Does vitamin D reduce mortality?

Taking vitamin D could increase your lifespan, reported The Daily Mail. It said that researchers had pooled the results of 18 individual trials of vitamin D supplements and found that taking them reduced the risk of death by 7%. Vitamin D is “being credited with reducing the risk of death from any causes,” the Mail said.

The reports are based on a good quality systematic review. Although this study has shown that vitamin D supplementation may reduce the risk of death over a certain period, it does not tell us how much longer people might live if they take vitamin D, or what causes of death vitamin D might be preventing. The authors of this research have called for further randomised controlled trials to confirm their conclusions.

Where did the story come from?

Philippe Autier and Sara Gandini from the International Agency for Research on Cancer in France, and the European Institute of Oncology in Italy carried out this research. No external funding for this study was reported. It was published in the peer-reviewed medical journal, Archives of Internal Medicine.

What kind of scientific study was this?

This was a systematic review and meta-analysis of the results from randomised controlled trials of vitamin D supplementation.

The researchers searched databases of scientific and medical literature in November 2006 for any randomised controlled trials comparing vitamin D supplementation (either vitamin D2 or D3) to control (no treatment or a placebo) for any health condition, that reported how many people died in each group.

The researchers then pooled the data on deaths using complex statistical methods, to see whether there were any differences between the vitamin D and control groups. They also used statistical methods to see whether results varied depending on dose of vitamin D received, whether participants also received calcium supplements, or how long participants were followed up for.

What were the results of the study?

The researchers identified 18 randomised controlled trials, which enrolled 57,311 people and followed them up for about six years on average. People in the vitamin D groups in these trials received an average daily dose of 528 units of vitamin D; in most trials, people were taking vitamin D to reduce their risk of a fracture.

Overall, the researchers found that vitamin D supplementation reduced the relative risk of death by 7% compared with the control group. This result did not seem to vary with vitamin D doses, whether people received calcium supplements, or how long participants were followed up for.

What interpretations did the researchers draw from these results?

The researchers concluded that taking “ordinary” doses of vitamin D reduce overall mortality. They suggested that large placebo-controlled randomised trials looking specifically at mortality should be conducted to confirm these findings.

What does the NHS Knowledge Service make of this study?

This was a well-conducted systematic review and meta-analysis, whose results seem reliable. When interpreting the study some points should be considered:

• Most of the people in these studies were middle aged or older: elderly people are often found to have low vitamin D levels due to various causes, e.g. differences in dietary absorption, poorer diet and less daylight exposure. Deficiency is less common in younger people; therefore it is unclear what effect vitamin D supplementation would have on mortality among younger people.
• Although the relative risk of death is reduced by 7%, the absolute reduction in the risk of death is relatively small: about 85 in every 1,000 people in the control group died compared with about 82 in every 1,000 people the vitamin D group, this is equivalent to 3 in 1,000 fewer deaths for those who took vitamin D.
• Most of the studies included in this analysis were carried out in countries where the majority of the population is Caucasian (Europe, the US, Australia and New Zealand). It is not certain whether similar results would be seen in people of different ethnic origins.
• The study did not look at what people died from; therefore it cannot be concluded from this study what sorts of deaths vitamin D might be preventing (e.g. deaths from cancer, heart disease, or fracture-related death in the elderly). Also, how much longer people who took vitamin lived cannot be determined.
• Vitamin D is found in some foods, but this study does not examine whether eating more vitamin-containing foods in the diet might have similar effects. Vitamin D is also made by the skin in response to sunlight, but the dangers of skin cancer from prolonged exposure outweigh any reduction in overall mortality that might be seen.
• The risks from taking too much vitamin D, for example, that it may cause excessively high calcium levels in the blood, have not been considered in this study. Because of these risks, the recommended daily dose of vitamin D should not be exceeded.

Sir Muir Gray adds...

The evidence about the benefits of vitamin D is getting stronger and there is little evidence of harm. More research is urgently needed.

However, I have decided to start taking vitamin D, as well as more exercise by walking my extra 3000 steps every day; these steps I call my vital steps because they are of importance to my life and health.