The fat-soluble vitamin D’s most important role is maintaining blood levels of calcium, which it accomplishes by increasing
absorption of calcium from food and reducing urinary calcium loss. Both effects keep calcium
in the body and therefore spare the calcium that is stored in bones. When necessary, vitamin D
transfers calcium from the bone into the bloodstream, which does not benefit bones. Although
the overall effect of vitamin D on the bones is complicated, some vitamin D is necessary for
healthy bones and teeth.
plays a role in immunity and
blood cell formation and also helps cells "differentiate"—a process that may reduce the
risk of cancer. From animal and human studies,
researchers have hypothesized that vitamin D may protect people from multiple sclerosis,1 autoimmune arthritis,
and juvenile diabetes.2
is also needed for adequate blood levels of insulin.3 receptors have been
found in the pancreas where insulin is made, and preliminary evidence suggests that
supplementation may increase insulin secretion for some people with adult-onset (type 2) diabetes.4
Where is Vitamin D found?
Cod liver oil is an excellent dietary
source of vitamin D, as are vitamin D-fortified foods. Traces of vitamin D are found in egg yolks and butter. However, the majority of vitamin D in the body
is created during a chemical reaction that starts with sunlight exposure to the skin.
Cholecalciferol (vitamin D3) is the animal form of this vitamin.
Who is likely to be deficient of Vitamin D?
In adults, vitamin D deficiency may result in a softening of the bones known as osteomalacia. This condition is treated with vitamin
D, sometimes in combination with calcium
supplements. Osteomalacia should be diagnosed, and its treatment monitored, by a doctor. In
people of any age, vitamin D deficiency causes abnormal bone formation. It occurs more
commonly following winter, owing to restricted sunlight exposure during that season. Living in
an area with a lot of atmospheric pollution, which can block the sun's ultraviolet rays, also
appears to increase the risk of vitamin D deficiency.5
deficiency is more common in strict
vegetarians (who avoid vitamin D-fortified
dairy foods), dark-skinned people,6 alcoholics, and people with liver or kidney
disease. People with liver and kidney disease can make vitamin D but cannot activate it.
deficiency is more common in people suffering from intestinal malabsorption, which may have occurred following
previous intestinal surgeries, or from celiac
disease.7 People with insufficient pancreatic function (e.g., those with
pancreatitis or cystic fibrosis) tend to be
deficient in vitamin D. deficiency is also common in individuals with
hyperthyroidism (Graves' disease), particularly women.8
In children, vitamin D deficiency is called
rickets and causes a bowing of bones not seen in adults with vitamin D deficiency. Vitamin
D deficiency is common among people with hyperparathyroidism, a condition in which the
parathyroid gland is overactive. In a study of 124 people with mild hyperparathyroidism,
vitamin D levels were below normal in 7% of them and suboptimal in 53% of them.9
deficiency is also common in men with advanced prostate cancer. In one study, 44% of 16 men with
advanced prostate cancer had decreased blood levels of vitamin D.10
One in seven adults has been reported to be deficient in vitamin D.11 In one
study, 42% of hospitalized patients under age 65 were reported to be vitamin D
deficient.12 In this same study, 37% of the people were found to be deficient in
vitamin D, despite the fact they were eating the currently recommended amount of this
nutrient. deficiency is particularly common among the elderly. Age-related decline
in vitamin D status may be due to reduced absorption, transport, or liver metabolism of
vitamin D.13
How much Vitamin D is usually taken?
People who get plenty of sun exposure do not require supplemental vitamin D, since sunlight
increases vitamin D synthesis when it strikes bare skin. Although the recommended dietary
allowance for vitamin D is 200 IU per day for adults, there is some evidence that elderly
people need 800 to 1,000 IU per day for maximum effects on preserving bone density and
preventing fractures.14151617 Sun-deprived
people should take no less than 600 IU per day and ideally around 1,000 IU per
day.18
Are there any side effects or interactions with Vitamin D?
People with hyperparathyroidism should not take vitamin D without consulting a physician.
People with sarcoidosis should not supplement with vitamin D, unless a doctor has determined
that their calcium levels are not elevated.
Too much vitamin D taken for long periods of time may lead to headaches, weight loss, and kidney stones. Rarely, excessive vitamin D may even
lead to deafness, blindness, increased thirst, increased urination, diarrhea, irritability, children’s failure to
gain weight, or death.
Most people take 400 IU per day, a safe amount for adults. Some researchers believe that
amounts up to 10,000 IU per day are safe for the average healthy adult, although adverse
effects may occur even at lower levels among people with hypersensitivity to vitamin D (e.g.
hyperparathyroidism).19 In fact, of all published cases of vitamin D toxicity for
which a vitamin D amount is known, only one occurred at a level of intake under 40,000 IU per
day.20 Nevertheless, people wishing to take more than 1,000 IU per day for long
periods of time should consult a physician. People should remember the total daily intake of
vitamin D includes vitamin D from fortified
milk and other fortified foods, cod liver
oil, supplements that contain vitamin D, and sunlight. People who receive adequate
sunlight exposure do not need as much vitamin D in their diet as do people who receive minimal
sunlight exposure.
increases both calcium and
phosphorus absorption and has also been reported to increase absorption of aluminum. Increased
blood levels of calcium (which may be a marker for vitamin D status) have been linked to heart disease.21 Some,22
but not all,23 research suggests that vitamin D may slightly raise blood levels of
cholesterol in humans.
Are there any drug
interactions?
Certain medicines may interact with vitamin D. Refer to drug interactions for a list of those medicines.
The information presented in Healthnotes is for informational
purposes only. It is based on scientific studies (human, animal, or in
vitro), clinical experience, or traditional usage as cited in each article.
The results reported may not necessarily occur in all individuals. For many of
the conditions discussed, treatment with prescription or over the counter
medication is also available. Consult your doctor, practitioner, and/or
pharmacist for any health problem and before using any supplements or before
making any changes in prescribed medications.
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