Vitamin A covers a wide variety of compounds that can exhibit the activity of retinol. For most animals there are two main forms of vitamin A that we find in food: pre-formed vitamin A and provitamin A. A provitamin is a compound that can be converted to a biologically active vitamin.
Preformed vitamin A is in the form of retinyl esters which are transformed into retinol by enzymes from the pancreas or the intestines at the mucosal brush border. Retinol is one of the active forms of vitamin A.
Carotenoids are some of the provitamins of vitamin A. In nature there are over 600 different carotenoids that we are aware of and about ten percent of these are considered provitamin A. Some of the most common ones include α-carotene, β-carotene, and β-cryptoxanthine.
Cats and Carotenoids Cats can absorb carotenoids like beta-carotene and utilize their antioxidant effects but they cannot transform them into biologically active vitamin A. Therefore they must ingest active vitamin A. The best source of this is animal tissues. Liver is a high very high in vitamin A.
Absorbing Vitamin A As both retinyl esters and carotenoids are fat soluble (hydrophobic) and thus require bile salts to aid in their absorption. In general vitamin A esters are well absorbed at 80 - 90%1. These are most often found in animal tissues.
Carotenoids however, the provitamins found in plant tissues have a variable level of absorption. In general, they are absorbed only half as much as the retinyl esters. However the higher concentration of carotenoids in the food allows a higher percentage to be absorbed. These are also more dependent upon bile acids for absorption.
Vitamin A has a wide variety of functions in the body:
Within the eye, the form of vitamin A that is used is the retinal form. Within the rods and cones (cell types that recognize light), 11-cis-retinal is bound to proteins (rhodopsin in rods and iodopsin in cones). When light enters the eye, this retinal changes form (specifically from 11-cis-retinal to 11-trans-retinal). This change sets of a series of steps which leads to nerve stimulation within the visual cortex of the brain and allows this light to be "seen." 11-cis-retinal is also used to help maintain rhodopsin. Rods are used to see black and white and for night vision. Thus without vitamin A, one of the symptoms that is unique to vitamin A deficiency is night blindness
Reproduction: vitamin A is necessary for the production of sperm.
Skin, growth and immune system: needed for the transcription and regulation of many genes.
Where do we find Vitamin A?
Vitamin A Sources
Liver has the highest source of vitamin A. This includes not only pork, beef, and chicken liver but that of fish as well. Cod liver oil is an excellent source of vitamin A
The incredible, edible egg
Dairy products such as milk and butter
Beta Carotene Sources
Plants such as carrots, corn, broccoli, kale, spinach, pumpkin, peas and collard greens
The difficulty with these sources The levels of vitamin A in both animal tissues as well as plants can be highly variable. For plants, soil conditions, location, time and method of harvesting, processing, etc can highly affect the levels of carotenes in the plants. The concentration in animal tissues will of course depend upon the amount of vitamin A that the animal has ingested. So just because a food has the right ingredients does not mean that it has the right nutrients. Furthermore, when we are vitamin A deficient, the body has a more difficult time
Deficiency and Toxicity of Vitamin A
As per this site, we will place both NRC and AAFCO levels.
What is Bioavailability? Just because a nutrient is present in a food, does not mean that the nutrient can be properly absorbed by the pet. Bioavailability refers to the ability of the nutrient to be actually absorbed by the pet. For example, many grains have rich stores of carbohydrates, protein, omega fatty acids and minerals. For most of these grains, if they are not properly cooked or ground, these nutrients are not available to the animal eating it.
NRC NRC is the National Research Council. They have published the Nutrient Requirements of Dogs and Cats. The values they provide are based upon highly digestible nutrients in a 4,000 ME kcal diet with exceptional digestibility. The look at the best case scenario with food and assume excellent bioavailability of the nutrients. Since many of the foods have been formulated rather than tested and many are based off of new theories, these levels might not apply to many of the diets over the counter. For over the counter diets, I would look to AAFCO. As this site contains both academic as well as practical information, it is important to keep the NRC levels. There are many diets for which the NRC is highly applicable and practical.
AAFCO AAFCO is the Association of America Feed Control Officials. AAFCO assumes less than ideal circumstances and bioavailability with diets. As most commercial pet foods are formulated to AAFCO standards as opposed to undergoing feeding trials for AAFCO (average cost of a feeding trial is over $30,000.00 dollars per food), they want to ensure that in the worst case scenario, the pet is receiving proper nutrition.
