Urtica dioica; Urtica urens

Alternative Medicine Review, Sept, 2007

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Description

Urtica dioica (stinging nettle) and Urtica urens (dwarf nettle) are members of the Urticaceae family native to Eurasia, and are considered therapeutically interchangeable. (1) The term nettle is used in this article to refer simultaneously to stinging and dwarf nettle.

Urtica prefers wet, rich soil and tends to grow in large patches. Stinging nettle is taller than dwarf nettle and is perennial; dwarf nettle is an annual. Both plants have fleshy, drooping, serrated, roughly heart-shaped leaves. The leaves and stems are covered with stinging hairs (dwarf nettle leaves are smooth and more delicate). Both produce inconspicuous green-white flowers in late spring or summer. The leaf, flower, seed, and root of nettle are used differently and contain different chemical constituents.

Constituents and Mechanisms of Actions

Like all green vegetables, nettle leaf is a micronutrient dense, nutritious food; however, it should be steamed or cooked before ingestion to destroy the stinging hairs, which contain histamine, formic acid, acetylcholine, acetic acid, butyric acid, leukotrienes, 5-hydroxytryptamine, and other irritants. (1,3) Contact with the hairs leads to a mildly painful sting, development of an erythematous macule, and itching or numbness for a period lasting from minutes to days. Medicinal extracts of nettle do not cause this reaction as the hairs are destroyed in processing.

The hydrophilic components of nettle, including lectins and polysaccharides, appear to be important, particularly in prostate disease; (4) however, hydrophobic constituents have not been ruled entirely unimportant. (5) Each constituent may have individual effects, with the combination acting differently than any one constituent in isolation.

The importance of nettle root lignans, such as (-)-3,4-divanillyltetrahydrofuran, in benign prostatic hyperplasia (BPH) and other androgen- and estrogen-sensitive conditions may be due to interference with binding of sex hormone binding globulin (SHBG) to testosterone, the testosterone receptor, and/or the SHBG receptor. (6,7) Nettle lectins and sterols did not show anti-SHBG effects in these studies.

The steroidal compounds stigmasterol, stimast-4-en-3-one, and campesterol have been shown to inhibit the prostatic sodium/potassium pump, which might contribute to nettles effects in BPH. (5) The small quantity of betasitosterol in nettle root (<0.01% of total mass) is unlikely to have an effect on BPH, given that 60 mg beta-sitosterol daily is the usual amount necessary to reduce symptoms. (8)

Urtica dioica agglutinin (UDA) is a heat- and acid-resistant lectin found in stinging nettle, primarily the root. UDA induces a pattern of T-lymphocyte activity not seen with any other known plant lectin. (9) UDA appears to prevent formation of a systemic lupus erythematosus-like condition in mice, and has diverse antiviral effects in vitro. (10,11) UDA antagonizes the epidermal growth factor receptor, an effect that may be of benefit in interfering with the pathogenesis of BPH. (12)

Testosterone, its metabolite dihydrotestosterone (DHT), and estrogen are hormones commonly associated with prostate disease. Testosterone is converted to estrogen via the enzyme aromatase. Aromatase levels increase as men age, testosterone levels decline (as higher aromatase levels convert testosterone to estrogen), and the prostate gland becomes enlarged. Aromatase inhibitors are used in BPH to prevent the age-related rise in estrogen and associated prostate enlargement. 9-hydroxy-10-trans-12-cis-octadecadienic acid (HOA) from nettle root inhibits aromatase in prostate tissue. (13) A botanical formula consisting of whole herb Urtica dioica and Pygeum africanum bark was found to inhibit both aromatase and 5-alpha reductase. (14) Inhibition of 5-alpha reductase interferes with conversion of testosterone to DHT, high levels of which are associated with BPH and prostate cancer.

Polysaccharides and caffeic malic acid (CMA) are both found to some extent in all parts of nettle. Polysaccharides stimulated T-lymphocyte activity and complement activation in vitro. (2) Urtica polysaccharides and CMA demonstrated anti-inflammatory activity in vitro and in animal studies, via cyclooxygenase and lipoxygenase inhibition. (2,15) Isolated polysaccharides promoted tumor necrosis factor (TNF) production in vitro, while whole plant extracts inhibited TNF. (2,16)

Clinical Indications

Benign Prostatic Hyperplasia

The best-researched indication for nettle is use of the root for men with symptomatic BPH, something nettle is not known for traditionally. At least four double-blind clinical trials confirm the efficacy of nettle root for BPH symptoms alone or in combination. Earlier research on a combination with Pygeum extract at two dose levels, a combination with alpha-adrenergic antagonists, and a combination with Serenoa repens (saw palmetto) fruit extracts all show the benefits of nettle root. (17-19) In perhaps the most persuasive trial, a combination of nettle and saw palmetto was just as effective as finasteride (Proscar[R]) in improving symptoms of BPH in a 48-week, double-blind trial, with fewer and milder adverse effects in the herbal group than the drug group. (20) Uncontrolled trials have also demonstrated nettles effectiveness for BPH. (21)