D
(Vitamin D)
DESCRIPTION:
Vitamin D is technically a hormone
as it is formed in one place but carries out its action in
another. The vitamin is made in the skin by the action of
sunlight on cholesterol.
Vitamin D is measured in µg or i.u.
with the conversion factor being: 40i.u. = 1µg
STABILITY
Vitamin D is stable to normal
cooking procedures.
FUNCTIONS
Vitamin D is converted in the body
to an important Calcium - controlling
hormone (1,25-dihydroxycholecalciferol) and all its functions
are related to this hormone’s effects.
The principle action of vitamin D
hormone is to increase the level of Calcium
in the blood by promoting the absorption of Calcium
and Phosphorus from the intestine and
promoting the release of these minerals by bone. To maintain the
body’s physiological ratio of Calcium to
Phosphorus vitamin D hormone also
increases the excretion of Phosphorus -
but not Calcium - from the kidney.
Rickets:
Deficiency of vitamin D during childhood leads to the
development of rickets. Rickets may show itself as early as two
months of age, when "craniotabes" (areas of softening
on the skull) are noted. Production of the first teeth may be
delayed and the posture affected. Rickets also produces
enlargement at the end of long bones, resulting in
characteristic bowing of the legs when the child starts to walk.
Osteomalacia:
In adults, vitamin D deficiency leads to osteomalacia. The
disease is essentially the same as rickets, but the symptoms are
slightly different because the bones are not still developing.
In osteomalacia, there is thinning and weakening of the bone and
spontaneous fractures may occur.
Osteoporosis
and Hip Fracture:
Incidence of Osteoporosis (brittle bones)
and hip fracture may be increased in
postmenopausal women who have an inadequate vitamin D supply.
Upper safe level for daily
supplementation = 10µg
Recommended Daily Allowance = 5µg
There are certain categories of
people who are theoretically much more likely to be at risk of
vitamin D deficiency and who may therefore need to supplement
with this nutrient. These include:
- Vegetarians and especially
vegans (1). (Vitamin D is found mostly in animal and dairy
products).
- Asian women and children who
may not eat many vitamin D containing foods and who choose
to cover up their skin
(2).
- Lactating women whose
breast-milk can be low in vitamin D especially during the
winter (3).
- The housebound elderly with a
tendency to eat poorly (4).
There are some reports of
hypercalcaemia (high blood Calcium
levels) occurring in infants at an intake of 50µg (2000i.u.)
vitamin D a day. In adults, symptoms of vitamin D toxicity have
been reported at daily intakes of 625µg (25000i.u.).
However, there is also some early
evidence that vitamin D at lower levels (around 1000 µg) may
have adverse effects unrelated to hypercalcaemia.
INTERACTIONS
AND CONTRA-INDICATIONS
Heart Drugs:
When digotoxin and certain other cardiac glycoside heart drugs
are taken in combination with vitamin D, there is a slight risk
of abnormal heart rhythm.
Thiazide Diuretics:
It may also be unwise to take substantial amounts of vitamin D
with thiazide diuretics as there have been some reports of high
blood Calcium
when these are taken in combination.
Food (i.u./100g) (µg/100g)
Cod liver oil 8500 212.5
Herring and kipper 896 22.4
Salmon, canned 500 12.5
Milk, evaporated 160 4.0
Eggs 65 1.6
Butter 32 0.8
Liver 32 0.8
Cheese, cheddar 12 0.3
Milk, whole 1.2 0.03
Milk, skimmed 0 0
REFERENCES:
1. Parsons TJ. et al. Reduced bone
mass in Dutch adolescents fed a macrobiotic diet in early life.
J Bone Miner Res,12;9:1486-1494, 1997.
2. "Dietary Reference Values for Food, Energy and Nutrients
for the United Kingdom", Dept. of Health, HMSO, 1991.
3. Specker BL, Nutritional concerns of lactating women consuming
vegetarian diets. Am J Clin Nutr, 59;5 suppl:1182S-1186S, 1994.
4. Russell RM. New views on the RDAs for older adults. J AM Diet
Assoc, 97;5:515-518, 1997.
Related News Stories
Smoking cigarettes can lower vitamin
D and calcium metabolism
Scientists question whether the RDA
for vitamin D is sufficient.
A study into children's intakes of
vitamin D in different UK regions suggests this varies from
region to region.
A study of vitamin D levels in UK
Asian children reveal lower levels than in their white British
counterparts.
The importance of vitamin D and a
suggested increase in the RDA for elderly people.
The effects of calcium supplements
on bone mineral density
The results of trials on
postmenopausal women suggest they may benefit from increased
intake of certain nutrients to protect against bone loss
Studies suggest MS is more in
evidence where Vitamin D levels are low.
It is common for smokers to have a
lower bone mineral density than non-smokers and various
suggestions have been put forward to explain this. However the
association is still not well understood.
Studies have shown that overweight
or obese individuals have reduced plasma concentrations of
vitamin D.
Low bone mineral density in Cystic
Fibrosis patients may be contributed to by vitamin D deficiency,
caused by poor absorption. There is an increased need for
vitamin D supplementation in patients.
Researchers in a recent study
provided strong evidence on why older adults should consume at
least 1300mg calcium daily.
The buffering effect of essential minerals is illustrated.
Multivitamins
and cancer
Studies support the use of a
multivitamin supplement in reducing the risk of colon cancer.
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