It is for the above reasons, that commercial dog and cat foods must abide by AAFCO standards and their reference of NRC is inappropriate.
Note: these numbers are placed here for the reasons of interest only. It is not the position of this website or myself that any pet owner attempt to supplement their pets or to make their own foods without the specific instructions from their veterinarian. Improper supplementation of any nutrients such as vitamins can lead to serious health effects and harm to a pet. Minimums Dogs
NRC: 1,000 RE or 3,333 IU/kg DMB for growth and maintenance. 2,000 RE or 6,667 IU/kg DMB for gestation and lactation
AAFCO: 5,000 IU/kg DMB for maintenance. 9,000 IU/kg DMB for growth and reproduction
NRC: 15,000 RE or 50,000 IU/kg for growing puppies or for lactating and gestating dogs. 64,000 RE or 213,333 IU/kg for adult dogs
AAFCO: 250,000 IU/kg DM
NRC: 80,000 RE or 266,667 IU/kg DMB for growing kittens. 100,00 RE or 333,333 IU/kg DMB for maintenance.
AAFCO: 750,000 IU/kg DMB
Note: because NRC is focused on highly digestible food sources with high bioavailability the numbers will be lower than those of AAFCO.
Source: Small Animal Clinical Nutrition Fifth Edition pg 127
What is Vitamin D?
The main dietary forms of Vitamin D are:
D2 - Ergocalciferol
D3 - Cholecalciferol
Calcitriol Neither however are biologically active. The active form is called Calcitriol in a pathway that involves, first the liver, and then the kidneys. In cats, ergocalcitrol is less efficiently transformed to calcitriol. As carnivores, cats have less need of being able to utilize the most common plant form of vitamin D.
In many animals, vitamin D can be synthesized in the skin with the use of ultraviolet light. This pathway does not work well in dogs or cats. For this reason, they both must ingest vitamin D.
What does Vitamin D do?
The main function of Vitamin D is the balance of calcium and phosphorus. It affects both the intestinal absorption of calcium and phosphorus as well as the balance of these ions within the bone. The interaction with calcium and phosphorus also involves calcitonin which is made by the thyroid gland and parathyroid hormone or PTH (which, if you can tell by the name, is made by the parathyroid glands). The parathyroid glands are small glands that sit next to the thyroid glands in the neck region. Vitamin D also has functions within the immune system and heart. In humans, there is a information with its prevention of certain types of cancer as well.
The Potential Link Between Vitamin D Excess and Feline Stomatitis
There has been some concern that high dietary levels of vitamin D may be related to a serious disease among cats called Feline Odontoclastic Resorptive Lesions (FORL). FORL is an extremely painful dental disease in cats. It is associated with bone loss (resorptive) lesions on the teeth at the gum line. The damage is permanent and often results in the loss (often surgically necessitated) of some or all of the teeth. There are two types of FORL disease. The first is associated with severe inflammation either from stomatitis/gingivitis or dental disease. Certain cells called odontoclasts (these cells typically only have one function in a mammal's life - to destroy and weaken the baby teeth so that they fall out so that the adult teeth may emerge) become activated from the inflammation and begin to resorb (eat away) the teeth again. The second type of FORL results for causes unknown but may have a link to Vitamin D. A theory with Type 2 FORL disease The theory is that excessive Vitamin D may interfere with normal bone repair mechanisms. Cementum is normally secreted by tooth cells (cementoblasts) to fill microscopic defects that can occur. Typically these never lead to problems due to the filling of cementum but it is theorized that excess vitamin D may interfere with this.
Where is the data? In a study by Dr. A.M. Reiter from the Veterinary Teaching Hospital of Pennsylvania, there was shown in a group of cats with FORL to have higher serum Vitamin D levels than normal cats. While this study does show the need for further study with regard to Vitamin D as a possible risk factor or even cause for FORL. Other studies support a potential link but unfortunately there are no conclusive studies that high levels of vitamin D definitively cause or predispose cats to FORL.
