Lactose (mal)digestion evaluated by the 13C-lactose digestion test

Lactase deficiency and lactose intolerance in a multiracial Asian population in Malaysia

Background and Aims

There have been few reports on lactase deficiency (LD) and lactose intolerance (LI) in Malaysia, which has a peculiar mix of three distinct major Asian races-Malay, Chinese, and Indian. The aim of this study was to determine the prevalence of LD and LI in a young multiethnic Malaysian population.

Methods

Lactase activity was measured with a ¹³CO₂ lactose breath test using an infrared spectrometer. Each subject took 25 g of lactose naturally enriched in ¹³CO₂ together with 250 mL of water after an overnight fast. Breath samples were collected at baseline and at 15-min intervals for 180 min. Subjects were asked to report gastrointestinal (GI) symptoms following ingestion of the lactose test meal.

Results

Of the 248 subjects tested, 216 (87.1%) were lactase deficient. We found no significant differences in the presentation of LD between gender and races. LD was found in 87.5% of males and 86.8% of females (P = 0.975) and in different races: Chinese (88.5%) versus Malay (83.1%) (P = 0.399), Indian (90.5%) versus Malay (P = 0.295), and Chinese versus Indian (P = 0.902). LI was diagnosed in only 49 (19.8%) subjects; 35 patients had diarrhea, while the remainder had at least two other GI symptoms after the lactose meal.

Conclusion

The prevalence of LD was high in all three major ethnic groups-Malays, Chinese, and Indians. Ironically, the prevalence of LI was low overall.

Lactose in milk replacer can partly be replaced by glucose, fructose, or glycerol without affecting insulin sensitivity in veal calves

Calf milk replacer (MR) contains 40 to 50% lactose. Lactose strongly fluctuates in price and alternatives are desired. Also, problems with glucose homeostasis and insulin sensitivity (i.e., high incidence of hyperglycemia and hyperinsulinemia) have been described for heavy veal calves (body weight >100 kg). Replacement of lactose by other dietary substrates can be economically attractive, and may also positively (or negatively) affect the risk of developing problems with glucose metabolism. An experiment was designed to study the effects of replacing one third of the dietary lactose by glucose, fructose, or glycerol on glucose homeostasis and insulin sensitivity in veal calves. Forty male Holstein-Friesian (body weight=114 ± 2.4 kg; age=97 ± 1.4 d) calves were fed an MR containing 462 g of lactose/kg (CON), or an MR in which 150 g of lactose/kg of MR was replaced by glucose (GLU), fructose (FRU), or glycerol (GLY). During the first 10d of the trial, all calves received CON. The CON group remained on this diet and the other groups received their experimental diets for a period of 8 wk. Measurements were conducted during the first (baseline) and last week of the trial. A frequently sampled intravenous glucose tolerance test was performed to assess insulin sensitivity and 24 h of urine was collected to measure glucose excretion. During the last week of the trial, a bolus of 1.5 g of [U-(13)C] substrates was added to their respective meals and plasma glucose, insulin, and (13)C-glucose responses were measured. Insulin sensitivity was low at the start of the trial and remained low [1.2 ± 0.1 and 1.0 ± 0.1 (mU/L)(-1) × min(-1)], and no treatment effect was noted. Glucose excretion was low at the start of the trial (3.4 ± 1.0 g/d), but increased in CON and GLU calves (26.9 ± 3.9 and 43.0 ± 10.6g/d) but not in FRU and GLY calves. Postprandial glucose was higher in GLU, lower in FRU, and similar in GLY compared with CON calves. Postprandial insulin was lower in FRU and GLY and similar in GLU compared with CON calves. Postprandial (13)C-glucose increased substantially in FRU and GLY calves, indicating that calves are able to partially convert these substrates to glucose. We concluded that replacing one third of lactose in MR by glucose, fructose, or glycerol in MR differentially influences postprandial glucose homeostasis but does not affect insulin sensitivity in veal calves.

Agreement between indirect calorimetry and traditional tests of lactose malabsorption

BACKGROUND

Lactose malabsorption occurs frequently and the variable consequent intolerance may seriously impair quality of life. No reliable and convenient test method is in routine clinical practice. A recent animal study showed that the respiratory quotient changed significantly after ingestion of sucrose and lactose in naturally lactase-deficient rats.

AIMS

This exploratory study evaluated the relevance of monitoring the respiratory quotient after lactose ingestion to detect malabsorption.

