While a study out of Denmark reports, in an observational study, that both high and low vitamin D levels are associated with an increase in mortality, a study published just last week out of Israel reports that only low vitamin D levels are associated with increased risk of mortality.
Published in the Journal of Endocrinology and Metabolism, lead investigator Dr Walid Saliba of the Carmel Medical Center found that in an analysis of 182,152 subjects, vitamin D levels above 20 ng/ml were better than levels below 20 ng/ml.
The researchers took a look at vitamin D levels of 182,152 patients from Clalit Health Services in 2008, the largest nonprofit health care provider in Israel. The researchers then took a look at mortality outcomes through August 31, 2011, which is a 32 to 44 month observational period.
In their analysis, they split their subjects into quartiles with the following vitamin D levels:
- 0-13.5 ng/ml. Mean level was 9.3 ng/ml.
- 13.5-19.8 ng/ml. Mean level was 16.8 ng/ml.
- 19.8-26.1 ng/ml. Mean level was 22.8 ng/ml.
- >26.1 ng/ml. Mean level was 32.6 ng/ml.
Sun exposure habits seem to have played a big role in vitamin D levels as opposed to supplement use. Roughly 60% of the levels in the lowest quartile were drawn in the winter, while roughly 60% of levels in the highest quartile were drawn in the summer. Jewish people made up about 80% of the lowest quartile and 97% of the highest quartile, while Arab people made up 20% of the lowest quartile and just 3% of the highest quartile. Women, too, were much more likely to have low levels of vitamin D compared to men. There was not a big difference in reported vitamin D supplement use, where 2.2% used vitamin D supplements in the lowest quartile versus 4.3% in the highest quartile.
People in the highest quartile (Q4, greater than 26.1 ng/ml) were the least likely to die. In this quartile, 2.98% died, versus 6% in Q1, 3.84% in Q2 and 3.07% in Q3. When the investigators began adjusting for a variety of factors like age, gender, seasonality, disease history, HDL, LDL and many more factors, Q3 and Q4 were about equally protective.
The authors note that given the short-term observation period, these study’s findings show that more than anything, these vitamin D levels may reflect the presence of an existing disease rather than the initiation of a new disease.
Once again, this observational mortality study should be considered interesting, but very far from conclusive. The problem with observational mortality studies is twofold:
- If the observation period post-vitamin D level was short (like this study), vitamin D levels may just reflect presence (or absence) of disease.
- If the observation period post-vitamin D level was long, vitamin D levels may become less and less reliable as a marker for vitamin D health. In the modern era, unless you make a conscious effort to maintain your vitamin D levels, they may fluctuate based on where you live, your outdoor habits of a particular season and year. In other words, my vitamin D level was probably different 10 years ago than it is today.
What this study does show is that we need a large randomized controlled trial that looks at broad scale mortality outcomes using vitamin D and placebo. As many readers know, this type of study is on its way, led by Drs. JoAnne Manson and Julie Buring and their VITAL study. Given the promising results of smaller scale, vitamin D randomized controlled trials, the Vitamin D Council finds it more than likely that 2,000 IU/day (like in the VITAL study) will reduce all-cause mortality. And we think 5,000 IU/day would do even better.