What is a safe level of vitamin D for cats? First of all, it should be restated that it is not known if vitamin D causes FORL. With that said, some have recommended that diets do not exceed 1,500 IU's/kg DMB (dry matter basis). Recent Food Recalls There have been a couple of companies who have had recalls due to excess vitamin D. The levels were higher than the AAFCO allowances and thus far higher than the level stated above and into dangerous levels. One of the companies had over eleven separate recalls in a years time due to excessive vitamin D.
Where do we find Vitamin D?
Vitamin D Sources
Marine Fish and Fish Oils have the highest levels
Fresh water fish
While it is good to have a source of vitamin D, it is also important not to have too much. Having large amounts of vitamin D rich food sources can potentially lead to excess levels if not careful.
Deficiency and Toxicity of Vitamin D
What is an IU? An IU is an International Unit. 1 IU of vitamin D is equivalent to 0.025 micrograms of cholecalciferol (D3) Minimums Dog
NRC: 552 IU/kg DMB
AAFCO: 500 IU/kg DMB
NRC: 250 IU/kg DMB for growth; 280 IU/kg DMB for maintenance and reproduction.
AAFCO: 750 IU/kg DMB for growth and reproduction; 500 IU/kg DMB for maintenance
NRC: 3,200 IU/kg
AAFCO: 5,000 IU/kg
NRC: 10,000 IU/kg
AAFCO: 30,000 IU/kg
Note: for the concerns with potential FORL in cats (see above), some specialists recommend to have levels below 1,500 IU/kg DMB for cats.
What is Vitamin E?
As with many vitamins, "Vitamin E" is not a single chemical but rather a group of compounds. The most biologically potent form of Vitamin E is α-tocopherol. Thus any compound with a biologic effect of α-tocopherol is considered a Vitamin E. The two main forms found in nature are tocopherols and tocotrienols. Tocopherols generally have better absorption ability than tocotrienols. In nature there are four main forms of each and are designated α-, β-, γ-, δ- (alpha, beta, gamma, delta).
Tocopherols and Pet Food Mixed tocopherols (α-, β-, γ-, δ- and their variations) have not only an antioxidant for the body but also for pet food. Mixed tocopherols are often utilized to prevent fat rancidity of pet foods. While α-tocopherol is the most active in the body ,γ-tocopherol is the most active in food.
Do not pour dog or cat food into a plastic bin It is important to note, that all fat will eventually turn rancid; that is, fat will break down into smaller reactive molecules. These breakdown products lead not only to an unpleasant "spoiled" smell but also are not healthy to be ingested. This breakdown process can occur via several mechanisms such as oxidation and by bacteria. (hydrolysis is the third basic method). The reactive breakdown products have another effect, they further the speed of rancidity via oxidation.
When you pour pet food into a bin, the bin gets covered with food particles which contain fat. This fat over time will turn rancid and leave reactive compounds. Thus as the food runs out, the rancid particles in the dust then come into contact with the new food that is poured in. This increased the breakdown and rancidity of the new food. As time goes by, there can be increased levels of rancid particles lining or sometimes even imbedded into the plastic. It can be difficult to get rid of them even when diligently washed.
Thus it is recommended that keep the pet food in the disposable bag that it comes in. Many people will place the bag into the plastic can or bin which is fine. Either way, when the food runs out the bag and associated dust particles is discarded and does not come in direct contact with the new food.
What does Vitamin E do?
Vitamin E is one of the most important antioxidants of the body. In fact it is the first line of defense against oxidation of the compounds that make up cell membranes (the phospholipids). Why do we need antioxidants? We need antioxidants because of free radicals which, technically defined, are oxygen based compounds with an unpaired electron. Electrons like to be in pairs and their pairing is what can define a bond between two atoms to make molecules. When a compound has this unpaired electron, it will react to whatever molecule is closest that it can "pair" its electron to and create a bond. When a reactive compound attaches onto another molecule, they form a new molecule. As this is a random union and not controlled or planned by the body, these new molecules lose their original function. This is what is called oxidative damage and can lead to cell death.
An antioxidant stops the destructive oxidative process of free radicals. The body faces over ten thousand free radicals every day, with over ninety percent of them being generated by the body. Many are made by the mitochondria as it makes energy for the body within the cells. As long we are alive, we will continue to make and come in contact with free radicals so having a steady supply of antioxidants like Vitamin E help keep the body safe.
So how does Vitamin E stop the oxidative damage of free radicals? Glutathione is the major antioxidant that is made