Increasing C[O.sub.2] threatens human nutrition

Dietary deficiencies of zinc and iron are a substantial global public health problem. An estimated two billion people suffer these deficiencies (1), causing a loss of 63 million life-years annually (2,3). Most of these people depend on [C.sub.3] grains and legumes as their primary dietary source of zinc and iron. Here we report that [C.sub.3] grains and legumes have lower concentrations of zinc and iron when grown under field conditions at the elevated atmospheric C[O.sub.2] concentration predicted for the middle of this century. [C.sub.3] crops other than legumes also have lower concentrations of protein, whereas [C.sub.4] crops seem to be less affected. Differences between cultivars of a single crop suggest that breeding for decreased sensitivity to atmospheric C[O.sub.2] concentration could partly address these new challenges to global health.