In work aimed at developing methodologies for validation of estimates of the dietary intake of free-living individuals, Bingham and colleagues have examined the use of urinary nitrogen (UN) excretion as an index of protein intake (Bingham & Cummings, 1985; Bingham, 1994; Bingham et al. 1995). The basis of this approach is that in subjects in N equilibrium, N intake is assumed equal to N excretion. Thus, if N excretion is measured it should indicate N and dietary protein intakes. In practice, since most N excretion occurs via the urine, which is relatively simple to collect, Bingham and colleagues examined the relationship between 24 h UN and N intake (DN). They argued that in subjects on typical UK diets UN should bear a fixed relationship to DN, and measured it carefully in a group of subjects. They showed that in a group of eight individuals in which intake and N excretion were measured meticulously, UN was 81% DN (SD 2, range 78–83%). They argued that measurement of this ratio could be used to assess the validity of food intake measurement and concluded ‘In healthy individuals eating normal western diets, 24 h urine N from an 8 day collection verified for its completeness by the PABA check method, should establish urine N to within 81(SD 5)% of the habitual dietary intake, range 70–90%. If the dietary assessment from 18 days of records or 24 h recalls or the diet history falls within these limits, it can be stated that there is no evidence of interference with normal dietary habits, or of reporting errors….’ (Bingham & Cummings, 1985).