As a member of the Cucurbitaceae family of plants, cucumbers are a rich source of the triterpene phytonutrients called cucurbitacins. Cucurbitacins A, B, C, D and E are all contained in fresh cucumber. They have been the subject of active and ongoing research to determine the extent and nature of their anti-cancer properties. Scientists have already determined that several different signaling pathways (such as the JAK-STAT and MAPK pathways) required for cancer cell development and survival can be blocked by the activity of cucurbitacins. Several other bioactive compounds have been isolated from cucumber including cucumegastigmanes I and II, cucumerin A and B, vitexin, orientin, isoscoparin 2″-O-(6?-(E)-p-coumaroyl) glucoside and apigenin 7-O-(6″-O-p-coumaroylglucoside). Despite huge exploration of cucumber from an agricultural standpoint, comparatively few studies have been published about its chemical profile and its therapeutic potential. 
 
The presence of several vitamins like A, B and C, high water content and various minerals like potassium, magnesium and silicon make cucumber a beneficial skincare ingredient for formulators. Cucumber contains high quantities of silicon and sulphur which provide the nourishment and nutrients necessary for healthy hair. In addition, cucumbers are a valuable source of conventional antioxidant nutrients including vitamin C, beta-carotene, and manganese. Fresh extracts from cucumber have been shown to provide specific antioxidant benefits as well as to reduce inflammation. These components make cucumber a kind of all-purpose ingredient with benefits that can extend across most cosmetic product categories.
 
More than 96% of edible unpeeled cucumber is water. Other constituents of Cucumis sativus are vitamins, minerals, amino acids, phytosterols, phenolic acids, fatty acids, and cucurbitacins. Traces of essential oil, pectins, starch, sugars, and vitamin C are also found in cucumbers. While glycosides, flavonoids, carbohydrates, terpenoids, and tannins have been identified in aqueous extracts of the cucumber fruit. According to a 2010 animal study published in the Journal of Young Pharmacists, fresh extracts from cucumber showed increased scavenging of free radicals. Free radicals are associated with a variety of human diseases, but can sometimes be held in check by antioxidants. 
 
There is research showing that the lutein (a carotenoid) component of cucumber can help suppress melanogenesis, the process that leads to skin discolorations. And, there is in vitro research showing that the constituents in cucumber may help protect skin against carcinogenic substances that cause tumors. Cucumber does not contain any fragrant components known to be irritating to skin and has been deemed safe for use in cosmetics ranging from facial toners, moisturizers, masks and more with little to no risk of allergic reactions or sensitization.
 
References:

1. Phytochemical and therapeutic potential of cucumber. Mukherjee PK, Nema NK, Maity N, Sarkar BK. Fitoterapia. 2013 Jan;84:227-36. doi: 10.1016/j.fitote.2012.10.003. Epub 2012 Oct 23. Review. PMID: 23098877
2. Vora,J., Rane, L., Kumar, S.A.. Biochemical, Anti-Microbial and Organoleptic Studies of Cucumber (Cucumis Sativus). International Journal of Science and Research (IJSR). ISSN (Online): 2319-7064
3. Tentative Safety Assessment: Cucumis Sativus (Cucumber)-Derived Ingredients as Used in Cosmetics. Cosmetic Ingredient Review (CIR). 2012.https://www.cir-safety.org/sites/default/files/cucum032012tent.pdf
4. Free Radical Scavenging and Analgesic Activities of Cucumis sativus L. Fruit Extract. Kumar D, Kumar S, Singh J, Narender, Rashmi, Vashistha B, Singh N. J Young Pharm. 2010 Oct;2(4):365-8. doi: 10.4103/0975-1483.71627.
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