Dredging up a post by Titanium from 2011:
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Apologies in advance, still on deadline and can't spare time for a long discussion on yet another a Vitamin D thread. But just in case it can help some reader, a few tips.

1) Vitamin D is fat-soluble. If you take it in a dry multivitamin, make sure to take it with food with some fat/oil content so the D has a chance of being absorbed. It is absolutely possible to be "taking" Vitamin D in dry form and NOT absorb it. So if you think you're "covered" for Vitamin D by popping a multivitamin down the hatch with your morning black coffee, ain't necessarily so. I would recommend D in oil, or in a sublingually-absorbable form. (Or just get lots of sunlight if your schedule and latitude permits!)

2) One reference I've seen asserts that D is absorbed at the tips of the intestinal villi. Those villi are decimated in people with the medical condition called "celiac sprue", leading to various malabsorption sequelae including those of D deficiency. Some people have less than full-blown celiac disease but may still have the D absorption problem. Now, the next sentence, on this board, will assuredly turn this thread into a shooting gallery, but here goes. The accepted medical treatment for celiac disease is to abstain from, yes, "gluten". So the conclusion is, in people with less-than-full-blown celiac sensitivity, removing gluten from the diet may improve nutrient absorption. The Paleo Diet folks have tons of testimonials of the benefits of getting off grains, and this might be one of the underlying mechanisms.

3) The past decade or two of skin-cancer scare/awareness conditioned people to avoid the sun and wear sunblock. Vitamin D is made in the skin in response to the UV component in sunlight, but this is screened out by sunblock. Long hours at all-indoor occupations have a similar D-depleting effect -- no sun. Note: most window glass does not transmit much UV (quartz glass is typically required if you want UV transmission). So being indoors under windows may still not 'cut it'.

As referred-to by a previous poster, D affects calcium absorption, perceived energy levels, sugar utilization, and immune function -- it is a *very* important contributor to thriving health, and something to be sure you're getting enough of, through adequate sunlight or properly absorbed via ingestion.

Vitamin A is also important, and an "old time" nutritional supplement for both Vitamin A and D used to be "cod liver oil" -- still available at health food stores. Much easier than eating seal liver, since you first have to get the liver out of the seal, and they tend to object.

Hoping this helps someone. Best wishes with all components of your training, which necessarily includes ensuring good nutrition and adequately absorbing the nutrients.
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So I'll be taking mine with cod liver oil, which I hope will augment absorption, and I won't have to "mega dose".
I scan old threads a lot.
The problem with cod liver oil seems to be you get too much vitamin A.