Fate of soluble carbohydrate in the colon of rats and man

The fate of glucose in the colon of rats and man was investigated by measuring breath ¹⁴CO₂ and fecal ¹⁴C after direct instillation of ¹⁴C labeled glucose, acetate, and lactate into the cecum. For the 6 h after administration of as much as 400 mg of [U ¹⁴C] glucose to the rat and 12.5 g to man, ¹⁴CO₂ excretion was as rapid after intracecal as after intragastric instillation. Less than 20% of ¹⁴C instilled into the cecum as glucose was recovered in feces and only about 15% of this fecal ¹⁴C was in a dialyzable form. The conversion of intracecally administered glucose to CO₂ was dependent upon the presence of the colonic flora, as evidenced by the minimal excretion of ¹⁴CO₂ after administration of [¹⁴C]glucose to germ free rats. In contrast, acetate and lactate, fermentation products of glucose, were converted to CO₂ as rapidly in germ free rats as in their conventional counterparts. Measurements of O₂ availability in the colonic lumen indicated that insufficient O₂ was available for the aerobic metabolism of glucose by the colonic bacteria. These experiments suggest that the colon bacteria anaerobically metabolize most of the glucose to short chain fatty acids, which are absorbed and oxidized by the host. Most of the remaining fecal glucose is converted to a larger molecular form that has limited osmotic activity. Thus, the colonic flora benefits the host by reducing the osmotic load of nonabsorbed carbohydrate and by making possible the salvage of a large percentage of the calories of carbohydrate, which is not absorbed in the small bowel.