Given the elevation in plasma nitrite levels observed at the higher dose of nitrate supplementation, it is possible that the rise in Hb at this dose was accompanied by enhanced local formation of methemoglobin (MetHb) via an interaction between Hb and nitrite; however, plasma concentrations of nitrate and nitrite, even at this highest dose, remain below those required to induce methemoglobinemia. The gene discussed is GSTM1; the disease is methemoglobinemia.