METHODS

Healthy volunteers were identified and classified lactose absorbers and malabsorbers by a lactose tolerance test (25 g). After an overnight fast, a second lactose challenge was performed to monitor hydrogen excretion and respiratory quotient kinetics over 4h. Participants also completed questionnaires to score and localise their gastrointestinal symptoms.

RESULTS

20 subjects were enrolled (10 per group, 60% males, mean age 34 ± 4 years). Respiratory quotient kinetics were different between absorbers and malabsorbers during the first 100 min after lactose ingestion (p<0.01) and during the initial 30-50 min period. Respiratory quotient was significantly, positively correlated to peak glycaemia (R=0.74) and negatively correlated to hydrogen excretion (R=-0.51) and symptoms score (R=-0.46).

CONCLUSIONS

Indirect calorimetry could improve the reliability of lactose malabsorption diagnosis. Studies on larger populations are needed to confirm the validity of this test and propose a simplified measurement.

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.

Slowly and rapidly digestible starchy foods can elicit a similar glycemic response because of differential tissue glucose uptake in healthy men

BACKGROUND

Previously we observed that the consumption of pasta and bread resulted in a similar glycemic response, despite a slower intestinal influx rate of glucose from the pasta. Underlying mechanisms of this effect were not clear.

OBJECTIVE

The objective was to investigate the differences in glucose kinetics and hormonal response after consumption of products with slow and rapid in vivo starch digestibility but with a similar glycemic response.

DESIGN

Ten healthy male volunteers participated in a crossover study and consumed (13)C-enriched wheat bread or pasta while receiving a primed-continuous D-[6,6-(2)H(2)]glucose infusion. The dual-isotope technique enabled calculation of the following glucose kinetics: rate of appearance of exogenous glucose (RaE), endogenous glucose production, and glucose clearance rate (GCR). In addition, postprandial plasma concentrations of glucose, insulin, glucagon, and glucose-dependent insulinotropic polypeptide (GIP) were analyzed.

RESULTS

GIP concentrations after pasta consumption were lower than after bread consumption and strongly correlated with the RaE (r = 0.82, P < 0.01). The insulin response was also lower after pasta consumption (P < 0.01). In accordance with the low insulin response, the GCR was lower after pasta consumption, which explained the high glycemic response despite a low RaE.

CONCLUSIONS

Slower intestinal uptake of glucose from a starchy food product can result in lower postprandial insulin and GIP concentrations, but not necessarily in a lower glycemic response, because of a slower GCR. Even without being able to reduce postprandial glycemia, products with slowly digestible starch can have beneficial long-term effects. These types of starchy products cannot be identified by using the glycemic index and therefore another classification system may be necessary. This trial was registered at controlled-trials.com as ISRCTN42106325.

The glycemic response does not reflect the in vivo starch digestibility of fiber-rich wheat products in healthy men

Starchy food products differ in the rate of starch digestion, which can affect their metabolic impact. In this study, we examined how the in vivo starch digestibility is reflected by the glycemic response, because this response is often used to predict starch digestibility. Ten healthy male volunteers [age 21 ± 0.5 y, BMI 23 ± 0.6 kg/m² (mean ± SEM)] participated in a cross-over study, receiving three different meals: pasta with normal wheat bran (PA) and bread with normal (CB) or purple wheat bran (PBB). Purple wheat bran was added in an attempt to decrease the rate of starch digestion. The meals were enriched in ¹³C and the dual isotope technique was applied to calculate the rate of appearance of exogenous glucose (RaE). The ¹³C-isotopic enrichment of glucose in plasma was measured with GC/combustion/isotope ratio MS (IRMS) and liquid chromatography/IRMS. Both IRMS techniques gave similar results. Plasma glucose concentrations [2-h incremental AUC (iAUC)] did not differ between the test meals. The RaE was similar after consumption of CB and PBB, showing that purple wheat bran in bread does not affect in vivo starch digestibility. However, the iAUC of RaE after men consumed PA was less than after they consumed CB (P < 0.0001) despite the similar glucose response. To conclude, the glycemic response does not always reflect the in vivo starch digestibility. This could have implications for intervention studies in which the glycemic response is used to characterize test products.

