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Low vitamin D is a marker — not a cause — of disease, research review suggests

vitamin d pills
Although vitamin D is essential for healthy bones, taking vitamin D supplements has not been shown to protect against bone fractures.

A large review of all major studies of vitamin D has found that taking vitamin D supplements does not offer protection against acute respiratory infections or chronic diseases, including cancer, heart disease, diabetes, multiple sclerosis, rheumatoid arthritis, dementia, depression and Parkinson’s disease.

“It looks increasingly likely that low vitamin D is not a cause but a consequence of ill health,” conclude the editors of the medical journal The Lancet Diabetes & Endocrinology, which published the review late last week.

This finding won’t come as any surprise to regular Second Opinion readers. I’ve written many times in recent years (such as here and here) about studies that have debunked the claim that vitamin D supplements lower the risk of various diseases.

In fact, although vitamin D is essential for healthy bones, taking vitamin D supplements has not been shown to protect against bone fractures. Nor have they been found to reduce the pain or progression of knee arthritis.

How the study was done

As the authors of this recent review point out, most of the studies that have suggested a link between inadequate blood levels of vitamin D and a greater risk for disease have been observational. But such studies can suggest only a correlation, not a cause-and-effect, between two things. To prove that one thing causes another — say, that vitamin D supplementation prevents heart attacks — scientists must conduct clinical trials in which people are randomly assigned to receive either a treatment (vitamin D supplementation) or a placebo.

For the current review, a team of researchers, led by Dr. Philippe Autier of the International Prevention Research Institute in Lyon, France, compared the findings of 290 observational vitamin D studies with those of 172 randomized clinical trials. They found that the randomized studies did not confirm the findings of the observational ones.

In other words, giving people vitamin D supplements to raise their blood levels of the vitamin did not reduce their risk of developing either acute or chronic disease, even when they had relatively low vitamin D levels at the start of the randomized trial. Nor did the supplements have any effect on the severity or course of disease.

Autier and his colleagues also assessed the effect of vitamin D supplements on glycosylated hemoglobin (HbA1c), a type of blood protein that is monitored in people with diabetes. High levels of HbA1c are associated with diabetes-related medical complications.

Although observational studies have reported a correlation between type 2 diabetes and low vitamin D levels, the review’s analysis of 16 randomized trials found that vitamin D supplementation did not reduce HbA1c.

 “What this discrepancy suggests,” said Autier in a press statement released with the study, “is that decreases in vitamin D levels are a marker of deteriorating health. Aging and inflammatory processes involved in disease occurrence and clinical course reduce vitamin D concentrations, which would explain why vitamin D deficiency is reported in a wide range of disorders.”

Few Americans need supplements

I’m not sure even this thorough examination of the evidence will deter millions of 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 notion that our northern latitude robs us from getting sufficient vitamin D from the sun.

But as the Institute of Medicine (IOM) reported a few years ago, most Americans under the age of 70 need no more than 600 IUs of vitamin D daily to keep their bones healthy — an amount, say the IOM experts, that seldom requires supplementation.

That’s because almost all of us — yes, including those of living on the Northern Plains — get plenty of the vitamin from our food and from natural sunlight.

You’ll find an abstract of the study and the full accompanying editorial on the Lancet Diabetes & Endocrinology website.

Comments (1)

  1. Submitted by Paul Brandon on 12/09/2013 - 10:18 am.

    Good points!

    This illustrates the weakness of marker studies in general.

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