kaolin. Naturally occurring clay mineral (silicate of aluminum) that is used in cosmetics for its absorbent properties.

Kathon CG. See methylchloroisothiazolinone.

kava-kava extract. Extract of the Piper methysticum plant that has analgesic (anti-inflammatory) properties, but can also cause skin irritation and dermatitis (Sources: Alternative Medicine Review, December 1998, pages 458–460; and Clinical Experimental Pharmacology and Physiology, July 1990, pages 495–507).

kawa extract. See kava-kava extract.

kelp extract. See algae.

Kigelia africana extract. Extract of African plant commonly known as the sausage tree. The African lore about this extract is that it can firm breast tissue, but there is no supporting research for this myth. The research on this ingredient is limited and mostly in vitro, but it does appear to have anti-inflammatory and antioxidant properties (Sources: Experimental and Toxicologic Pathology, August 2007, pages 433–438; and Journal of Natural Products, November 2005, pages 1610–1614).

kinetin. Trade name N6-furfuryladenine, a plant hormone responsible for cell division. As a “natural” skin-care ingredient, it is promoted primarily as having been clinically proven to reduce the signs of aging, improve sun damage, reduce surfaced capillaries, and offer many other skin benefits of particular interest to aging baby boomers. There is a good deal of research on kinetin when it comes to plants or in test tubes (in vitro), with cells, and even on flies, but there is no published research on kinetin’s topical effect, either on animal or human skin (Source: Dermatologic Clinics, October 2000, pages 609–615).

Although there are two unpublished clinical studies responsible for much of the attention kinetin is getting, both were sponsored by Senetek, the company that licenses the use of kinetin. On a closer look, according to MedFaq.com (a now-defunct Internet source that evaluated the legitimacy of medical research), the data are far less convincing than Senetek wants you to know. These studies, paid for by Senetek, were both performed by Dr. Jerry L. McCullough, Professor of Dermatology, University of California, Irvine. According to MedFaq, “The first study was well-designed—there was a control group and [it was done] double-blind…. After 24 weeks, a good response was noted in 30% of the subjects treated with kinetin … [but] there was no statistically significant difference between the people taking kinetin and the people just getting the placebo.” Another study was then performed that did not use a placebo control group, but in which everyone was using a product that contained some amount of kinetin. Not surprisingly, in this protocol the results for skin were much better. “Essentially all of the subjects reported improvement after 24 weeks …” regardless of how much kinetin the product contained. As MedFaq states, “This outcome could also have a variety of causes unrelated to kinetin: It could reflect an improvement over time, a change across seasons, the subjects’ enthusiasm, or it could have been caused by the cream or lotion the kinetin is in. In the first study, all of the subjects followed ‘a standard skin-care regimen consisting of a gentle-skin cleanser and daily use of sunscreen.’ If that regimen was followed in the second experiment, it too might explain the improvement.”

Recent studies indicate that kinetin can help increase cell differentiation (turnover rate) and that it works best in the presence of calcium as an inducing agent, but that combination is not what is being used in skin-care products that contain kinetin (Source: Annals of the New York Academy of Sciences, May 2006, pages 332–336). Kinetin may have benefit as a cell-communicating ingredient, but this has been demonstrated only in vitro (Source: Proteonomics, February 2006, pages 1351–1361).

kiwi fruit extract. As a food, kiwi has significant antioxidant properties that may even be greater than those of vitamin C (Source: Nutrition and Cancer, 2001, volume 39, number 1, pages 148–153). Whether that benefit translates into its use on skin has not been demonstrated. The acid component of the kiwi can be a skin irritant.

kojic acid. By-product of the fermentation process of malting rice for use in the manufacture of sake, Japanese rice wine. There is definitely convincing research, both in vitro and in vivo and in animal studies, showing that kojic acid is effective for inhibiting melanin production (Sources: Biological and Pharmaceutical Bulletin, August 2002, pages 1045–1048; Analytical Biochemistry, June 2002, pages 260–268; Cellular Signaling, September 2002, pages 779–785; American Journal of Clinical Dermatology, September–October 2000, pages 261–268; and Archives of Pharmacal Research, August 2001, pages 307–311). Both glycolic acid and kojic acid, as well as glycolic acid with hydroquinone, are highly effective in reducing the pigment in melasma patients (Source: Dermatological Surgery, May 1996, pages 443–447). So why aren’t there more products available containing kojic acid? Kojic acid is an extremely unstable ingredient in cosmetic formulations. Upon exposure to air or sunlight it turns a strange shade of brown and loses its efficacy. Many cosmetics companies use kojic dipalmitate as an alternative because it is far more stable in formulations. However, there is no research showing that kojic dipalmitate is as effective as kojic acid, though it is a good antioxidant. There is a small amount of research showing that kojic acid is a skin irritant (Source: www.emedicine.com, “Skin Lightening/Depigmenting Agents,” November 5, 2001).

kola nut. One of the major components of the kola nut is caffeine, which can be a skin irritant. However, kola nut also has a primary amine content that can form nitrosamines, which are potential carcinogens (Source: Food and Chemical Toxicology, August 1995, pages 625–630). See caffeine.

kudzu root. Source of isoflavone, genistein, and daidzein, all plant estrogens (Sources: Phytochemistry, June 2002, pages 205–211; and Journal of Alternative Complementary Medicine, spring 1997, pages 7–12). It can be a potent antioxidant.

kukui nut oil. Non-volatile oil from a plant native to Hawaii; it has emollient properties for skin (Source: Journal of the Society of Cosmetic Chemists, September–October 1993).