Association does not equal causation.
It’s always a good idea to remember that handy phrase when reading screamy nutrition-related headlines. Such as the one today:
If you get your nutrition information from headlines, you’d easily conclude that taking vitamin D while pregnant will prevent autism. And possibly that giving vitamin D to an autistic child will cure him. Wrong conclusions.
Not that the study in question isn’t interesting. It was done in the Netherlands, a northern country known for long dreary dark winters, meaning much less chance for sunlight-induced vitamin D production in skin. More than 4200 pregnant women were followed. Their blood levels of vitamin D was measured at 20 weeks, and again at birth. The children were assessed for autism spectrum disorder symptoms at 6 years of age. Results: the children of women with deficient vitamin D levels had higher scores on the autism spectrum assessment tests.
Well that sounds pretty conclusive, no? Just give women vitamin D supplements and autism goes away. Let’s look at the evidence.
- The definition of “deficient” for this study was pretty darn deficient. The blood level cut off was 25 nmol/l (nanomole per liter) or 10 ng/ml to use the values common to many US labs. Either way, this is a very low number. The Vitamin D Council considers this “severely deficient” while deficient is 10-20 ng/ml. The study authors decided that levels above 50 nmol/l (or 20 ng/ml) were “sufficient”; the Vitamin D Council disagrees with that. Which bring up one problem: there’s not a lot of consensus about the definition of vitamin D deficiency, let along agreement on what levels are sufficient or optimal. But in any event, in this study, the women in question were really deficient.
- So they were deficient. Why? That the only thing different about their nutrient status or lifestyle was low vitamin D? How does a person get low vitamin D? From a certain type of diet. From lack of sun exposure. Which means there could be lifestyle factors that were not identified, that led to low vitamin D, that also impact autism risk. The study only looked at vitamin D. In fact, the study did not look at diet. Vitamin D intake was not assessed at all. Perhaps some women do not absorb it properly, due to some other issue that in fact affects autism risk. Well, we just don’t know, but I hope you get the drift here. Vitamin D level could be a direct cause, or it could just be a sign of some other process.
- Vitamin D was not given to the subjects. We don’t know anything about potential effects (good or bad) of supplementing pregnant women. And we certainly don’t have any information from this study about the impact of supplementing children with autism.
- There is evidence that vitamin D plays many roles in fetal neurological development, not just autism. So it would seem very prudent for pregnant women to have sufficient blood levels. If nothing else, having sufficient vitamin D means one less possible risk for problems. Doctors should test pregnant women and advise on increasing vitamin D intake accordingly. But as I noted previously, there’s still disagreement about what “sufficient” means. There’s even less agreement about what “optimal” levels might be. One study I read suggested that health benefits for non-pregnant adults were most pronounced in the 30-40 ng/ml range, and higher levels brought no additional benefit. And of course, there’s such as thing as vitamin D excess. It’s a fat soluble vitamin, easy to absorb, hard to get rid of excesses. Simply dosing yourself with vitamin D is a bad idea if you don’t know your starting level. And a really bad idea to give it to a child with autism, expecting miraculous results.
What do I think?
I think there’s something to this, but exactly what is not known at the moment. It would be great if it turns out fixing deficient vitamin D eliminates autism. It could be that simple, or it could be one piece of a very complex puzzle, helpful but not the whole answer.
Vitamin D-linked health problems have increased as people live more indoor lives, slather themselves with anti-vitamin D sunscreen and avoid the few foods high in this vitamin, such as liver, egg yolk and fatty fish. Diagnosis of Autism Spectrum Disorder has been increasing steadily. If there’s a vitamin D connection, I wouldn’t be surprised.