The advice given on Vitamin D these days is a mess.

Technically, vitamin D is not a vitamin, since the body can manufacture
all it needs, although it rarely does in winter months.  Also, unlike
true vitamins, you can't get what you need from natural food.  To
complicate matters: this "vitamin" is a steroid hormone.

Vitamin D3 (cholecalciferol) is manufactured when the skin is exposed
to sunlight.  The skin contains a form of cholesterol,
7-dehydrocholesterol, which absorbs short-wave ultraviolet radiation
[UVB], to generate cholecalciferol.  (Most sunscreens are good at
blocking UVB).  UVB exposure can lead to sunburn; excessive exposure
may result in skin cancer.

The more damaging long-wave form (UVA) can cause subcutaneous DNA
damage, and ultimately cancer.  Many sunscreens offer inadequate UVA
protection.

Ironically, sunscreens empower prople to spend longer periods in the
sun - avoiding sunburn but incurring DNA damage (the Law of Unintended
Consequences strikes again!)

Sun exposure has not been known to produce toxic amounts of Vitamin D.

Melanin blocks UV rays - the more you have the more you block.
Presumably, Africans in Africa developed a balance of cancer protection
& vitamin D production.  This balance is lost in North America &
Europe.

The fact that ancient people who settled in northern lattitudes evolved
a way of losing melanin in periods of weak sunlight exposure attests to
the survival importance of D.  The fact that they also retained the
ability to produce melanin attests to the survival danger of unfiltered
UV.

Almost all Americans are D-deficient, with African-Americans more so.

Giovannucci, et al, 2006, found that a vitamin D increment of 25 nmol/L
was associated with a reduced risk of total cancer incidence (risk =
0.83), total cancer mortality (risk = 0.71) and digestive system cancer
mortality (risk = 0.55).  The supplementation needed to achieve
protection was estimated to be at least 1500 IU/day.

The recommended daily allowance [RDA] for men is 200 IU, rising to 400
IU after age 50, and 600 IU at age 70.  Many experts believe the RDA to
be too low.  A supplementation level of 1,000 IU is sometimes
suggested.  Men should be aware that sustained very high intakes of
Vitamin D can result in toxicity.  However, this is not at all likely
below a 2,000 IU per day level.  A more realistic safe limit may be
5,000 IU.  [A young, near-naked, white-skinned male will produce 20,000
IUs during the first 20 minutes of summer sun exposure.]

A vitamin D-deficiency connection with prostate cancer, was only
postulated in 1990, by Schwartz, et al.

Cholecalciferol is not biologically active and must be converted to the
active hormonal form.  The first step of the conversion occurs in the
liver, where cholecalciferol is turned into the pro-hormone, calcidiol.
The production of calcidiol is loosely controlled.  Since the
half-life of calcidiol in the blood can be several weeks, this is where
a possibility of massive over-supplementation, over an extended period
of time, might lead to toxic levels.

The second vitamin D conversion step occurs in the kidneys, where
calcidiol becomes the active secosteroid hormone, calcitriol. Levels of
calcitriol circulating in the blood are highly regulated.  Blood levels
of calcitriol are small, compared to calcidiol - perhaps a thousand
times smaller.  In contrast with calcidiol, the half-life of calcitriol
is measured in hours.  Calcitriol is a hormone, because the
parathyroids cease to stimulate its production when calcium blood
levels have been regulated.

The primary purpose of calcitriol is to assist in the absorption of
calcium and phosphate.  Calcitriol levels therefore rise and fall as
the parathyroid glands exert their stabilizing influence.

The epithelial cells of the prostate have vitamin D receptors that have
a particular affinity for calcitriol.  Calcitriol is important in the
development of the prostate and plays an important part in controlling
cell division.  Calcitriol has been shown to be able to regulate the
growth and differentiation of prostate cancer cells and even induce
programmed cell death.

Although the kidneys are the primary source of calcitriol, prostate
cells are also able to convert calcidiol into calcitriol, for local
use.  Since calcitriol levels fluctuate, a sufficient level of
circulating calcidiol is required to ensure that the prostate has the
calcitriol it needs.

The fact that prostate cells can convert calcidiol to calcitriol,
indicates the importance of a steady supply of calcitriol to the
prostate.  As prostate cancer cells evolve, multiple regulatory systems
fail.  Loss of the conversion capability is common.  Maintaining a
healthy level of calcidiol is then, by itself, not sufficient.  At this
stage, it is important that the kidneys produce calcitriol on a regular
basis.  This does not occur when the diet contains excessive levels of
calcium.  This perhaps explains the association between high calcium
intake & advanced disease.

Calcitriol is not available as a supplement.  It is a powerful hormone,
and supplementation would totally disrupt the system that governs
calcium levels in blood and bone.  Although some laboratory studies
have used physiological levels of calcitriol, many others have used
levels that are simply impracticable in the body.  With such doses, the
patient would inevitably experience hypercalciuria or hypercalcemia.
Because of this, there has been much research into producing a
lookalike that lacks the calcemic properties of calcitriol.  {As of
2002, according to Mehta, almost 400 analogs had been synthesized.}

Vitamin D3 is, by definition, an animal product.  Virtually all of the
vitamin D added to food is synthetic vitamin D3, which is
animal-derived.  The vegetarian alternative is vitamin D2, which can be
processed by the body.

Ergocalciferol is the plant form of vitamin D.  The yeast form is
ergosterol, which can be converted to ergocalciferol.  Ergocalciferol
can be converted to calcidiol.

Synthetic Vitamin D3 is is the form that is added to milk and fortified
cereals.  It is generally made by exposing 7-dehydrocholesterol from
animal skins to ultraviolet radiation.  An alternative process starts
with cholesterol, obtainable from the lanolin in sheep's wool, to
create the 7-dehydrocholesterol.  Regardless, the final product is pure
vitamin D3.

A pint of unprocessed milk may contain up to 35 IU of vitamin D.  An
additional 200 IU of synthetic vitamin D is added.  However, milk
intake is associated with an increased risk of advanced PC.  This may
be due to the calcium intake, or to growth factors (bovine IGF-I is
identical to human IGF-I & dairy cattle receiving growth hormones to
boost milk yield, produce more IGF-I.)

{Personal note: I prefer not to take fish oil supplements.  I do take a
good omega-3 supplement, but cod liver oil is not a good way to obtain
high levels of D3.  The synthetic D3 is pure & exactly what the body
produces.  Unfortunately, you have to search for it.  Another problem
with fish oil, apart from the amount of oil you might ingest, is that
vitamin D is coupled with vitamin A.  The latter is an issue for men
with PC.)