Researchers at Harvard University and Massachusetts General Hospital have found a new link between low vitamin D levels and immune functioning in infants. These latest findings add to the now rapidly growing body of evidence suggesting that extra amounts of this vitamin, naturally obtained principally through sun exposure, may be needed by many of us to promote optimal health. Other studies have highlighted the importance of vitamin D in a number of critical health areas, including bone health, weight management, immune functioning, brain health, and even cancer and multiple sclerosis prevention. Read more about the new study at Vitamin D & Infants.
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Here is another good summary of the research: http://news.harvard.edu/gazette/story/2010/12/infants-need-their-d/