Aloe

Scientific Name(s): Aloe vera L., A. perryi Baker (Zanzibar or Socotrine aloe), A. barbadensis Miller (also called A. vera Tournefort ex Linne or A. vulgaris Lamark; Curacao or Barbados aloe), or A. ferox Miller (Cape aloe). A. vera Miller and A. vera L. may not be the same species. Family: Liliaceae

Common Name(s): Cape , Zanzibar , Socotrine , Curacao , or Barbados aloes , aloe vera

Clinical Overview

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Uses of Aloe

Aloe appears to inhibit infection and promote healing of minor burns and wounds, frostbite, and possibly of skin affected by diseases such as psoriasis and seborrheic dermatitis. Dried aloe latex is used, with caution, as a drastic cathartic.

Aloe Dosing

As a gel, A. vera may be applied externally ad lib. The resin product is cathartic at doses of 250 mg and is not recommended for internal use.

Contraindications

Its use is contraindicated in pregnant and nursing women, children younger than 12 years of age, patients with inflammatory bowel disease, and elderly patients with suspected intestinal obstruction.

Pregnancy/Lactation

Documented adverse effects. Cathartic, reputed abortifacient. Avoid use.

Aloe Interactions

None well documented.

Aloe Adverse Reactions

There has been 1 report that using the gel as standard wound therapy delayed healing. The gel may cause burning sensations in dermabraded skin, and redness and itching also can occur. Use caution with cosmetic products containing A. vera gel.

Toxicology

The resin product is cathartic at doses of 250 mg and is not recommended for internal use.

Botany

Aloes, of which there are approximately 500 species, belong to the family Liliaceae. 1 The name, meaning “bitter and shiny substance,” derives from the Arabic “alloeh.” Indigenous to the Cape of Good Hope, these perennial succulents grow throughout most of Africa, southern Arabia and Madagascar, and are cultivated in Japan, North and South America, and in the Caribbean and Mediterranean regions. They do not grow in rain forests or arid deserts. Often attractive ornamental plants, their fleshy leaves are stiff and spiny along the edges and grow in a rosette. Each plant has 15 to 30 tapering leaves, each up to 0.5 meters long and 8 to 10 cm wide. Beneath the thick cuticle of the epidermis lies the chlorenchyma. Between this layer and the colorless mucilaginous pulp containing the aloe gel are numerous vascular bundles and inner bundle sheath cells, from which a bitter yellow sap exudes when the leaves are cut. 2

History

Drawings of aloe have been found in the wall carvings of Egyptian temples erected in the fourth millennium BC. Called the “Plant of Immortality,” it was a traditional funerary gift for the pharaohs. The Egyptian Book of Remedies (ca. 1500 BC) notes the use of aloe in curing infections, treating the skin, and preparing drugs that were chiefly used as laxatives. The Bible (John 19:39-40) says that Nicodemus brought a mixture of myrrh and aloes for the preparation of Christ's body. Alexander is said to have conquered the island of Socotra to obtain control of it. The Greek physician Dioscorides, in 74 AD, recorded its use to heal wounds, stop hair loss, treat genital ulcers, and eliminate hemorrhoids. In the 6th century AD, Arab traders carried it to Asia. From the Mediterranean region, it was carried to the New World in the 16th century by the Spaniards. In the modern era, its clinical use began in the 1930s as a treatment for roentgen dermatitis. 2

Chemistry

The aloe yields 2 commercially important products. “Aloe resin” is the solid residue obtained by evaporating the latex obtained from the pericyclic cells beneath the skin. 3 The bitter yellow latex contains the anthraquinone barbaloin (a glucoside of aloe-emodin) and iso-barbaloin in addition to a series of O-glycosides of barbaloin, called aloinosides, chrysophanic acid, and up to 63% resin. Filtering out resins from the exudate and concentrating the remaining anthraglycoside material (which is up to 25% barbaloin) into crystalline form produces aloin. Aloin is a mixture of water-soluble glycosides obtained from aloe.

A second product, aloe gel, is a clear, thin, gelatinous material obtained by crushing the mucilaginous cells found in the inner tissue of the leaf. The gel is the product used most frequently in the cosmetic and health food industries. It is generally devoid of anthraquinone glycosides. The gel contains a polysaccharide glucomannan, similar to guar gum. It is this component that is believed to contribute mostly to the emollient effect of the gel. “Aloe vera gel extract” is not actually an extract, but rather the pulverized whole leaves of the plant.

