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Vitamin D and Cancer: A Sunny Follow-Up Story
National Public Radio and AP echo concerns over extreme sun avoidance 
by Randy Hartnell and Craig Weatherby 

We received many responses to last week's article about the anti-cancer powers of vitamin D. Then, as if they'd been reading our newsletter, National Public Radio (NPR) covered the same topic this past Monday, June 6, on “Morning Edition,” in a segment titled” Research Suggests Need for Balance in Sunscreen Use.” 


And, unnoticed by us, the Associated Press distributed a wire service story in late May titled “Vitamin D research may have doctors prescribing sunshine,” which painted a similarly positive picture (The Web addresses of the NPR and AP reports can be found in the Sources list at the end of this article).


The NPR and AP reports both support the points made in our article, and their high-profile coverage prompted us to further explore the risks and rewards of sun exposure and sunscreen, and the pros and cons of sunlight—versus foods and supplements—as sources of vitamin D.


Sunlight and seafood: the superior D sources

As we noted last issue, the best food sources of vitamin D by far are fish rich in oil (and omega-3s), such as salmon, sardines, sablefish, and tuna. A single 3.5 ounce serving of salmon (360 IU) or sardines (500 IU) provides amounts of vitamin D close to the recommended daily allowance (RDA) of 400 IUs, while a serving of tuna provides a bit more than half the RDA (233 IU). 


Dietary supplements are often inferior to food in terms of the effectiveness of their vitamin D. Instead of the D-3 form used by the body, and found in fish, most mass market brands contain 400 IU of the D-2 form which is biologically inefficient.  In addition, many multivitamins also contain vitamin A, which interferes with the body's ability to absorb vitamin D.


The obvious solution is to eat plenty of vitamin-D-rich foods—a category in which oily fish have no peers—and spend adequate time in the sun.  It takes only 20 minutes in full sunlight—the time it takes to develop slight redness on pale skin—for the body to make 20,000 IUs of vitamin D. 


Because the melanin that makes sun-tanned skin brown blocks UVB rays, a white-skinned person with a substantial tan will make about 10,000 units in the same 20 minutes of sun exposure, while a person with very dark brown skin might make only about 5,000 IU.


The good news is that you can't overdose on sun-generated vitamin D, since the body starts destroying any excess vitamin D after production of 20,000 IU. In other words, sun exposure serves both to ensure adequate body levels of vitamin D, and to ensure that any excess is destroyed.


The sun debate: risks versus rewards

The proven anti-cancer powers of vitamin D are myriad, and becoming increasingly clear. The AP quoted this cogent comment by Harvard's Edward Giovannucci, M.D., addressed to fellow researchers at the April, 2005 meeting of the American Association for Cancer Research: "I would challenge anyone to find an area or nutrient or any factor that has such consistent anti-cancer benefits as vitamin D. The data are really quite remarkable."


Dr. Giovannucci told NPR's Patricia Neighmond in this week's story, an estimated 30 people may die of cancers related to vitamin D deficiency for every person who dies from skin cancers caused by excessive sun exposure. 


Yet, Michael Holick, M.D. of Boston University—perhaps the foremost vitamin D expert in the country—lost his titular role in BU's dermatology department for publishing a book that anticipated the opinion expressed in April by Dr. Giovannucci and increasingly shared by other biomedical researchers. 


As the AP reported last month, some dermatologists are now persuaded that the anti-cancer benefits of sun outweigh the cancer-promoting risks. Their story included this quote from Memorial Sloan-Kettering Cancer Center's chief dermatologist, Allan Halpern, M.D.: "I find the evidence [that the benefits of sun outweigh the risks] to be mounting and increasingly compelling.”


However, while the anti-sun emperors who still dominate dermatology seem increasingly bereft of clothes, they want us to keep ours on outdoors—and apply sunscreen obsessively—as though sunlight were highly toxic. Their view downplays the serious downside of sunscreen, and overlooks the reality that the human body evolved in the context of far more sun exposure than most contemporary Americans get.


This is not to say that you should take sunburns lightly, since they can create pre-cancerous changes in skin tissues.  The worst risk—that of developing a deadly melanoma—appears related to severe sunburns in childhood. Still, all burns are to be avoided, nor is it wise to use tanning booths other than very sparingly. 


Sun and cancer

Two types of skin cancer—deadly melanomas and relatively benign carcinomas—are linked to sun exposure, to widely varying degrees.


Melanomas afflict about 60,000 Americans each year, and kill about one in seven (8,000) of those diagnosed. However, their link to sun exposure remains unclear, since melanomas often appear in areas rarely exposed to sun. It is thought that a severe sunburn in childhood may be a risk factor for melanoma.


By contrast, there are more than 15 times as many cases—one million annually—of the distinctly less dangerous, more clearly sun-related basal cell or squamous cell carcinomas. These non-melanoma cancers are highly curable if treated early, and cause only about 1,500 deaths each year, or just 0.15 percent of diagnosed cases. (Note: Men are much more likely than women to get non-melanoma cancers.)


The American Cancer Society Web site offers up to date information about deadly melanoma cancer and the less dangerous but caution-worthy non-melanoma carcinomas.


The biggest risk factors for melanomas are fair skin, family history, the presence of moles that are large in numbers or size, and severe sunburns in childhood. 


For the less-threatening basal or squamous cell carcinomas, the biggest risk factor is a fair complexion that resists tanning.  If this describes you, it's wise to limit your exposure to full, midday sun to the 20 minutes per day recommended for vitamin D production, apply sunscreen when you must spend more time in the sun, and enjoy plenty of vitamin D-rich fish to make up for lack of sun-induced vitamin D. 


