Zwart SR, Laurie SS, Chen JJ, et al. - Given that participants exposed to carbon dioxide, 0.5%, for 30 days, exhibited development of optic disc edema and increased total retinal thickness in a recent strict 6° head-down tilt bed rest (HDTBR) study, researchers used a cohort of healthy volunteers to investigate if there exists an association of peripapillary retinal thickness measures, obtained from optical coherence tomography images during HDTBR and carbon dioxide, 0.5%, exposure, with B vitamin status and single-nucleotide polymorphisms implicated in folate-dependent and vitamin B12–dependent 1-carbon metabolism pathways. This investigation was run at the Institute of Aerospace Medicine at the German Aerospace Center in Cologne, Germany. A greater increase in total retinal thickness and baseline retinal nerve fiber layer thickness during 30 days of head-down tilt bed rest with carbon dioxide, 0.5%, exposure, was found in correlation with the presence of more 5-methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR) 66G and serine hydroxymethyltransferase 1 (SHMT1) 1420C alleles. Additionally, B vitamin status was identified as a contributing factor.