We've reported on some of the many studies that have associated higher vitamin D intakes or blood levels with reduced rates of breast, colon, and other cancers.
Now the coincidence of two simultaneous studies explains why the evidence remains mixed, albeit mostly positive.
One, from the New England Journal of Medicine, was a review of the abundant epidemiological evidence – and much smaller amount of clinical evidence – that vitamin D intake affects cancer risk.
As the authors wrote, there's good reason to think that vitamin D should help prevent cancer: “The theory that vitamin D can help prevent cancer is biologically plausible.” (Manson JE et al. 2011)
Fish fit the vitamin D bill; Sockeye salmon stand out
In addition to getting vitamin D from supplements, certain fish rank among the very few substantial food sources of vitamin D, far outranking milk and other D-fortified foods.
Among fish, wild Sockeye Salmon may be the richest source of all, with a single 3.5 ounce serving surpassing the US RDA of 400 IU by about 70 percent:
Vitamin D per 3.5 ounce serving*
Sockeye Salmon 687 IU
Albacore Tuna 544 IU
Silver Salmon 430 IU
King Salmon 236 IU
Sardines 222 IU
Sablefish 169 IU
Halibut 162 IU
*For our full test results, click here.
But as they also noted, some studies find no link between higher vitamin D levels and lower cancer risk, and that having low vitamin D blood levels could logically be associated with also having one of more of four risk factors for cancer:
Obesity (vitamin D becomes sequestered in fatty tissue)
Lack of physical activity (correlates with less time outdoors and less sun exposure).
Dark skin (meaning less skin synthesis of vitamin D in response to sunrays).
Generally unhealthful, nutrient-poor diets based on processed foods.
The other study, by leading vitamin D researchers, was an analysis of how much dietary vitamin D is needed to reach blood levels associated with reduced risk of cancer in epidemiological and clinical studies (Garland CF et al. 2011).
Let's look at that study,which examined evidence related to minimum protective blood levels .. whose results would explain the mixed anti-cancer evidence cited in the New England Journal of Medicine paper.
Leading researchers find D intakes too low to consistently curb cancer
Compared to the intake levels thought adequate, researchers at the University of California and Omaha's Creighton University found that markedly higher intakes of vitamin D are needed to reach blood levels that might cut the incidence of major cancers and other diseases.
While these levels are higher than traditional intakes, they are largely in a range deemed safe for daily use in a December 2010 report from the National Academy of Sciences Institute of Medicine.
“We found that daily intakes of vitamin D by adults in the range of 4000-8000 IU are needed to maintain blood levels of vitamin D metabolites in the range needed to reduce by about half the risk of several diseases – breast cancer, colon cancer, multiple sclerosis, and type 1 diabetes,” said Cedric Garland, DrPH, professor of family and preventive medicine at UC San Diego's Moores Cancer Center. (UCSD 2011)
“I was surprised to find that the intakes required to maintain vitamin D status for disease prevention were so high – much higher than the minimal intake of vitamin D of 400 IU/day that was needed to defeat rickets in the 20th century.” (UCSD 2011)
“This result was what our dose-response studies predicted, but it took a study such as this, of people leading their everyday lives, to confirm it”, said Robert P. Heaney, MD, of Creighton University, a distinguished scientist who's studied vitamin D needs for decades. (UCSD 2011)
Garland, Heaney, and their co-authors analyzed data from a survey of several thousand volunteers who were taking vitamin D supplements in the dosage range from 1000 to 10,000 IU/day. Blood studies were conducted to determine the level of vitamin D circulating in their blood.
“Most scientists who are actively working with vitamin D now believe that 40 to 60 ng/ml is the appropriate target concentration of 25-vitamin D in the blood for preventing the major vitamin D-deficiency related diseases, and have joined in a letter on this topic,” said Garland.
“Unfortunately,” he added, “according a recent National Health and Nutrition Examination Survey, only 10 percent of the US population has levels in this range, mainly people who work outdoors.” (UCSD 2011)
Recommended daily doses found too low ... even after being raised
Last year, a U.S. Institute of Medicine (IOM) committee identified 4000 IU per day as a safe supplemental intake level for adults and children nine years and older … with intakes in the range of 1000-3000 IU per day safe for infants and children through eight years of age.
While the IOM committee declared 4000 IU per day a safe dosage, their recommended minimum daily intake was only 600 IU.
“Now that the results of this study are in, it will become common for almost every adult to take 4000 IU per day,” Garland said. “This is comfortably under the 10,000 IU per day that the IOM Committee Report considers as the lower limit of risk, and the benefits are substantial.”(UCSD 2011)
Garland added that people who may have contraindications should discuss their vitamin D needs with their family doctor.
“Now is the time for virtually everyone to take more vitamin D to help prevent some major types of cancer, several other serious illnesses, and fractures,” said Heaney. (UCSD 2011)
Other co-authors of the article were Leo Baggerly, Ph.D., and Christine French.
We recommend viewing the video interviews conducted by breast cancer survivor Carole Baggerly, founder of Grassroots Health … a non-profit organization that collects and disseminates scientifically accurate information on vitamin D.
Grassroots Health is advised by the world's leading vitamin D researchers, including scientists from the University of California, Emory University, Harvard University, Boston University, University of Toronto, and more.
Mixed evidence is due to participants' relatively low vitamin D intakes
When you examine the two studies side-by-side, it seems apparent that the amount of dietary vitamin D needed to affect cancer risk is much higher than previously thought, and much higher than the amounts consumed by people in most epidemiological and clinical studies.
In other words, it's not much of a surprise that the New England Journal of Medicine review found the evidence mixed and inconclusive.
This is not to say that the available evidence, while compelling, is conclusive, because it is not.
However, few of the many populations studied consumed enough vitamin D to reach the blood levels associated with reduced cancer risk.
So it's no surprise that the evidence from epidemiological and clinical studies, while generally positive, is mixed.
If you presume that vitamin D exerts real anti-cancer effects, people in most studies just aren't getting enough vitamin D from their diets or sun exposure to consistently produce reduced cancer risk.
When it comes to epidemiological evidence, it's critical to observe the central scientific caveat, “correlation is not causation”.
Just because a particular lifestyle factor or blood nutrient level is statistically associated with lower rates of a particular disease does not mean it is responsible for that risk reduction.
That said, when a large amount of epidemiological evidence is accompanied by lab evidence showing why a food or nutrient could prevent cancer, it becomes harder to dismiss the association as mere chance.
And the landmark findings by Dr. Garland and his colleagues would explain why the evidence is mixed.
Garland CF, French CB, Baggerly LL, Heaney RP. Vitamin d supplement doses and serum 25-hydroxyvitamin d in the range associated with cancer prevention. Anticancer Res. 2011 Feb;31(2):607-11. Accessed at http://www.iiar-anticancer.org/openAR/journals/index.php/anticancer/article/view/215
Manson JE, Mayne ST, Clinton SK. Vitamin D and Prevention of Cancer - Ready for Prime Time? N Engl J Med. 2011 Mar 23. [Epub ahead of print]
University of California - San Diego (UCSD). Higher vitamin D intake needed to reduce cancer risk
Feburary 22, 2011. Accessed at http://www.eurekalert.org/pub_releases/2011-02/uoc--hvd022211.php