The dark side of sunscreen

There is some evidence that sunscreens help inhibit formation of relatively unthreatening squamous-cell carcinomas.  Otherwise, it remains unclear how much cancer protection they provide.


Drs. Cedric and Frank Garland of the University of California were among the first scientists to oppose use of chemical sunscreens, both because there is no proof that they protect against melanoma or basal cell carcinoma in humans, and because people using sunscreen are likely stay longer in the sun.


As the Garland brothers postulated, the greatest increases in melanoma rates have occurred in countries like Australia, where chemical sunscreens have been most heavily promoted.


While they are useful for preventing sunburn, sunscreens pose two problems vis a vis cancer prevention:

  1. Sunscreens block vitamin D-producing UVB rays.
  2. Sunscreens produce a false sense of sun security that encourages lengthier-than-normal sun exposure, which exposes skin to far more UVA radiation than people would otherwise undergo.  Recent work by Australian and U.S. researchers indicates that UVA radiation plays a bigger role in causing skin cancer than previously thought.  UVA rays cause cancer by creating clouds of cancer-causing free radicals, striking deep into the pigment cells involved in melanoma cancers, and suppressing aspects of the immune system.

As early as 1994, researchers at the University of Texas' famed M. D. Anderson Cancer Center drew these conclusions from their animal studies:  “Sunscreen protection against UV radiation-induced inflammation [sunburn] may encourage prolonged exposure to UV radiation and thus may actually increase the risk of melanoma development.”


In conclusion, it's worth quoting the American Cancer Society, whose Web site advises, “Don't forget that sunscreen is used to protect from the sun's rays during normal activity. It should not be used to allow you to spend more time in the sun.”


Sunscreens and SPF numbers

By law, sunscreens are labeled with a “sun protection factor” or SPF number, which reflects the amount of added sun exposure you can bear before your skin would burn. Any given individual's “time before burning” varies by the fairness of their skin, which is a function of the amount of melanin (pigment) it contains. Thus, an SPF of 15 means that you can expose yourself to the sun 15 times longer without burning your skin, compared to the time it would take to burn without any sunscreen.


This does not mean that it is safe, in cancer-prevention terms, to spend 15, 30, or 45 times as long in the sun. In fact, research has not determined the exact relationship between sun exposure time, skin type, and other factors. 


And, SPF numbers refer only to protection against UVB radiation: the type that burns skin and produces both cancer-inhibiting vitamin D and raises the risk of non-melanoma cancers. As we've seen, UVA radiation probably poses much greater cancer risks than once believed.


Many sunscreens also contain chemicals that block UVA rays to some extent, but the amount of protection they provide remains unclear, and no UVA-specific SPF number is allowed on such products. 



  • Agar NS, Halliday GM, Barnetson RS, Ananthaswamy HN, Wheeler M, Jones AM. The basal layer in human squamous tumors harbors more UVA than UVB fingerprint mutations: a role for UVA in human skin carcinogenesis. Proc Natl Acad Sci U S A. 2004 Apr 6;101(14):4954-9. Epub 2004 Mar 23.
  • Ainsleigh HG. Beneficial effects of sun exposure on cancer mortality. Preventive Medicine, Vol. 22, February 1993, pp. 132-40
  • Burren R, Scaletta C, Frenk E, Panizzon RG, Applegate LA. Sunlight and carcinogenesis: expression of p53 and pyrimidine dimers in human skin following UVA I, UVA I + II and solar simulating radiations. Int J Cancer. 1998 Apr 13;76(2):201-6. 
  •  de Gruijl FR. Photocarcinogenesis: UVA vs. UVB radiation. Skin Pharmacol Appl Skin Physiol. 2002 Sep-Oct;15(5):316-20. Review. 
  • Dover JS, Arndt KA. Dermatology. Journal of the American Medical Association, Vol. 271, No. 21, June 1, 1994, pp. 1662-63
  • Garland CF, et al. Effect of sunscreens on UV radiation-induced enhancement of melanoma growth in mice. Journal of the National Cancer Institute, Vol. 86, No. 10, May 18, 1994, pp. 798-801
  • Garland CF, Garland FC. Could sunscreens increase melanoma risk? American Journal of Public Health, Vol. 82, No. 4, April 1992, pp. 614-15
  • Garland FC., et al. Geographic variation in breast cancer mortality in the United States: a hypothesis involving exposure to solar radiation. Preventive Medicine, Vol. 19, 1990, pp. 614-22
  • Haywood R, Wardman P, Sanders R, Linge C. Sunscreens inadequately protect against ultraviolet-A-induced free radicals in skin: implications for skin aging and melanoma?  Invest Dermatol. 2003 Oct;121(4):862-8. 
  • Koh HK, Lew RA. Sunscreens and melanoma: implications for prevention. Journal of the National Cancer Institute, Vol. 86, No. 2, January 19, 1994, pp. 78-9
  • Neighmond P. “Research Suggests Need for Balance in Sunscreen Use.” National Public Radio, Monday, June 6, 2005. Accessed online June 6, 2005 at
  • Reynolds T. Sun plays havoc with light skin down under. Journal of the National Cancer Institute, Vol. 84, No. 18, September 16, 1992, pp. 1392-94
  • Skolnick AA. Revised regulations for sunscreen labelling expected soon from FDA. Journal of the American Medical Association, Vol. 265, No. 24, June 26, 1991, pp. 3217-20
  • The Associated Press. Vitamin D research may have doctors prescribing sunshine. May 21, 2005. Accessed online June 6, 2005 at
  • Wright B. Sunscreens and the protection racket. New Scientist, January 22, 1994, pp. 21-2


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