ObjectivesIn 2005, researchers observed a link between infants’ access to thiamine (vitamin B1) and their subsequent language functioning: infants fed formula that inadvertently lacked thiamine later displayed significant delays in language and motor development. In the context of a double-blind, randomized, controlled trial, the present research provided the first experimental test of a relationship between early life thiamine exposure and infants’ early language processing. Methods335 lactating mothers were randomly assigned to receive capsules containing either 0, 1.2, 2.4, or 10 mg of thiamine daily, from 2 weeks postpartum until infants were 24 weeks. We assessed the integrity of 24-week-old infants’ developing system for language processing by measuring the extent to which they displayed attentional enhancement in response to infant-directed speech (IDS) relative to adult-directed speech (ADS). Such IDS-related attentional enhancement implies that infants can differentiate IDS from ADS, which requires underlying skills for analysis of the complex stream of sound that human speech embodies. Of the 335 infants in the study, analyzable data for the IDS preference task were available for 251. ResultsAs predicted, a greater-than-chance percentage of all infants displayed enhanced attention to IDS relative to ADS, replicating prior research (one-sample t(250) = 3.06, P = 0.002). The magnitude of the IDS-elicited attentional-enhancement increased significantly in relation to the thiamine dose that lactating mothers received (beta-weight 0.007, t = 2.57, P = 0.011). Notably, however, when supplementation groups were examined separately, only infants whose mothers received the 10 mg daily supplement displayed a statistically significant IDS-related attentional enhancement (10 mg group: one-sample t(63) = 4.14, P = 0.000; all other groups t’s < 1.41, P’s > 0.16). ConclusionsTaken together, these findings confirm a link between infants’ post-natal access to thiamine and their language processing facility at 24 weeks. The findings both showcase the value of the IDS preference task for monitoring the integrity of infants’ language processing, and underscore the importance of adequate thiamine for ensuring infants’ optimal language development. Funding SourcesBill & Melinda Gates Foundation and the New York Academy of Sciences.