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In March 1998, the New England Journal of Medicine
published the surprising results of a study by researchers from
Harvard, which found a prevalence of vitamin D deficiency not
only among elderly people with low vitamin D intake, but in patients
less than 65 years of age without any apparent risk factors for
hypovitaminosis D. Hypovitaminosis D was found in 42% of 1546
African American women who participated in the study. Even among
243 subjects who consumed 200 IU daily from supplements, the prevalence
of hypovitaminosis D was 28%. It has been estimated that as much
as 40% of the U.S. population may be deficient in vitamin D, and
this deficiency is prevalent in much of the world, in the young
and old. It appears that vitamin D deficiency is so common because
of both increasingly limited exposure to sunlight and inadequate
dietary intake of vitamin D.
The primary source of vitamin D3 is sunlight, which converts 7-dehydrocholesterol
in the skin to vitamin D3 via photochemical reactions using ultraviolet
B radiation (UV-B). Numerous factors such as latitude, season,
air pollution, pigmentation of skin, and sunscreen affect UV-ray
exposure. The US is between 30° and 45° latitude, and
six months or more of each year has insufficient UV-B sunlight
to produce optimal D levels. People with dark skin produce less
vitamin D since melanin, a skin pigment, acts as a natural sunscreen,
absorbing UV radiation. Only a few foods naturally contain vitamin
D: fatty fish, fish liver oils and eggs from hens that have been
fed vitamin D.
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Fortified foods, such as milk dietary vitamin D intake.
The term vitamin D refers to vitamin D2, vitamin D3 as well as
to their metabolites and analogues. Vitamin D2 (ergocalciferol)
is derived from fungal and plant sources; vitamin D3 (cholecalciferol)
is derived from animal sources. Super D3 is derived from lanolin,
from sheep's wool. Vitamin D usually acts as a hormone precursor,
requiring two stages of metabolism before reaching hormonal form.
Following absorption or endogenous synthesis, it is metabolized
to 25- hydroxycholecalciferol (25(OH)D) in the hepatocytes. 25(OH)D
is the major circulating form of vitamin D, but is not biologically
active. Then, the biologically active hormone form, 1 alpha, 25-dihydroxycholecalciferol
(1,25(OH)2D) is produced in the kidneys. Vitamin D receptors exist
in a wide variety of tissues and organs.
The principal direct toxic effects of too much vitamin D are excessive
absorption of calcium and phosphorus from the intestine and resorption
of calcium and phosphorus from bone. The prolonged ingestion of
excessive amounts of vitamin D and accompanying hypercalcemia
can result in metastatic calcification of soft tissues, including
the kidney, blood vessels, heart and lungs. There is a wide variation
among individuals in their tolerance to toxicity. In general,
chronic ingestion of 50,000 IU to 100,000 IU daily of vitamin
D is required to produce hypercalcemia.
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The Reference Daily Intake (RDI) for vitamin D is 400 IU. The
U.S. National Academy of Sciences has recommended the following
adequate intakes for vitamin D: 200 IU for ages 0-50, 400 IU for
ages 51-69, and 600 IU for ages greater than 70. They have also
defined a tolerable upper limit intake level (UL) that is unlikely
to pose risk to the general population: infants 0-12 months, 1000
IU/day; children and adults, including pregnant and lactating
women, 2000 IU/day. However, some researchers argue that the RDIs
and ULs are too limited. Testing of plasma 25(OH)D concentration
is considered the best way to determine the level of vitamin D
and to monitor vitamin D supplementation
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Vitamin D is the principal regulator of calcium homeostasis in
the body, and it is essential for skeletal development and for
bone mineralization.* A number of studies have reported significant
benefits from supplemental vitamin D in supporting bone health.*
Along with the classic role it plays in regulation of bone mineralization
and serum calcium levels, vitamin D has been increasingly recognized
to play important roles in the regulation of blood sugar within
normal levels, and supporting normal muscle strength, the health
of connective tissue, and the gastrointestinal, immune, circulatory,
and respiratory systems.*
Allergy Research Group®
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