She’s Not a Reptile

April 22, 2009 at 9:55 pm Leave a comment

Despite her penchant for basking in the sun I’ve discovered that Zip isn’t a reptile after all.


 EurekAlert reports:

Keeping warm isn’t the only reason lizards and other cold-blooded critters bask in the sun. According to a study published in the May/June issue of Physiological and Biochemical Zoology, chameleons alter their sunbathing behavior based on their need for vitamin D.

“It’s a longstanding assumption that thermoregulation is the only reason that lizards bask,” says Kristopher Karsten, a biologist at Texas Christian University who led the study. “Our results suggest that in addition to thermoregulation, vitamin D regulation appears to have a significant impact on basking behavior as well.”

Chameleons, like humans and most other vertebrates, get vitamin D in two ways: They can absorb it from food, and they can produce it in their skin. In order to produce vitamin D, however, the skin must be exposed to UV radiation.

But – unlike most of her vertebrate cousins Mrs. Zippy can’t synthesize vitamin D3 by basking in the sun.  Like all dogs and cats Zip has to get her vitamin D from eating meat, fish, egg yolks or additives in commercial pet food.

Vitamin D isn’t a single chemical, it’s a group of compounds that can be synthesized in most animals’ bodies from a parent compound  that’s structurally similar to cholesterol.  In fact – today many scientists recognize the biologically active form of vitamin D as a type of steroid hormone (a seco-steriod) rather than a vitamin.

Pro-vitamin D3 or 7-dehydrocholesterol is produced in relatively large quantities in the skin of vertebrate animals like humans, sheep, cattle, horses and poultry.  When the skin of these animals is exposed to sunlight, the 7-dehydrocholesterol in their epidermal and dermal cells absorbs ultraviolet B (UVB) radiation and a series of chemical changes resulting in the formation of pre-vitamin D3 are triggered.  Pre-vitamin D3 is thermodynamically unstable and  it spontaneously breaks down into the far more thermodynamically stable vitamin D3 structure.  Vitamin D3 is a prohormone, not nutritionally important on its own. It has to be metabolized into other active forms in the liver and kidneys.  Thirty-seven forms of Vitamin D3 have been isolated and characterized.

Unlike humans and chameleons, the skin of dogs and cats doesn’t produce significant quantities of pre-vitamin D3 when it’s exposed to UVB radiation.  Their skin contains much lower concentrations of 7-dehydrocholesterol than the skins of species that can synthesize vitamin D so they need to fultill their need for vitamin D3 nutritionally, not through thermoisomerisation.

Some researchers have hypothesized that carnivores like dogs and cats evolved without the need to produce their own vitamin D because the fat, liver and blood of their prey fulfilled their needs adequately.  Vitamin D is also present in commercial dog foods, fish, egg yolks and fortified dairy products – so even if they don’t eat raw fat, liver and blood; most modern dogs still get plenty of vitamin D in their diet.  And because the dietary forms of vitamin D are very stable, fat soluble and easily stored in the body, deficiencies typically only develop when an animal either eats an extremely restricted diet or has a metabolic disorder.

Vitamin D was classified as a vitamin in a somewhat ironic turn of history. In 1920 Sir Edward Mellanby conducted research on rickets using dogs.  Mellanby fed the dogs a diet consisting only of oat porridge (the staple food of his Scottish homeland at the time).  The dogs developed rickets while on the oats-only diet and Mellanby was able to cure them by adding cod-liver oil to their diet.  He assumed that the substance that cured the dogs of rickets was a vital nutritional compound present in the cod-liver oil and he referred to it as “vitamin D”.

Scientists later discovered that vitamin D can be consumed in the diet or produced naturally by our skin when it’s exposed to sunlight.  Though Mellanby’s dogs were raised in an indoor laboratory away from sunlight, we now know that this wasn’t a significant factor in his research. Mellanby’s findings were somewhat  ironic because – unlike other vitamins – vitamin D can be synthesized in the bodies of most animals and because the species he selected for his research was one of only a few species that isn’t able to synthesize the vitamin D on its own.

So, unlike a chameleon, Zip doesn’t bask to generate vitamin D.  She’s just an Australian sun goddess.

Entry filed under: dogs, health, science. Tags: .

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