From: sbhattac@rnd.GBA.NYU.EDU (Shankar Bhattacharyya)
Newsgroups: rec.food.veg
Subject: Re: Dietary vitamin D unnecessary?
Date: 11 Oct 92 17:14:44 GMT
Organization: NYU Stern School of Business

In article <NSINGH.92Oct8144019@hqsun5.oracle> nsingh@hqsun5.oracle (Naunihal Singh) writes:

>Rickets is due to lack of vitamin C which is caused by insufficient
>vegetables and fruits in your diet. British sailors used to eat lemons
>/ limes to combat this while at sea, hence the name limey.
>
>I was taught in intro high school bio that it only took 15 minutes of
>direct sunlight on the back of your hands to generate all of the
>vitamin D you need - on the other hand (no pun intended) compugeeks
>like me do not often see the light of day ..........


Vitamin C deficiency causes scurvy, not rickets.

As Rich Young has already pointed out, we are really talking about
vitamin D, not vitamin C.

That said, much of what one reads about vitamin D and sunlight in the
popular press is based on wild extrapolation of a less than casual
acquaintance with the photochemistry. I will guarantee, in writing if
necessary, that it takes more than an undergraduate course in organic
chemistry and/or biochemistry to understand this well. I don't intend
that as an elitist crack, but this is profound chemistry.

The rest of this is not aimed at Naunihal Singh, but at the rather casual
published sources on which many people appear to base serious judgments.

It is technically correct that exposure to sunlight produces vitamin D in
the body. It is a mite misleading to extrapolate that and say that because
of this the body does not need dietary sources of vitamin D. If you tried
saying that to an organic chemist you would stimulate a laugh. We nerds
would ask, "What does that have to do with anything?" It makes a debating
point, provided the audience is sufficiently ignorant, but it does not
address the basic question.

The energy from sunlight causes photochemically induced cyclization of the
precursors to the D vitamins (there is more than one). So, the functional
question reduces to the following: Can the body manufacture the precursors,
or does it need to get these pre-vitamin D products from the diet? That is
an extremely specific question, to which I have no answer.

My impression is that the body does not make these precursors, but I am no
expert on the biosynthesis of isoprenoids in the human body, though I know
the basics. I've even read the original papers, but that ain't expertise.
And that was a long time ago.

If my half-baked presumption is correct, the body must get its pre-vitamin D
compounds from the diet. I am not claiming that I am right in this
presumption. I am claiming only that this is precisely the question. No
precursors, no D vitamins, never mind how long you spend in the sun.

Further, I express, emphatically, no opinion on whether these precursors
are or are not available from vegetable sources. In an earlier discussion
on this subject Michael Traub said that the precursors are indeed
available from vegetable sources, and I have no reason to doubt that. And I
am not sufficiently interested in this to go look it up. My judgment is
that it would take more study than might seem likely at first thought.

Incidentally, the biosynthesis of isoprenoids, chiefly the steroids, of
which class the D vitamins are members, is sufficiently beautiful chemistry
that every chemist should be familiar with the basics. Certainly, anyone
with a background in organic or biochemistry who has managed not to absorb
this stuff should be put up against a wall and shot. This is really
utterly, utterly exquisite biochemistry. It is every bit as compelling as
van Gogh's Starry Night, or Rubenstein playing Chopin.

Further, the cyclization of the pre-vitamin D compounds to the D vitamins,
and many related reactions, are examined in a classic book called
"Conservation of Orbital Symmetry", by Woodward and Hoffman, published in
the early 70's.  That, too, is extraordinarily lovely chemistry, and any
organic chemist who does not know that book from cover to cover should
be taken out and shot. I'm a pretty tolerant fellow, but some things are
simply intolerable. I am reliably informed that St. Peter, and his
equivalents in other religions, ask candidates if they have read the book.

Both threads of research have earned Nobel prizes, the biosynthesis work
in the sixties, the orbital symmetry work in the seventies, if I remember
correctly.

- Shankar