Vitamin D deficiency can be a negative health consequence of breastfeeding when either you or your infant are not properly supplemented. Don't ignore this common problem that can be solved safely and easily! Some feel that any discussion of Vitamin D deficiency (or anything else that nursing moms should be warned about) is somehow insulting to breastmilk. Others bristle at the word "supplement," but remember that you can take Vitamin D yourself if you feel uncomfortable giving it to your infant directly.
Why is the problem of Vitamin D deficiency getting worse? Our bodies depend on UVB rays in sunlight to make Vitamin D. People spend a lot less time in the sun these days than we used to because we spend more time indoors and protect our skin when we do go out (for good reason). To make up for this lack of UVB exposure, we must rely on dietary sources and/or vitamin supplements.
Unless you just love herring or spend lots of time tanning, you may not be getting the 2000 IU of Vitamin D recommended for breastfeeding mothers by the Canadian Pediatric Society. The American Association of Pediatrics recommends supplementing your infant directly with 400 IU, typically in the form of drops from birth to age 1. Whichever you decide to do, know your Vitamin D levels and your baby's. As clearly demonstrated in the above review from JABFM, making sure that your baby has adequate Vitamin D intake won't just spare them from rickets (the sad consequence of severe Vitamin D deficiency) - it will also help them to avoid other health problems throughout life including cancer, Alzheimer's, multiple sclerosis (MS), cardiovascular disease, and diabetes.