j. Soc. Cosmet. Chem., 44, 239-247 (September/October 1993) Emollient action of kukui nut oil H. AKO, L. FUJIKAWA, and D. GRAY, Department of Environmental Biochemistry, University of Hawaii, Honolulu, HI 96822 (H.A., L.F.), and Hawaiian Kukui Nut Company, P.O. Box 685, Waialua, HI 96791 (D.G.). Synopsis Dry skin drives the search for effective emollients. This report searches for an explanation of the superb skin feel of this native Hawaiian oil. Using skin stripping technology and gas chromatography, we demonstrated kukui oil's ability to penetrate the stratum corneum, leaving a triglyceride barrier containing a mixture of saturated and unsaturated fatty acids. A kukui oil-based lotion was also very effective in laying down an emollient layer of lipids. However, skin penetration did not distinguish kukui oil from oils containing saturated fatty acids. It is speculated that the skin feel of kukui oil may be due to the fact that it lays down a mixture of saturated and unsaturated fatty acids, forming a semipermeable barrier that would protect the skin from further drying and would allow dry skin to heal naturally. It was found that kukui oil is quite stable as judged functionally by the degradation of linolenate (C18:3n-3). INTRODUCTION Kukui oil is produced in Hawaii by cold pressing the oil from the nut of the kukui tree (Alleurites moluccana). Hundreds of years ago, Hawaiians used it to protect the skin of babies. It is used as a body oil by modern-day Pacific Islanders. Cosmetic chemists report that kukui oil has an excellent skin feel. They say the kukui oil seems to be readily absorbed into the skin (it does not leave a greasy film). It seems to make chapped or rough dry skin feel smooth, silky, and soft. It seems to prevent scarring when applied to abrasions. A major purpose of the present work is to explain these reported properties. A number of hypotheses were entertained. We do not prefer the metabolic explanation of dry skin. Based on the work of Burr and Burr (1), nutritional essential fatty acid deficiency (EFAD) has been cited as a cause of dry skin. They maintained weaned rats on a totally fat-free diet for months before signs of scaly feet, dandruff, and slow growth were seen. These symptoms were cured with small amounts of dietary essential fatty acids. The extreme conditions required to induce dry skin and the ease of curing dry skin nutritionally suggest to us that dry skin normally seen in the population may not be due to EFAD. Goodgame, Lowry, and Brennan (2) studied a 13-year-old in a coma with signs of EFAD due to absence of essential fatty acids in intravenous feedings. Cutaneous application of essential fatty acids showed a slight decrease in his triene:tetraene ratio, a biochemical test for EFAD. However, cutaneous application did not bring the ratio within normal range. Essential fatty acid infusion three times per week with a product called "Intra- 239

240 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lipid" at 5% of total caloric intake brought his triene:tetraene ratio into normal range in one week. This case contrasted topical application of EFAs with infusion of small amounts of EFAs. With an average intake of Americans at 30% kcal from fat, EFAD is not something one would expect to see in Americans except under very unusual circumstances. In this work we exclude the possibility that kukui oil's superior skin feel is due to its penetrating ability. We demonstrate that coconut oil, a saturated fat, penetrates the skin as well as kukui oil. Thus we will be left with the hypothesis that kukui oil forms a semipermeable barrier that protects dry skin from further damage and permits dry skin to heal naturally. While the definition of dry skin is beyond the scope of this paper, it may be assumed that dry skin is characterized by apparent dryness, scaling, cracking, etc. These con- ditions may be induced by the loss of natural lipids in the stratum corneum. Such could occur by swabbing with acetone (3). This treatment decreases the barrier function of the skin, as may be seen by the increase in transepidermal water loss (TEWL). There is a linear relationship between lipid removal and the increase in TEWL. It follows that use of household chemicals such as soaps, or extreme exposure to the wind, sun, and sea such as occurs commonly in Hawaii, may also cause dry skin. The same condition is often reported by people who live in very cold environments (where barrier lipid fluidity may be inappropriate to prevent TEWL) or very dry environments (where TEWL would be extremely high). We also note that dry skin heals naturally. Unless further damage occurs, barrier function is restored to dry skin within 48 hours (3). Intervention in the healing process is not straightforward. On the one hand, some TEWL must occur because no lipid biosynthesis or recovery of the normal rate of TEWL occurs if the skin is protected occlusively with films such as latex. On the other hand, unprotected skin may be damaged further. We propose that lipids composed of a mixture of saturated and unsaturated fatty acids could provide a protective barrier against excessive TEWL and yet promote healing. Perhaps this explains the emollient properties of kukui oil. Kukui oil contains 8.1% saturated fatty acids, 15.3% monounsaturated fatty acids, 43% linoleate (C18:2n-6, a diunsaturated fatty acid), and 33% linolenate (C18:3n-3, a triunsaturated fatty acid). Another purpose of the present study is to determine the stability of kukui oil. Kukui oil has the reputation of being highly susceptible to rancidity. It normally leaves the factory with a peroxide value of 2. Sometimes batches are rejected because peroxide values have risen to 10 or more. METHODS SAMPLES Coconut oil was purchased commercially. Kukui oil samples with and without antiox- idants were obtained from the production line of the Hawaiian Kukui Nut Company. Kukui lotion was obtained similarly. It contained water, 10% kukui oil, 3% macadamia nut oil, and an unextraordinary mixture of buffers and emulsifying agents.