Disorders of intestinal secretion and absorption

The gastrointestinal tract possesses a huge epithelial surface area and performs many different tasks. Amongst them are the digestive and absorptive functions. Disorders of intestinal absorption and secretion comprise a variety of different diseases, e.g. coeliac disease, lactase deficiency or Whipple's disease. In principle, impaired small intestinal function can occur with or without morphological alterations of the intestinal mucosa. Therefore, in the work up of a malabsorptive syndrome an early small intestinal biopsy is encouraged in conjunction with breath tests and stool analysis to guide further management. In addition, there is an array of functional tests, the clinical availability of which becomes more and more limited. In any case, early diagnosis of the underlying pathophysiology is most important, in order to initiate proper therapy. In this chapter, diagnostic procedure of malabsorption is discussed with special attention to specific disease like coeliac disease, Whipple's disease, giardiasis and short bowel syndrome. Furthermore, bacterial overgrowth, carbohydrate malabsorption and specific nutrient malabsorption (e.g. for iron or vitamins) and protein-losing enteropathy are presented with obligatory and optional tests as used in the clinical setting.

13C-breath tests: current state of the art and future directions

13C-breath tests provide a non-invasive diagnostic method with high patient acceptance. In vivo, human and also bacterial enzyme activities, organ functions and transport processes can be assessed semiquantitatively using breath tests. As the samples can directly be analysed using non-dispersive isotope selective infrared spectrometers or sent to analytical centres by normal mail breath tests can be easily performed also in primary care settings. The 13C-urea breath test which detects a Helicobacter pylori infection of the stomach is the most prominent application of stable isotopes. Determination of gastric emptying using test meals labelled with 13C-octanoic or 13C-acetic acid provide reliable results compared to scintigraphy. The clinical use of 13C-breath tests for the diagnosis of exocrine pancreatic insufficiency is still limited due to expensive substrates and long test periods with many samples. However, the quantification of liver function using hepatically metabolised 13C-substrates is clinically helpful in special indications. The stable isotope technique presents an elegant, non-invasive diagnostic tool promising further options of clinical applications. This review is aimed at providing an overview on the relevant clinical applications of 13C-breath tests.

Intestinal capacity to digest and absorb carbohydrates is maintained in a rat model of cholestasis

Cholestasis is associated with systemic accumulation of bile salts and with deficiency of bile in the intestinal lumen. During the past years bile salts have been identified as signaling molecules that regulate lipid, glucose, and energy metabolism. Bile salts have also been shown to activate signaling routes leading to proliferation, apoptosis, or differentiation. It is unclear, however, whether cholestasis affects the constitution and absorptive capacity of the intestinal epithelium in vivo. We studied small intestinal morphology, proliferation, apoptosis, expression of intestine-specific genes, and carbohydrate absorption in cholestatic (1 wk bile duct ligation), bile-deficient (1 wk bile diversion), and control (sham) rats. Absorptive capacity was assessed by determination of plasma [(2)H]- and [(13)C]glucose concentrations after intraduodenal administration of [(2)H]glucose and naturally enriched [(13)C]sucrose, respectively. Small intestinal morphology, proliferation, apoptosis, and gene expression of intestinal transcription factors (mRNA levels of Cdx-2, Gata-4, and Hnf-1alpha, and Cdx-2 protein levels) were similar in cholestatic, bile-deficient, and control rats. The (unlabeled) blood glucose response after intraduodenal administration was delayed in cholestatic animals, but the absorption over 180 min was quantitatively similar between the groups. Plasma concentrations of [(2)H]glucose and [(13)C]glucose peaked to similar extents in all groups within 7.5 and 30 min, respectively. Absorption of [(2)H]glucose and [(13)C]glucose in plasma was similar in all groups. The present data indicate that neither accumulation of bile salts in the body, nor their intestinal deficiency, two characteristic features of cholestasis, affect rat small intestinal proliferation, differentiation, apoptosis, or its capacity to digest and absorb carbohydrates.

Effect of lactose on oro-cecal transit in lactose digesters and maldigesters

BACKGROUND

The transit time of the small intestine, in addition to lactase activity, may influence lactose digestion and thus play a role in the occurrence of lactose intolerance. The objectives of this study were to investigate the effect of lactose on the oro-cecal transit time (OCTT) in lactose digesters and maldigesters as well as the possible mechanisms underlying these effects.

MATERIALS AND METHODS

Twenty-eight Chinese subjects and 16 Dutch subjects underwent one glucose and one lactose challenge in two single-blinded tests. Twenty of the Chinese subjects without complaints after the challenge then underwent another lactose challenge. A 6-h symptom score (SSC) was recorded, breath-hydrogen concentration was measured and OCTT after consumption of glucose and lactose was determined with the lactose-[13C] ureide breath test. The lactose digestion index (LDI) was determined in both the Dutch and 20 Chinese subjects with the 13C/2H-glucose test.

