Much has been written lately about the role of vitamin D in the prevention of many cancers, including prostate cancer (PCa) and also in potentially slowing cancer progression. However, before discussing this further, it should be noted that there are countervailing studies that cast doubt on the efficacy of vitamin D and at least one study that claims it can actually be harmful.

Before going further, let's define what vitamin D is. From the Wiki article (https://en.wikipedia.org/wiki/Vitamin_D):

Vitamin D refers to a group of fat-soluble secosteroids responsible for enhancing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc. In humans, the most important compounds in this group are vitamin D3 (also known as cholecalciferol) and vitamin D2 (ergocalciferol).[1] Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements.[1][2][3] Very few foods contain vitamin D; synthesis of vitamin D (specifically cholecalciferol) in the skin is the major natural source of the vitamin. Dermal synthesis of vitamin D from cholesterol is dependent on sun exposure (specifically UVB radiation).

Vitamin D from the diet or dermal synthesis from sunlight is biologically inactive; activation requires enzymatic conversion (hydroxylation) in the liver and kidney. In the liver, cholecalciferol (vitamin D3) is converted to calcidiol, which is also known as calcifediol (INN), 25-hydroxycholecalciferol (aka 25-hydroxyvitamin D3 — abbreviated 25(OH)D3). Ergocalciferol (vitamin D2) is converted in the liver to 25-hydroxyergocalciferol (aka 25-hydroxyvitamin D2 — abbreviated 25(OH)D2). These two specific vitamin D metabolites are measured in serum to determine a person's vitamin D status.

The possible relationship between vitamin D and PCa was highlighted in a recently published study conducted at Northwestern University (http://jco.ascopubs.org/content/early/2016/02/17/JCO.2015.65.1463.abstract) and summarized in their press release (http://www.news-medical.net/news/20160302/New-study-finds-link-between-vitamin-D-deficiency-and-aggressive-prostate-cancer.aspx):

A new study provides a major link between low levels of vitamin D and aggressive prostate cancer. Northwestern Medicine research showed deficient vitamin D blood levels in men can predict aggressive prostate cancer identified at the time of surgery.

The finding is important because it can offer guidance to men and their doctors who may be considering active surveillance, in which they monitor the cancer rather than remove the prostate.

"Vitamin D deficiency may predict aggressive prostate cancer as a biomarker," said lead investigator Dr. Adam Murphy, an assistant professor of urology at Northwestern University Feinberg School of Medicine and a Northwestern Medicine urologist. "Men with dark skin, low vitamin D intake or low sun exposure should be tested for vitamin D deficiency when they are diagnosed with an elevated PSA or prostate cancer. Then a deficiency should be corrected with supplements."

Previous studies showing an association between vitamin D levels and aggressive prostate cancer were based on blood drawn well before treatment. The new Northwestern study provides a more direct correlation because it measured D levels within a couple of months before the tumor was visually identified as aggressive during surgery to remove the prostate (radical prostatectomy).

The relationship between vitamin D and prostate cancer may explain some disparities seen in prostate cancer, especially among African American men. Prior research by Murphy and colleagues showed African American men who live in low sunlight locations are up to 1½ times more likely to have vitamin D deficiency than Caucasian men.

But because vitamin D is a biomarker for bone health and aggressiveness of other diseases, all men should check their levels, Murphy said.

A big promoter of vitamin D as a cancer preventer is Dr. Bruce W. Hollis, PhD, a professor at the Medical University of South Carolina. In this Youtube vid he shares the results of a recent trial including identifying the vitamin D level needed to protect the prostate gland: https://www.youtube.com/watch?v=QrU1yrmNIqc. In a nutshell, Dr. Hollis believes our vit. D levels should be above 60 ng/ml.

On the other hand, the Wiki article states the following: "It is unclear, however, if taking additional vitamin D in the diet or as supplements affects the risk of cancer. Reviews have described the evidence as being "inconsistent, inconclusive as to causality, and insufficient to inform nutritional requirements"[59] and "not sufficiently robust to draw conclusions".[77]

Snce there is so much recent evidence in favor of Vitamin D, we should pay attention at a minimum.