Cho said too much preformed vitamin A (typically from supplements and some animal foods) increases the risk of osteoporosis and hip fractures.
People with higher levels of vitamin A tended to be older.
It has long been known that vitamin A affects immune status and it has been asked whether vitamin supplementation can affect the progression of AIDS or its transmission from mother to infant.
Overall this is an impressive collection updating us on many aspects of vitamin A's function and the background molecular biology and biochemistry.
Nine participants in the vitamin A
group responded, compared with one in the placebo group.
"Our team has shown that vitamin A influences the types and amounts of immune cells--such as T-helper cells--and immune system molecules, called interferons and interleukins, that your body produces in response to infection," Stephensen says.
For the research, Stephensen exposed mouse immune-system cells, in petri dishes, to a form of vitamin A known as retinoic acid, along with a protein that mimics an infectious microbe.
Sipes and his co-workers compared that damage to what they saw in animals who received up to 250,000 international units of vitamin A
(retinol) per kilogram of body weight daily
To make the water-soluble forms, Barua attached either glucose or glucoronic acid to vitamin A
or its metabolite, retinoic acid.
Studying bovine papillomaviruses in mouse-cell cultures, the Vancouver group also discovered that a vitamin A
relative called retinoic acid reduces viral DNA inside cells.