Birth dimensions have been associated with increased risk of both, severe mental illness and type 2 diabetes in adulthood, however, any influence on their co-occurrence has never been examined. This cohort study examine whether birth weight/ponderal index explain or modify the later association between severe mental illness and risk of type 2 diabetes.

The Metropolit cohort included 10,863 Danish men born in 1953 with information from age at conscription (between1971-84) until February 15^th, 2018. Severe mental illness was defined as the exposure and information was retrieved from the national Danish health registries. Information on type 2 diabetes diagnosis or oral antidiabetic prescriptions was also obtained, as they were the outcome of interest. Information on birth weight/ponderal index was available from birth certificates. Cox proportional hazards regression models were used to estimate the associations and interactions were tested.

After 47.1 years of follow-up, 848 (7.8%) and 1320 (12.2%) men developed a severe mental illness or diabetes, respectively. Men with severe mental illness presented higher risk of subsequent diabetes (HR = 1.92; 95%CI, 1.61–2.30). This association was stronger in severe mental ill men with low birth weight (HR = 3.58; 95%CI, 2.11–6.07), than in those normal birth weight (HR = 1.79; 95%CI, 1.45–2.20). This effect modification was most evident for men diagnosed with schizophrenia.

Birth information on birth weight/ponderal index could be of interest in diabetes screening on severe mental ill populations (especially in schizophrenia) since they might play a critical role in the increased risk of type 2 diabetes following severe mental illness.