For an exclusively breastfed infant, vitamin D supplementation (400 IU/day) is recommended starting within the first few days of life: breast milk alone doesn't provide enough on its own, and this is one of the few supplement recommendations in pediatrics with genuine international consensus.
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Source: guidelines from the American Academy of Pediatrics (2008, reaffirmed), the CDC, and the WHO. Evidence type: public-health recommendations built on historical clinical trials and rickets-prevalence data. Breast milk naturally contains very little vitamin D, and unsupplemented, exclusively breastfed infants show insufficient blood levels, particularly in winter or when the mother's own status is low. A very low calcium level from severe vitamin D shortfall can genuinely come with seizures in infants — that's documented in the pediatric literature, not an exaggeration on the post's part. The consensus (AAP, CDC, WHO) converges on 400 IU/day starting within the first days of life for any breastfed infant, regardless of any supplemental formula. Direction: causal, grounded in trials and well-established case literature.
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