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Gulf War and Health: Volume 1. Depleted Uranium, Sarin, Pyridostigmine Bromide, Vaccines
The committee was asked to examine the literature on potential health effects of squalene since it has been raised as an issue of concern to Gulf War veterans. A recent GAO (1999a) report found that at the time of the Gulf War, DoD had concerns about having a sufficient quantity of the anthrax vaccine and sufficient time to fully immunize the troops (GAO, 1999a). However, DoD has stated that it decided against the use of novel adjuvant formulations (e.g., formulations with squalene) because of lengthy FDA relicensure requirements (GAO, 1999a). The following section provides a brief overview of the animal and human studies that have been conducted and concludes with the committee’s thoughts on directions for future research. The committee was not asked to draw conclusions on the strength of the evidence for an association between exposure to squalene and adverse health effects.
Squalene14 is a polyunsaturated terpene hydrocarbon that is widely distributed in nature. It is found in human sebum (a skin surface lipid) and is a precursor in the synthesis of human cholesterol (Final Report, 1982; Kelly, 1999). Its name stems from its abundance in shark (Squalus spp.) liver oil,15 from which it was first isolated (Liu et al., 1976). Squalene also is found in other fish oils, olive oil (0.7 percent), wheat germ oil, rice bran oil, and many other foods.
For more than 25 years, squalene has been used commercially as an emollient for topical application of more than 300 cosmetic formulations, including suntan preparations, bath oil, eye makeup, hair preparations, lipstick, baby powder, and skin care preparations. Cosmetic concentrations range from less than 0.1 to more than 50 percent. Squalene also is available as a dietary supplement; as a constituent of certain pharmaceuticals, including suppositories; and as a carrier of lipid-soluble drugs (Final Report, 1982; Kelly, 1999). As described below, squalene is being investigated for a number of potential medical purposes.
Dietary Intake, Absorption, Distribution, and Metabolism
In the 1970s, the average dietary intake of squalene in the United States was calculated at 24 mg per day (given a daily dietary intake of 2,000 calories) (Liu et al., 1976). Olive oil is particularly rich in squalene. Among Asians, shark liver oil supplements containing squalene are popular over-the-counter folk remedies (Asnis et al., 1993). The average total squalene exposure of humans from all routes of administration does not appear to have been studied. In case studies, excessive ingestion of squalene from dietary supplements has led to lipoid pneumonia (Asnis et al., 1993).
Its chemical formula is 2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene.
Squalene concentrations in shark liver oil range between 50 and 80 percent (Liu et al., 1976).