Biomedical Engineering Reference
In-Depth Information
products; in particular of a calcium binding protein. By this
mechanism, vitamin D regulates calcium and phosphate
absorption in the intestine and reabsorption in the kidney
tubules, thus both increasing calcium and phosphate uptake
into circulation and decreasing their excretion via urine.
However, 1,25 dihydroxyvitamin D has other actions as
well, such as regulating the balance between osteoclasts
and osteoblasts in bone, skin cell differentiation, and in
immune cell production and function (
Power et al., 1999
).
There are two forms of vitamin D: vitamin D
2
(ergo-
calciferal) is the plant form of the vitamin and vitamin D
3
(cholecalciferal) is the form produced by animals. Vitamin
D
3
is the preferred form for diets and is required for New
World monkeys, who appear to utilize vitamin D
2
very
poorly (
National Research Council, 2003
). Standard prac-
tice in diet formulation now is to use vitamin D
3
, so the
form of vitamin D in any manufactured diet should not be
a major concern.
Animals exposed to adequate unfiltered sunlight may
produce enough endogenous vitamin D to satisfy require-
ments; however, all primate diets should contain vitamin
D
3
. Circulating levels of 25 hydroxyvitamin D are the best
diagnostic tool for vitamin D status. In humans, circulating
levels greater than 30 ng/ml are considered adequate and
levels below 10 ng/ml are considered deficiency (
Power
et al., 1999
).
It has been suggested that the callitrichid primates
(marmosets and tamarins) have higher vitamin D require-
ments than do other monkeys. This hypothesis was sug-
gested due to several characteristic findings in marmosets
and tamarins. First, circulating 1,25 dihydroxyvitamin D
levels are significantly higher in these species compared
with humans. Also, marmosets and tamarins appear to be
tolerant of very high circulating levels of 25 hydroxy-
vitamin D and of high vitamin D
3
levels in the diet that
would be potentially toxic in other animals. However, the
only data from free-living callitrichid monkeys producing
their vitamin D through sunlight does not fully support this
contention (
Power et al., 1997
). Wild cotton-top tamarins
(Saguinus oedipus) in Colombia exhibited a mean (70 ng/
ml) and range (25.5
e
120 ng/ml) of 25 hydroxyvitamin D
levels that is not particularly different from values from
humans exposed to large amounts of sunlight. Levels of 25
hydroxyvitamin D in captive Callithrix pennicilatta with
full sunlight exposure ranged from 100
e
140 ng/ml, again
suggesting that though circulating levels are high compared
to modern humans they are not extraordinarily so (
Teixeira
et al., 2010
).
The vitamin D receptor in callitrichid primates has been
shown to bind 1,25 dihydroxyvitamin D as efficiently as
does the human vitamin D receptor, implying that any
vitamin D resistance is not due to receptor binding (
Chun
et al., 2001
). However, in callitrichid primates there is
a highly expressed intracellular binding protein that
competes with ligand-complexed vitamin D receptor at
vitamin D response elements, thus reducing 1,25 dihy-
droxyvitamin D signaling (
Chun et al., 2008
). It is not clear
that this translates into a higher vitamin D requirement for
these species, however. Cotton top tamarins have been
successfully maintained on diets with vitamin D at
2500 IU/kg (
Ullrey et al., 1999
). At the same time, in
colonies of cotton top tamarins and common marmosets fed
diets with much higher levels of dietary vitamin D there
have been instances of frank vitamin D deficiency. Dietary
vitamin D appears to result in variable vitamin D status in
captive callitrichid primates, possibly due to individual
health conditions relating to the intestinal tract that can
affect nutrient absorption.
Solely breast fed infants without exposure to natural
sunlight or another source of UV-B radiation are at risk for
vitamin D deficiency. Vitamin D deficiency in a nursing red
howler infant (
Ullrey, 1986
) and three juvenile colobus
monkeys (
Morrisey et al., 1995
) was suggested to have
occurred because the young animals received almost all
their nutrition from mother's milk and had no access to UV-
B radiation. Breast milk is generally deficient in vitamin D.
From an evolutionary perspective this is unsurprising, as
infants would normally produce their own vitamin D
through photosynthesis. Interestingly, vitamin D deficiency
in callitrichid infants is of less concern now that their need
for the animal form of vitamin D is understood. This is
likely due to the fact that callitrichid infants begin feeding
on the adult diet at an early age (by 1 month in the common
marmoset) and are weaned by 2
e
3 months.
Minerals
Minerals fulfill many roles in living organisms. They are
important in structural components (e.g. bone), serve as
cofactors in enzyme and hormone systems, help maintain
osmotic pressure and acid-base balance, affect membrane
permeability, and act in muscle fiber contractions. Minerals
are generally divided into the essential macrominerals
(calcium, phosphorus, magnesium, potassium, sodium,
chloride, and sulfur) whose concentration in diets is usually
measured as a percentage of the diet, and the essential trace
elements (iron, copper, manganese, zinc, iodine, selenium,
and chromium). Trace elements are usually expressed as
parts-per-million (ppm) or mg/kg of diet. Finally, there are
also toxic minerals (e.g.
lead, cadmium) whose intake
should be avoided.
Deficiencies of sodium, chloride, potassium, and sulfur
are unlikely to occur if animals are fed natural ingredient
diets or artificial diets with appropriate mineral mixes.
Strict herbivores fed diets with no manufactured food items
(i.e. all browse and produce) may be at some risk of sodium
deficiency (
National Research Council, 2003
). Magnesium
deficiency is also rare and, like calcium and phosphorus, is
