During the last two decades, the lifestyles and dietary habits of Americans have changed considerably. The average American spends 93% of his or her time indoors-more time spent in front of the television, the video game console, or the computer. Because this change has a dramatic effect on vitamin D levels there has been a greater incidence of rickets (a medical condition which affects bone calcification), a disease that was more common in the past.
Many people think they can get enough vitamin D from fortified foods like milk, and yogurt. But these products provide relatively small amounts of vitamin D. One cup of milk provides 100 IU of vitamin D, whereas just 10 minutes of full-body sunlight exposure during summer months provides 10,000 to 20,000 IU of vitamin D within 24 hours.
For nursing mothers, if their vitamin D levels are low her milk will also be low in vitamin D. One study conducted in Iowa found that 78 % of breastfeeding children were deficient in Vitamin D in the winter months. So nursing mothers living in more northerly latitudes should consider supplementing their diet with at least 6,000 IU/day of vitamin D.