I can attest to with some authority because I benefited from Vitamin D. It is the single biggest factor for me after air, water and my ovo-lacto vegetarian diet.

If you are Black, Asian and your ancestors in the last 500 years lived close to the equator; you must get your and your childrens Vitamin D levels tested. Frankly, if you are not giving adequate Vitamin D to your kids no matter what else you do it does not matter.

I lived in three continents by the age of 22 and I am nearing 50. I can say with certainity pretty much every ailment of human body is connected to low Vitamin D levels. Remember dark skinned people are impacted at least ten times more.

Circulating 25-Hydroxyvitamin D Levels in Fully Breastfed Infants on Oral Vitamin D Supplementation

Given the fact human skin produces 10,000iu~20,000iu D3 given a few minutes sun exposure and naturally attains and maintains (when near full body sun exposure is available) a status between 60~80ng/ml it’s not surprising that primitive level 0f 25(OH)D3 is associated with least incidence of chronic illness.

A critical review of Vitamin D and Cancer A report of the IARC Working Group William B Grant

How strong is the evidence that solar ultraviolet B and vitamin D reduce the risk of cancer? An examination using Hill’s criteria for causality

Shining light on the vitamin D Holick

Levels of vitamin D and cardiometabolic disordersHigh levels of vitamin D among middle-age and elderly populations are associated with a substantial decrease in cardiovascular disease, type 2 diabetes and metabolic syndrome. If the relationship proves to be causal, interventions targeting vitamin D deficiency in adult populations could potentially slow the current epidemics of cardiometabolic disorders.

Alternative view of Vitamin D. (scroll down a bit)

Black-White Differences in Cancer Risk and the Vitamin D Hypothesis

We also know high-fibre diet reduces plasma half-life of 25(OH)D3.

Vitamin D3 is fat soluble. Raised insulin levels lead to fat storage. As fat molecules are stored in fat cells so Vitamin D3 is taken out of circulation as adipose tissue increases, Vitamin D3 insufficiency is associated with obesity.

The process of photosynthesis of Vitamin D3 from cholesterol near skin surface uses UVA and UVB. Although UVA helps produce previtamin D and also processes unabsorbed vitamin D3 into suprasterols the body doesn’t use UVB (heat) is required to process previtamin d into vitamin D3.
It follows that UVA exposure without UVB (as through sunny window, car windscreen, covered shopping mall) degrades D3 near skin surface into those suprasterols you body doesn’t use. This may have been a useful strategy to prevent vitamin D3 toxicity at a time prolonged outdoor exposure was required for hunting/gathering but now being exposed to more UVA exposure than UVB exposure leads to declining 25(OH)D levels. Sunscreen use, cosmetic use, time in front of TV/PC, as well as clothing all contribute to lower levels.
Fortunately up to 10,000iu/daily D3 (the amount skin naturally makes in a few minutes full body exposure) is both cheap and safe.