Our objectives are 1) to describe the relationship between erythropoietin and iron nutrition status; 2) to describe the influence of obesity and inflammation on erythropoietin concentration; and 3) to describe how pregnancy, obesity and erythropoietin concentrations influence red cell indices, taking into consideration folic acid and vitamin B12 nutrition status. The gene discussed is EPO; the disease is obesity due to melanocortin 4 receptor deficiency.