Calcium is a vital nutrient for growth and sustained pet
health. It is a principal structural component of bone and teeth, facilitates blood
clotting binding-proteins, serves as a key conductor of nerve signals, initiates
muscle contractions, activates select physiological enzymes and buffers pH
changes.
Because of these critical roles, it is important that the
diet contain sufficient amounts of calcium. However, the calcium content of
common ingredients used in pet diets (outside of incidental bone) is often
inadequate. This makes supplementation necessary. The ingredient most often used
to fill the gap is calcium carbonate.
What is calcium carbonate? It is one of those invisible low-tech
components of our modern world that is used in a broad array of everyday
products with little fanfare. While the name—calcium carbonate—may belie its
origin, it is no “cooked-in-the-lab” synthetic compound. Rather, it is a
readily available, inexpensive, naturally occurring mineral that literally lies
right under our feet.
At the elemental level, calcium carbonate (CaCO3)
is a strangely unique polymorphous molecule that can occur in at least three
different crystalline forms (i.e., calcite forms a trigonal crystal, aragonite
a rhomboid and vaterite a hexagonal crystal). The calcite form is the dominant structure
in nature and found mostly in chalk, limestone or marble. While chalk and
marble have their limited applications, limestone is the form with the greatest
array of uses.
Limestone is a sedimentary rock formed either from direct precipitation
of calcium carbonate from (sea) water or from ancient oceanic organisms (corals,
sponges, foraminifers and algae) that settled to the sea floor. These calcium
carbonate deposits accumulated into layers and were compressed by billions of
pounds of pressure over millions of years until they became rock. Today, limestone
is found in relative abundance throughout the world in stratified rock
formations.
Calcium carbonate from limestone has a number of industrial
applications in paint, paper, ceramics and metals. It is the starting material
for the production of other calcium products such as “lime” (CaO), hyrated lime
(CaOH), calcium chloride (CaCl) and calcium sulfate (CaSO4). It is used to modify soil pH in farming and is
the reason your water may be “hard.”
It is one of those invisible low-tech components of our modern world that is used in a broad array of everyday products.Calcium carbonate has pharmaceutical applications in antacids
(e.g., Tums) and as a phosphate binder for the treatment of conditions such as hyperphosphatemia.
Also, when finely ground, it is the abrasive used in toothpaste and is an oft
used tableting aid for dietary supplements and medicinal pills. So, calcium
carbonate is an ingredient you experience almost every day in one form or
another.
Calcium allowances in dog and cat diets are recommended from
as low as 0.29% for maintenance to as high as 1.2% for growth and reproduction
(dry matter 4,000 kcal diet; NRC, 2006). Further, the ratio of calcium to phosphorus
should be between 1:1 and 2:1 to avoid imbalances and antagonisms. Calcium is
relatively non-toxic in high doses, but imbalances relative to other minerals
can lead to depressed intake and growth in cats and skeletal developmental
issues in dogs when Ca levels exceed >2.3% of the diet (NRC, 2005).
Calcium absorption and maintenance of circulating levels is tightly
controlled by the actions of vitamin D and parathyroid hormone. Utilization of
calcium sources is further influenced by phosphorus, gastric pH, age, calcium
source, phytates, oxalates, fatty acyl “soaps” and imbalances and (or) interference
from other minerals such as iron, aluminum and magnesium (McDowell, 1992).
So, a number of non-ingredient factors can influence the utilization
of dietary calcium. While there is only limited data available regarding supplemental
calcium sources for companion animals, there is ample applicable data in swine,
where bioavailability of calcium from ground limestone and (or) calcium
carbonate is effectively 100% (Soares, 1995).
In the regulatory world, “ground limestone” and “calcite”
are defined as acceptable sources of calcium carbonate with a minimum calcium
composition of 33% and “calcium carbonate” (the ingredient) is defined as more
than 38% calcium (AAFCO, 2010). Since ground limestone is commonly merchandised
for pharmaceuticals, food and feed with more than 98.5% calcium carbonate, the
yield of calcium exceeds the 38%. So, while they may be defined differently with
regard to calcium content (33 vs. 38%), “ground limestone” and “calcium
carbonate” are for all practical purposes the same ingredient.
But, not so quick! They aren’t exclusively calcium and
carbonate. Because ground limestone and (or) calcium carbonate are earthen
compounds, they are not “chemically pure” like synthetic compounds. As such,
they may contain trace amounts of other nutritionally important minerals such
as sodium (~0.06%), magnesium (~0.05%), manganese (~2,779 ppm), iron (336 ppm),
copper (24 ppm) and selenium (0.07 ppm). In addition, it is incumbent upon suppliers
to provide assurance that arsenic, fluoride, lead and other potentially toxic
minerals are below harmful levels.
Depending on the calcium level of the core ingredients in
the formula, the amount of calcium carbonate and (or) ground limestone added to
petfoods generally falls into the range of 0.1 to 1.5%. In the market, it is
available as white to gray in color and found in various particle sizes from
coarse granules to a flour-fine powder. It has good flow characteristics and handling
is straight forward.
While the name may belie its origin, it is no “cooked-in-the-lab” synthetic compound.Calcium carbonate from ground limestone is an easy-to-use,
safe, effective and economical source of calcium fortification for most dog and
cat diets (and is good for their humans, too).