The study, published in the Archives of Internal Medicine, highlights the importance of maintaining healthy vitamin D levels.
“Further observational studies are needed to confirm these findings and establish the mechanisms underlying these observations. If confirmed, randomized clinical trials will be needed to determine whether vitamin D supplementation at higher doses could have any potential benefit in reducing future mortality risk in those with 25(OH)D deficiency,” wrote lead author Michal Melamed from the Albert Einstein College of Medicine.
The vitamin is produced in the body on exposure to sunlight, but increasing vitamin D levels via sunlight or supplements has been a source of ongoing debate.
In the US, where over 1.5 million people are diagnosed with skin cancer every year, experts are pushing supplements, claiming recommendations for sun exposure are "highly irresponsible".
Another push for supplements comes from the fact that intakes are low from dietary sources coupled with a lack of sunshine in northern climates, has led to estimates that as much as 60 per cent of northern populations may be vitamin D deficient.
In adults, vitamin D deficiency may precipitate or exacerbate osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
Study details
Melamind and co-workers from John Hopkins University used data from the Third National Health and Nutritional Examination Survey (NHANES III). Vitamin D levels were analysed in 13,331 healthy men and women
According previous results from the same nationwide survey, the optimum blood level of 25[OH]D is 30 nanograms per millilitre (ng/ml) or higher. However, about 41 per cent of American men and 53 per cent of American women have levels lower than 28 ng/ml. An unhealthy deficiency is considered blood levels of 17.8 ng/ml or lower.
Over the course of 8.7 years of follow-up, the researchers documented 1,806 deaths. Of these, nearly 700 deaths were associated with some form of heart disease, with 400 of these being deficient in vitamin D.
When the researchers compared people with the lowest and highest average 25(OH)D levels, they found that low levels were associated with a 26 per cent decreased rate of death from any cause.
No significant associations were found when the researchers assessed vitamin D levels and risk of death from cardiovascular disease or cancer alone.
"Our results make it much more clear that all men and women concerned about their overall health should more closely monitor their blood levels of vitamin D, and make sure they have enough," said study researcher Erin Michos.
"We think we have additional evidence to consider adding vitamin D deficiency as a distinct and separate risk factor for death from cardiovascular disease, putting it alongside much better known and understood risk factors, such as age, gender, family history, smoking, high blood cholesterol levels, high blood pressure, lack of exercise, obesity and diabetes," she added.
Considerations
“This is the first study, to our knowledge, to explore the association between 25(OH)D levels and mortality in the general population,” wrote the authors.
They note, however, several limitations to the study, most importantly that it is an observational study and therefore cannot prove if the role of vitamin D is causal.
“Several lines of evidence suggest that vitamin D deficiency may be a risk factor for cardiovascular, cancer, and all-cause mortality,” they added.
“Ecological studies reveal that CVD events are higher in the winter when vitamin D levels are lower and that cancer survival is better if the cancer is diagnosed in the summer when vitamin D levels are higher.”
Source: Archives of Internal MedicineVolume 168, Issue 15, Pages 1629-1637“25-Hydroxyvitamin D Levels and the Risk of Mortality in the General Population”Authors: M.L. Melamed, E.D. Michos, W. Post, B. Astor