Our study showed that (1) anemia was a risk factor for AF development, (2) Hb levels showed a U-shaped relationship with the risk of incident AF after adjusting for various demographic and cardiovascular risk factors, and (3) maintaining Hb levels within normal range conferred the lowest risk of AF, while maintaining high Hb levels or increase to high Hb levels posed the highest risk of AF. This evidence concerns the gene GSTM1 and anemia (phenotype).