In an age of extremes and absolutes, vitamin A serves as a
potent example of the necessity of balance in diet and
nutrition. This is an important vitamin that has a direct
effect on vision, the endocrine system in many ways and gene
expression modulation. A deficiency can lead to blindness, skin
lesions, reproductive issues and pneumonia, while an excess may
result in anorexia, diarrhea, bone deformities and cancer. So,
a more-is-better approach is as detrimental as having none at
all.
In other words, achieving just the right amount is all
important, especially considering that vitamin A is derived
solely from the diet. That puts the full responsibility for
getting it right squarely on our shoulders.
Many standard animal- and fish-based petfood ingredients
like beef liver, chicken meal and fish oil contain measurable
amounts of vitamin A. However, the level in the total mix of
ingredients in a petfood may not be fully adequate to support
the animals' needs. In addition, processing can destroy vitamin
A and more may be depleted during storage. So, supplementation
is typically required. To fill the gap, almost every commercial
dog and cat food is supplemented with ingredients such as
"Vitamin A Supplement," "Vitamin A Acetate" or some other form
of vitamin A.
While there is plenty of information in standard nutrition
texts regarding the biochemistry of this nutrient and plenty of
details regarding food sources of vitamin A, there is very
little information about the commercially produced supplemental
vitamin A compounds used to fortify petfood products.
Considering how common these supplemental ingredients are,
maybe it's time to do some digging to better understand and use
them.
Vitamin A was one of the first essential micro-nutrients to
be officially recognized nearly a century ago, in 1915 by Elmer
McCollum. It was originally isolated from fish liver oils and
called "fat soluble A," thus the name. Today, vitamin A
describes a class of fat soluble compounds with retinol
bio-activity. These "retinoids" include pre-formed vitamin A
(retinyl esters and retinol-found in organ meats like liver,
eggs and dairy) and the pro-vitamin A carotenoids (β-carotene,
α-carotene and β-cryptozanthin-found in green, yellow and
orange fruits and vegetables).
Just for clarity, plants do not produce vitamin A. Dogs, but
not cats, can convert β-carotene into vitamin A by cleaving
this molecule in half at the intestinal mucosa through the
enzymatic activity of 15,15′-dioxygenase.
Vitamin A is produced commercially through exothermic
carbon-carbon synthetic processes involving metal catalysts.
The starting materials are basic carbon compounds such as
acetone or formaldehyde, which are formed into 5-carbon
isoprene subunits. From two of these subunits, a ring structure
(β-ionone) is formed, and then a third isoprene subunit is
added to elongate the molecule.
Just for clarity, plants do not produce vitamin A.This 15-carbon vinyl-β-ionol is then combined with the last
isoprene subunit to form the 20-carbon trans-retinol molecule.
This last isoprene subunit is commonly esterified to a
palmitate or acetate (e.g., retinyl acetate) to help protect it
from destruction via oxidation. The resulting product is a
resinous oil identical to that occurring in nature.
Commercial products are commonly processed further to
improve their handling and storage qualities. As an example,
because vitamin A is extremely sensitive to oxidation, most
commercial vitamin A supplements used in petfoods are
encapsulated. Various approaches are used, but the general idea
is to atomize the retinyl acetate in combination with powdered
gelatin, starch, sugars or alginates. These may then be heated
to form maillard reaction products or other suitable cements
that lock the retinyl acetate in a fixed matrix.
If we looked at a cross-section of one of these "beadlets"
under magnification, the vitamin A would look like berries
trapped in a muffin. The net result is a physical barrier for
the vitamin A to protect it from oxidation. In addition, large
quantities of antioxidants are included to serve as further
protection. The final products are commonly sold in strengths
of 500,000 to 1 million international units (IU) per gram
through vitamin premixes.
In its last publication on dog and cat nutrient requirements
(2006), the
National Research Council
changed the units used to express vitamin A requirements
from IU to retinol equivalents. This update factors for the
bioavailability of vitamin A from various sources. For example,
1 mg of retinol is equal to 3,333 IU of vitamin A, whereas (in
the dog specifically) 1 mg of β-carotene is equal to 833 IU of
vitamin A.
The challenge is that the 2008 guidelines from the European
Pet Food Industry (known as
FEDIAF
), the 2010 nutrient profiles from the Association of American
Feed Control Officials, most vitamin and ingredient supplier
specifications and the older ingredient composition tables all
still use IU. Reconciling one with the other simply requires
some math.
The exception is new reference tables such as the
US Department of Agriculture
nutrient database for standard reference (NDB SR22),
which provides retinol, retinol equivalents and a computed
vitamin A value. Caution should be exercised to use only the
reported retinol concentrations because the vitamin A values
have been computed using equations applicable to humans and
would lead to gross overestimations of vitamin A content if
applied to dogs and cats.
In addition to converting on potency, it is common to
overformulate vitamin A to compensate for the losses during
food production and storage. No food process is immune. For
example, something as simple as boiling vegetables can destroy
67% of the retinol.
For the synthetic vitamins, it has been reported that
storage in vitamin premixes can drop vitamin A levels from 3.9%
to 20% per month. Extrusion processing can account for a 40% to
60% loss of vitamin A (at 141-145˚C depending on preparation)
and drying can destroy another 25% to 40% (at 201-205˚C
depending on preparation). Plus, vitamin A can be lost during
finished product storage in the warehouse or on the store shelf
at a rate of 8% to 30% per month (BASF, 2000).
Processing can destroy vitamin A and more may be depleted during storage.In canned foods, loss of vitamin A is not commonly an issue
because these diets usually contain large quantities of liver,
which may push levels near the recognized upper limits.
To achieve the necessary vitamin A fortification in complete
pet diets, supplementation with synthetic vitamin A is almost
mandatory. These commercial forms are identical to natural
vitamin A, perform in the same manner and are subject to the
same process and environmental losses. The real trick to
effective vitamin A fortification is getting all the inputs on
a consistent unit of potency, factoring for all the processing
and storage losses and then supplementing with the most
protected form of vitamin A available to assure enough, but not
too much, is in the diet the day it is eaten.