RESULTS

Lactose digesters (n = 13) and maldigesters (n = 20) were classified based on the results of the LDI and the breath-hydrogen test. The OCTT after the lactose and glucose challenges did not differ in the digesters, whereas in the maldigesters the OCTT, after the lactose challenge, was shorter than that after glucose. There was no difference in OCTT after the glucose challenge between the maldigesters and the digesters. However, the OCTT after the lactose challenge in the maldigesters was shorter than that in the digesters. The LDI of the digesters was significantly higher than that of the maldigesters. The OCTT after the lactose challenge was not correlated to the LDI in the maldigesters nor in the digesters. Based on the SSC after the one glucose and two lactose challenges, a tolerant (n = 7) and an intolerant (n = 5) group were classified in the Chinese subjects. The two groups did not differ in their LDI or OCTT after the lactose challenge. The OCTT after the lactose challenge was not correlated to the SSC or the LDI.

CONCLUSIONS

Lactose triggers a faster oro-cecal transit in lactose maldigesters, but not in digesters. However, this could not be explained by intestinal distension resulting from the osmotic load posed by maldigested lactose, and thus suggests a direct effect of lactose on intrinsic factors regulating intestinal motility.

Lactose intolerance in infants, children, and adolescents

The American Academy of Pediatrics Committee on Nutrition presents an updated review of lactose intolerance in infants, children, and adolescents. Differences between primary, secondary, congenital, and developmental lactase deficiency that may result in lactose intolerance are discussed. Children with suspected lactose intolerance can be assessed clinically by dietary lactose elimination or by tests including noninvasive hydrogen breath testing or invasive intestinal biopsy determination of lactase (and other disaccharidase) concentrations. Treatment consists of use of lactase-treated dairy products or oral lactase supplementation, limitation of lactose-containing foods, or dairy elimination. The American Academy of Pediatrics supports use of dairy foods as an important source of calcium for bone mineral health and of other nutrients that facilitate growth in children and adolescents. If dairy products are eliminated, other dietary sources of calcium or calcium supplements need to be provided.

Management and treatment of lactose malabsorption

Lactose malabsorption is a very common condition characterized by intestinal lactase deficiency. Primary lactose malabsorption is an inherited deficit present in the majority of the world’s population, while secondary hypolactasia can be the consequence of an intestinal disease. The presence of malabsorbed lactose in the colonic lumen causes gastrointestinal symptoms. The condition is known as lactose intolerance. In patients with lactase nonpersistence, treatment should be considered exclusively if intolerance symptoms are present. In the absence of guidelines, the common therapeutic approach tends to exclude milk and dairy products from the diet. However, this strategy may have serious nutritional disadvantages. Several studies have been carried out to find alternative approaches, such as exogenous β-galactosidase, yogurt and probiotics for their bacterial lactase activity, pharmacological and non pharmacological strategies that can prolong contact time between enzyme and substrate delaying gastrointestinal transit time, and chronic lactose ingestion to enhance colonic adaptation. In this review the usefulness of these approaches is discussed and a therapeutic management with a flow chart is proposed.

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.

Analytical techniques in biomedical stable isotope applications: (isotope ratio) mass spectrometry or infrared spectrometry?

An overview is presented of biomedical applications of stable isotopes in general, but mainly focused on the activities of the Center for Liver, Digestive and Metabolic Diseases of the University Medical Center Groningen. The aims of metabolic studies in the areas of glucose, fat, cholesterol and protein metabolism are briefly explained, as well as the principle of breath testing and the techniques to study body composition and energy expenditure. Much attention is paid to the analytical considerations based upon metabolite concentrations, sample size restrictions, the availability of stable isotope labelled substrates and dose requirements in relation to compound-specific isotope analysis. The instrumental advantages and limitations of the generally used techniques gas chromatography/reaction/isotope ratio mass spectrometry and gas chromatography/mass spectrometry are described as well as the novelties of the recently commercialised liquid chromatography/combustion/isotope ratio mass spectrometry. The present use and future perspective of infrared (IR) spectrometry for clinical and biomedical stable isotope applications are reviewed. In this respect, the analytical demands on IR spectrometry are discussed to enable replacement of isotope ratio mass spectrometry by IR spectrometry, in particular, for the purpose of compound-specific isotope ratio analysis in biological matrices.

