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Vitamin D

Updated: September 16, 2013

A substance gets labeled a vitamin (vita = life, when discovered all were thought to be amines) when it is essential for survival but cannot be created by human metabolism.

Vitamin D is actually a group of vitamins (D2 and D3). Vitamin D3 is synthesized in skin by exposure to direct sunlight and obtained in the diet chiefly in fish liver oils and salt water fish. Fish such as salmon, tuna, and mackerel are very good sources of vitamin D. Small amounts of vitamin D are also found in beef liver and egg yolks according to the National Institutes of Health, Office of Dietary Supplements.

Actions of Vitamin D
Vitamin D is involved in numerous human systems, based on widespread distribution of the Vitamin D receptor. Below are some known actions:
Bone: Promotes bone formation by maintaining calcium and phosphorus levels.
Pancreas:  Stimulates insulin production.
Immune system: Stimulates the immune response and anti-tumor action. Decreases risk of autoimmune disorders (e.g. type 1 diabetes, rheumatoid arthritis).
Intestine:  Enhances Calcium and phosphorus absorption.
Kidneys:  Enhances Calcium retention. Suppresses the renin-angiotensin system (having an effect on blood pressure and heart disease).
Parathyroid glands:  Inhibits parathyroid hormone secretion.

What if there is not enough vitamin D?
Low vitamin D levels have been associated with many common and some uncommon chronic illnesses. These include (but not limited to) the risk of fractures in the elderly; hypertension (high blood pressure); numerous cancers; multiple sclerosis; diabetes; schizophrenia and depression. Some foods are fortified with vitamin D and supplemental vitamin D has been associated with real benefit, especially in the elderly relating to fractures.

Can vitamin D prevent diabetes?
This is a facinating subject for which there is some evidence and a lot of speculation. There is a popular UTube dance demonstration by graduate student: Sue Lynn Lau, depicting the role of vitamin D on beta cell function.

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In this dance sequence, the Sun (guy with the headlight) activates vitamin d in the skin (girl in pink dress) who then make insulin producing beta cells active (dancers with ß on the t-shirt). Very cute. To see this on UTube, click here.

The suggestive evidence for a role of vitamin d in diabetes includes the following:

  • Blood sugar control in type 2 diabetes depends on the season, with the lowest A1c (HbA1c) levels during summer when there is increased Sun exposure and increased vitamin D levels.
  • In healthy older adullts with high risk for type 2 diabetes (pre-diabetes), supplimentaiton with calcium and vitamin D slows the expected increases in blood glucose (sugar) with age.
  • One study showed that a combined daily intake of 1200 mg of calcium and 800 IU of vitamin D lowered the risk of type 2 diabetes by 33%.
  • Over 10,000 children in Finland were given 2000 IU of vitamin D3 per day during their first year of life and followed for 31 years, the risk of type 1 diabetes was reduced by approximately 80
    Among children with vitamin D deficiency the risk of type 1 diabetes was increased by approximately 200%.
  • How much vitamin D is needed?
    It has been suggested that those at risk of poor vitamin D status (low dietary intake, little Sun exposure, dark skin) ingest at least 400 to 800 IU vitamin D with at least 1200 mg of elemental calcium daily in the diet or as a supplement. There is no need to take added Vitamin D if the blood level is normal. Fortunately the vitamin D level can be measured with a simple blood test.


    How much vitamin D is too much?
    Vitamin D is fat soluble. It therefore tends to persist in fat tissue and remains in the human system longer than the water soluble vitamins. Too much Vitamin D causes high blood calcium levels. Symptoms of elevated calcium levels include loss of appetite, nausea, and vomiting. Later increased thirst and urination, weakness, nervousness, itching, and eventually kidney failure occurs if the process continues. Kidney stones are also a risk. Vitamin D toxicity usually occurs only if excessive doses (prescription or megavitamin) are taken. In adults, taking 1250 μg (50,000 IU)/day for several months can produce toxicity.
    See more on vitamin D at UPTODATE.com

    Vitamin D in African Americans.
    In the southern states, 53% – 76% of African Americans (depending on sex and age) have low vitamin D levels (25-OH D below 50 nmol/L) compared with 8% – 33% of non-Hispanic whites in the winter. The lower stored form of vitamin D (25[OH]D) is likely related to a reduction of skin production caused by pigmentation. Individuals with darker skin can produce high 25(OH)D levels given sufficient Sun (ultraviolet) exposure but, at most latitudes in North America, even young, healthy blacks do not attain optimal 25(OH)D concentrations at any time of year.

    National surveys (NHANES) indicate a lower vitamin D intake in African Americans. Several authors attribute the low intake to lactose intolerance causing a reduction in consumption of milk, milk products.


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    Vitamin D and African Americans.Susan S. Harris. Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111-1524. American Journal of Nutrition. 0022-3166/06 2006



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