Although vitamin D is obtained from diet and dietary supplements, the main source of vitamin D is production in our skin under the influence of solar radiation.
As sun exposure varies with the latitude of regions and during the year, so vitamin D concentrations of populations vary accordingly. Furthermore, women are generally more prone to low vitamin D concentrations than men. Vitamin D deficiency is especially common among the elderly who often have less sun exposure because of reduced outdoor activity and reduced capacity of the skin to produce vitamin D.
Research on Vitamin D’s Protective Role
We conducted an international study of eight populations from Europe and the United States to investigate the association of level of vitamin D in blood and deaths from all-causes, cardiovascular diseases, and cancer mortality, published in the British Medical Journal in June 2014. We paid attention to differences between countries, sexes, age groups and seasons of blood sampling. The study was conducted with the CHANCES Project, a research initiative funded by the European Commission and coordinated by scientists at the Hellenic Health Foundation in Athens, Greece, and Mount Sinai School of Medicine in New York, and primary authors were from the German Cancer Research Center, in Heidelberg.
A total of 26,018 people had their vitamin D level measured in blood samples collected at enrolment in the study, when they were ages 50 to 79, and were followed for an average of 4 to 16 years, depending on the population. During this period 6,695 of them died, including 2,624 from cardiovascular disease and 2,227 from cancer. We compared the risk of dying between the group with highest vitamin D level and those with lower levels by dividing each study population in five groups of equal size based on their baseline vitamin D level.
Increased Deaths in People With Low Vitamin D Levels
Compared with the group with the highest vitamin D, people in the group with lowest vitamin D had a 57 percent increased mortality for all causes. They also had a 41 percent increased mortality from heart disease for those who were free from heart disease at the study’s start. In addition, individuals with history of cardiovascular disease had a 65 percent increased risk. People with a history of cancer had a 70 percent increased risk. However, for cancer mortality, we saw no benefit in those who were free from cancer at the start.
For all these associations, the three groups with intermediate vitamin D level were at intermediate risk, with a dose-response relation. These associations did not vary by sex, age group, or season of blood collection.
For the US population in the study, the comparison was between people with level of 25-hydroxyvitamin D (the main metabolite of vitamin D, and the active compound measured in blood) just above 85 nmol/L on the high end, and people with a level below about 42 nmol/L on the lowest. In that population, the difference in mortality was 42 percent for all causes and 26 percent for cardiovascular diseases.
Since ours an observational study, it cannot directly address the issue of causality. In other words, it is possible that blood vitamin D is a marker of some other characteristics causally linked to disease and mortality, like general good or poor health status. Controlled vitamin D trials, in which individuals are randomly allocated to vitamin D supplement or placebo, are needed to disentangle the effect of vitamin D from that of other factors. However, such studies can only address the effect of dietary supplementation, not of normal dietary intake and sun-related endogenous production. Prior trials showed a protective effect of vitamin D on mortality, albeit of smaller magnitude than observational studies, including ours.
Why Vitamin D Is Good for the Heart
If indeed the protective effect of vitamin D on mortality is real, our study provides important evidence that the effect is present for cardiovascular disease — but not for cancer. The protective effect is present at all ages and in both sexes. Unfortunately, our study did not include a sufficient number of people of different racial and ethnic groups to provide information on this important issue.
An important question is the identification of who should increase their vitamin D status using supplements. There is no consensus on ideal cut-off values for vitamin D deficiency. Our observation that the use of cut-off levels which were specific for country, age, sex, or season produced comparable results, raises the question whether such factors should be considered for defining vitamin D deficiency.
Future population-based studies with standardized vitamin D measurements are needed to fully resolve this issue; in the meantime, caution should be used in extrapolating results across regions with different latitudes.
Despite these uncertainties, I recommend moderate sun exposure, in particular in the elderly. The amount of sun exposure depends on the type of skin (15-30 minutes are sufficient for a fair skinned person, while one or two hours are needed for dark skinned person) and on the latitude the person lives (more time is needed in Northern regions, in particular in winter). If sun exposure is not sufficient, people can consider taking supplements, after talking with their doctor.
Paolo Boffetta, MD, MPH is a chronic disease epidemiologist who is Director of the Institute for Translational Epidemiology and Associate Director for Population Sciences of the Tisch Cancer Institute at the Icahn School of Medicine, which is part of Mount Sinai Medical Center in New York City. He is also Adjunct Professor in the Department of Medicine at Vanderbilt University in Nashville, Tenn., and in the Department of Epidemiology at the Harvard School of Public Health. He has edited 12 books and published more than 1,000 peer-reviewed scientific articles.
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