Hb is a convenient and widely used biomarker in nutrition and health research, however it remains unclear if the associations with adverse outcomes are being driven by direct alterations in functional Hb for essential functions or by the indirect underlying causes of anemia (e.g. iron deficiency vs. hemoglobinopathy) or high Hb (e.g. excess iron vs. failure of plasma volume expansion). This evidence concerns the gene GSTM1 and hemoglobinopathy.