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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 clcium 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.
DEFICIENCY
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.
REQUIREMENTS
Upper safe level for daily
supplementation = 10µg
Recommended Daily Allowance
= 5µg
SUPPLEMENTAL USES
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).
SAFETY
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
The following list is not
exhaustive and anyone taking medication should consult their
general practitioner before using vitamin D supplements.
Adverse reactions for vitamin
D have been associated with certain heart drugs such as Verapamil.
Combined Oestrogens can
elevate blood levels of vitamin D and so vitamin D supplements
should be avoided by people using these drugs.
It has been suggested that
vitamin D supplements can increase the activity of anticoagulants
such as Warfarin. The evidence for this interaction is contradictory
but it would be prudent for people using anticoagulant drugs
to avoid vitamin D supplements as a precaution.
Vitamin D should not be
taken at the same time as thiazide diuretics as there is some
evidence to suggest that this combination might raise blood
levels of Calcium.
FOOD SOURCES
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.
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