What is vitamin A? Where does it come from?
Vitamin A is not a single compound. Rather, it is the name given to a group of fat-soluble compounds that includes retinol, retinoids (retinol metabolites) and provitamin carotenoids.
The human body does not produce vitamin A, so there are two main forms available through the diet. One is preformed vitamin A (retinol and retinyl esters) consumed from animal foods, and the other is provitamin A carotenoids, which are obtained from plant-based materials. Converts to retinol.
Each of these different forms of vitamin A is solubilized in the intestinal lumen, absorbed and converted to retinol, which is then oxidized to retinal and retinoic acid. These are the two main active forms of vitamin A metabolites in the body.
Recommended dietary allowance
The recommended dietary intake of vitamin A is 900 μg retinol active equivalent (RAE) per day for men and 700 μg/RAE for women.
Research has demonstrated that vitamin A is important for the healthy functioning of the immune system, vision, gene expression, reproduction, embryonic development, and a variety of other biological processes.
However, there are still many unknowns when it comes to vitamin A, including how vitamin A levels are maintained throughout the body and its role in brain health and cell repair. Here we highlight some of the latest advances in vitamin A research that are working to address these questions.
Evidence for vitamin A sensors within the hypothalamus
Researchers at the University of Aberdeen last year published the first evidence of the brain’s potential role in regulating vitamin A.
Vitamin A is rare compared to other vitamins and is widely stored throughout the body. This means that unless dietary deficiencies are chronic, there is always an uninterrupted supply of vitamin A to all cells. It is well known that vitamin A is stored in the liver, but “a fundamental unanswered question is how vitamin A levels in the circulation are maintained”, says Professor Peter McCaffery. said researchers led by Dr. Peter Ikianosimhe Imoesi.
“It is important to understand the regulation of vitamin A balance in the body, given that both deficiency and excess are detrimental to human health,” says Imoesi.
Imoesi and colleagues investigated whether the hypothalamus, a deep brain structure that helps regulate the body’s homeostasis, might play a role. Their research is science.
Using a rat model and human tissue samples, they administered vitamin A directly to the hypothalamus in the form of retinol and retinoic acid, retinol binding protein 4 (Rbp4) gene had been knocked down. Rbp4 It encodes a protein responsible for transporting vitamin A throughout the body.
Taken together, Imoesi et al.’s experiments affected the amount of vitamin A stored in the liver and the amount distributed to other cells in the body through the bloodstream. The researchers believe this evidence suggests that a vitamin A sensor system exists within the hypothalamus that regulates how vitamin A is distributed in the body. Cells in the rat hypothalamus that may form this sensor system are also present in the human hypothalamus, suggesting that this system may also be applicable to humans.
“What we discovered was fundamentally new. No one had previously suggested that the brain might control vitamin balance in the body, and this suggests that the hypothalamus’ “This is the first study to suggest a role for ‘vitamin suppression’,” Imoesi explained.
“Our results show that vitamin A imbalances are not simply due to irregular intake; abnormalities in hypothalamic function due to disease or inflammation can lead to insufficient supply of vitamin A to the body. ,” Imoesi said. Therefore, diseases affecting the hypothalamus may present with symptoms resulting from disturbances in circulating vitamin A levels. “Measuring vitamin A levels in the blood may be a guide to whether the hypothalamus is functioning properly,” Imoesi concluded.
Exploring the genetics of circulating vitamin A
At Newcastle University, Professor Murray Cairns’ research investigates the molecular structure of complex diseases. His team investigated the role of vitamin A in psychiatric disorders such as schizophrenia because previous research suggested that changes in vitamin A levels can affect neural connections in psychiatric disorders such as schizophrenia. I became interested in the potential role played by
in nature communications, Cairns and colleagues conducted the largest genome-wide association study (GWAS) of circulating retinol to date. “Our new study by William Ray and colleagues combines summary statistics from thousands of individual genomes to reveal what genetic factors regulate retinol levels in the blood.” said Dr Cairns. “We essentially matched retinol levels with genetic variations to better understand the genes involved in retinol absorption and blood transport.”
