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ydrogen breath test in the diagnosis of lactosealabsorption: Accuracy of new versus

onventional criteria

ICHELE DI STEFANO, ANTONIO MISSANELLI, EMANUELA MICELI, ALESSANDRA STROCCHI,nd GINO ROBERTO CORAZZA

AVIA, ITALY

Increased hydrogen excretion in the breath after carbohydrate ingestion, an ex-pression of carbohydrate malabsorption, represents the pathophysiologic basis ofthe hydrogen breath test, a simple, noninvasive, reproducible test for the diagnosisof this condition. Few data are available concerning the breath hydrogen responsethat most accurately identifies carbohydrate malabsorption. In this article we reportour application for the first time in clinical practice of 2 recently described, moreaccurate criteria for the diagnosis of lactose malabsorption (ie, breath hydrogenexcretion value > 6 parts per million (ppm) 6 hours after carbohydrate load and asum greater than 15 ppm for the breath hydrogen values obtained 5, 6, and 7 hoursafter carbohydrate load). On 3 separate days, we subjected 84 consecutive pa-tients with functional or organic gastrointestinal disease to measurement of hydro-gen excretion in the breath after the administration of lactose to test for lactosemalabsorption; after the administration of lactulose as a means of ruling out false-negative results resulting from hydrogen-nonproducer status; and after the admin-istration of a nonabsorbable electrolyte solution as a means of ruling out false-positive results caused by the mixing of intestinal content and release of preformedhydrogen trapped in the feces. According to the conventional criterion, 51% of thesubjects proved to be lactose malabsorbers. Positive results were obtained with theuse of the 6th-hour criterion in 76% of patients, and positive results were found in 80%when the sum of the excretion at hours 5, 6, and 7 was used as the criterion (P < .05for both comparisons). Ten of 21 and 11 of 25, respectively, showed intolerancesymptoms. It was therefore possible for us to diagnose lactose malabsorption in 24%and 27% of patients, respectively, who tested negative according to the conven-tional criterion. The conventional criterion must be abandoned and these newcriteria adopted. (J Lab Clin Med 2004;144:313-8)

Abbreviation: ppm � parts per million

ITbtwttth

rom the Gastroenterology Unit, University of Pavia, Istituto di Ricov-ro e Cuza a Carattere Scientifico “S Matteo” Hospital, Pavia, Italy.

upported in part by a grant from the Istituto di Ricovero e Cuza aarattere Scientifico “S Matteo” Hospital on colonic fermentationnd irritable bowel syndrome.

ubmitted for publication July 12, 2004; revision submitted Septem-er 24, 2004; accepted for publication October 8, 2004.

eprint requests: Professor Gino Roberto Corazza, Gastroenterologynit, IRCCS “S Matteo” Hospital, Viale C Golgi 19, 27100 Pavia,

taly; e-mail: gr.corazza@smatteo.pv.it.

022-2143/$ – see front matter

2004 Elsevier Inc. All rights reserved.

foi:10.1016/j.lab.2004.10.005

ncreased excretion of hydrogen in the breath aftercarbohydrate ingestion is considered a consequenceof carbohydrate bacterial fermentation in the colon.1

his mechanism represents the basis of the hydrogenreath test, a simple, cheap, noninvasive, repeatable testo detect carbohydrate malabsorption. Despite theidespread use of this test, data concerning the criteria

hat most accurately identify carbohydrate malabsorp-ion are scant and limited. As for lactose malabsorption,he commonly used criterion (ie, an increase in breathydrogen � 20 ppm over fasting values) was derived

rom a study involving a 50-g dose of lactose,2 the

313

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mount contained in roughly 1 L of milk. A moreecent study, in which breath hydrogen excretion wasvaluated after low and more physiologic doses ofarbohydrates, showed that this classical criterion hadn almost absolute specificity but a sensitivity of lesshan 60%.3 On the other hand, both breath hydrogenxcretion greater than 6 ppm at the 6th hour and theum of breath hydrogen values obtained at the 5th, 6thnd 7th hours greater than 15 ppm showed a muchetter sensitivity without sacrificing specificity.3 Thisrocedure has been deemed considered a useful andasy-to-use tool with which to evaluate the accuracy ofhe diagnostic criteria,4 and the results have been con-rmed in a subsequent multicentric prospective study.5

o far, few data are available regarding the adoption ofhese more accurate criteria in clinical practice.6

These new criteria make possible the use of a sim-lified methodology because just one or three breathamples, depending on the criterion, must be analyzedo diagnose carbohydrate malabsorption. However, theiagnostic accuracy of these new criteria has only beenvaluated under rigid experimental conditions, andoubts may be raised as to their application and accu-acy in clinical practice. Therefore in this study weought to apply them in the diagnosis of lactose mal-bsorption as a means of determining whether it isossible to recognize this condition with greater accu-acy.

