A Complete Look at Vitamin D

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Vitamin D helps maintain the normal levels of calcium and phosphorus in the bloodstream. It is a fat-soluble steroid hormone that is actually not a vitamin since human skin can manufacture it. It is however referred to as one, and is also known as calciferol.

The natural human form of vitamin D is Vitamin D3 which is also known as cholecalciferol. It is made in the skin when cholesterol via 7-dehydrocholesterol reacts with ultraviolet light in the skin. Ultraviolet light (UVB) is found in sunlight when the sun is high enough above the horizon for UVB to penetrate the atmosphere. Cholecalciferol is transported to the liver where it is hydroxylated to calcidiol or 25-hydroxy-vitamin D, the storage form of the vitamin.

Some parts of the world have widespread vitamin D deficiency, due to higher latitudes and winter conditions. Foods may be fortified with vitamin D2 or vitamin D3, but milk only contains 100 IU per glass. Severe deficiency can lead to a disease known as rickets in children, where the bones are softened due to faulty mineralization. Similar conditions can develop in adults known as osteomalacia. Other recent studies have linked vitamin D deficiencies to forms of cancer, heart disease, diabetes, depression, hypertension, periodontal disease, autoimmune disease and obesity. A deficiency can be determined by a blood calcidiol level. Healthy levels should be between 40 and 60 ng/ml.

Calcitriol is the most active form of the vitamin. It is synthesized from calcidiol in the kidneys to perform its endocrine function of maintaining the calcium levels. Calcitriol binds to a transportation factor which then regulates gene expression. As a result, the calcium and phosphorus levels in the bone and blood are maintained with the assistance of parathyroid hormone and calcitonin.

Some tissues throughout the human body can make and regulate their own calcitriol, which is the autocrine, and perhaps paracrine, functions of the vitamin D steroid hormone system. It is these autocrine and paracrine functions of vitamin D which may explain vitamin D’s association with many chronic diseases

Production of Vitamin D

Human skin exposed to sunlight can when conditions are right, produce large quantities of vitamin D. In just a few minutes, up to 20,000 IU could be produced. This is easily enough to avoid deficiency. Exposure to sunlight can also destroy vitamin D; so long term exposure can cause toxicity.

Just being exposed to sunlight does not automatically trigger production of vitamin D. Only the UVB in the sunlight triggers the production. UVB reaches ground level when the sun is high in the sky which is usually a few hours around midday. At higher latitudes, the sun is only high in the summer. In the US, those living north of San Francisco to Atlanta will not be able to produce it at all for 3 to 6 months of the year.
From the end of summertime to the next spring, most humans run on stored amounts of vitamin D which gradually deplete. Suntan lotions also block production.

Vitamin D Deficiency

People over the age of fifty have a higher risk for a vitamin D deficiency. The ability of the skin to convert 7-dehydrocholesterol to pre-vitamin D3 is decreased in older people. In addition in older people, the kidneys do not function as well, so calcidiol is not converted as well to its active form. Many older people need a vitamin D supplement.

In addition newborn infants who are breastfeeding may require a supplement, since breast milk does not contain significant levels of the vitamin and newborns are not exposed to direct sun. Infant formula is fortified with vitamin D.

Some evidence also suggests that obese people have lower levels of vitamin D due to it being deposited in body fat compartments. Those not exposed to summer midday sunshine may also require a supplement. Patients with chronic liver disease and intestinal malabsorption may require larger doses of vitamin D (up to 40,000 IU or 1 mg a day)

Deficiency can lead to several bone diseases due to insufficient calcium or phosphate in bones. Rickets is a childhood disease caused by a failure of growth and deformity of long bones. In adults osteoporosis is a condition characterized by fragile bones. Osteomalacia is a bone-thinning disorder in adults where there is proximal muscle weakness and bone fragility.

Deficiency has also been linked to chronic diseases such as cancer (breast, prostate, ovarian, colon, lung and skin) chronic pain, weakness, fatigue, autoimmune diseases (multiple sclerosis and Type I diabetes), high blood pressure, mental illness, heart disease, psoriasis, tuberculosis, periodontal disease, rheumatoid arthritis and inflammatory bowel disease.

Overdose of Vitamin D

There is no recommended dietary allowance for vitamin D in North America. 200 IU/day is the adequate intake officially recommended up to age 50. The safety of vitamin D intake can not be assured beyond 50 micrograms/day (2000 IU/day). Current official advice about vitamin D is out of date and Vitamin D dietary guidelines are among the next in line for review by the FDA.

Overdose is rare, deficiency is more common. The body has a large storage capacity for vitamin D and most foods and supplements contain too small of an amount of vitamin D for an overdose to occur. Oral overdoses have however been reported due to manufacturing and industrial accidents and leads to hypercalcaemia and atherosclerosis and ultimately death.

The exact long term safe dose is not known. It appears there are chemical processes that destroy excess vitamin D even when taken orally, although the processes have not been identified. Normal food and supplements have concentration levels too low to be toxic.

Sources of Vitamin D

Only a few foods naturally contain significant amounts of vitamin D including: Fish liver oils such as cod liver, fatty fish (salmon, mackerel, sardines, and eel), one whole egg, beef liver and one vitamin pill.

Fortified foods are the major dietary sources of this vitamin. In the US milk is fortified with 10 micrograms (400 IU) of vitamin D per quart. One cup of vitamin D fortified milk supplies about one forth the estimated adequate intake of vitamin D for adults over 50. Dairy products like cheese, yogurt and ice cream are not fortified with vitamin D.

Vitamin D Research

Recent studies indicate cancer patients who have surgery or treatment in the summer where they can get more vitamin D from the sun, have a higher chance of survival than those who get treatment in the winter.

A study was done in the US and releases in 2005 published in the American Journal of Public Health. It indicated there was a connection between Vitamin D intake and the prevention of cancer. The scientists claimed that taking 1,000 IU or 25 micrograms of the vitamin daily could lower an individual’s risk of cancer by 50% in colon cancer and by 30% in breast and ovarian cancer. Cancer experts however indicate much further research is needed to provide proof of this claim.

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