There was something rather timely about the publication of the result by Guifeng Xu and colleagues  observing that: "In a nationally representative sample of US children, a significant and positive association of common allergic conditions, in particular food allergy, with ASD [autism spectrum disorder] was found."
Timely, because earlier the same day I read a rather disparaging piece titled 'oversold diets' *trying* to condense down the findings reported by Gogou & Kolios . This duo looked at the "current literature data about the effect of therapeutic diets on autism spectrum disorder", where certain diets are often put in place because of a perceived allergy or intolerance to certain foodstuffs and effects on behaviour and physiology. Gogou & Kolios concluded that: (a) "No serious adverse events have been reported" ('first, do no harm' and all that) and (b) the currently available peer-reviewed data on things like the use of a ketogenic diet and/or a gluten- and casein-free (GFCF) diet 'for' autism is promising but: "More research is needed to provide sounder scientific evidence." I agree that lots more resources and money need to be put into the scientific examination of such diets in the context of autism (see here and see here) including identifying potential best-responders . I'd also like to see gastrointestinal (GI) issues included in the research agenda, in light of other promising peer-reviewed data too on a potential tie-up with said diets and autism (see here)...
Anyhow, back to the Xu findings, which have seemingly attracted some media attention (see here for example). The source of the data was the "National Health Interview Survey [NHIS] collected between 1997 and 2016", a US initiative designed to 'monitor the health of the United States population through the collection and analysis of data on a broad range of health topics'. Including parent or guardian responses on a questionnaire used as part of the NHIS, and specifically the questions: "during the past 12 months, has your child had (1) any kind of food or digestive allergy; (2) any kind of respiratory allergy; (3) eczema or any kind of skin allergy?", data was crunched for nearly 200,000 children.
Authors reported that as a total group (N=199,520), the weighted prevalence of food allergy being reported was about 4%. For a respiratory allergy, the prevalence came in at about 12% and 9% for a skin allergy.
Then to the autism vs. not-autism comparisons where: "A diagnosis of ASD was reported in 1868 children (weighted prevalence, 0.95%; 95% CI, 0.89%-1.01%)." Parents/guardians of those diagnosed with autism were significantly more likely to report all of those various allergies compared to the not-autism group. The figures were: food allergy (autism: 11.25% vs. not-autism: 4.25%), respiratory allergy (autism: 18.7% vs. not-autism: 12%) and skin allergy (autism: 16.8% vs. not-autism: 9.8%). Further: "After adjustment for age, sex, race/ethnicity, family highest education level, family income level, and geographical region, the OR [odds ratio] of ASD was more than doubled (OR, 2.72; 95% CI, 2.26-3.28; P < .001) among children with food allergy compared with those without food allergy." This last observation basically said that there was more chance of autism being reported among those with a food allergy than those without a food allergy.
Caveats? Well, a few to mention. First of all, you'll have noted the words 'parent or guardian responses on a questionnaire' being used, which need to be taken into consideration. No, I'm not casting any aspersions on reporting accuracy of anything like that (parents can be very sensitive to lots of things), but this was not a study of hospital records and tests or anything like that. Second is the use of the word 'allergy'. The medical definition of allergy (see here) is something like 'a biological response to a normally harmless substance'. Allergy is obviously immune-related and typically involves something called IgE (immunoglobulin E) among other things. Whilst some people might think they have an allergy to this, that or t'other, there is still some confusion about whether 'allergy' is the most suitable word in that context without for example, suitable allergy testing being undertaken. I might be being a bit pedantic, but words count...
That all being said, the Xu findings are not the first time that allergy and autism has been raised as being *associated* (see here and see here for examples). Indeed, allergic illness such as something like asthma, has quite a long association with autism (see here) and hints at a possible 'immune-related' connection to at least 'some' autism. That also one of the most common 'comorbid' conditions over-represented in relation to autism - attention-deficit hyperactivity disorder (ADHD) - seems to have an even stronger connection with allergy (see here) is also important to mention, and further strengthens some immune system involvement where there is diagnostic intersection. It also *might* have intervention implications too (see here) (with no medical advice given or intended)...
 Xu G. et al. Association of Food Allergy and Other Allergic Conditions With Autism Spectrum Disorder in Children. JAMA Network Open. 2018; 1: e180279.
 Gogou M. & Kolios G. Are therapeutic diets an emerging additional choice in autism spectrum disorder management? World Journal of Pediatrics. 2018. May 30.
 Whiteley P. Nutritional management of (some) autism: a case for gluten- and casein-free diets? Proc Nutr Soc. 2015 Aug;74(3):202-7.