Vitamin D is an essential vitamin which assists in the maintenance of proper calcium and phosphorus levels within the body, and is very important for overall health. For instance, Vitamin D builds strong bones because it helps the body use calcium from the diet.

Vitamin D deficiency causes rickets, in which the bone tissue doesn't mineralize, leading to soft bones and skeletal deformities. Increasingly, emerging research is starting to reveal that vitamin D helps in protecting against many other health problems as well.

Vitamin D deficiency is probably the most common medical problem worldwide. According to some health experts, being vitamin D sufficient can reduce the risk of having a first heart attack and developing peripheral vascular disease, along with lowering risk for cancers such as prostate, colon, and breast cancer. Increasing vitamin D intake to about 800 international units (IU) per day has been reported to reduce the risk of developing type 2 diabetes by as much as a third.

Vitamin D deficiency can occur for many reasons:

Following a strict vegetarian diet - most natural sources of this vitamin are animal-based, including fish and fish oils, egg yolks, cheese, fortified milk, and beef liver.

Limited exposure to sunlight - the body makes vitamin D when skin is exposed to sunlight. People who have dark skin, or are homebound, live in northernlatitudes, wear long robes or head coverings for religious reasons, or have an occupation that prevents sun exposure are all at risk.

Poorly functioning kidneys - as people age, their kidneys are less able to convert vitamin D to its active form.

Inability to absorb vitamin D - medical conditions such as Crohn's disease, cystic fibrosis, and celiac disease can affect the intestine's ability to absorb dietary vitamin D.

Being overweight or obese - vitamin D is extracted from the blood by fat cells, lowering its levels in the circulation. People with a BMI of 30 or greater often have low blood levels of vitamin D.

Observational data suggests that low levels of 25-OHD are associated with diabetes mellitus, hypertension, increased risk of death from cardiovascular disease, cognitive impairment and some cancers. However, it is unknown whether low blood 25-OHD levels are associated with mortality in the general population - which is why researchers from the Albert Einstein College of Medicine tested the association of low 25-OHD levels with all-cause, cancer, and cardiovascular disease mortality in 13,331 adults 20 years or older from the Third National Health and Nutrition Examination Survey (NHANES III) study.

The study authors found that 25-OHD deficiency (lowest quartile, (17.8 ng/mL) was independently associated with a 26% higher risk of all-cause mortality. The estimated increased risk of mortality due to heart disease was similar, although not statistically significant. They did not find an association with cancer mortality or other causes of death.

Their findings are supported by ecological studies, which reveal that cardiovascular death events are higher in the winter when vitamin D levels are lower and that cancer survival is better if the cancer is diagnosed in the summer, when there is more sunlight. In addition, it is well known that the use of activated vitamin D therapy in patients with end-stage renal disease is associated with decreased mortality.

Low 25-OHD levels are also associated with hypertension, diabetes mellitus, insulin resistance, and an elevated BMI, all of which are risk factors for death from heart disease and all-cause mortality. The association of 25-OHD deficiency with obesity, glucose intolerance and the metabolic syndrome is yet another potential mechanism for increased risk of cardiovascular death.

In support of the NHANES III data, a meta-analysis of 18 randomized clinical trials showed that participants randomized to vitamin D supplementation experienced fewer deaths compared with those randomized to placebo - however, the meta-analysis was similarly unable to pinpoint the specific cause of death responsible for lower mortality.

The NHANES III study is an observational study, and therefore causality cannot be inferred. Excess mortality observed in the lowest quartile may be because of the participant's overall poor condition or a yet unknown confounder.