Mean concentration of haemoglobin in healthy white participants as a function of: (a) age, stratified as 26-35, 36-45, 46-55, 56-65, 66-75, and 76-99 years, plotted at the interval midpoint, number of participants shown; (b) per cent transferrin saturation in 26-55 year olds; (c) serum ferritin concentration in 26-55 year olds; and (d) per cent transferrin saturation in >55 year olds, divided into tenths and plotted at the mean for each tenth. Error bars show the standard error in the mean. Group sizes are given on bmj.com