Dietary fibre metabolism and colon function
Dietary fibre is a complex material made up of plant cell polymers which are not digested by gastrointestinal enzymes of man but partially fermented by the commensal bacteria in the colon. Since in man most of the colon is not readily accessible, indirect methods must be sought to study dietary fibre metabolism and its effects on colon function. In this project gas chromatographic measurements of two of the end products of fermentation, H2 and CH^, which are expired in the breath were used to monitor fibre degradation in the colon. Effects of fibre on colon function were assessed by measuring changes in stool weight, faecal constituents, transit time and serum lipids. The effect of dietary fibre ingestion on bile acid metabolism and the role of the colon in this process was further studied in the rat. Preliminary studies on the normal excretion pattern of the two gases showed that breath H2 was excreted at a level of less than 1.0umol/l and follows a regular daily pattern of excretion. The concentration was decreased by fasting. CK^ excretion was limited to certain individuals, about a third of the subjects studied, though the proportion of excretors increased with age. In those who excreted CH^ there was no regular daily variation in the excretion and levels were unaffected by diet. Test meals of the dietary fibre components, in the manner administered here, reached the caecum within three hours. Short term separate administration of chemical isolates of fibre resulted in raffinose, stachyose and hemicellulose increasing total breath H2 excretion from a base line level of 1.73 + O.73umol/l to levels of up to l6.36umol/l. Cellulose, pectin and lignin did not alter the excretion level. CH^ excretion was unaffected by any of the polymers. Difference in physical properties of the same chemical polymer appear to have no influence on the or CH^ production. The ingestion of 200g raw carrot daily for three weeks increased ^ evolution significantly but had no effect on the CH^. There was a lag period of a few days between the start of carrot intake and the increase in breath H2 excretion. Carrot decreased the serum cholesterol concentration from a control level of 6.6+0.5 to 5.9 + 0.3mmol/l (P<0.05) and more than doubled bile acid excretion, particularly primary bile acids. The effect on cholesterol and bile acids remained the same three weeks after cessation of carrot intake. The effect on stool weight was modest and made no impact on the transit time. The addition of cereal fibre to the diet of rats affected the metabolism of bile acids in the gastrointestinal tract both quantitatively and qualitatively and their synthesis in the liver. It is concluded that dietary fibre because of its physical properties of absorption, adsorption and cation exchange and the partial degradation of some of its component polymers by bacteria in the colon, alters the distribution of organic and inorganic substances in the gut which change the luminal content of the colon. This change in luminal content affects the activity of the bacteria and influence some physiological functions of the colon like motility, water and electrolyte absorption and excretion, bile acid metabolism and reabsorption and cholesterol degradation|§nd excretion. Different polymers of dietary fibre have different effects on each of these variables. It is possible to .examine indirectly fibre metabolism and its effects on colon function by measuring expired gases and stool constituents.