The BBC headlined it, “Low vitamin D boosts dementia risk.” And countless blogs and Twitter messages parroted that same line. Erroneously.
Because, as the BBC story itself went on to explain:
“We need to be cautious at this early stage and our latest results do not demonstrate that low vitamin D levels cause dementia,” (the lead researcher said).
And if you haven’t proven cause-and-effect, you can’t say that x boosts the risk of y. With an observational study such as this, you can say there’s a statistical association, but it’s inaccurate to use causal verbs like “boosts.”
So, these headlines were wrong as well:
In the US, a Washington Post story was confusing with this line:
The researchers acknowledged the possibility of reverse causation – that is, that having dementia might alter a person’s behavior or diet in such a way as to contribute to Vitamin D deficiency – but suggested that the makeup of the study made that unlikely.
Wait a minute. Before you start talking about reverse causation, you better make clear that this study did not establish causation of any sort. And this story did not do that – not with a headline that stated “Vitamin D deficiency raises Alzheimer’s risk,” and not anywhere in the rest of the story.
It doesn’t take much time or many words to get this right.
On the CBS Evening News, Dr. Jonathan LaPook said, succinctly:
“Just because low vitamin D is linked to a condition doesn’t mean that low vitamin D is causing that condition.”
“This shows you there is a link between vitamin D and the development of Alzheimer’s,” said Keith Fargo, director of scientific programs and outreach for the Alzheimer’s Association, one of several funding sources for the study. “What it doesn’t show you is that [cause-and-effect] link.”
Whether dietary changes or getting more sun exposure would help isn’t known, Fargo said. “We don’t know if increasing vitamin D levels would decrease the risk of Alzheimer’s,” he added.
A Reuters Health story concluded:
Many other observational studies have tied low vitamin D levels to increased risk of any type of disease, said Dr. Philippe Autier, but that by no means implies that low vitamin D causes those diseases.
Autier coauthored a systemic review of vitamin D and ill health published in The Lancet in 2013 and studies the question at the International Prevention Research Institute in Lyon, France.
“Our paper in the Lancet showed that low vitamin D status would rather be the consequence of systemic inflammatory processes that are common in many chronic conditions, infectious diseases, aging, dementia and even depression,” Autier said. “Inflammatory processes are strongly involved in Alzheimer’s disease, and these processes are likely to be present well before Alzheimer’s disease is clinically detectable.”
The one randomized trial that has investigated vitamin D levels and dementia did not find a causal connection, he noted.
We’ve offered this many times before. If you’re not aware of it, we offer a primer: “Does the Language Fit the Evidence? – Association Versus Causation.”
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