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Why do animals seem to need more vitamin D than humans

From Merck Veterinary Manual 1998

At 400 IU/day the FDA appears to believe that humans require less than the majority of animals in the Merck Vet Manual, and far less than the non-human primates (2,500 IU/kg).

A great many animals have their vitamin D requirements specified in terms of IU per kilogram of dry feed.
Often there is a reduced requirement if the animal has access to sunshine.

I wonder if vets in the northern latitudes or cloudy areas increase the vitamin D in the feed.
I also wonder if the vitamin D levels in the feed are increased in the winter months.

Psittacines (large seed eating – macaw, parrot, cockatoo) 2000 IU/ kg of dry feed pg 1850
Ratites (emu, cassowary, ostrich, rhea) 1,500-2,500 IU/kg of dry feed pg 1851
Non-human primates 2,500 IU/kg of dry feed pg 1858; marmosets require up to 4 times that amount = 10,000 IU/kg of dry feed pg 1859
Reptiles 500-1,000 IU/kg of dry feed pg 1863
Turtles 1,000 IU/kg of dry feed pg 1865
Horses 800 -1,000 IU/kg (if not have >4 hours per day of sunshine or have sun-cured hay) pg 1879 -later growth 500 IU / kg
Pigs 200 IU/kg if < 10 kg in weight and 150 IU/kg if > 20 kg in weight pg 1891 - “they seem to preferentially use D3” over D2 pg 1896
Dogs 500 IU/kg of dry feed pg 1915 minimum
Cats 700 IU/kg growth and reproduction minimum, 500 IU/kg adult maximum 10,000 IU/kg pg 1916
Pekin Ducks 400 IU/kg of dry feed < 7 weeks, 900 IU/kg of dry feed breeding pg 2315
Geese 200 IU/kg pg 2315
Leghorn-type chickens 200 IU/kg of dry feed until first egg, then 300 IU/kg pg 2316
Turkeys 1,100 IU/kg of dry feed pg 2318

Note: Assume* adults eat about 1 kg of dry food per day, the FDA RDA of 400 IU is less than the RDA for the majority of animals listed.

This edition is on-line at http://www.merckvetmanual.com/mvm/index.jsp

1 kg of dry weight per day = 365 kg/ year. = 803 pounds/year.

Internet indicates that people eat 1800 pounds of (wet) food per year.
So I will guess that humans might eat 1kg of dry food per day
The FDA says that we need should have 400 IU/day

I was unable to find dry weight food a person consumes in a short search of the web.

See also VitaminDWiki