Mostly about the Finnish and US CARET studies on Carotenes
This does show the extreme difficulty of trying to make blanket pronouncements about anything to do with metabolism.
So many different intersecting and overlapping probability curves involved, in so many different domains, toxicity, metabolic cofactors, stress responses, environmental and dietary factors, etc.
Most vitamins have quite narrow ranges in which they are beneficial (too much or too little being toxic), the standout exception being vitamin C, which is actually a rather simple sugar with a very shallow toxicity curve, that in times of stress we need massive amounts of (a goat of about human mass makes upwards of 100g per day under stress of disease). Humans seem to have similar optimal response curves, but we no longer have the enzyme that allows us to make it, so we need to supplement (or eat a lot of raw green stuff) when we are sick. The major action of vit C in most instances seems to be in the HIF regulation pathway that allows phagocytes to clean up neutrophils that have done their thing.
And every individual is different in so many ways, genetic, behavioural, dietary. Personal optimisation is something that takes awareness. There will come a time not too many years away when we will have personal instrumentation that will allow us to accurately monitor and supplement our immune and metabolic systems as needed, but that time is not here yet (unless you have millions to spend).
Personally I supplement with about 9g of vit C twice daily, but I was given less than a year to live over 5 years ago – so I had a substantial incentive to follow evidence rather than dogma, culture, or other people’s interpretation of what evidence might mean 😉