Vitamin D was first discovered at the beginning of the 20th century, as a missing nutrient in children causing severe bone demineralization, a disease called rickets. Vitamin D plays a pivotal role in metabolizing calcium for healthy bones. Insufficient vitamin D makes bones thin and brittle causing rickets in children and osteomalacia in adults. Vitamin D has multiple non-skeletal actions besides its major role in calcium homeostasis and bone metabolism.
The term "vitamin D" refers to two different forms; (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D2 is synthesized by plants and vitamin D3 is synthesized by humans in the skin when it is exposed to ultraviolet-B (UVB) rays from sunlight. Vitamin D, as either D3 or D2, does not have significant biological activity it must be metabolized within the body to the hormonally-active form known as 1, 25-dihydroxycholecalciferol.
The major sources of vitamin D are our diet, supplementation and sun exposure. The good food sources of Vitamin D are fish liver oils, fatty fish, fortified milk products, fortified cereals. However, vitamin D is found in very few foods naturally (e.g. fish, eggs and cod liver oil); while foods such as milk, orange juice, yogurts and some breakfast foods are fortified with Vitamin D. Foods may be fortified with vitamin D2 or D3. The fortification level is aimed at about 400 IU per day.
Vitamin D is formed naturally as a result of sunlight exposure; as little as 10 minutes of sun exposure is thought to be enough to prevent vitamin D deficiencies. Lack of proper sun exposure or inadequate Vitamin D3 supplement increases body’s odds to vitamin D deficiency or insufficiency.
The Recommended Daily Amount (RDA) or Adequate of Vitamin D for adults’ age 18-50 years, pregnant and breastfeeding women is 5 micrograms/day. The RDA for adults’ age 51-70 years is 10 micrograms/day, while for adults over age 70 years RDA is 15 micrograms/day. Upper limit or the highest amount of Vitamin D taken without risk is said to be 50 micrograms/day. (The recommended daily intake of vitamin D is 200 international units (IU) for people up to 50 years old and 400 IU for people 51 - 70 years old and 600 IU for people over 70 years old.)
The Vitamin D blood levels should be around 50-80 ng/mL (125-200 nmol/L) for optimum health, these values are relevant to both children and adults. While, most doctors still consider a result of 30 ng/mL (75 nmol/L) to be sufficient; when it is not. Total serum 25(OH) D (25-hydroxyvitamin D) is the total amount of vitamin D in the blood, both vitamin D3 and vitamin D2. The only way to determine the body Vitamin D status is by testing Vitamin D blood levels. Vitamin D test tells us how much of the storage form of vitamin D, known as 25(OH) D is present in our blood serum. Vitamin D test can be obtained by asking the doctor for a 25-hydroxyvitamin D [5(OH) D] test. We need to be careful while ordering the right test a 25-hydroxyvitamin D test NOT a 1, 25-dihydroxyvitamin D test. We can also perform the test oneself by purchasing an in-home test kit.
Vitamin D deficiency is a highly prevalent condition. In spite of foods fortified with vitamin D and wide supplement intake, it is estimated that ~1 billion people have insufficient levels of vitamin D. According to Anthony Norman, an international expert on vitamin D, half the people in North America and Western Europe get insufficient amounts of vitamin D. Anthony Norman recommends a daily intake of 2000 international units for most adults. He says only eating vitamin D-rich foods are not adequate to solve the problem for most adults.
Lately, epidemiological data has shown prominent role of Vitamin D in autoimmune and cardiovascular disease as well as in cancer therefore interest in mechanisms of vitamin D action is growing persistently. Century later we are now aware of Vitamin D non-skeletal actions. Vitamin D has effect in target organ damage; Vitamin D receptor (VDR) is present in many tissues including immune cells and cardiovascular (CV) target organs.
Vitamin D ameliorates the target organ damage by enhancement of immune suppression/ modulation of immune responses; downregulates renin-angiotensin system and inhibits cardiac hypertrophy.
Several experimental data point towards beneficial effects of vitamin D in cardiovascular disease as it is said to have a novel role as a negative modulator of renin synthesis. One of the studies done by Wang et al., (2010) assessed whether vitamin D and calcium supplements reduced the risk for cardiovascular events in adults. They used data sources from clinical studies and trails from 1966 to July 2009. The findings showed consistent reductions in cardiovascular disease (CVD) mortality among adults who received vitamin D supplements in five prospective studies of patients receiving dialysis and one study involving a general population. However, study found no differences in incidence of CVD between calcium supplement recipients and nonrecipients in four prospective studies of initially healthy persons. Similarly, the results of secondary analyses in 8 randomized trials showed a slight but statistically nonsignificant reduction in CVD risk with vitamin D supplementation at moderate to high doses but not with calcium supplementation or a combination of vitamin D and calcium supplementation compared with placebo. The results suggest that vitamin D supplements at moderate to high doses may reduce CVD risk, whereas calcium supplements have minimal cardiovascular effects.
Likewise, several observational studies have suggested anti-inflammatory and immunosuppressive properties of vitamin D. Results have shown that vitamin D deficiency is associated with higher incidence of autoimmune diseases such as multiple sclerosis and type 1 diabetes mellitus. Evidences from a number of studies have reported substantial reductions in incidence of breast cancer, colon cancer and Type 1 diabetes in association with adequate intake of vitamin D. These positive effects were reported to have occurred within five years of initiation of adequate vitamin D intake.
Vitamin D has much potential health benefits, research so far suggest vitamin D provides protection from osteoporosis, hypertension (high blood pressure), cancer, and several autoimmune diseases. However, more research needs to be done in some areas to make it even more comprehensible.
References:
Holick, M. F. (2007). Vitamin D deficiency. N Engl J Med, 357, 266–281.
Munger, K. L., Levin, L. I., Hollis, B.W., Howard, N. S., Ascherio, A. (2006). Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis. JAMA, 296, 2832–2838.
Wang, L., Manson, J. E., Song, Y., Sesso, H. D. ( 2010). Systematic review: Vitamin D and calcium supplementation in prevention of cardiovascular events. Ann Intern Med, 152, 315– 323.
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