Two more reviews of vitamin D studies were published on Tuesday in BMJ (formerly the British Medical Journal). Both came to similar conclusions: No studies to date have provided good, reliable evidence that vitamin D supplements are beneficial to health.
As I’ve said here before, I’m not sure any examination of the evidence will deter Americans from spending an estimated $600 million on vitamin D supplements every year — especially here in Minnesota, where many people seem to have bought into the overhyped notion that our northern latitude keeps us from getting sufficient vitamin D from the sun.
But I’ll keep reporting on the evidence, nevertheless.
Study No. 1
For the first study, a team of European and U.S. researchers conducted an “umbrella” review of data from more than 180 vitamin-D-related studies previously analyzed in other review articles and meta-analyses. Some of the studies had investigated whether there was a connection between disease and blood levels of vitamin D. Others had tried to determine if the taking of vitamin D supplements improves health outcomes.
The studies had linked vitamin D levels to an astounding 137 different health outcomes, including those related to heart disease, cancer, autoimmune disorders, infectious diseases and osteoporosis.
Only 10 studies, however, had been tested in well-designed clinical trials, and only one showed evidence of a health benefit. That study suggested that pregnant women with high levels of vitamin D circulating in their bodies in their final trimester were less likely to have a baby with a low birth weight.
The authors of the umbrella review also note that vitamin D levels have a “probable” association with tooth cavities in children and with thyroid-hormone levels in patients with kidney disease that requires dialysis.
But even those findings were ambiguous. None of the studies, the researchers conclude, offer “highly convincing evidence of a clear role of vitamin D with highly significant results” for any health outcome, including those related to bone health.
Any new vitamin-D-related research, they stress, must involve rigorous, well-designed studies.
Study No. 2
For the second study, an international team of researchers conducted a meta-analysis of data collected from 95 observational studies and clinical trials that had investigated vitamin D supplementation and the risk of death from all causes.
Fourteen of those studies (all randomized controlled trials) were deemed reliable. Together, they revealed that taking vitamin D3 supplements was associated with an 11 percent reduced risk of death, at least within the three- to seven-year follow-up periods of the studies.
But they also found a slight increase (about 4 percent) in the death rate among people taking vitamin D2 supplements.
Vitamin D2 (ergocalciferol) is found in plants, while natural sources of vitamin D3 (chlolecalciferol) primarily include sunlight and fatty fish, such as salmon and tuna. Vitamin D2 supplements are made synthetically by irradiating yeast. Supplements of vitamin D3 are also made through a synthetic process — by irradiating lanolin extracted from sheep’s wool. Other research suggests that the human body absorbs the synthetic form of vitamin D3 more efficiently than that of vitamin D2.
The authors of the meta-analysis warn that their findings regarding both forms of vitamin D must be interpreted with caution, for the 14 studies were small and involved elderly populations, who have many health conditions that might have influenced the results.
Like the first group of researchers, this second group also calls for more rigorous vitamin-D-related studies.
Until those studies are done (one is in the works now and is projected to be completed by 2017), physicians should avoid recommending vitamin D supplements as prevention against disease unless there is a clear and diagnosable reason for it, write Paul Welsh and Naveed Sattar of the British Heart Foundation’s Glasgow Cardiovascular Research Center in an accompanying BMJ editorial.
They point out that the virtues of other supplements, specifically vitamins E, C and beta-carotene (vitamin A), were once widely recommended to patients — until they were shown in large, well-designed clinical trials to have no effect or even a harmful effect on health outcomes. Beta-carotene supplements have been found, for example, to increase the risk of lung cancer, particularly among smokers. And vitamin E supplements have been linked to an increased risk of prostate cancer.
“To improve health and prevent chronic disease,” write Welsh and Sattar, “we should stick to what is proven: encourage better lifestyles in general and target established risk factors in people at elevated risk.”
You’ll find links to the two studies and the editorial on the BMJ website.