Sci was a little startled recently when she saw "the latest study" on ADHD splashed across the frontpage of Yahoo. You can see it here on Reuters.
PESTICIDES TIED TO ADHD.
(Run for the hills, indeed. Or maybe run AWAY from the hills, since they might have pesticides)
However, the story broke a good TWO DAYS in advance of the paper actually coming out, and so Sci was forced to possess her soul in patience until she had access.
But she's got it now! And let's take a look at this thing.
But first, I want us to all breathe in together and say: "Correlation is not causation"
Say it with me: "Correlation is not causation"
(Behold Sci contemplating the science of the universe)
All right, here we go.
Bouchard, et al. "Attention-Deficit/Hyperactivity Disorder and Urinary Metabolites of Organophosphate Pesticides" Pediatrics, 2010.
We all know that there are a lot of pesticides used in the US today. And some people believe that attention deficit/hyperactivity disorder (ADHD) is on the rise. Sci isn't so sure about this one herself. ADHD that meets diagnostic criteria is VERY easy to find. I'm not saying that ADHD doesn't exist, of course it does. The question is whether or not there's an increase because there's an increase, whether we're seeing an increase because we are looking for it (and people certainly are), or whether we're seeing an increase because it's seen as a very easy and good thing to treat for the sanity of the parents and teacher (the child might be hyperactive, and certainly can be treated with Ritalin, but is the child ADHD? Often, yes, but not always). But certainly the DIAGNOSES of ADHD are on the rise. And of course people are looking for a cause.
Pesticides aren't necessarily the first thing you'd look at, but they aren't the last thing either. Most of the time, when people look at studies with pesticides and humans, they look at people exposed to high doses of pesticides, rather than the lower daily doses that most Americans are exposed to. And high doses of pesticides have been linked with changes in brain development in children. So actually, it's not that big of a leap to hypothesize that lower levels of pesticides might cause smaller changes in brain development, like those associated with ADHD.
So for this study, the authors surveyed a total of almost 4000 children, or rather, they did an analysis of data collected from the National Health and Nutrition Examination Survey. Of those, they managed to grab a little over 1000 of them (boys and girls) and get a urine sample, which they tested for metabolites associated with pesticides. They then called up the parents, and ran a questionnaire by them which contained questions to diagnose ADHD via the DSM IV. They then checked the urinary metabolite levels against the questionnaire results for the child.
What they got was all in table form and unpleasing to the eye, but has some important stuff in it. They found that
There was a 55% to 72% increase in the
odds of ADHD for a 10-fold increase in
DMAP concentration, depending on the
criteria used for case identification.
Basically, of the kids that had urinary metabolites of pesticides that were detectable (not all were detectable, in fact it appears that for some of the metabolites, levels were below detection limit about half the time), it appears that the incidence of ADHD correlated with an increased level of organophosphates in the urine. This means that the more pesticides you had in your urine, the more likely you were to have ADHD. They controlled for age and socio-economic status.
And of course everyone in the media read this study and started freaking out about how you should feed your kids only organic fruit.
Hold your horses, please.
The authors themselves admit that this is a correlation, not a causation. It's a good correlation, and they did a great job with controls for things like economic class, race, etc, (well done!) but it is NOT causation. And it raises far more questions than it answers. Here are a few Sci can think of off the top of her head:
1) Is it the pesticides in the CHILDREN, or could it be exposure to pesticides in utero? Could it be exposure over one period of the lifespan?
2) What was the ADHD status of the parents of each of the children who came out positive? Is there a genetic correlation to account for some of this?
3) Did they ask the parents WHAT they fed their kids? Did they ask the children WITHOUT any pesticide metabolites in their urine if THEY had ADHD? Why didn't they? The study only covers children with measurable metabolites. What if there is a difference? What if there ISN'T?
4) Does taking Ritalin (a bunch of the kids were on Ritalin) change the concentration of pesticides in the urine? Possibly not, but if both chemicals are metabolized similarly there could be issues there.
So as you can see, there are way more questions than there are answers. As to the organic fruit? Sci says meh. Washing your fruit well is ALWAYS a good idea and should always be done, but if this is something in neural development, and your kid already has ADHD, washing the fruit now is VERY unlikely to help. Organic fruit may help, but pesticides spread.
And let's not run for the hills just yet. There are still many things to be accounted for, including genetics (which may play a VERY big role), environment, timing of exposure (if pesticides are the cause), and a huge control (kids with undetectable metabolites) that they missed. Not only that, Sci thinks they could have used a control group of kids fed mostly organic food and done a comparison. It would surprise me if someone has it planned. I would also like to see an animal study with human-comparable levels of pesticide exposure on neural development over the lifespan (start with fetal exposure, go up to exposure through mother's milk, etc., and behaviorally test for deficits later. Not a hard study to do, actually...)
In the meantime wash your fruit (you should already be washing your fruit). Buy organic if it makes you feel better. But don't run for the hills just yet, the science isn't done. It's a nice correlation, but it isn't causation yet.
Bouchard, M., Bellinger, D., Wright, R., & Weisskopf, M. (2010). Attention-Deficit/Hyperactivity Disorder and Urinary Metabolites of Organophosphate Pesticides PEDIATRICS DOI: 10.1542/peds.2009-3058
EDIT: Reader Alex pointed out that there are some points that Sci missed. They DID look at undetectable levels of pesticides, and they DID look at kids treated with Ritalin. But I still think we can't say much until we can show a group of kids without pesticide exposure at all (or extremely minimal). I also would love a really long longitudinal study of women and their diets in utero through the kids growing up, as I'll bet many of the big changes would be in utero or during nursing.