Limonene is found in orange peel, caraway, celery seed and nutmeg.
Everyone knows herbs add incredible flavor to food. Many of the same compounds that give herbs their intense flavors also have antioxidant, anti-inflammatory or other healthful properties. New research suggests limonene, the volatile compound most often associated with “citrus” fragrance and flavor, could be useful against asthma.
In recent years, scientists discovered that inflammatory lesions and immune-system processes within the body produce ozone (a well-known pollutant) as a byproduct of chemical reactions. Some hypothesized that the inflammatory process of asthma involves a vicious cycle of ozone production in the lungs, where white blood cells are marshaled to help defeat inflammation, in turn producing even more ozone.
Based on this knowledge, an Israeli research group predicted that compounds that act as ozone scavengers could help prevent asthma. The researchers looked at limonene—a component in the essential oils of citrus rinds and many herbs—which is known for its ozone-protective effect. They theorized that saturating the lung membranes with limonene could protect against the ozone produced by the body as well as ozone from external sources. In tests with animals, limonene inhalation significantly prevented bronchial obstruction, while also providing an anti-inflammatory effect.
Previous research has shown limonene to have many beneficial effects. In animal studies, it has had anti-cancer effects, increasing liver enzymes that help render various carcinogens inactive. It also has reduced carcinogenicity by acting on enzymes in the small intestine.
But citrus peels are not the most abundant source of limonene, according to medicinal plant expert James Duke. On a weight-for-weight basis, the best sources are caraway (3 percent dry weight) and celery seed (2.5 percent). Oranges and tangerines come in at about 1 percent limonene. Other herbs whose essential oils contain appreciable amounts of limonene include cardamom, fennel, spearmint, nutmeg, thyme and ginger. Perhaps scientists will extract the limonene from these popular herbs to make a fragrant, new treatment for asthma. Meanwhile, enjoy their flavor.
For more information, see Bioorganic & Medicinal Chemistry 13(2):557-62.2005; Handbook of Phytochemical Constituents of GRAS Herbs and Other Economic Plants by James A. Duke; and Duke’s Phytochemical Database at www.ars-grin.gov/duke .
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