Rachel wrote on my
>>Iceberg salad is astonishingly high in vitamin A.

Rachel, it really seems that that's your "pet peeve"
because it's you second posting complaining about
the Vitamin A in Lettuce.
If you've been deficient, you may be sensitive to the theme, but that
doesn't give you the right to spread mis-informations.
You tell:
>Eat lettuce till the cows come home but you cannot get any VitaminA.
>Try cod liver oil, liver, other organ meats, etc.
>We need to eat animal sorce
>food to get our essential As!

This is not true and you should know it.
Or how do you explain that so many vegetarians and also meat-eaters
which never eat liver or dairy (animal Vitamin A donators)
can ever survive for longer than half a year (vit A storage time)?

It's no secret, that only animal stuff contains the ready-made
Vitamin-A. And by no surprise "real" Vitamin A and its precursors
(carotenes) are considered together, because Vitamin A alone
isn't a Vitamin. It can be built from carotenes
and so one of the both must be supplied.

>Many people mistakenly believe that beta-carotene is the same as

vitamin A..
Carotenes have been discovered because they *can* cure the same
diseases as pure Vitamin-A does.
Carotenes have to be rebuilt into real Vitamin a to work, this
conversion has a certain degree of effectiveness (as you describe).
This is the reason, why Vitamin A requirements are expressed
in the scale of IU, a unit in which carotenes and ready vitamin A
have different adjustment factors.
Therefore IU-measurements are already adjusted for
the different effectivenesses of different carotenes and ready

vitamin.

Vitamin A (as a term , including carotenes) from plants aren't as
perfectly digested as animal vitamin A is.
This may be a disadvantage when the supply is low.
However the same effect *is* an advantage when the supply is high.
The body has a mechanism to adjust the intake of carotenes.
But it can't hinder the intake of vitamin A.
Vitamin A can become toxic, so,
if you will eat much of liver every day, then you will
become ill by the vitamin a.
Paleolithic sceletons have been found, where obvoiusly hunting
humans show the charasteristic misformings of vitamin A
toxicity.
You will never get too much of carotene vitamin-A-IU's but
possibly from animal sources.

BTW:
Vitamin A and the topic of meat beeing a natural food:
In the beyondveg site one thing is mentioned as to
show that animal stuff must have been found in the human food chain
for a very long adaption time.
Because (as one study found) humans have receptors in the gut, which
are *specialized* in absorbing *only* animal vitamin A.
These would be useless in the absence of animal-Vitamin A
an so couldn't have evolved.
If the study is right, then I conclude from this two things:
1. Animal Vitamin A must have been in the food long enough
  to evolve such receptors. They need to be present in lactation
  time for the first years of children. Because milk supplies
  the vitamin A too (of course). But the receptors might
  regress in adulthood, because milk isn't an adults food.
2.Since the receptors can't stop vitamin A ingestion,
  animal vitamin A intake must have been rather low
  to reward (evulutionary) the developement of such an organism.

  If gorillas had these receptors, the evolutionary "reward"
  of such a receptor may be to expliot the little animal vitamin A
  from the few ingestet insectes.

Vitamin A supply in todays society is probably less than
optimal, because plant sources (dark green or red vegetables)
are too less consumed and liver isn't popular.
Muscle meat itself contains also too small amounts
(in 1 kg only 9%RDA).
Many don't eat liver and fish (both evironment poison accumulating).
But liver, fish sources, dairy sources (butter) or supplements
are the most effective sources for a deficient.
Not to forget that fat *is* necessary.

>They have also been led to believe that sunlight can provide all the
>necessary vitamin D.
Thats annother point.
Vitamin D is made in the sun. 17 minutes daylight on hands and
face are sufficient (if the precursors are available).
The only ready-made Vitamin D sources worth to mention are fish
and mushrooms. Mushrooms may be a important key factor,
(also if you consider Vitamin B2).
But fish? Requires you to support some aquatic human developenent
theory. Do you? Or does this include too much doubts with you.
I personally like to swim, but I don't see fins on my hands....

Amadeus