There is some evidence that lower serum retinol concentrations in ELBW infants are due to inflammation rather than vitamin A deficiency, because transport proteins of retinol — retinol-binding protein (RBP) and transthyretin (TTR) — are reduced by inflammation, and infants who later develop bronchopulmonary dysplasia have a preserved RBP–TTR ratio rather than a low RBP–TTR ratio as would be seen with vitamin A deficiency.17 Here, TTR is linked to vitamin A deficiency.