Human milk, in most nursing mothers, contains very little vitamin D. Infant formulas, from various manufacturers, all contain some added vitamin D in amounts calculated to be sufficient to meet an infant's needs. But extensive studies during the first year of life reveal that less than one-fifth of all infants ever get as much as the recommended 400 IU/d from any source, and fewer than one out of 10 breast-fed infants meet the requirement.
... in addition to its role in calcium metabolism, this critical nutrient reduces both the risk of current infections and the late-life development of such autoimmune diseases as multiple sclerosis and type 1 diabetes.
How much vitamin D does the mother need so as to ensure an adequate amount in her milk? As with everything else related to vitamin D, there is a lot of individual variation, but it appears that the daily intake must be in the range of 5,000-6,000 IUs.
This breast-feeding example is not a special case; it is just one of the many pieces of evidence that point to the fact that current vitamin D recommendations for adults are too low -- way too low. [emphasis mine]