A study in Nigeria found that
calcium and potassium levels in human milk varied by a factor of 2, magnesium and copper by a factor of 3,
chloride levels by a factor of 4, iron and selenium by almost 5, iodine and sodium by almost 7, and zinc, which is vital to the nervous system, by
over 7.15 In other words, some mothers had seven times more zinc in their milk than others.