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In Vitro Measurement of the Lactase Activity and the Fermentation Products of Lactose in the Cecal and Colonic Contents of Rats Fed a Control or 30% Lactose Diet¹

Departments of Meat and Animal Science, and Nutritional Sciences, University of Wisconsin, Madison, Wisconsin 53706

Cecal and colonic contents of rats fed a control or a 30% lactose diet for 18 days were collected at 0, 3, and 6 hours following the initiation of a 1-hour feeding, and incubated anaerobically at 37° for 30 minutes with 0.1 M phosphate buffer (pH 6.5), 25 mM lactose and [U-¹⁴C]-lactose to measure lactase (EC 3.2.1.23) activity and lactose fermentation products. The feeding of the 30% lactose diet over the 18-day experiment depressed final body weight and increased the weight of wet tissue and wet contents of the cecum + colon as compared with feeding the control diet. The lactase activity of the cecal + colonic contents of rats fed the 30% lactose diet was three times (per g of contents) or nine times (per rat) as much as that of the controls. The radioactivity recovered in 15,000 x g supernatants obtained from the incubations was decreased with the length of incubation while that in the precipitate and CO₂ was increased. As with lactase activity, lactose fermentation by the contents of rats fed the lactose diet was much higher than that of the controls, resulting in a higher (17 times on a per g of contents basis) production of lactate. The rate of lactose fermentation tended to be decreased with time after feeding in rats fed the lactose diet. The recovery of carbon from fermented lactose in ethanol, acetate, propionate, lactate, butyrate, and CO₂ was approximately 80%. No significant differences between the two dietary groups were observed. Fifty and 60% of the ATP theoretically available from direct catabolism of lactose by the rat was recovered in the fermentation products of rats fed the control and lactose diets, respectively. These results suggest that feeding a high level of lactose greatly enhances the capability of hydrolyzing and fermenting lactose in the large intestine, and approximately one-half of the lactose energy (as ATP equivalents) which enters the large intestine may be available to the animal as fermentation products.

KEY WORDS: • lactase • lactose fermentation • VFA • lactate • ethanol • cecum • colon

¹ Supported in part by the Graduate School and the College of Agricultural and Life Sciences, University of Wisconsin.

Manuscript received 18 August 1978.