Limonene is a terpene that also goes by the names ᴅ-limonene and ʟ-limonene. Terpenes are aromatic compounds found in hundreds of foods and plant species (including Cannabis sativa). ᴅ-limonene is one of the most common terpenes in nature thanks to its prevalence in the essential oils of citrus fruits. Once isolated, limonene is used in perfumes, cleaners, and as a flavouring agent for food.
ᴅ-limonene has a prominent orange scent, while ʟ-limonene provides a piney and turpentine like aroma.
Orange, lemon, mandarin, lime, grapefruit, and juniper.
Despite limonene’s relative chemical stability, studies into its effects are limited. A handful of preclinical studies have been performed. It is suggested that limonene may have anti-inflammatory, antioxidant, and cardioprotective qualities.
The Department of Medicinal Chemistry and Natural Products in Israel conducted an in vitro study using citrus oil in 2006. They concluded that a “mixture of citrus oil and MgCl₂ could be used as a natural antibacterial and anti-inflammatory agent” (Mizrahi, Shapira, Domb, & Houri-Haddad, 2006).
A second in vitro study, this time published by the Journal of Oleo Science, found that limonene inhibited proinflammatory cytokines. Although proinflammatory cytokines are a necessary part of the body's inflammatory response, a buildup can lead to instances of chronic inflammation. The authors suggest that “ᴅ-limonene may be considered a potential anti-inflammatory candidate" (Yoon, Lee, & Hyun, 2010).
Following reports of ᴅ-limonene influencing fat retained by the liver, researchers in China examined the compound using an animal model of high-fat, diet-induced obesity. They concluded that limonene might have uses as a dietary supplement, “preventing and ameliorating metabolic disorders” (Jing et al., 2013).
A 2018 review published by the journal Experimental and Therapeutic Medicine observed the impact of ᴅ-limonene on “stroke-prone spontaneously hypertensive rats” (SHRsp). ᴅ-limonene was found to decrease the “systolic blood pressure of SHRsp rats following stroke” while protecting against memory and cognitive impairment (Wang, Li, & Shen, 2017).
Limonene has been approved as a safe food additive and flavouring by the Food and Drug Administration since 1994. When assessing the toxicity of the compound, they found that it has “relatively low acute toxicity taken orally”. The only reported side effect was the risk of skin irritation after using limonene-infused shampoos, creams, or sprays.