Vitamin A and brain damage at high doses in animals and people
What it found
High doses of vitamin A caused signs of brain cell damage in rats and nerve-related side effects in people.
The exact way this happens is not yet clear.
What they found
Studies in people
Nerve symptoms in people
People who took in very large amounts of vitamin A reported headaches, confusion, anxiety, depression, blurred vision, and poor muscle control. In one case, eating bear liver caused sudden behavior changes that were compared to schizophrenia.
Animal and lab studies
Brain cell stress in rats
Rats given 1,000 to 9,000 IU per kg each day for up to 28 days showed more oxidative damage in several brain areas. Their mitochondria, which power the cells, worked less well after the treatment.
Lower movement in rats
After 28 days of vitamin A supplements, rats moved around less and explored their surroundings less in an open test area. Rats given pharmacological doses also showed anxious behavior.
Cell death signals in labs
In lab tests, exposing rat cells to retinol raised levels of reactive oxygen molecules and lowered cell survival. Vitamin A also triggered a cell death enzyme in the outer layer of rat brains.
Memory protein fell in rats
Giving rats 1,000 to 9,000 IU per day for 28 days lowered levels of BDNF, a protein needed for learning and memory, in the hippocampus. Lower doses of 100 to 500 IU per kg each day for two months did not show antioxidant benefits in female rats.
Other findings
How vitamin A works
Vitamin A comes from plants as beta-carotene or from animal foods as retinol esters. The body turns it into active forms like retinoic acid, which then bind to receptors inside cells to change how genes work.
What the authors conclude
- They advise caution when giving vitamin A to people who are not deficient, especially those with a family history of brain diseases.
- They note that while vitamin A is essential for normal brain growth, the central nervous system is clearly affected by too much of it.
- They call for more research to understand exactly how vitamin A and its derivatives affect brain cells.
How it was done
More people now get extra vitamin A from fortified foods and supplements, so the author wanted to check whether amounts above normal needs could harm the brain.
The author gathered and examined published research on vitamin A and related compounds called retinoids. The work looked at studies in people, rats, mice, and cells grown in labs. Doses tested in animals ranged from 100 to 300,000 IU per kg each day for up to two months.
What it can’t tell you
- Most of the detailed findings come from rats and lab-grown cells, so they cannot show what happens in human brains at everyday supplement doses.
- Reports of side effects in people came from extreme intakes, such as eating bear liver or taking medical doses far above normal nutrition, rather than typical food or supplement use.
- Because this work gathered past studies instead of running a new trial, it can point out patterns but cannot prove that vitamin A directly causes brain damage in people.
The paper
- Title
- The neurotoxic effects of vitamin A and retinoids
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Studies in people · animals · lab
- Summarised from
- Full text
- Cite
- Oliveira MR (2015). The neurotoxic effects of vitamin A and retinoids. Anais da Academia Brasileira de Ciencias. doi:10.1590/0001-3765201520140677Free full textPubMed 26247148DOI
Summary written 1 Oct 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error