Dietary intake and faecal excretion of carbohydrate by Australians: importance of achieving stool weights greater than 150 g to improve faecal markers relevant to colon cancer risk

Abstract

Objectives: This study investigated, on 53 Australians consuming a typical Western diet, the relationship between dietary intake, faecal excretion of carbohydrate and changes in faecal markers believed to be relevant to colon cancer risk, for example faecal output, transit time and concentrations of phenols, ammonia and butyrate. Design: Fifty-three subjects consuming their usual diet were asked to record and weigh all food consumed for a seven day period, and to collect faeces for three days during this period. Setting: Geelong, Victoria, Australia. Subjects: All volunteers were either staff and students of the university, or associates of the authors. Interventions: None. Results: Volunteers had the following dietary intakes of carbohydrate (g/d; mean±s.d.); starch 131±41 (including resistant starch (RS), 5±2), sugar 108±37 and non-starch polysaccharides (NSP) 14±7. Daily faecal output was 127±70 g and transit time 47±19 h. Analysis of faecal samples found 0.8±1.2 g RS and 5.6±3.6 g NSP were excreted daily. Dietary starch intake was the only dietary carbohydrate to show a significant relationship with the concentration (mmol/L) of butyrate excreted in faeces (r=0.34, P<0.05). Dietary intake of RS was associated with higher concentrations of faecal ammonia (r=0.34, P<0.05), but this association was reversed when RS was combined with NSP in the diet (r=0.07, NS). In contrast to dietary intake, the faecal excretion of RS was negatively related to faecal ammonia concentration (r=−0.40, P<0.01) and positively related to faecal output (r=0.64, P<0.01). Individuals who consumed more NSP in their diet (19±7 g/d) excreted more than 150 g faeces per day and had higher quantities of faecal-RS and -NSP; faster transit times; higher concentrations of short chain fatty acids and lower concentrations of potentially harmful ammonia and phenols. Conclusions: The combination of RS and NSP in the colon may be required to achieve an optimal luminal environment conducive to ‘colonic health’. The results also support the suggestion that faecal output (< or > 150 g/d) may provide a useful index of colon cancer risk. High faecal outputs are achieved through higher intakes of NSP (the major component of dietary fibre). Sponsorship: This work was supported by a scholarship to A. Birkett from the Australian Postgraduate Research Council.