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Interestingly, new research shows that carnivorous animals who cannot make vitamin D in their skin actually get all of the vitamin D they need from the meat they eat. This finding has led to improved understanding of food-based vitamin D. For the longest time, meat was not considered a good source of vitamin D, primarily because it was so difficult to measure that we didn’t think it contained useful amounts.
"Most of the assays that have been published up until very recently listed no vitamin D content for beef, pork, poultry, etc. Things are changing in lots of ways, [for starters], we have a much better way of measuring vitamin D than we did back then. We're now finding vitamin D present in [food] at quantities that are biologically meaningful...
My best guess – and I'm going to qualify that by saying 'guess,' because this is a moving target and we don't have all the data we need yet – is that the average adult today living in the central part of the US is getting 1,500 to 2,000 international units from food—unrecognized sources of food. They get less than we use to think from the sun... Most of us – professionals, wage slaves, workers of one kind or another – we're indoors. Not very many of us work outdoors and not very many of us get sun exposure in mid-day, and that's when you make your vitamin D."
http://articles.mercola.com/sites/articles/archive/2015/02/01/daily-vitamin-d.aspx?e_cid=20150201Z1_SNL_RTL_C_art_1&utm_source=snl&utm_medium=email&utm_content=art1&utm_campaign=20150201Z1_RTL_C&et_cid=DM68190&et_rid=826736719