The long-lasting effects of thiamine deficiency in infancy on language: A study of a minimal-pair of twins

Thiamine, vitamin B1, is a crucial component in brain development. This study examined the role thiamine plays in the development of language, by examining the long-term effects of thiamine deficiency in infancy. The participants were a young adult who had consumed a thiamine-deficient baby formula at age 1;0–1;5, and her non-identical twin sister, who had consumed a non-deficient formula. We conducted a comprehensive assessment of various language abilities, including syntax, morphology, lexical encoding and retrieval, word and nonword reading, and phonological working memory, most of which have not been previously tested in individuals who had thiamine deficiency in infancy. The twin who had thiamine deficiency showed selective deficits in various language domains, including syntactic movement, morphology, and lexical abilities (which also caused surface dyslexia in reading aloud). She also showed impaired input and output phonological working memory and impaired reading aloud of nonwords (involving voicing errors, morphological errors, and lexicalizations). Her twin sister, who did not have thiamine deficiency, showed typical language abilities. The findings show for the first time that language disorders due to thiamine deficiency in infancy persist into adulthood. In light of previous literature of adults whose thiamine deficiency took place in adulthood, who do not show language impairments, we suggest that thiamine is crucial for language development during the critical period for first language acquisition in the first years of life. Thiamine deficiency during the critical period may cause long-lasting impairments in syntax, morphology, reading, phonological working memory, and lexical abilities.