Allantoin is a primary mucilaginous substance in aloe and is an important proliferating agent.

Other compounds such as tannins, polysaccharides, organic acids, enzymes, vitamins, and steroids have been identified. 4 Aloe contains bradykininase, which relieves pain and decreases swelling and redness. Magnesium lactate, by blocking histamine production, may contribute to the antipruritic effect of aloe. An antiprostaglandin that reduces inflammation also has been isolated. The anthraquinones are local irritants in the GI tract and have been used in treating certain skin diseases such as psoriasis.

Chemical composition differs among the species of aloe. For example, A. barbadensis Miller may contain 2.5 times the aloe-emodin of A. ferox Miller, and the time of harvest is a further factor in composition.

Aloe Uses and Pharmacology

Aloe latex has been used for centuries as a potent cathartic. The aloinosides exert strong purgative effects by irritating the large intestine.

The most common use of the gel remains in the treatment of minor burns and skin irritation. Early studies of its use generally were poorly controlled, and the data were incomplete and conflicting. These reports described the use of aloe in the treatment of radiation-induced dermatitis. 5

A. barbadensis extracts, in a murine model, have been shown to prevent ultraviolet radiation-induced suppression of contact and delayed hypersensitivity by reducing the production of interleukin-10. 6 , 7 However, a small study in 10 healthy volunteers did not support this observation. 8 Subsequent reports of the use of topical aloe in treating human and animal radiation burns suggested that although healing occurred, a clear advantage over aggressive wound care could not be established. However, in a study of 27 patients, aloe vera gel-treated partial thickness burn wounds healed in an average of 12 days as opposed to the vaseline-gauze-treated area of the same burn, which healed in an average of 18 days. 9

The activity of aloe in treating burns may stem from its moisturizing effect, which prevents air from drying the wound. 10 Its activity also has been ascribed to its chlorophyll content and that of other minor components, but this has not been adequately substantiated. Current theory suggests that healing is stimulated by mucopolysaccharides in combination with sulfur derivatives and nitrogen compounds. Topical aloe treatment for burns has not been adequately documented. Two FDA advisory panels found insufficient evidence to show that A. vera is useful in the treatment of minor burns and cuts or vaginal irritations.

Other studies generally have found preparations containing aloe to accelerate wound healing.

In patients who underwent dermabrasion, aloe accelerated skin healing by about 72 hours compared with polyethylene oxide gel dressing, 11 and aloe has been found to accelerate wound healing in patients with frostbite. 12 Aloe vera applied to debrided white or clear blisters and to intact hemorrhagic blisters every 6 hours, is part of the treatment protocol for frostbite. 13 , 14 Addition of oral pentoxifylline, in a study with rabbits, showed additional improvement in tissue survival after frostbite injury. 15 However, at least 1 study found that aloe applied as standard wound therapy delayed wound healing significantly (83 vs 53 days). 16

Studies of the antibacterial activity of aloe have yielded conflicting results. One study using A. vera gel 17 found no activity against Staphylococcus aureus and Escherichia coli . Other tests found that A. chinensis inhibited growth of S. aureus , E. coli , and Mycobacterium tuberculosis , but that A. vera was inactive. 18 Further, these extracts lost their in vitro activity when mixed with blood. The latex has shown some activity against pathogenic strains. 19 Two commercial preparations exerted antimicrobial activity against gram-negative and gram-positive bacteria as well as Candida albicans when used in concentrations greater than 90%. 20

Aloe has been found to be more effective than sulfadiazine and salicylic acid creams in promoting wound healing and as effective as silver sulfadiazine in reducing wound bacterial counts. 21

Research reveals no clinical data regarding the use of aloe as an antibacterial.

Research reveals no animal data regarding the use of aloe for the treatment of seborrheic dermatitis or psoriasis.

