Observational studies find that vitamin A status, as measured by serum concentrations of retinol, retinoic acid, or retinyl esters, is associated with several health conditions. For example, lower serum vitamin A concentrations are found in people with asthma compared to healthy controls,[1] and low serum vitamin A concentrations are associated with a greater risk of stroke.[2] Other studies find complicated relationships. For example, a U-shaped relationship exists between vitamin A status and the risk of hip fracture: both lower-than-normal and higher-than-normal serum concentrations are correlated with a greater risk of fracture.[3] Furthermore, vitamin A status has been associated with cardiometabolic diseases like cardiovascular disease, obesity, and type 2 diabetes, but the direction of association varies depending on which vitamin A metabolite is measured.[4]
Consequently, it is difficult to determine causality from such observations. Furthermore, the measurement of vitamin A metabolites in the blood is not a good biomarker for bodily vitamin A stores or dietary intake.[5][6][7][8][9] The better approach is the retinol isotope dilution method, which is considered the gold standard since it correlates strongly with liver stores and indicates vitamin A status in deficiency and in excess.[10] However, this method is not used in the above-described observational studies. Therefore, further research, ideally with dose-response and/or randomized controlled trial designs, is needed to clarify the causal effect of vitamin A status on health conditions.
References
- ^Hu J, Sang J, Hao F, Liu LAssociation between vitamin A and asthma: A meta-analysis with trial sequential analysis.Front Pharmacol.(2023)
- ^Farashi S, Shahidi S, Sarihi A, Zarei MAssociation of vitamin A and its organic compounds with stroke - a systematic review and meta-analysis.Nutr Neurosci.(2023-Oct)
- ^Wu AM, Huang CQ, Lin ZK, Tian NF, Ni WF, Wang XY, Xu HZ, Chi YLThe relationship between vitamin A and risk of fracture: meta-analysis of prospective studies.J Bone Miner Res.(2014-Sep)
- ^Olsen T, Blomhoff RRetinol, Retinoic Acid, and Retinol-Binding Protein 4 are Differentially Associated with Cardiovascular Disease, Type 2 Diabetes, and Obesity: An Overview of Human Studies.Adv Nutr.(2020-May-01)
- ^Dewett D, Lam-Kamath K, Poupault C, Khurana H, Rister JMechanisms of vitamin A metabolism and deficiency in the mammalian and fly visual system.Dev Biol.(2021-Aug)
- ^de Souza Mesquita LM, Mennitti LV, de Rosso VV, Pisani LPThe role of vitamin A and its pro-vitamin carotenoids in fetal and neonatal programming: gaps in knowledge and metabolic pathways.Nutr Rev.(2021-Jan-01)
- ^Bohn T, Desmarchelier C, El SN, Keijer J, van Schothorst E, Rühl R, Borel Pβ-Carotene in the human body: metabolic bioactivation pathways - from digestion to tissue distribution and excretion.Proc Nutr Soc.(2019-Feb)
- ^Olson CR, Mello CVSignificance of vitamin A to brain function, behavior and learning.Mol Nutr Food Res.(2010-Apr)
- ^Harrison EHMechanisms of digestion and absorption of dietary vitamin A.Annu Rev Nutr.(2005)
- ^Green MH, Ford JL, Green JB, Berry P, Boddy AV, Oxley A, Lietz GA Retinol Isotope Dilution Equation Predicts Both Group and Individual Total Body Vitamin A Stores in Adults Based on Data from an Early Postdosing Blood Sample.J Nutr.(2016-Oct)