D for dogs, C for yourself:
The debate is raging again, stirred up by new findings related to the function and health benefits of Vitamin D in human metabolism and physiology.
One writer suggests that baldness might be an evolutionary adaptation to compensate for lower Vitamin D conversion or utilization in older men. Is there no evolutionary advantage to older women’s survival?
The team doctor for the Chicago Blackhawks hockey team had the players’ blood levels tested and ordered supplementation for those who were deficient. In fact, all were deficient and the minimum supplement prescribed was 5000 IU daily. The Blackhawks won the Stanley Cup the following year. (NB that IU is much less than a mg.)
One major US laboratory was claimed to use an inaccurate (low) reference or standard blood level for their Vitamin D tests, leading to many deficient patients not receiving treatment.
Who is winning the argument about safe Vitamin D supplementation doses? Sams Club in the USA now sells a bottle of 400 10,000 IU capsules for $12. Used to be that the FDA restricted the max capsule to 400 IU because of concern about toxicity.
Moderate exposure to sunlight on bare skin can generate 100 000 IU or more in an hour, so what about toxicity? Is there a mechanism that prevents the Vit D from sunlight and possibly from oral supplementation from being converted to the active and potentially toxic form?
The skin secretions are activated by sunlight, but that may take a period of several hours or days before enough can be reabsorbed back into the skin. People who wash too often may be shorting themselves Vit D from this source.
Cod liver oil was a common dietary supplement for humans and dogs, with the warning that excess can be toxic. Now appears it is more likely that Vitamin A, not Vitamin D, is the more toxic component in cod liver oil.
If the questions are ever resolved for humans it will not be conclusive or compelling evidence for dogs.
Dogs’ metabolism is different in kind and in quantity from humans. For example, it is conventional wisdom that dogs can synthesize Vitamin C and do not require any dietary source of this nutrient. Some believe that supplementary Vit C in food can be helpful in particular circumstances or conditions: the familiar necessary and sufficient is not optimum theme.
Dogs are covered with hair… are they more reliant on skin lipids conversion to Vit D precursors or does their Vit D come mostly from dietary sources?
Does dog hair block or allow passage of the ultraviolet rays needed for Vit D conversion in/on the skin?
Another possibility, do dogs acquire Vit D by grooming, licking and absorbing the lipids excreted from the skin on their hair which has been converted there to Vit D precursors?
Waiting for the answers, I started to give several dogs with skin and coat conditions a 5000 unit capsule twice a week. Vit D is stored, unlike the water soluble vitamins, so maybe I could save money by giving each dog a 10,000 unit cap each week, since the 400 capsule bottles of 5K units cost almost as much as the 10K units bottles. But the 10K unit bottles are a new product so I still have a bottle of the 5K dose on hand.
Sams Club and Walmart health supplement product shelves are a reflection of what is mainstream in nutrition science.