CANCER
March 9, 2011

Confusing D

There's more evidence that at high doses vitamin D greatly reduces cancer risk. But what about the risk of toxicity?

A recent study showing that high levels of vitamin D appear to help reduce the risk of a variety of diseases again calls into question the Recommended Dietary Allowance (RDA) for vitamin D set last December.

When it was issued in 2010, the government's new RDA for vitamin D was already at odds with the many research studies published over the past few years that have suggested people aren’t getting enough of the vitamin. The Institute of Medicine (IOM) raised the RDA only slightly: from 400 international units (IUs) a day to 600 IUs a day. Some scientists had wanted to see the levels set at 1,000 to 2,000 IUs a day.

At this point, even nutrition professionals are confused about vitamin D. For years, textbooks have taught that in excess, vitamin D is the most toxic of all the vitamins, and that taking supplements of vitamin D in concentrated form is risky.

The new study suggests that substantially higher levels of vitamin D (4,000 to 8,000 IUs) are needed to prevent or greatly reduce the risk of colon cancer, breast cancer and several other diseases than once thought. The reason behind the government's fairly low RDAs has to do with the longstanding belief that at high doses, vitamin D can be harmful.

The data for the study was collected from thousands of volunteers who were taking supplements containing 1,000 to 10,000 IU of vitamin D per day. The study participants were recruited from those who attended a Vitamin D Seminar in December of 2008 and supplemented with Internet invitations. No one was excluded from the study, so people of both genders, all ages, nationalities and levels of health status were included. Blood tests were used to determine the level of 25-vitamin D, the form of vitamin D that circulates in the blood, and they were repeated throughout the study. No evidence of clinical toxicity was evident.

The authors of the study, from the University of California, San Diego School of Medicine and Creighton University School of Medicine in Omaha, note that the strengths of the study were the community base, the size of the sample, and the completeness of pertinent data. Weaknesses of the study, as reported by the researchers, included the fact that no standard vitamin D product was used, the products used were not evaluated for vitamin D content, and the doses were self-reported. In addition, the cohort was self-selected for health consciousness, thus the participants could not be considered representative of the general population given their likely high degree of adherence to taking a supplement.

'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.'

In a press release from UC San Diego, Cedric Garland, DPh, an adjunct professor of family and preventive medicine at Moores Cancer Center said, "We found that daily intakes of vitamin D by adults in the range of 4,000 to 8,000 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. 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."

When will this conundrum end? At this point, even nutrition professionals are confused about vitamin D. For years, textbooks have taught that in excess, vitamin D is the most toxic of all the vitamins, and that taking supplements of vitamin D in concentrated form is risky.

The Tolerable Upper Intake Level as set by the IOM is 4,000 IU per day. According to the Office of Dietary Supplements of the National Institutes of Health, "Vitamin D toxicity can cause non-specific symptoms such as anorexia, weight loss, polyuria, and heart arrhythmias. More seriously, it can also raise blood levels of calcium which leads to vascular and tissue calcification, with subsequent damage to the heart, blood vessels, and kidneys." Further, the ODS states that while symptoms of toxicity are unlikely at daily intakes below 10,000 IU a day, emerging science from national survey data, observational studies, and clinical trials suggest that even lower doses of vitamin D might have adverse health effects over time. The ODS also points out that a high intake of vitamin D is associated with increases in all-cause mortality, a greater risk of cancer at some sites like the pancreas, a greater risk of cardiovascular events, and more falls and fractures among the elderly.

Both vitamins C and E have had their day in the sun, but the purported benefits have been, for the most part, disproven. Now it is vitamin D’s turn in the spotlight but who’s right and who’s wrong, and to what do we owe all the confusion? Most importantly, how and when will we get it right?

The study was published February 21, 2011 in the journal Anticancer Research.

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