Lactose digestion by human jejunal biopsies: the relationship between hydrolysis and absorption

Heat Treatment of Bovine Milk Influences Gastric Emptying of Lactose but not its Apparent Small Intestinal Disappearance in the Growing Pig as a Model for the Adult Human

BACKGROUND

Heat treatment influences gastric emptying of proteins and lipids in bovine milk. Whether heat treatment influences lactose gastric emptying and small intestinal lactose disappearance remains unknown.

OBJECTIVES

This study aimed to determine the gastric emptying of lactose and its disappearance from the small intestine of the growing pig as a model for the adult human.

METHODS

After 10 d of adaptation, fasted 9-wk-old pigs (n = 48) consumed 500 mL of pasteurized or ultra-high-temperature (UHT) bovine milk before being killed at 20, 60, 120, or 180 min post-feeding. Gastric, small, and large intestinal contents were collected to determine the gastric emptying of lactose, its apparent small intestinal disappearance, and its release into the large intestine. Portal vein blood samples were also collected to measure galactose concentration over time. Data were analyzed using nonlinear and polynomial models.

RESULTS

The gastric emptying of lactose was faster for pigs fed pasteurized milk than those fed UHT milk (P ≤ 0.05). For example, 14 ± 2.9% and 24 ± 2.2% (mean ± standard error) of lactose, respectively, were retained in the stomach at 60 min post-feeding. The apparent small intestinal disappearance of lactose increased to 88 ± 2.4% over time, but it did not differ between milk types. Similarly, the plasma galactose concentration in the portal vein increased during the first post-feeding hour but did not differ between milk types. Lactose reached the large intestine during the first 20 min post-feeding, and the amount of lactose released into the large intestine did not change over time (523 μmol on average across post-feeding times after back transformation of 6.26 ± 0.37) or between milk types.

CONCLUSIONS

Heat treatment influenced the gastric emptying of lactose but did not impact subsequent small intestinal apparent disappearance and portal blood plasma galactose appearance.

Phase I and phase IB clinical trials for the noninvasive evaluation of intestinal lactase with 4-galactosylxylose (gaxilose)

GOALS AND BACKGROUND

Hypolactasia is widespread, yet reliable diagnostic tests are lacking. A new test based on oral administration of 4-galactosylxylose (gaxilose) and urine or serum measurement of D-xylose after cleavage by intestinal lactase is under clinical development. We investigated the optimal dose of gaxilose and calculate cutoff values of D-xylose for that dose.

STUDY

In the randomized, dose-finding, phase I study, urine and serum pharmacokinetics of D-xylose were determined after oral administration of 6 ascending doses of gaxilose (and placebo) to 12 healthy adult volunteers. In the open, parallel, phase Ib study, 30 volunteers received the doses established for the urine and blood tests and D-xylose was measured. Cutoff values were calculated as 1.96 × SD below the mean value. Safety was assessed through reporting of adverse events.

RESULTS

Gaxilose administration showed a progressive, dose-dependent increase in D-xylose in urine and serum. An optimal gaxilose dose of 0.45 g and urine collection periods of 4 and 5 hours were selected for further studies. For the blood test, a 2.7 g dose was selected and C max measured at 90 minutes. The calculated cutoff values of D-xylose for normal lactase activity were 27.58 and 37.87 mg for the 4- and 5-hour urine tests, respectively, and 0.97 mg/dL for the blood test. There were no treatment-related adverse events.

CONCLUSIONS

The methodology described provides a simple, safe test for the evaluation of lactase activity in vivo. Further evaluation of the test as a noninvasive diagnosis of hypolactasia is ongoing in patients with lactose intolerance.

Noninvasive evaluation of intestinal lactase with 4-galactosylxylose: comparison with 3- and 2-galactosylxylose and optimization of the method in rats

BACKGROUND

Urinary excretion of D-xylose by suckling rats after ingestion of a mixture of 4-, 3-, and 2-galactosylxyloses reflects lactase activity in vivo. We aimed to select the most convenient of these disaccharides for detecting changes of the enzyme activity in vivo and to optimize the method.

