Faecal water genotoxicity, determined using HT-29 cells and the alkaline comet assay, is not associated with changes in meat associated endogenous N-nitroso compound levels in human faeces

Increased haem intake through red meat consumption causes a significant increase in faecal N-nitroso compounds (NOC), measured as apparent total N-nitroso compound levels (ATNC)1. Many NOC are known to be carcinogens. To investigate possible genotoxic effects of these NOC, the alkaline single cell gel electrophoresis (SCGE) or comet assay, was used to determine genotoxic potential of human faecal water samples using two different protocols. The first protocol incubated HT-29 cells for 5 min at 4C, whereas the second protocol used an extended exposure time of 30 min but at 37C. DNA strand breaks were quantified by tail moment (the fractional amount of DNA in the comet tail multiplied by the length of the comet tail from the centre of the head of the comet to the end of the tail). The faecal water samples were obtained from 16 individuals fed controlled diets containing different amounts of red meat, protein, haem and iron2. Results from the two methods used were significantly correlated (r 5 0.352, p 5 0.003). However, there was no significant correlation between concentrations of ATNC in the faecal homogenate and genotoxic potential of faecal water using either method (r 5 0.041, p 5 0.7 for the first method, r 5 0.211, p 5 0.09 for the second method). Although there were significant differences in ATNC levels between different dietary periods2 there were no significant differences in mean tail moment values during different dietary periods (p 5 40.05). Analysis of variance suggested an individual response in tail moment independent of diet using both methods (p 5 0.0005 for the first method, p 5 50.0002 for the second method).