Page 112 - Small Animal Clinical Nutrition 5th Edition

Minerals and Vitamins 113

Active calcium absorption is affected by the physiology of the
VetBooks.ir host (i.e., calcium and vitamin D status, age, pregnancy and
lactation).
The other pathways of calcium absorption are facilitated and
passive absorption, which are important in the distal gastroin-
testinal (GI) tract. Passive absorption is a nonsaturable, paracel-
lular route that is independent of vitamin D regulation. The
amount of calcium absorbed in this way depends primarily on
quantity and availability of calcium in the food. No matter
where absorption takes place, vitamin D is the most important
regulator of calcium absorption (Birge and Avioli, 1990). Renal
handling of calcium is also modulated by PTH and calcitonin
but not as much by vitamin D.
Deficiencies and excesses of calcium, as well as calcium-
phosphorus imbalances, should be avoided in dogs and cats
(Box 6-3). A food grossly deficient in calcium, but adequate
in phosphorus can cause secondary hyperparathyroidism. An
all-meat diet devoid of bones, for example, is a very poor
source of calcium. Inadequate calcium intake produces
hypocalcemia, which stimulates release of PTH, which in
turn stimulates production of 1,25-(OH) -D , resulting in a
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higher fractional absorption of calcium and phosphate, and
lower calcium but higher phosphate concentration in urine.
PTH acts with vitamin D to promote bone resorption and
turnover, which may lead to pathologic fractures.
Hypocalcemia is a common problem in diseased states
(chronic or acute renal failure, pancreatitis, eclampsia, etc.),
and parenteral supplementation of calcium and/or calcitriol
(1,25-(OH) -D ) is sometimes warranted (Chew and Figure 6-2. Calcium absorption by the intestine and bone resorp-
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Carothers, 1995). Calcium excess is probably more detrimen- tion and reabsorption in the kidney are closely regulated by calci-
tal in rapidly growing animals than in adults, especially large- um-regulating hormones: parathyroid hormone, calcitonin and 1,25-
and giant-breed puppies (Chapter 33). Table 6-1 describes (OH) -D . See text for details.
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signs of calcium deficiency and excess.
Research suggests that the dietary requirement of calcium for 0.63% [Morris and Earle, 1996] and 0.36 and 0.28%, respec-
growing puppies (especially large breeds) is higher at 1.2% dry tively [Pastoor, 1993]). These investigators concluded that the
matter (DM) (Hazewinkel et al, 1991; Schoenmakers et al, 1% DM calcium recommended by AAFCO (2007) for kittens
1999; Nap et al, 1993) than the previous recommendation of is excessive and that the NRC recommendation (2006) of 0.8%
0.59% (NRC, 1985). The NRC (2006) recommended DM calcium was a more defensible allowance for kittens fed
allowance for adult dogs and growing puppies after weaning is typical moist foods. The current AAFCO (2007) canine and
0.40 and 0.59% DM calcium (both based on foods containing feline recommendations for calcium are 1.0% for growth/repro-
4,000 kcal/kg), and for large- and giant-breed puppies at risk duction and 0.6% for adult maintenance (DM for both values).
for developmental orthopedic disease, the recommendation is For dogs, this calcium requirement is based on an energy den-
0.7 to 1.2% DM calcium (based on foods containing 3,800 sity of 3.5 kcal/g metabolizable energy (ME), whereas an ener-
kcal/kg) (Chapter 33). gy density of 4.0 kcal/g ME is assumed for cats (AAFCO,
A balance study was conducted to determine the calcium 2007). Foods with increased energy densities should have a
requirement of adult cats (Pastoor et al, 1994). Four levels of proportionally increased amount of calcium to account for
calcium (CaCO ) ranging from 0.27 to 1.62% DM were eval- decreased food consumption.
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uated. The minimum level evaluated (0.27% calcium) resulted These meat meals are rich sources of calcium because of their
in positive mineral balance with no adverse effect on serum bone content: poultry by-product meal, lamb meal and fish
phosphorus, calcium, magnesium and alkaline phosphatase meal. Grains (corn, rice, etc.) are generally poor sources of cal-
concentrations. This level is less than half that of current cium. Soybean meal and flaxseed have calcium contents
AAFCO (2007) recommendations (i.e., 0.6% calcium). between those of meat meals and grains. Meats without bone
Likewise, two groups of investigators conducting studies in kit- are poor sources of calcium.The most common calcium supple-
tens demonstrated lower calcium and phosphorus requirements ments used in pet foods are limestone (calcium carbonate), cal-
than those currently recommended by AAFCO for growth cium sulfate, calcium chloride, calcium phosphate and bone
(i.e., requirements for calcium and phosphorus were 0.5 and meal, ranging in calcium from 16 to 39% (Table 6-2).