Despite many studies linking higher blood levels of vitamin D to fewer heart attacks and deaths, a new trial found giving older women daily D supplements didn’t cut their heart-related risks.

The women’s cholesterol, blood pressure and blood sugar was no lower after a year of taking one of two doses of vitamin D, compared to those who took vitamin-free placebo pills.

So-called observational studies – which measure vitamin D in people’s blood and then follow them over time – have tended to find a link between vitamin levels and heart health.

But those types of trials can’t prove cause-and-effect because it’s impossible to take into account all the possible diet and lifestyle differences between people with lower and higher vitamin D levels. Randomized controlled trials, considered the “gold standard” of medical studies, aim to get around that hurdle. By randomly dividing a group of people and giving some but not others a particular treatment, researchers can better hone in on its specific effects. For the new trial, Adrian Wood from the University of Aberdeen in the UK and colleagues split 305 women in their sixties into three groups. Each morning for one year, the women took either 400 international units (IU) or 1000 IU of vitamin D or a placebo. The participants returned to the lab every two months for a panel of heart-related tests. At the start of the study, women in each group were similar on most of those health measures. Women in the placebo group, for example, had an average weight of 152 pounds, a blood pressure of 128/78 and total cholesterol of 238. Over the next year, cholesterol and blood pressure varied by season – but not based on whether women were taking extra vitamin D.

In the end, there was no clear difference between the three groups in changes among any of the heart-related markers, Wood’s team reported in the Journal of Clinical Endocrinology & Metabolism. The study didn’t look far enough out, or have enough women, to determine whether vitamin D might affect heart attacks or deaths. Food sources of vitamin D include cod liver oil and other fish as well as fortified juice and dairy products. The Institute of Medicine recommends that most adults get 600 IU of vitamin D per day. In a 2010 report it concluded there is strong evidence connecting both vitamin D and calcium to better bone health but that other proposed benefits, including for blood pressure and cardiovascular disease, are shakier. Still, some researchers believe taking extra vitamin D – especially for people who have very low levels to begin with – may help prevent body-wide inflammation and play a role in regulating blood sugar levels.

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