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Vitamin D cancer merits questioned

Wednesday 31 October 2007

A study has found that Vitamin D, “once heralded as a major weapon in the war on cancer” actually does little to cut the risk, the Daily Mail reported today. However, the newspaper added that although the study found no benefit for cancer in general, it did find that people with high levels of Vitamin D were 72 per cent less likely to die of bowel cancer.

The study is a well conducted cohort study, however it has a couple of important limitations. These include the fact that the people who took part in the study only had their levels of vitamin D measured once, and that only a small number of them died from cancer.

Although the study appeared to indicate that vitamin D has no effect on cancer in general, it still showed that people who had high levels of it have less risk of bowel cancer, the second biggest cause of cancer death in the UK. This result, combined with the fact that vitamin D is an essential nutrient for our body that is obtained naturally though diet and sunlight anyway, suggests that we should not change our intake of vitamin D.

Where did the story come from?

Dr Michael Freedman and colleagues of National Cancer Institute and Institutes of Health in Maryland, USA. The study was funded by Intramural Research Programme of National Institutes of Health, National Cancer Insititute and US public Health Service of Department of Health and Human Services. The study was published in the peer-reviewed Journal National Cancer Institute.

What kind of scientific study was this?

This was a cohort study which examined participants of the Third National Health and Nutrition Examination Survey, a larger survey designed to examine the health and nutrition of the US population.

Between 1988 and 1994, 16,818 people over the age of 17 were enrolled in the study, had samples of their blood taken, and were followed until the the year 2000.

Using the blood samples, the researchers measured the candidates levels of 25(OH)D; the substance that is the main form of Vitamin D in the body.

The blood samples were taken at different times of the year depending on which area the participant was from, with collection in southern areas in the winter months, and collection in northern areas in the summer months. At the end of the study period, the researchers looked at deaths in the cohort, and in particular cancer related deaths. The relationship between cancer and 25(OH)D levels were examined taking into account other potentially contributing factors such as age, ethnicity, retinol (vitamin A) and calcium levels and a variety of other personal and social factors.

What were the results of the study?

The researchers found there were 536 deaths due to cancer up until the year 2000. They found no link between levels of vitamin D and deaths due to cancer, although they did find there to be a reduction in the risk of death due to bowel cancer in people with higher Vitamin D levels.

There was also no variation in risk when considering the season or latitude that the sample was collected from, the participants levels of vitamin A, or by looking at men and women and different ethnic groups separately. They found that blood 25(OH)D levels varied significantly according to personal characteristics of ethnicity, sex, age, education, smoking, alcohol, BMI, level of exercise, and vitamin and calcium intakes in food.

What interpretations did the researchers draw from these results?

The authors conclude that they found no link between 25(OH)D levels and cancer risk, although there was some evidence of protection against bowel cancer. They say that, in order to confirm their findings, further studies where 25(OH)D is measured at multiple time points and compared to cancer mortality will be needed.

What does the NHS Knowledge Service make of this study?
This is a well-conducted study. There are a couple of points to consider when interpreting the findings:

  • Although there is no evidence from this study that vitamin D reduces overall cancer mortality, we should note that vitamin D (obtained through our diet and through sunlight) remains a vital nutrient for our health that is essential for the body’s regulation of calcium levels.
  • The numbers of people who actually died from cancer during the follow up period was relatively small. As a result, the study is unlikely to have had the power to identify the effect of vitamin D on specific kinds of cancer. It presents the results mainly as the effects on all cancer deaths together.
  • There are many factors that have been linked to increases in the risk of cancer death. Although adjustments have been made for certain identified risk factors, it is not possible to consider every possible confounding factor.
  • Also, as the researchers themselves highlight, the study relied on a single blood reading, which may not provide an accurate reflection of normal levels.

There continues to be speculation over and consideration of the health benefits of vitamin D and we look forward to further large studies to provide more data on the issue.

Analysis by Bazian
Edited by NHS Website

Links to the headlines

Vitamin D downgrade as scientists advise there is no real proof it fights cancer.

Daily Mail, 31 October 2007

Links to the science

Freedman DM, Looker AC, Chang S-C, Graubard BI.

Prospective Study of Serum Vitamin D and Cancer Mortality in the United States.

J Natl Cancer Inst 2007; Advance Access published online on October 30