	A prior diagnosis of asthma increases the risk of subsequent type 1 diabetes mellitus (DM), but a type 1 DM diagnosis first decreases the risk of subsequent asthma, according to a new study.

	Previous studies into the association between the two immune-mediated diseases had yielded inconsistent results. Evidence from observational studies shows inverse, direct and null associations.

	Finnish researchers, led by Johanna Metsälä, PhD, an epidemiologist at the National Institute for Health and Welfare, Helsinki, Finland, set out to assess the association between asthma and type 1 DM among children and adolescents, taking into account the time sequence of disease diagnoses and potential confounders in a large case-cohort study.

	The researchers reported their results at the European Academy of Allergy and Clinical Immunology Congress 2017 in Helsinki (Abstract 0204).

	In an initial cohort of all children born between 1981 and 2008 in Finland, the researchers found 81,473 had been diagnosed with asthma and 9,541 had been diagnosed with type 1 DM up to age 16 by the end of 2009. They used 171,138 patients derived from a 10% random sample from each birth year cohort as a reference cohort.

	Children were identified on the basis of insulin and anti-asthmatic drug purchases from Finland's drug prescription register. Data on maternal background and perinatal factors came from the medical birth register.

	The researchers used a multistate modelling approach between asthma and type 1 DM to estimate transition rates between healthy and disease states, asthma, and type 1 DM. The children were divided into groups by birth year (ages 0 - 3, 4 - 7, 8 - 11, and 12 - 16 years) to analyze transition rates.

	Altogether 80,871 children had only asthma, 8,939 children had only type 1 DM, and 602 children had both diseases. After adjusting for sex and birth decade, prior diagnosis of asthma increased the risk of subsequent type 1 DM on average by 41%. However, prior diagnosis of type 1 DM decreased the risk of subsequent asthma on average by 18%.

	“Results from the present study's novel approach to the association between asthma and type 1 diabetes indicate that the direction of the association between asthma and type 1 diabetes may depend on the sequential occurrence of the diseases,” the researchers stated. “The association between the diseases is complex, not just inverse.”

	They suggested that additional studies that take into account the sequential appearance of the diseases are warranted to further clarify the association between asthma, other allergic diseases, and type 1 DM. “Elucidation of biological mechanisms underpinning the association between asthma and type 1 diabetes may give further insights into pathogenesis of both of these diseases,” the researchers stated.

	The study has some limitations. The researchers were unable to account for when each disease process was initiated, and they were unable to confirm all the diagnoses. Also, they lacked data on potential confounders, such as dietary factors or infections that may have been relevant to both diseases.

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Opinion

Association between the diseases “is complex, not just inverse,” according to researchers.