Use of dual isotope tracers in biomedical research

Biomedical stable isotope studies involve administration of tracer and measurement of isotope enrichment in blood, urine, feces or breath. The aim of the studies is to gather quantitative information about a specific metabolic function. However, the measured isotope enrichment may be affected by other metabolic events than only this function. In this case, a correction is necessary. The best approach is to add a second tracer simultaneously which is known to be metabolised by all interfering metabolic events but not by the function of interest. This dual isotope approach also enables simultaneous measurement of two interrelated functions. A summary of selected applications involving dual isotope tracer studies is presented. The applications deal with energy expenditure (doubly labelled water technique), cholesterol absorption, starch and lactose digestion, fat digestion, bile acid metabolism and the combination of stable and radioactive carbon isotopes in breath testing.

Biologic and methodologic issues for nutritional biomarkers

Nutritional biomarkers are used for a variety of purposes in large-scale population surveys and epidemiologic studies as well as smaller clinical studies. The main reasons for using nutritional biomarkers are to provide measures of nutritional status that have less error than dietary data, nutrient status for nutrients with inadequate dietary data, to obtain a more proximal and integrated assessment of nutrient status that incorporates metabolism, to assess dietary change and compliance in intervention studies, and dietary intake for the validation of dietary questionnaires. However, often there is oversight by the investigators regarding biologic and laboratory issues, which have implications for the utility of nutritional biomarkers. This article reviews some of the physiologic issues that contribute to between-person variability in nutrient status and the utility and meaning of specimens from various body compartments. Issues related to the collection and storage of biologic specimens are addressed, although it is recommended that investigators contact laboratory colleagues at the beginning of any study for updated information. The necessity for blind quality surveillance of laboratory analyses beyond the normal procedures employed by collaborating laboratories also is addressed. The advantages and disadvantages of nutritional biomarkers are reviewed, especially in comparison with using dietary methodology.

Lactose intolerance: analysis of underlying factors

BACKGROUND

We studied the degree of lactose digestion and orocecal transit time (OCTT) as possible causes for the variability of symptoms of lactose intolerance (LI) in a sample of a population with genetically determined low lactase activity.

METHODS

Lactose digestion index (LDI) was measured by the recently developed 13C-lactose/2H-glucose test. The OCTT was determined using the breath hydrogen test. Based on a 6-h symptom score (SSC) after a challenge dose of 25 g of lactose the subjects were divided into a tolerant group (T: n= 15; SSC = 0) and an intolerant group (IT: n= 28; SSC 1-40). The intolerant group was subdivided according to the severity of symptoms: group ITa (n = 17; mild symptoms without diarrhoea) and group ITb (n = 11; with diarrhoea).

RESULTS

The LDI was lower in the intolerant group (0.34 +/- 0.14) (mean +/- SD) than in the tolerant group (0.47 +/- 0.14) (P = 0.008). The OCTT of group IT (60, 30-90 min) (median, quartiles) was significantly shorter than that of group T (105, 60-120 min) (P = 0.003) and was positively correlated with the LDI (P = 0.050). In groups ITa and ITb the OCTT (60, 30-90 min; 60, 26-83 min) and LDI (0.30 +/- 0.14; 0.39 +/- 0.14) were similar.

CONCLUSIONS

Lactose digestion capacity, which is determined by small intestinal lactase activity as well as by OCTT, affects the occurrence of lactose intolerance. However, the major difference in intolerance symptoms is caused by differences in the colonic processing of maldigested lactose.

The 13C/2H-glucose test for determination of small intestinal lactase activity

To diagnose hypolactasia, determination of lactase enzyme activity in small intestinal biopsy material is considered to be the golden standard. Because of its strongly invasive character and the sampling problems, alternative methods have been looked for. We analysed the 13C-glucose response in serum after consumption of 25 g of naturally enriched 13C-lactose. As an internal standard, 0.5 g of 2H-glucose was added and the 2H-glucose response in serum was measured simultaneously. The studies were performed in healthy volunteers with a background of genetically determined lactase nonpersistence (n = 12; low lactase activity) and lactase persistence (n = 27; high lactase activity). The results were compared with those of the lactose hydrogen breath test, the lactose 13CO2 breath test and the previously described 13C-lactose digestion test. After consumption of 13C-lactose and 2H-glucose, the mean ratio 13C-glucose/2H-glucose concentration in serum at 45-75 min was 0.26 +/- 0.09 in the low lactase activity group and 0.93 +/- 0.17 in the high lactase activity group (P < 0.01). Threshold of the ratio between digesters and maldigesters was calculated as 0.46. Accuracy of the new test was superior to all other tests. We conclude that the 13C/2H-glucose test has the potential of determining the small intestinal lactase activity in vivo and of estimating the amount of lactose which is digested in the small intestine.