There are nutrients that are good for the skin, such as
lecithin (in apples, for example, if I remember right)
and omega-3 and omega-6 essential fatty acids, and
vitamins C and E. It seems likely that
increasing these nutrients in the diet can help heal
cracked nipples. Many people have diets deficient in
omega-3, which is found in walnuts, pecans, flax seeds
and flax seed oil, and to a lesser extent in soybeans
or soybean oil and wheat germ.

Udo Erasmus, an expert on essential fatty acids, says that
cracks in the skin of the feet that many people get in the winter can
be avoided by adding plenty of omega-3 and omega-6 essential
fatty acids to the diet. I suppose cracked nipples are
similar. His website is http://www.udoerasmus.com

The essential fatty acids are also good for baby's
brain development.