From: timi@mendel.berkeley.edu ( Tim Ikeda)
Newsgroups: rec.food.veg
Subject: Re: Well, I have a question for Ted. . .
Date: 23 Nov 92 22:08:03 GMT
Organization: /etc/organization
NNTP-Posting-Host: mendel.berkeley.edu


In <altar.722486223@sfu.ca>, Ted Altar writes:
>Norman then presents evidence as to how vitamin D is best
>understood and classified as a "steroid hormone produced by the
>endocrine organ, the kidney, and which a variety of other target
>organs elicits the characteristic responses formally attributed
>to vitamin D" (p. 205).
>
>Norman then proceeds how vitamin D does indeed meet this
>characterization and others of a steroid hormone.  Some 40 pages
>later, Norman concludes:
>
>    This chapter has attempted to summarize the current evidence
>    that supports the steroid hormone concept for the mode of
>    action of 1,25-(OH)2-D3 as a biologically active form of
>    vitamin D.  The key element of data which supports this
>    concept is the nuclear localization of 1,25-(OH)2-D3 in the
>    target tissues, particularly the intestinal columnar
>    epithelial cells vis-a-vis stimulation of intestinal calcium
>    absorption.
>    . . .
>    While some attention has been directed toward models of
>    posttranscriptional control for steroid hormone-induced
>    protein synthesis, it now seems abundantly clear on the
>    basis of data obtained from all steroid hormones that they
>    exert their primary effects at the level of gene
>    transcription.  Administration of aldosteroine,
>    glucocorticoid, estrogen, progesterone, dihydrocortison,
>    1,25-(OH)2-D3, or thyroid hormones, under conditions that
>    permit localization of the hormones in vivo in their target
>    tissues, results in measurable increases in RNA synthesis.


I think an important aspect of hormone function has been completely
overlooked in the current discussion - that is, hormone specificity.
To a person unfamiliar with biochemistry or cellular metabolism,
the revelation that vitamin D belongs to the class of molecules known
as steroid hormones may provoke an unjustified fear of the vitamin.
In the public mind, steroid hormones are strongly associated with
images of beefed-up bodybuilders and (formerly), Soviet-bloc athletes.

While the *mode* of vitamin D action may be similar to that of
many other steroid hormones, the *specificity* of its action and the 
*specific* regulatory processes being controlled are very distinct.
Members of the steroid hormone class have distinct targets and
elicit distinct responses in different tissues. This is by design.
For instance, if our body's tissues could not recognize the difference
between estrogen and testosterone, puberty would be even more 
"interesting" than it is now (Side note: There are genetic diseases
linked to hormone action which affect sexual dimorphism).

The reason why a cell can tell the difference between the various
hormones is that it uses different receptors for each. It does not
matter that the steroid hormones share a common cholesterol-based
skeleton; they are all chemically distinguishable by the cell.

For this discussion thread, it is largely irrelevant whether vitamin D
acts like a steroid hormone.  Vitamin D acts only like vitamin D.
A certain subset of tissues have specific receptors for this regulatory
molecule. In turn, a distinct subset of genes are regulated by these
receptors.  The central question should instead be: Whether
vitamin D supplementation is hazardous.  I believe this is the
central thrust of Steve Dryer's previous posts.  The transcription
of Dr. Norman's article did not include this subject.  Is his opinion
different from that of Dr. Moon's?

Regards, 
Tim Ikeda