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Vitamin D: Sources, Health Benefits, Deficiency and Overdose

Vitamin D (or calciferol) is a fat-soluble secosteroid required to build and maintain strong bones. The human body is able to produce vitamin D, the so-called “sunshine vitamin” through exposure to sunlight. Like all fat-soluble vitamins, this vitamin also requires specific proteins to allow it to move through the body and interact with cells. Vitamin D can be stored in the body, with excess amounts found in the liver and fat tissues.

The body can usually produce enough vitamin D to fulfil its requirements. Ultraviolet radiation from sunlight causes skin cells to change a metabolite of cholesterol into an inactive form of vitamin D, called D3. This inactive vitamin then undergoes chemical transformation in the liver and kidneys to become the active form of vitamin D that the body can use. Some foods also provide inactive forms of vitamin D, which can be converted to the active form in the liver and kidneys.

Active form of vitamin D behaves as a hormone which helps regulate the amount of calcium and phosphorus in the bloodstream. The release of vitamin D into the bloodstream causes more of these two minerals to be absorbed by the intestines from food and less to be excreted by the kidneys. When needed, vitamin D causes calcium and phosphorus to move from the bloodstream into bones. In addition to keeping bones strong, there is enough evidence of vitamin D’s involvement in maintaining healthy immune and reproductive systems as well as the muscles, skin and pancreas.

The recommended dietary allowances (RDAs) for vitamin D are 600 IU or 15mcg per day for adolescents and adults and 800 IU or 20mcg per day for the elderly. However, research suggests that many people living in northern countries are not getting enough vitamin D.

Sources of Vitamin D

Vitamin D is unique among the vitamins because the human body can make enough to satisfy its own needs. This, however, requires adequate nutritional status and sun exposure. In addition, vitamin D is also available from some foods. But, compared to most other vitamins, there are not too many foods that naturally contain large amounts of vitamin D. Fish liver oils and fatty fish like mackerel, salmon and tuna are among the few sources that provide significant amounts of inactive forms of vitamin D. Small quantities are also present in cheese, egg yolks and liver. Additional dietary options include fortified foods.

For many people, especially those living in the regions with lots of sunshine, sunlight provides most, if not all the vitamin D their body needs and dietary intake is less important. For light-skinned people, exposing the arms, legs and face for about 10 minutes a day should provide sufficient vitamin D. The length of exposure required increases with darker skin color. Moreover, the skin’s ability to make vitamin D diminishes with age. Generally, people living in the northern regions with long winter nights do not get adequate sunlight to produce vitamin D in the winter. Sometimes, a doctor may recommend vitamin D supplementation. Always ask your doctor before taking vitamin D supplements due to the possible health risks that may occur from overdosing.

Potential Health Benefits of Vitamin D

Vitamin D is probably best known for its ability to maintain healthy bones. When taken together with calcium it can help reduce the incidence of osteoporosis and hip fractures. Furthermore, vitamin D supplements can be used for a variety of other conditions, such as:

  • Familial hypophosphatemia. A combination of vitamin D and phosphate supplements can be prescribed to treat this disorder.
  • Psoriasis. Topical medications containing vitamin D may be prescribed to reduce symptoms of psoriasis.

Other potential applications of vitamin D supplements include the prevention and treatment of the following conditions:

  • High blood pressure. Research shows that increasing vitamin D intake may help reduce blood pressure and thus reduce the risk of stroke, heart attack and other cardiovascular events.
  • Cancer. Increased intake of vitamin D is associated with lower risk of developing certain types of cancer, including colorectal, esophagus, prostate, ovaries and breast cancer. Although sun exposure can help improve vitamin D levels, excessive sun exposure happens to be a leading cause of skin cancer.
  • Type 1 diabetes. Some studies have found a link between low levels of vitamin D and increased risk of diabetes.
  • Alzheimer’s disease. Studies have found that vitamin D may significantly improve the rate of cellular repair in the brain.
  • Multiple sclerosis. There is some evidence of an existing link between increased intake of vitamin D and a reduced incidence of multiple sclerosis.
  • Age-related macular degeneration. Research shows that people with higher levels of vitamin D in their blood have a lower risk of developing the early stages of age-related macular degeneration.
  • Muscle and bone pain. Studies also suggest that there is a link between vitamin D deficiency and other conditions such as muscle and bone pain.

Deficiency and Overdose of Vitamin D

Too little or too much of vitamin D may lead to serious, life-threatening medical conditions. The conditions related to deficiencies or overdoses of vitamin D mostly reflect the relationship between the mineral calcium and this vitamin. Therefore, diseases linked to vitamin D deficiency are usually characterized by weakened bones. They may include:

  • Osteoporosis. Often associated with calcium deficiency, osteoporosis may also result from insufficient intake of vitamin D, which reduces calcium absorption and leads to a loss of bone density and increased risk of fractures.
  • Rickets. Softening of bones slows growth and causes bowed legs and arms and skeletal abnormalities in children.
  • Osteomalacia. This disease causes soft, painful bones and muscle weakness and may lead to a stooped posture.

Increasing the intake of vitamin D and, in some cases, calcium and phosphorus may help to treat the aforementioned diseases. However, there are certain avoidable as well as unavoidable factors that may increase the risk of developing vitamin D deficiency, including:

  • Geographic location. During the winter months, sunlight is not strong enough to cause skin to create enough vitamin D in people living in northern regions.
  • Urban environment. The smog present in large cities blocks the ultraviolet light needed for the synthesis of vitamin D.
  • Skin cover. Clothing and sunscreen used to protect the skin from the sun also interfere with the process of vitamin D synthesis.
  • Dark skin. Melanin reduces the rate of vitamin D synthesis. Therefore, people with dark skin living in northern climates are at greater risk than light-skinned people for having low levels of vitamin D.
  • Age. As people age, the ability of their skin, liver and kidneys to make and activate vitamin D declines. This often happens in conjunction with reduced exposure to the sun while a diet poor in vitamin D may be an additional contributing factor.
  • Liver and kidney disease. Liver and kidneys convert inactive forms of vitamin D to forms the body can use. Any condition that affects liver or kidney function can reduce the amount of active vitamin D in the body.
  • Fat malabsorption. Any disease, such as cystic fibrosis, gluten intolerance, liver disease or pancreatic enzyme deficiency, which limits fat absorption, may also reduce the amount of fat-soluble vitamin D usable from foods.
  • Breastfeeding infants. Infants over 6 months of age who are fed a diet consisting exclusively of breast milk may develop vitamin D deficiency.
  • Gastric bypass. Weight loss surgery to reduce the size of the stomach can also increase the risk of vitamin D deficiency.
  • Hereditary vitamin D resistance. This is an extremely rare genetic condition which prevents the body from using vitamin D.

Although vitamin D deficiency can cause serious medical conditions, overdosing with vitamin D supplements can also lead to severe health problems related to too much calcium entering the bloodstream, a condition called hypercalcemia. Over time, elevated levels of vitamin D in the body can lead to serious diseases. Calcium may leave the blood and build up in soft tissues, for example causing kidney stones. Besides kidneys, damage may also occur in the blood vessels, heart and lungs.

The primary cause of vitamin D toxicity is the overuse of supplements, though this rarely happens. Staying in the sun for too long does not lead to vitamin D toxicity but it may increase the risk of developing skin cancer.

Where to Find More Information: Vitamin D Council