Why Does My Dog Need Vitamins?  Part One – The Fat-Soluble Vitamins

When we talk about nutrition we tend to focus on the protein and fat requirements of our pets (the carbohydrate requirement gets us all a little hot under the collar) and it stands to reason because they are macronutrients.  What this means is that they are required in larger quantities for optimal bodily function.  But we also have micronutrients, and despite them being required in lower quantities, they are still critical to health.

Under the umbrella of micronutrients, we have vitamins and minerals.  We have some helpful blogs on minerals here:

Why Does My Dog Need Minerals – Part One

Why Does My Dog Need Minerals – Part Two

But, as vitamins are just as important, here at My Pet Nutritionist we thought we’d pop together a blog on why our dogs need them too!

What Are Vitamins?

Vitamins are defined by their physical and physiological characteristics.

Vitamins are needed in minute quantities to function as essential enzymes, enzyme precursors or coenzymes in many of the bodies metabolic processes.

Generally, vitamins are not synthesised by the body and must therefore be provided by food (but our canine companions have a trick up their sleeve with vitamin C for example).

Fat vs. Water Soluble Vitamins

Vitamins are split into fat soluble and water-soluble vitamins. In this blog, we will focus on the fat-soluble vitamins, with the water-soluble vitamins to follow.

Fat soluble vitamins can be stored in the body’s lipid deposits, making them more resistant to deficiency but also more likely to result in toxicity.

Fat soluble vitamins require bile salts and fat to form micelles for absorption.  They are then passively absorbed through the lacteals (the lymphatic vessels of the small intestine which absorb digested fats), usually in the duodenum and ileum and transported with chylomicrons to the liver via the lymphatic system.

Synthetic and naturally made vitamins are used in the body in the same way but they do have different availabilities.

The Fat-Soluble Vitamins

Vitamin A

Vitamin A is the term which describes several compounds which biologically, have the activity of the parent compound retinol.  This form of the vitamin and its derivatives are found only in animal tissues, whereas plants contain precursors in the form of the carotenoids. These are the yellow/orange pigments found in carrots and many other vegetables.  The most widely abundant precursor of vitamin A is carotene, and most animals can convert this into the vitamin itself.

The well-known function of vitamin A is its role in vision.  Vitamin A is a precursor of rhodopsin, the photopigment found in rods within the retina of the eye that helps us and our pets to see at night.  One manifestation of vitamin A deficiency is slow, dark adaptation progressing to night blindness.

Vitamin A is also part of the bone formation and bone resorption equation. It influences both osteoblast and osteoclast function. Much data is now suggesting higher vitamin A levels are associated with lower bone density so balancing levels is essential.

Vitamin A plays a role in maintaining healthy endothelial cells which are those lining the body’s interior surfaces.  As we know, in the gut, they play a role controlling the passage of antigens and commensal gut microbiota from the intestine into the bloodstream.

Another key role of Vitamin A is supporting immune function.  It helps make white blood cells which circulate in the body, searching for foreign invaders and cell irregularities.

Sources of Vitamin A:
Carotenoids: sweet potatoes, dark leafy greens, yellow/orange/red fruits/vegetables.
Retinoids: animal meat; liver, fish oil.

Vitamin D

Vitamin D is actually a hormone that promotes calcium absorption.  In human health, you will have heard it referenced as the sunshine vitamin as it is produced in the skin in response to sunlight (UV) exposure.

Unlike herbivores and omnivores, cats and dogs are unable to synthesize Vitamin D adequately in the skin.  There is no seasonal change in concentrations and studies have shown that when fed a diet deficient in Vitamin D, puppies develop rickets which could not be prevented by exposure to UVB light.

Findings Here

This means that cats and dogs are dependent on dietary sources of Vitamin D.

There are two forms of Vitamin D.  Vitamin D2 is also known as ergocalciferol which occurs in plants and Vitamin D3, known as cholecalciferol, is created in the skin during exposure to UV light and occurs in animals.  Cholecalciferol (D3) is of greatest nutritional importance to both cats and dogs and luckily it can be obtained from the consumption of animal products that contain it.

Once vitamin D3 is formed in the skin (of animals who can do so), it binds and is then either stored or transported to the liver.  Both D2 and D3 are biologically inactive and must undergo two more steps to be activated.  This involves both the liver and the kidneys.

Vitamin D plays an indirect role in bone health by managing calcium levels in the body.  It controls absorption of calcium in the intestine and the amount of calcium excreted by the kidneys.  If Vitamin D levels are low, then the intestines struggle to absorb calcium.

Vitamin D is also vital in immune function.  We find vitamin D receptors on a range of immune cells, meaning there needs to be sufficient levels of it in the body for the cells to do their job.

Vitamin D can help modulate inflammatory responses and is also required to produce natural killer cells which are known for killing virally infective cells.

Findings Here

The Importance of Vitamin D for Cats and Dogs

Vitamin E

Vitamin E includes several compounds, of which the most biologically active and widely distributed is alpha tocopherol.

Vitamin E functions as an important antioxidant within cells, protecting lipids, particularly the polyunsaturated fatty acids in cell membranes against oxidative damage caused by free radicals and active forms of oxygen that may be generated during metabolic processes.

The dietary requirement for vitamin E is influenced by the intake of selenium because of its role in glutathione peroxidase, with one nutrient, partially able to spare a deficiency of the other. It is also influenced by the PUFA content of the diet and increasing this leads to an increase in vitamin E requirement. It has been recommended the dietary ratio of alpha tocopherol:PUFA (mg/g) of 0.6:1 is maintained as a minimum to protect against PUFA peroxidation.

Rancid fats are particularly destructive of vitamin E, so these should be avoided in diet.

Vitamin E is absorbed from the small intestine by non-saturable, passive diffusion into the intestinal lacteals and is transported via the lymphatics to general circulation.

Sources of Vitamin E:
Plant based oils
Dark green veggies
Red bell pepper

Deficiency may be implicated in digestive disorders where fat isn’t absorbed sufficiently.  Being a fat-soluble vitamin, it requires fat to be absorbed.  Signs of deficiency include; impaired vision, neuropathy, decreased immune function and loss of control of body movements.

Vitamin K

Vitamin K comprises a group of compounds called the quinones. Vitamin K1 occurs naturally in green leafy plants and vitamin K2 is synthesised by bacteria in the large intestine.  Vitamin K3 is the most common form of synthetic vitamin K.

Vitamin K is required for normal blood clotting and is also involved in the regulation of calcium phosphates in growing bone. The synthesis of vitamin K by intestinal bacteria of dogs contributes significantly to the requirements of vitamin K for the species.

Deficiency is usually due to intestinal malabsorption, as being a fat-soluble vitamin, these vitamins are generally low in cases of IBD or EPI.  However, ingestion of anticoagulants like rat poison also results in low levels along with the destruction of the gut microflora by antibiotic therapy.

Sources of Vitamin K:

Green leafy vegetables
Egg yolks

As you can see, fat-soluble vitamins have a range of important functions in our pet’s body.  We know the next question you will have is whether your dog may have a deficiency, so check out our blog here:

Does My Dog Have a Vitamin Deficiency?

If you would like to discuss any of your dog’s dietary requirements, then check out our services to see if we can help.

Thanks for reading,

MPN Team    

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