A study appearing in JAMA neurology links better Vitamin D level in pregnant women to a lower risk of multiple sclerosis in their offspring. There are some really impressive features of this study, but there are some equally impressive logical leaps that seem to defy the force of epidemiologic gravity. Let's give the study some sunlight.
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The study was run out of Finland, which is a country that figured it might be a good idea to keep track of the health of its citizens. In fact, since 1983, nearly every pregnant woman in Finland has been registered, and a blood sample sent to a deep freezer in a national biobank. The researchers identified 193 individuals with MS, and went back into that biobank to measure their moms' vitamin D levels during pregnancy. They did the same thing with 326 controls who were matched on their date of birth, mother's age, and region of Finland.
This is from the first line of their discussion:
Wow. 90%. That sounds scary. And the news outlets seem to think it is scary too. But that impressive result hides a lot of statistical skullduggery.
Here's the thing, Vitamin D level is what we call a continuous variable. Your level can be 5, 10, 17, 42, whatever – any number within a typical range. When you study a continuous variable, you have to make some decisions. Should you chop up the variable into categories that others have defined (like deficient, insufficient, normal), or should you chop it up into even-sized groups? Or should you not chop it up at all?
As a general rule, you have the most power to see an effect when you don't chop at all. Breaking a continuous variable into groups loses information.
When the Vitamin D level was treated as the continuous variable it is, there was no significant relationship between Vitamin D level in mom and MS in the child. When the researchers chopped it into 5 groups, no group showed a significantly higher risk of MS compared to the group with the highest level. Only when they chopped the data into 3 groups did they find that mom's who were vitamin D deficient had 1.9 times the risk of those that were insufficient. That's the 90% figure, but the confidence interval ran from 20% to 300%.
And did I mention there was no accounting for mothers BMI, smoking, activity level, genetic factors, sun exposure or income in any of these models? Despite that, the paper's conclusion states :
That statement should go right on the jump to conclusions mat.
Look, I'm not hating on Vitamin D. I actually think it's good for you. But research that adds more to the hype and less to the knowledge is most definitely not.