The need to transport oxygen and remove carbon dioxide from animal tissue is a fundamental requirement of life, independent of age or sex.1 The role of iron in humans and many other mammals is central to this process. 2 3 Haemoglobin concentration and red blood cell count are important diagnostic indicators for anaemia in humans and animals.

In prepubertal humans no major differences can be found between the sexes in red blood cell count or haemoglobin and serum ferritin concentrations.4 Only after the onset of menstruation does a difference emerge.4 Not until 10 years after the menopause does this situation revert in women, when the haemoglobin concentration becomes similar to that of aged matched men. 4 5 This situation is compounded by the fact that modern women have a different reproductive history from those in the past. They reach sexual maturity at an earlier age, have fewer pregnancies, and breast feed for shorter periods; as such they menstruate for more years than women in the past. Menstruation is the principal cause of iron loss in women.6^–8 Furthermore, 90% of UK females of childbearing age do not achieve the recommended daily intake of elemental iron (14.8 mg) from their diet.9 Evaluation of the haemoglobin concentration and red blood cell count of women from Canada, Central America, China, and the United States shows that this situation is widespread. 4 10^–14 Women worldwide are at risk of being in a negative iron balance, and by current criteria if their haemoglobin concentration is less than 115 g/l they are deemed to be anaemic, whereas in men the cut-off point is 130 g/l.15

As far as the authors are aware, of the primates only humans show a sex difference in haemoglobin concentration and red blood …