(RxWiki News) Vitamin D has been reported to decrease some bone diseases and reduce risk of other diseases like cancer and heart disease. But some of these claims may not be supported by research.
To find and compare the scientific evidence for the effects of vitamin D on health, a team of researchers evaluated over a hundred published research studies.
Analysis of the published studies showed some weak associations between vitamin D, either in the blood or taken as a supplement, and decreased risk of certain diseases. The only significant finding, however, was the link between vitamin D and reduced risk of low birth weight in babies.
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This research team was led by Evropi Theodoratou, PhD, from the Centre for Population Health Sciences at the University of Edinburgh in the UK.
The team analyzed published studies that were either research reviews, observational studies or clinical trials. Observational studies included studies in which researchers linked vitamin D blood levels with certain diseases or conditions. In clinical trials, subjects were given vitamin D and effects on certain conditions or diseases were measured.
Dr. Theodoratou and her team analyzed 107 research reviews, 74 observational studies and 87 clinical trials.
Evaluation of the research reviews revealed that only 8 percent of the studies concluded that there was an association between vitamin D and certain diseases. These studies showed associations between higher concentrations of vitamin D and lower risk of high blood pressure in children, falls in elderly people, bacterial vaginal disease in pregnant women, rheumatoid arthritis activity, colorectal cancer and rickets (a bone disease due to lack of vitamin D) in children.
Review of the observational studies showed a probable effect of vitamin D in 63 percent of the studies. These studies showed a possible, but not strong, link between increased vitamin D and decreased risk for low birth weight, some cancers, heart disease and diabetes.
Of the clinical trials, 23 percent showed a tendency for vitamin D supplements to impact health. In these clinical trials, vitamin D increased levels of high density lipoprotein (a “good” fat in the blood) and vitamin D in pregnant women's blood and decreased risk of low birth weight babies, risk of cavities in children and bone fractures in elderly people.
The study team offered some reasons for the lack of convincing scientific evidence in many of the studies. They noted that people have different abilities to absorb and metabolize vitamin D, and therefore, may have needed different doses to produce results.
Another important consideration the authors noted was that subjects participating in studies in which they were given vitamin D supplementation each started with a different baseline amount of vitamin D in their blood. The differences after vitamin D dosing could have depended on the starting amount of vitamin D in the blood.
Summing up their review, the authors noted, “[C]urrent recommendations on daily supplementation of vitamin D are largely expert driven, rather than evidence based.”
“Our overview of the evidence on vitamin D suggests that strong recommendations cannot be made regarding its supplementation,” they concluded.
In an editorial on the published findings of Dr. Theodoratou and her team, Dr. Paul Welsh and Prof. Naveed Sattar noted other complications in interpreting vitamin D studies. Study participants’ vitamin D levels can be lowered by lack of sun exposure, inflammation, smoking, obesity and poor diet. These factors can all influence the interpretation of study results, they wrote.
These editorial authors felt it was remarkable that out of 137 study results of vitamin D, only 10 had been tested in clinical trials, and only increased birth weight was a finding present in both observational studies and clinical trials.
“This pattern of findings should ring alarm bells,” these editorial authors wrote.
“Fortunately, new trials are under way,” they wrote.
The research review by Dr. Theodoratou and her team was published April 1 in BMJ.
There was no specific funding for the study.
The authors declared no conflicts of interest.