Zinc is an essential trace element, necessary for plants, animals, and microorganisms. It is "typically the second most abundant transition metal in organisms" after iron and it is the only metal which appears in all enzyme classes. There are 2–4 grams of zinc distributed throughout the human body, and it plays "ubiquitous biological roles". A 2006 study estimated that about 10% of human proteins (2800) potentially bind zinc, in addition to hundreds which transport and traffic zinc. In the U.S., the Recommended Dietary Allowance (RDA) is 8 mg/day for women and 11 mg/day for men. Harmful excessive supplementation may be a problem and should probably not exceed 20 mg/day in healthy people, although the U.S. National Research Council set a Tolerable Upper Intake of 40 mg/day.