METHODS

4-, 3-, and 2-galactosylxyloses were synthesized and purified, then orally administered to suckling rats of different ages. D-Xylose was measured colorimetrically by the phloroglucinol reaction in urine and plasma. Lactase activity was determined in extracts of small intestine mucosa with lactose, galactosylxyloses, and phlorizin as substrates.

RESULTS

D-Xylose appeared in the urine in a dose-dependent manner after ingestion of any of the 3 galactosylxylose disaccharides. Correlation between D-xylose elimination and intestinal lactase activity was highest with 4-galactosylxylose (r = 0.97; n = 24), lower with 2-galactosylxylose (r = 0.89; n = 24), and lowest with 3-galactosylxylose (r = 0.34; n = 23). The kinetic properties of intestinal lactase accounted for these differences. D-Xylose concentration in plasma after administration of 4-galactosylxylose also correlated with lactase activity (r = 0.93; n = 33).

CONCLUSIONS

4-Galactosylxylose is the most suitable compound for the evaluation of lactase activity in vivo. Measurement of the derived D-xylose in either urine or blood gives an estimate of the total lactose digestive capacity of the small intestine. The optimized method holds promise for development of a simple, low-cost, and reliable new test for the noninvasive diagnosis of hypolactasia.

Ontogeny of intestinal safety factors: lactase capacities and lactose loads

We measured intestinal safety factors (ratio of a physiological capacity to the load on it) for lactose digestion in developing rat pups. Specifically, we assessed the quantitative relationships between lactose load and the series capacities of lactase and the Na+-glucose cotransporter (SGLT-1). Both capacities increased significantly with age in suckling pups as a result of increasing intestinal mass and maintenance of mass-specific activities. The youngest pups examined (5 days) had surprisingly high safety factors of 8-13 for both lactase and SGLT-1, possibly because milk contains lactase substrates other than lactose; it also, however, suggests that their intestinal capacities were being prepared to meet future demands rather than just current ones. By day 10 (and also at day 15), increased lactose loads resulted in lower safety factors of 4-6, values more typical of adult intestines. The safety factor of SGLT-1 in day 30 (weanling) and day 100 (adult) rats was only approximately 1.0. This was initially unexpected, because most adult intestines maintain a modest reserve capacity beyond nutrient load values, but postweaning rats appear to use hindgut fermentation, assessed by gut morphology and hydrogen production assays, as a built-in reserve capacity. The series capacities of lactase and SGLT-1 varied in concert with each other over ontogeny and as lactose load was manipulated by experimental variation in litter size.

Determinants of lactose digestion in the miniature pig

Although lactose is an important nutrient in the diet of the infant and child, the factors contributing to its digestion have not been clarified adequately. We sought to determine the degree to which lactase activity and small intestinal transit explain lactose digestion, the average error (SEE) in estimating lactose digestion using these parameters, and the effect of age. We compared lactose digestion from both a 7% lactose-containing formula and a solution by determining lactose in ileostomy output in pig littermates at 10 days, 4 weeks, and 10 weeks of age. The entire small intestinal mucosa was assayed for lactase specific activity (micromol x min-1 x g protein-1), total activity (micromol x min-1), and whole-villus lactase activity. Transit time (min), and transit rate (cm/min) were measured. Meal type did not affect lactose digestion. Lactose digestion was explained best by lactase specific activity (formula, R2 = 0.73, SEE = 1.1; solution, R2 = 0.69, SEE = 1.0; P < 0.001). The next best parameter was total transit rate (formula, R2 = 0.69, SEE = 2.0; solution, R2 = 0.46, SEE = 1.3). The relationship with lactase specific activity was age related and there appeared to be a critical value of lactase specific activity above which essentially all the lactose was digested.