Everyone seems to agree that vitamin D is important throughout life. This is certainly as true in the first year of life as it is later on. For it is during the first year that, 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.
Both the Institute of Medicine (IOM) and the American Academy of Pediatrics (AAP) agree that vitamin D intake during the first year of life should be 400 IU/d. My own estimation of the requirement (for different ages and body sizes) is 65-75 IU/kg body weight per day. For average body weights in infants during the first year of life that rule of thumb computes to somewhere between 300 and 500 IU/d for infants. So, while there is still contention with respect to the optimal intake for adults, there really is no disagreement about how much is needed for infants, either among various authoritative sources or arising from different approaches to the evidence. With respect to infants, 400 IU/d seems to be just about right.
The question is, how is the infant to get that vitamin D? 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. As a result, the AAP urges that all infants, regardless of whether they are breast or formula fed, receive their 400 IU/d as pediatric drops. Unfortunately, this recommendation, while appropriate, is not often followed. Most babies are just not getting the vitamin D they need. The late-life consequences of this shortfall could be enormous.
It must seem strange that on the one hand we stress that human milk is the best source of nourishment for our babies, and on the other seem to ignore the fact that human milk doesn't contain the vitamin D those babies need. The explanation, very simply, is that the disconnect is artificial. Nursing mothers have so little vitamin D in their own bodies that there is little or none left over to put into their milk. But it has not always been this way. We know that the vitamin D blood concentrations that are regularly found today in Africans living ancestral lifestyles are high enough to support putting into breast milk all the vitamin D an infant needs. But the bulk of the world's population today is not living on the high equatorial plains of East Africa nor exposing much of its skin for most of the day.
Fortunately, we don't have to return to East Africa. It turns out that, if we give nursing mothers enough vitamin D to bring their blood levels up to the likely ancestral levels, then they automatically put all of the vitamin D their baby needs into their own milk, thereby ensuring that the infant gets total nutrition without the need to resort to vitamin D drops.
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. As no surprise, that's just about the amount needed to reproduce the vitamin D blood levels in persons living ancestral lifestyles today. And while 5,000-6,000 IU may initially seem high, it is important to remember how much the sun produces for us. A single 15 minute whole body exposure to sun at mid-day in summer produces well over 10,000 IU.
There is one important proviso for nursing mothers concerning the needed intake. Those who live in North America and have to rely on supplements should be certain that they take their supplements every day. While for other purposes it is possible to take vitamin D intermittently (e.g., once a week), that doesn't work for putting vitamin D into human milk. The residence time of vitamin D in the blood is so short that, if the mother stops taking her vitamin D supplement for a day or two, vitamin D in her milk will be low (or absent altogether) on the days she skips.
There is a glaring disconnect here between these well-attested physiological facts and the official IOM recommendation for nursing mothers, i.e., only 400 IU/d - the same intake for her as IOM recommends for her baby (whose body weight is less than 10% of her own). The IOM, if it were to be explicit about its current recommendations, would be telling nursing mothers something like this:
"The evidence we analyzed indicates that your own body needs only 400 IU of vitamin D each day. Unfortunately, that won't allow you to put any vitamin D into your breast milk. Sorry about that . . . So, if you want to ensure that your baby is adequately nourished, you are going to have to resort to giving your infant vitamin D drops."
That would be a hard message to sell, and clearly, it makes little sense on its face. As I have already noted, women living ancestral lifestyles (whether or not they are nursing an infant) have far higher blood levels of vitamin D than contemporary urban Americans. Milk production (and its optimal composition) are simply two of the many functions that vitamin D supports in a healthy adult. 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.
Vitamin D supplements - and in this case vitamin D drops - are literal lifesavers for infants today. But what about two or three generations back - before nutritional supplements come into existence, but long after migration out of Africa? Ninety years ago vitamin D had not yet been discovered, and there certainly were no vitamin D supplements that could have been used. How did we get by through those thousands of years? There are two answers. Most of us, living in temperate latitudes, got a lot more sun exposure than we do today, and of course there were no sunscreens, so there was no blocking of the solar radiation that produces vitamin D in our skin. Those of us living in far northern latitudes survived mostly by depending upon diets that were very high in seafood, which is naturally a rich source of vitamin D. And those of us who got vitamin D by neither route were at increased risk of a whole host of vitamin D-related disorders, most obvious and most easily recognized being rickets.
The bony deformities of rickets were common a century ago in Europe, North America, and East Asia, and were largely eradicated in growing children by use of cod liver oil and, in the US, by the introduction of vitamin D fortification of milk in the 1930s. Fortunately, growing children can repair some of the bone deformities of rickets if they are given vitamin D soon enough. But, repairing rickets, while a good and necessary thing to do, is not sufficient. It is too late, by the time we recognize the deformities of rickets, to ensure maximal protection against the autoimmune diseases (for example), for which susceptibility is mainly determined in the first year of life.
To sum up, we now better recognize the importance of vitamin D in the earliest stages of life. Furthermore, there is, as noted earlier, quite good agreement on how much an infant needs. Where we lack consensus is how to make certain that all of our babies get the amount they need. Why not rely on giving nursing infants vitamin D drops, as the AAP recommends? Two reasons: 1) It's been tried and has failed; and, 2) When it does work in individual infants, it provides no benefit for the mother. By contrast, ensuring an adequate vitamin D input to the mother during pregnancy and lactation is almost certainly the best way to meet the needs of both individuals.
An "adequate" intake for nursing mothers, as noted earlier, is not the 400 IU/d the IOM recommends, but is instead in the range of 5,000-6,000 IU/d, taken daily. If they get that much, they will meet not only their own needs, but their infant's as well. And they will have the satisfaction of knowing that they are supplying all their baby's needs, the natural way.