Nature's Pharmacopeia: A World of Medicinal Plants
The aloes comprise a group of hundreds of species of succulent plants adapted to survive in warm, dry conditions by storing water in thick, sword-shaped leaves, often protected by sharp, spiky leaf edges and tips (figure 14.26). The genus originated in Africa and diversified into shrub and tree forms now distributed throughout North and South Africa, Madagascar, the Arabian Peninsula, the Indian Ocean region, and the Mediterranean.203 Many species have been employed in medicine, most commonly Aloe vera, a perennial cultivated variety that produces fifteen to thirty leaves that can range in size from 5 to 50 centimeters long and up to 10 centimeters wide at the base.204 The pale-green leaves, sometimes bearing whitish spots and red streaks, and darkening with age, have been employed in medicine since ancient times.
FIGURE 14.26 Leaves of Aloe vera.
Aloe was mentioned in medical writings of the Sumerians of ancient Mesopotamia (ca. 2100 B.C.E.) and the Egyptians (ca. 1550 B.C.E.).205 When Pliny the Elder described aloe during the Roman period, he outlined an herb that he considered warming, “employed for numerous purposes, but principally as a purgative,” and noted that it was one of the few available drugs that acted in this way while also being good for the stomach. He also recommended it, rubbed on the skin or made into an herbal tea, to allay headaches, and mixed with wine and worked onto the scalp to prevent baldness. Pliny further listed a series of external ailments treatable with applied aloe, including wounds, hemorrhoids, genital warts, and sores.206 The Greek herbalist Dioscorides assured his readers that “it loosens the bowel and cleanses the stomach” when taken in water, is also useful for jaundice, and “is suitable for inflammations of the tonsils, for the gums, and for all conditions associated with the mouth.”207
Aloe maintained currency in medieval European herbal practice and appeared in numerous manuscripts of the era. By the fifteenth and sixteenth centuries, reinvigorated trade throughout Europe and the Near East made aloe more widely available to the inhabitants of the cooler countries of northern Europe. John Gerard in 1597 noted that the British sometimes kept aloe as a houseplant, “hanged on the seelings and upper posts of dining roomes,” and characterized its juice as “good for many things.” As a strong laxative, aloe was lauded by Gerard as “an enimie to all kinds of putrefactions, and defendeth the bodie from all manner of corruption,” particularly helpful to expel intestinal worms. Like his Greco-Roman forbears centuries earlier, the English author listed several ways to employ aloe on the surface of the body, including to staunch bleeding and to heal sores—rather useful on the sensitive skin of the buttocks and “secret partes.”208 Doctors and folk healers long ago discovered two of the principal properties of aloe that have since been explained in biomedical terms: its capacity to stimulate intestinal contractions and to promote wound healing.
These physiological effects can be attributed to chemical compounds in two types of leaf extract (figure 14.27). One type of leaf product is the latex, a yellowish fluid that exudes just underneath the outermost layer of the leaf and is enriched in an assortment of molecules called aloin.209 Intestinal bacteria metabolize aloin into its active form, aloeemodin, which irritates the colon and causes muscular contractions. Aloe-emodin also interferes with the normal mechanism of water regulation in the intestine, increasing the water content in the fecal mass. These mechanisms render aloe latex an effective laxative, for which it was widely used until recently (figure 14.28). Aloe-emodin is also under investigation for a number of possible effects on cell proliferation and inflammation.210 In 2002, the Food and Drug Administration banned the use of aloe latex in over-the-counter laxative preparations because of concerns about safety.211
In contrast to the latex, the clear, mucilaginous, juicy central portion of the leaf, called the leaf gel, is included in beverages and incorporated into lotions for external use. The active components of the gel appear to be the numerous complex carbohydrates that form a latticework inside and between the water-storing cells of leaf’s interior.212 Historically considered helpful in treating wounds, aloe leaf gel’s carbohydrates (polysaccharides such as acemannan) and other components have been shown in laboratory tests to stimulate the immune system, inhibit the growth of microbes, and thereby aid cuts, sores, and burns to heal more quickly.213 Some people consider that aloe leaf gel, when taken orally, might reduce inflammation caused by ulcerative colitis and gastric ulcers, for example, and treat diabetes.214 Since the methods by which aloe leaf gel is prepared can affect its chemical composition, and because clinical trials employ so many different testing protocols, the studies on human interventions using aloe have not generated consistent and easily compared data to demonstrate its therapeutic efficacy in wound healing and other applications.215
FIGURE 14.27 The active principles of aloe: aloe-emodin, a laxative compound generated in the intestine from aloin, a precursor in aloe latex; acemannan, one of the complex carbohydrates in aloe leaf gel.
Clinical data yet to come should address whether the healing properties seen in the laboratory can be observed in humans: whether, for example, wounds heal more quickly when dressed with a preparation containing aloe gel, as compared with an inert placebo lotion. As aloe is already widely accepted as an ingredient in skin-care products, generally believed to soften skin and improve its health, there might also be value in its hydrating capacity, its texture, and the way it makes people feel to apply such an ancient herb to take care of themselves. Aloe’s juice, Gerard wrote long ago, is “good for many things.”
FIGURE 14.28 An advertisement for Beecham’s Pills, a laxative that contained aloe. (From Illustrated Sporting and Dramatic News, March 3, 1888; National Library of Medicine, A028925)