Ask a Pharmacist: Vitamin D

Welcome to a new segment called Ask a Pharmacist. This is where I answer questions that nobody asked, but I think definitely should have been brought up. 
Today my imaginary friend Sally wants to know what all the excitement about Vitamin D means, and should she take a supplement?
Thanks for asking, Sally!
Vitamin D is produced by the body when UVB sunlight reacts with a cholesterol-based compound found in the lower levels of the epidermis. Fun fact: This process is dependent on both the liver (it requires an enzyme produced in the liver) and the kidney (the last step in activation). A disorder of either organ can affect vitamin D levels.
Vitamin D has one function in the body that we know of, but with many results. It binds to the vitamin D receptor. We usually associate vitamin D with increased absorption of calcium. This is because when vitamin D binds to its receptor (the "VDR") in the nuclei of cells, the VDR is activated and can regulate gene transcription (If your biology is rusty, this means that VDR now which proteins the genes are coded to make). VDR promotes the production of transport proteins that carry calcium into the blood stream from the intestine. This is why vitamin D is required for optimal calcium absorption.
Sally interrupted me just now to ask about other effects of vitamin D. It seems that she has heard a million stories about vitamin D preventing the flu or promoting healthy hair. 
Vitamin D is a subject of interest right now. There are several studies that indicate that activated VDR has other uses besides its calcium-related ones. There is evidence that vitamin D may:
- Increase the activity of the immune system. VDR has been shown to produce an antimicrobial peptide that triggers your macrophages to phagocytise invading pathogens; it also ups the activity of natural killer cells (a type of white blood cell that attacks invading organisms and causes their cells to burst).
 Low vitamin D increases respiratory infections and influenza.
- Decrease peripheral artery disease and cardiovascular risk . There are some studies which show an association between low levels of vitamin D and cardiovascular risk. However, these studies only show an association, not cause and effect and are lacking evidence.
- Prevent or delay cancer. There is evidence that the VDR in cancer cells may be altered of changed; there are several studies that link vitamin D supplements with decreased cancer diagnosis. Unfortunately at this time data are tenuous and mechanisms hazy, but keep your eye on this one.
- Vitamin D has been linked to healthy lungs in a number of studies which pointed to decreased asthma attacks or lowered respiratory infections with vitamin D supplementation.
Of course, we also know that vitamin D prevents rickets and osteomalacia and can treat and prevent psoriasis.
Why are there two kinds of vitamin D?
Most supplements contain cholecalciferol (or "D3"), which is produced by vertebrates. Ergocalciferol (D2) is usually obtained from fungi. Which is easier for us to use? Based on a variety of studies, no one knows.
How much vitamin D do I need?
Hm. Depends on who you ask. The recommended daily intake varies HUGELY based on which country's data you reference. The FDA recommends that adults take in 600 IU per day. You get 100 IU for every cup of milk; fatty fish contains 300-400 IU per serving; and up to 10,000 IU from ten minutes in the midday sun (varies greatly depending on skin tone and UVB content of sunlight).
So should I take a vitamin D supplement?
No. You should go outside everyday with no sunscreen. That's my opinion. But if you have osteomalacia or osteopenia or osteoporosis, you should take 600 IU per day.
If I take vitamin D, can I overdose? 
Yes, because vitamin D is fat soluble so it can't just dissolve in the urine and be flushed out. Your body stores excess instead. You can overdose by ingestion; usually toxicity is related to excess calcium and can be serious. In pregnant women, high doses will cause birth defects (mainly mental retardation). Pregnant woman should not supplement with vitamin D without medical supervision and labwork.
The chance of overdosing is slim as a study on the topic indicates that most people would need to take 50,000 IU daily for over a month to reach toxic levels.
Take home:
Evidence is abundant that we need healthy vitamin D levels. My firm recommendation is to increase sun exposure if you do not normally receive direct sunlight on your skin. Because dietary supplements tend to isolate vitamins outside of normal lifestyle ratios, I do not recommend supplementing unless you have a condition caused or worsened by decreased vitamin D.