dietary silicon comes from grains and other plant foods in which it is frequently bound in poorly assimilable forms. Up to 97% of the silicon in a high fiber diet remains undigested and is excreted in the stool. Silicic acid, one form in which silicon exists in equisetum and comfrey (Symphytun spp.), is readily soluble in water, readily absorbed in the digestive tract once dissolved, and readily diffuses to the extracellular fluid reservoir and connective tissues. Thus a small amount of infused herbal material containing soluble silicic acid may provide more physiologically available silicon than much larger amounts of food in which the silicon is bound by fiber or fails to be extracted into solution in the small volume of fluid in the stomach and intestine.

The physiological role of silicon, especially in the assimilable form of silicic acid, may explain the persistent traditional use of horsetail and comfrey for bone and connective tissue health. Comfrey contains from 50-80% of the silicic acid content of horsetail. Those who want to use comfrey to promote connective tissue healing might follow the method described above and make a long decoction of the leaves. Such a method, using mature leaves, would also minimize the exposure to the potentially hepatotoxic pyrrolizidine alkaloids in comfrey.

Medical Herbalism.

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