A double-blind, randomized, placebo-controlled trial demonstrated effectiveness of an aloe vera crude extract emulsion in reducing scaliness, pruritus, and the number of involved sites in 44 patients with seborrheic dermatitis. 22

A double-blind, randomized, placebo-controlled trial demonstrated effectiveness of topical A. vera extract 0.5% in a hydrophilic cream in curing 25/30 vs 2/30 placebo-treated patients with psoriasis. 23

Aloe-emodin is antileukemic in vitro 24 and has exhibited selective activity in vitro and in a murine model against neuroectodermal tumors (eg, neuroblastoma, Ewing sarcoma). 25 Other studies showed A. vera gel to be less cytotoxic 26 than indomethacin or prednisolone in tissue cultures. In vitro study of diethylhexylphthalate isolated from A. vera Linne demonstrated antileukemic and antimutagenic effects; however, results are conflicting when studied in rats. 27 , 28 The National Cancer Institute concluded that A. vera latex was not worthy of further study as a cancer cure. However, the US Department of Agriculture has approved A. vera as an adjunctive treatment for fibrosarcomas in dogs and cats. 29

Other health claims generally are poorly documented. An emulsion of the gel was reported to cure 17 of 18 patients with peptic ulcers, but no control agent was used in this study. 30 Additionally, pretreatment with an A. vera extract reduced aspirin-induced injury in a study with rats. 31 Further human studies are needed to establish this potential protective property.

A. vera extract 0.5% in a hydrophilic cream was shown in a placebo-controlled study to shorten time to healing in male patients with first episodes of genital herpes. 32

A gel containing A. vera extract 0.125%, allantoin 0.35%, and silicon dioxide was found effective in decreasing the duration of lesions associated with aphthous stomatitis. 33

Lyophilized A. barbadensis combined with zinc acetate has been studied in rabbits for use as a vaginal contraceptive. 34

In 1 laboratory experiment, rats injected with 1, 10, or 100 mg/kg SC doses of A. vera (without anthraquinones) daily for 7 days showed improved circulation and wound healing. 35 Arthritic mice were injected SC with a 150 mg/kg suspension of anthraquinones once a day for 13 days. This aloe extract caused a 48% inhibition of inflammation, caused by anthraquinone and cinnamic acid, and a 72% inhibition of arthritis, caused by anthranilic acid, which also had an anti-inflammatory effect. A. vera extracts have bradykininase activity in vitro. 36 Topical administration of aloe extracts reduced swelling in an animal model of inflammation by 29%. 37 Investigations have established that certain components of aloe inhibit the complement system, thereby reducing inflammatory responses. 38 , 39

A small study has found that parenteral administration of aloe extract protects the liver from chemical injury and has been shown to ameliorate ALT levels dramatically in patients with chronic hepatitis. 40

Only the dried latex is approved for internal use as a cathartic. In some cases, A. vera is sold as a food supplement, allegedly with FDA approval. FDA has only approved A. perryi , A. vera , A. ferox , and certain hybrids for use as natural food flavorings. 41

Dosage

As a gel, A. vera may be applied externally ad lib. The resin product is cathartic at doses of 250 mg and is not recommended for internal use. 42

Pregnancy/Lactation

Documented adverse effects. Cathartic, reputed abortifacient. Avoid use. 43

Interactions

None well documented.

Adverse Reactions

Because aloe is used extensively as a folk medicine, its adverse effects have been well documented. Except for the dried latex, aloe is not approved as an internal medication. The external use of aloe has not been associated with severe adverse reactions. The majority of adverse effects are relatively mild and reversible upon cessation of application. 44 Reports of burning skin following topical application of aloe gel to dermabraded skin have been described. 45 Contact dermatitis from the related A. arborescens has been reported. 1 Erythema, edema, urticaria, and eczematous rash have also been reported following A. vera application. 46 , 47

There has been 1 report that using the gel as standard wound therapy delayed healing. The gel may cause burning sensations in dermabraded skin, and redness and itching can also occur. Use caution with cosmetic products containing A. vera gel. 48

Toxicology

Anthranoid laxative (aloe-emodin) use has not been shown to be a risk factor for the development of melanosis coli, colorectal adenomas, or carcinomas. 49 Aloe-emodin and other anthraquinones may cause severe gastric cramping and aloe has been associated with congenital malformations; 50 thus, its use is contraindicated in pregnant and nursing women, 51 children younger than 12 years of age, 52 , 53 patients with inflammatory bowel disease, 53 and elderly patients with suspected intestinal obstruction. 53

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