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The roles of calcium and phosphorus

Lee Danks in this latest in a series of columns from Royal Canin focusing on innovations in small animal nutrition, looks at the roles of macronutrients calcium and phosphorus in veterinary diets.

IN terms of food, minerals are
simply the inorganic elements that
we nd listed collectively as “ash”
after removal of all organic matter.

They are naturally occurring
substances
which aren’t
produced by the body
but quite
a number
are used
and even
essential to
everyday function.

They may form the components
of bodily structures and fluids (think
electrolytes) or be involved in the
biochemical reactions which happen
in both normal physiological and
pathological states.

Given their important and often
essential roles, minerals are kept in
close homoeostatic check which quite
often involves significant mineral
interactions. The macronutrients are
a select group definitively required by the animal and therefore need to
be included in the diet within specific
reference ranges, which are established
by the regulatory and research
authorities in small animal feeding, in
the case of dogs and cats.

Two macronutrients which are in
close balance with one another are
calcium and phosphorus.

The mineral element found in the
greatest abundance in mammals is
calcium. It is vital for healthy bones
and teeth (where 90% of this mineral
is found) where along with phosphorus
it brings rigidity and shape to these
structures.

Calcium also plays a fundamental
role in the transfer of information
between cells and in the transmission
of nerve impulses. For example,
it plays essential roles in blood
coagulation and muscular contraction
as well as serving as a second
messenger in a host of intracellular
reactions.

Similar to calcium, a large proportion
of phosphorus (over 80%) is retained in the bones and teeth. Other roles
include its presence in cell membranes
(as phospholipids) and the provision of
energy at a cellular level. Phosphorus is
also a structural component of DNA and RNA, the molecules that carry the
cell’s genetic code.

Homoeostasis

Bodily calcium homoeostasis is a
complex process, involving several
organs. Concentration of free calcium
in the blood is the major initiator of
calcium regulatory mechanisms which
involves the interplay of parathyroid
hormone and calcitonin.

These stimulate a
number of metabolic
cascades which alters the
body’s selective uptake,
absorption, resorption
or excretion of calcium
which happens wherever
it’s present or processed
(namely from skeletal
stores, intestinal
contents and throughout
the renal system).

To add further
complexity, calcium requirements
change during growth, pregnancy and
lactation.

Everything working as it should,
calcium should be in equilibrium with
50% present in the body in its free
or ionised state, 40-45% in a protein-
bound state and 5-10% in a chelated
state, bound to other ions.

Diet, of course, plays a role in
calcium balance by way of affecting
intestinal availability. If de ciencies are
present, the pet becomes susceptible
to skeletal abnormalities, particularly
while in growth or lactation, when
calcium demands are greatest.

Achieving a nutritional balance
between calcium, phosphorus (described
below
) and energy in accordance
with the recommendations made
by regulating authorities (such as
NRC, AAFCO and FEDIAF) is of
key importance. Some companies
go one step beyond and assess the
macronutrient requirements of pets
according to their expected adult
bodyweight. This is one hallmark of a
“tailored nutrition” approach.

Where calcium is in excess in the
bloodstream and phosphorus is equally
high, soft tissue mineralisation is
likely. Any organ where an extensive microvascular network is present is
more at risk of damage should calcium
and phosphorus come together
to form an insoluble molecule.
Hypercalcaemia and the resultant
sequelae can occur in renal failure and
mineralisation can, of course, affect
the kidney.

Signs of hypercalcaemia include
anorexia, lethargy and weakness but
individuals can display remarkable
differences in their symptoms. They are
usually insidious and often not noticed
by owners and, being non-specific, the
electrolyte imbalance can sometimes be
difficult to elucidate.

Some signs may be referable to
the urinary system (e.g. PU/PD,
dehydration, haematuria/pollakiuria/
dysuria associated with urolithiasis), the
gastrointestinal system (e.g. vomiting,
constipation), neuromuscular system
(e.g. seizures, weakness) or cardiac
system (arrhythmias).

The differential diagnoses for
hypercalcaemia are many in number and
beyond the scope of
this article but further
reading is in the
reference list below.

Moving to
phosphorus, regulation
also requires the co-
ordinated efforts of the
kidneys and intestine.
Under conditions of
low dietary phosphorus intake, the intestine increases its
absorptive efficiency and the kidneys
increase phosphorus transport to
minimise urinary losses.

Conversely, under conditions of
dietary excess, the kidneys increase
excretion of minerals. These processes
are very much due to the influence of
the homoeostatic mechanisms which
regulate calcium.

Given their close homoeostatic
relationship, dietary phosphorus
content is usually governed by calcium
levels with a focus on achieving precisely the right Ca:P ratio for
each species, size and life-stage. That
said, ageing pets may demonstrate
sub-clinical renal insufficiency and
consequently be more susceptible to
the effects of excess phosphorus.

Here, ideal ratios can change and
senior diets are often formulated with
phosphorus levels at the lower end of
the reference range. When we get to
the point of CKD diagnosis, avoiding
dietary phosphorus to the best of our
ability will help to slow the progress of
kidney disease.

Of all of the macronutrient groups,
calcium and phosphorus interact
with one another more than most.
It’s imperative to get the nutritional
balance right and consider the
requirements of the individual dog or cat which depends on many a
physiological or pathological process.

  • For further reading go to vet portal.
    royalcanin.co.uk (or vetportal.royalcanin.ie for Ireland).

References and further reading:

  • Elliott, D. (2012) Nutritional considerations
    for optimal puppy growth. Veterinary Focus
    22 (2): 2-10.
    Galvao, J., et.al (2011) Hypercalcaemia:
    diagnosis and treatment options in the dog
    and cat. Veterinary Focus 21 (1): 27-34.
  • Grandjean, D. and Butterwick, R. [eds]
    (2009) Waltham pocket handbook of essential
    nutrition for dogs and cats: Waltham-on-the-
    Wolds, UK.
  • Hand, M., Thatcher, C., Remillard, R. and
    Roudebush, P. [eds] (2000) Small Animal
    Clinical Nutrition
    : Mark Morris Institute,
    Kansas, USA.
  • Kallfelz, F. (2004) Calcium and phosphorus
    requirements of puppies and kittens.
    Veterinary Focus 14 (3): 4-9.
    Martin, L. (2004) Classic pitfalls in puppy
    nutrition. Veterinary Focus 14 (3): 23-27.
    Nutrient Requirements of Dogs and Cats (2006):
    National Academies Press, Washington,
    USA.
  • Wills, J. and Simpson, K. [eds] (1994) The
    Waltham Book of Clinical Nutrition of the Dog
    and Cat
    : Kidlington, UK.