Why Sunscreens Do Not Fully Prevent Skin Cancer
By Dale Kiefer
What About Vitamin D?
Some health authorities have recently suggested that the campaign to encourage rigorous use of sunscreen may have an unforeseen side effect: an increase in vitamin D deficiency. It is true that occasional sun exposure is important for the body’s manufacture of vitamin D.28-30 In fact, 90-100% of the body’s requirement for vitamin D can be obtained by the action of UV light striking exposed skin.31 It is also well established that adequate vitamin D is essential for good bone health. Some scientists have even proposed that sunlight exposure confers a measure of protection against certain cancers—including, paradoxically, melanoma,32 possibly due to sunlight’s role in helping the body manufacture vitamin D.30,31
This issue, however, remains controversial. Dr. Coldiron believes that approximately 15 minutes of unprotected sun exposure per week is sufficient to meet the body’s requirements, given the level of vitamin D fortification in the food chain. Michael F. Holick, MD, PhD, a professor of medicine, physiology, and dermatology at Boston University Medical Center, agrees. In an article published in The American Journal of Clinical Nutrition, Dr. Holick writes: “Sensible sun exposure (usually 5-10 minutes of exposure of the arms and legs, or the hands, arms, and face, 2 or 3 times per week) and increased dietary and supplemental vitamin D intakes are reasonable approaches to guarantee vitamin D sufficiency.”33
While UV-mediated vitamin D manufacture is efficient, this important vitamin can also be obtained through dietary sources and supplements. Vitamin D is present in fatty fish, which likewise provide healthful omega-3 fatty acids, and as a dietary supplement and food additive. However, skin cancer is far more common than rickets, the soft bone disease that can result from vitamin D deficiency.34 Many aging baby boomers suffered severe sunburns as children, which places them at significantly greater risk of developing skin cancer as they approach the age of 50 and beyond.
Routinely applying sunscreen with an SPF of 15 or greater—and carefully re-applying it according to the manufacturer’s instructions—is essential to helping prevent skin cancer and premature skin aging. Although SPF represents the beginning of the modern sun-protection story, it is certainly not the end. As one team of scientists recently declared in the peer-reviewed medical journal Cutis, “Even those products with a very high sun-protection factor (SPF) and full-spectrum UVB and UVA protection may not prevent UV radiation-induced [damage to the immune system].”35
Recent research on cancer-fighting phytochemicals—potent, biologically active compounds derived from plants—indicates that a more proactive approach to skin health may be in order. By combining a variety of beneficial compounds with more mundane sun-blocking agents, it is possible to protect the skin from damage and actively repair some damage that already may have occurred.
Benefits of Topical Green Tea
Scientists have recently focused on several remarkable compounds that offer protection against photoaging and photodamage. Chief among these naturally occurring chemicals is epigallocatechin gallate (EGCG), a catechin polyphenol compound found in green tea. Studies have shown that EGCG provides broad-spectrum protection by preventing three of the pathological changes associated with sun damage: inflammation, DNA damage, and immune system deficits.36,37 According to one leading researcher, “Animal and human studies suggest that green tea polyphenols are photoprotective in nature.”38
Inflammation normally results with the exposure of skin to UV radiation. White blood cells known as leukocytes stream into irradiated tissues. Oxidative damage occurs and produces free radicals, thereby depleting immune system cells. Numerous studies have found that topical application of green tea extract prevents these pathological changes.38-45 As a result, researchers believe, green tea may prevent skin cancers that would normally be induced by exposure to solar radiation.38-45 Indeed, experiments on mice specially bred for their susceptibility to skin cancer demonstrate that topical application of green tea extract reduces tumor incidence and size following exposure to measured amounts of UV radiation.42,46 A leading dermatology research team concluded, “Treatment with EGCG . . . resulted in exceptionally high protection against photocarcinogenesis . . . ”42
Green tea’s benefits do not stop there. More recently, scientists examined green tea’s effects on normal, healthy skin in aging humans. Topically applied green tea extract stimulated the proliferation of skin cells known as keratinocytes. The increase in these structural support cells led to an increase in epidermal thickness, and subsequent UV exposure failed to destroy the cells, as would normally occur. Two of the hallmarks of aging skin are reduced thickness and keratinocyte destruction. Green tea reversed both of these markers of skin aging. The researchers concluded, “This study demonstrates that EGCG promotes keratinocyte survival and inhibits the UV-induced [cell death] via two mechanisms . . .”47
Earlier this year, researchers in England published the results of experiments in which human cell cultures were exposed to UV radiation with or without the presence of EGCG. Cells treated with the green tea compound experienced significant protection from UV-induced DNA damage. Scientists believe such damage may underlie the eventual development of cancer; it is likewise implicated in the development of visible signs of aging. Taking things a step further, human subjects were given green tea to drink, and their blood was collected before and after tea drinking. The blood cells were subsequently irradiated with UVA radiation. Blood cells drawn from subjects who drank green tea experienced significant protection from UV damage compared to those drawn before tea drinking.37
