http://donmatesz.blogspot.com/2009/11/another-vegetarian-fallacy-exposed.html

...at least 45 percent of adult men and women do not efficiently convert carotenes to vitamin A (retinol), making dietary retinol, found only in animal products, an essential nutrient for these folks [1, 2].

The vitamin A in liver and other organ meats is very easily absorbed though! :-)