Blood samples from 22,274 participants were analyzed and eight common genetic variation loci associated with retinol were identified, including genes not directly involved in the major retinol transport complex. “These genes were highly expressed in the liver and overrepresented in biological pathways including carbohydrate metabolism,” the authors said.
A phenomenon-wide Mendelian randomization study (MR-pheWAS) was conducted to investigate the causal relationship between circulating retinol and 20,000 clinical phenotypes. This data suggests that retinol may have causal effects on a wide range of biological processes, including but not limited to inflammation, the microbiome, magnetic resonance imaging-derived brain phenotypes, obesity, etc. doing.
“Using this approach, we can support the importance of retinol in inflammation, plasma lipids, obesity, vision, microbiome, brain structure/connectivity, asthma, COPD, and several other properties. ” said Cairns. explained. “This is important because we use synthetic retinoids as medicines and may guide their application through precision medicine approaches based on genetic information. For example, in patients suffering from autoimmune diseases People have low retinol levels.”
“This study replicates the known effects of retinol on ophthalmologic measures, innate and adaptive immune responses, and congenital heart malformations,” the authors said. “However, we also uncovered some less well-characterized relationships that may have direct clinical relevance. We quickly discovered that circulating retinol affects the thickness and surface area of several brain regions, and We highlight examples that are genetically predicted to influence indicators of brain connectivity.”
Retinoic acid is known to guide processes such as neuronal differentiation and adult neurogenesis. Therefore, while it makes sense that retinol can influence brain structure and connectivity throughout an individual’s lifespan, retinol’s effects on the brain regions identified in this study “are of no clinical significance.” Further investigation is needed,” Cairns and his research team said.
Role of retinoic acid in skin cell lineage plasticity
Dr. Matthew Tierney, a postdoctoral fellow at Rockefeller University, and colleagues recently studied how the body controls lineage plasticity and identified an interesting role for retinoic acid. Their research is science.
When you endure a skin injury such as a cut, scrape, or burn, your skin’s stem cells quickly generate new cells and repair the epidermis. During the healing process, our bodies play a neat trick called lineage plasticity. At this time, other stem cells, such as follicular stem cells, transform into epidermal stem cells and support these efforts. The hair follicle enters an intermediate state that has both follicular and epidermal stem cell transcription factors.
“This process is necessary to redirect stem cells to the parts of the tissue where they are most needed, but if left unchecked can leave those same tissues vulnerable to chronic reparative conditions and even some types of cancer. “It could be,” Tierney explained.
“Through our research, firstly, in vitro after that in vivo“We have discovered a previously unknown function of vitamin A, a molecule long known to have powerful but mysterious effects on the skin and many other organs.” said Professor Elaine Fuchs, senior author of the study and the Rebecca C. Lancefield Professor. Rockefeller University.
Researchers found that increasing or decreasing retinoic acid levels in vivo Affected how stem cells can respond to damage and hair growth. High levels of retinoic acid inhibit cell lineage plasticity and wound repair, while low levels of retinoic acid promote wound repair but limit hair regeneration.
This may explain why vitamin A’s effects on tissue biology are so “elusive,” Fuchs said. Topical retinoids have been shown to stimulate hair growth in wounds at certain concentrations, but they also appear to interfere with the hair cycle when used in excess.
“By defining the minimal requirements needed to form mature hair cell types from stem cells in vitro, this study has the potential to change the way we approach research in hair biology.” Professor Tierney said.Understanding how to guide stem cells to make the “right” decisions could also impact approaches to cancer treatment, Fuchs said: “Cancer stem cells never make the right choices. No. They’re always doing something outlandish. […] As we studied this condition in different types of stem cells, we began to realize that lineage plasticity, when left unchecked, is a major cause of cancer. ”