Silibinin, Silymarin Are Also Protective
The milk thistle plant (Silybum marianum) is another source of beneficial compounds. Silibinin and silymarin, flavonoid compounds extracted from milk thistle, have well-established antioxidant, anti-inflammatory, and immune-enhancing properties. Used clinically to treat liver toxicity in Europe and Asia, milk thistle flavonoids also combat carcinoma of the prostate and lungs, as well as other cancers.48-60
Aware that many antioxidants guard against tumor promotion, scientists at Case Western Reserve University wondered whether silymarin might also protect against skin cancer. In the late 1990s, they tested this hypothesis on mice bred for their susceptibility to skin cancer. After topical application of silymarin, scientists exposed the mice to chemical carcinogens known to elicit tumors. Tumor incidence, multiplicity, and volume were all reduced significantly. According to the research team, “These results suggest that silymarin possesses exceptionally high protective effects against tumor promotion . . .”49
In the last few years, similar research has confirmed and expanded on these findings. Scientists at the University of Colorado reported that silibinin, a major constituent of milk thistle, significantly reduced skin tumor multiplicity and volume when applied to the skin of mice bred to serve as a model of UV-induced human skin cancer. Tumor incidence was decreased moderately, and the mechanisms underlying these protective effects were elucidated. According to the researchers, “Together, these results show a strong preventive efficacy of silibinin against photocarcinogenesis, which involves the inhibition of DNA synthesis, cell proliferation, and cell cycle progression, and an induction of apoptosis.”52
Working with cultures of skin carcinoma cells, researchers examined the effects of EGCG and silibinin on molecular signaling events involved in the rampant proliferation of the aberrant cells. While treatment with both phytochemical agents resulted in “strong dose- and time-dependent cell growth inhibition,”57 the scientists noted that cancer-preventive effects were achieved through different molecular mechanisms—a finding that supports combining photoprotective phytochemicals for maximum efficacy.
Because milk thistle flavonoids prevent skin cancer through various mechanisms and are well tolerated, they are a natural choice for inclusion in broad-spectrum sunscreen formulations. “Silymarin may favorably supplement sunscreen protection and provide additional anti-photocarcinogenic protection,” wrote a leading researcher earlier this year.48
Grape Seed Polyphenols and Proanthocyanidins
Resveratrol, which is found in grape seeds and skins, has been hailed as a remarkable compound capable of exerting a wide range of beneficial effects and extending life span in a variety of organisms.61,62 Scientists classify resveratrol and related compounds as phytoalexins: natural antibiotics designed to protect a plant from attack by pathogens. Researchers also believe that resveratrol protects plants from UV damage. For instance, grapes grown at higher altitudes, where UV exposure is greater, tend to contain greater amounts of resveratrol.63
Another class of beneficial compounds concentrated in grape seed, the antioxidant proanthocyanidins, inhibit skin chemical carcinogenesis and photocarcinogenesis in mice through at least three pathways.64-67 Proanthocyanidins have demonstrated antioxidant power that is 20 times greater than that of vitamin E and 50 times greater than that of vitamin C. These powerful phenolic compounds protect the skin against sun damage while promoting the elasticity, flexibility, youthfulness, and health of skin cells.67
Turmeric Root Extract
Throughout history, the curry spice turmeric (Curcuma longa) has been valued as both a culinary and medicinal agent. Curcumin, a yellow pigment derived from turmeric root, exhibits antioxidant and anti-inflammatory effects, and may help promote wound healing.68 In an experimental animal model, topical curcumin application inhibited the initiation and promotion of skin tumors.69
While these and other studies suggest that topical curcumin may benefit the skin, curcumin’s yellow color makes it undesirable as a cosmetic agent. Fortunately, researchers have developed a colorless derivative of turmeric root called tetrahydrocurcumin that may allow people to benefit from this spice without undesirable cosmetic effects. Topical application of tetrahydrocurcumin helps to quench existing free radicals and prevent the formation of new ones. This dual action protects skin cells from UV light-induced damage and resulting inflammation and skin injury.70
Laboratory studies indicate that topical tetrahydrocurcumin is a safe and effective skin-lightening agent.71,72 Skin-lightening agents help to fade sun-induced areas of hyperpigmentation, or skin darkening. Many such agents work by inhibiting tyrosinase, a key enzyme involved in melanin synthesis. Thus, the colorless turmeric root derivative tetrahydrocurcumin may help protect the skin against detrimental effects of UV light and may help prevent (or fade) hyperpigmented areas of skin. While hyperpigmentation is not a medically harmful condition, it is always advisable to have a physician examine new brown spots to rule out skin cancers.