ETHODS

Patients. Eighty-four consecutive patients (58 women, 26en; mean age 38 � 4 years, range 24-57 years) took part in

he study. Forty-eight were affected by functional gastroin-estinal disorders such as irritable-bowel syndrome (n � 22),unctional dyspepsia (n � 20), or functional constipation (n �); the other 36 had received a final diagnosis of coloniciverticula (n � 12), Crohn’s disease in remission (n � 10),lcerative colitis in remission (n � 9) or peptic-ulcer diseasen � 5). All of the subjects were compliant with their treat-ent regimens and gave their informed consent to inclusion

n the study. Subjects with intestinal, liver, renal, chest, car-iac, metabolic, or neurologic disease and those who wereaking antibiotics, prokinetics, laxatives, or any other drugnown to influence colonic flora in the month preceding thetudy were excluded.

Breath testing. As a means of avoiding prolonged intesti-al gas production resulting from the presence of nonabsorb-ble or slowly fermentable material in the colonic lumen, werefaced the breath test with a preparation procedure based onhe consumption, the evening before the test day, of a mealonsisting of rice, meat, and olive oil.7 This meal was fol-owed by a 12-hour period of fasting. Breath testing was

tarted between 8:30 and 9:30 AM, after thorough mouthwash- b

ng with 40 mL of a 1% chlorhexidine solution.8 Smoking9

nd physical exercise10 were not allowed in the hour preced-ng the test or during the test.

We conducted sampling of alveolar air using a commercialevice (Gasampler; Quintron Instrument, Milwaukee, Wis)hat allows the first 500 mL of dead-space air to be separatednd discarded while the remaining 700 mL of end-alveolar airs collected in a gas-tight bag. Subjects were instructed tovoid deep inspiration and to not hyperventilate before exha-ation.11 A gas chromatograph dedicated to the detection ofydrogen and methane in air samples was used for breathample analysis (model DP12; Quintron Instrument). Theccuracy of the detector was �2 ppm, with a linear-responseange between 2 and 150 ppm of hydrogen and methane.

Hydrogen excretion in the breath was measured in allatients after the administration of 400 mL of semiskimmedilk, which contains 20 g of lactose. According to conven-

ional procedure, we collected breath samples for hydrogeneasurement during fasting and every 30 minutes for 4 hours

eriod. To evaluate the diagnostic accuracy of the new crite-ia, we also collected breath samples 5, 6, and 7 hours afterhe patients drank the milk. The positivity of each test result,onventional and new, on the basis of the absolute value ofreath hydrogen excretion greater than 6 ppm at hour 6 andhe absolute value of the sum of breath hydrogen excretion atours 5, 6, and 7. Breath methane excretion represents theost important metabolic pathways of hydrogen consump-

ion.12 However, because we have previously shown that theiagnostic accuracy of the new criteria is not modified byethane excretion, we do not consider it.3

To rule out the possibility of a false negative readingesulting from unknown hydrogen nonproducer status,13 weubjected all patients to breath hydrogen measurement afteractulose administration on a different day. Ten grams ofactulose in a 400-mL water solution was administered, afterhich air samples were collected every 15 minutes for 8ours. A patient was considered a hydrogen nonproducer ifone of his or her breath samples showed a breath hydrogenxcretion value greater than 20 ppm.

Moreover, it was necessary to rule out the possibility thatncrease in breath hydrogen excretion after lactose adminis-ration reflected the washout of previously ingested carbohy-rates from the small bowel14 or enhanced colonic stirring,hich would liberate hydrogen trapped in feces.12 As de-

cribed in previous articles by our group,12,13,15 a nonabsorb-ble electrolyte/polyethylene glycol solution (Colyte; Reed &arnrick, Piscataway, NJ) delivering a volume of fluid equiv-lent to that of lactose solution to the colon, was administeredo all patients. Breath hydrogen excretion was then measuredver the course of 8 hours.The three tests were performed in random order, with

ntervals of at least 4 weeks between them. During the pro-ocol period, subjects were asked to follow their usual dietaryegimen. Symptoms of intolerance were monitored during thehours of the test. Before administration of the test solution

nd every hour during the test, subjects were asked to indicatehe presence and severity of symptoms (abdominal pain,

loating, diarrhea, flatulence) as a cumulative score on a scale

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J Lab Clin MedVolume 144, Number 6 Di Stefano et al 315

anging from 0 to 5 on which 0 indicated an absence ofymptoms, 1 denoted trivial symptoms, 2 reflected mild (dis-inct but negligible) symptoms, 3 indicated moderate symp-oms, 4 reflected strong (annoying) symptoms, and 5 denotedevere (disabling) symptoms.16,17 A mean score for eachubject was then calculated.

Statistical analysis. Data are expressed as mean � SD.e used the �2 test to assess differences between proportions.values of less than .05 were considered significant.

ESULTS

The conventional criterion indicated lactose malab-orption in 43 patients (51%) (Table I). On the otherand, 64 (76%) were indicated to be positive for mal-bsorption according to the criterion of the 6th-houreading, and 68 (80%) were shown to be positive ac-ording to the criterion of the sum of breath hydrogenxcretion readings at hours 5, 6, and 7 after milkngestion (P � .05 vs the conventional criterion foroth criteria). Use of the new criteria resulted in aositive findings in 51% and 61% of formerly negativeatients, respectively. None of the patients was found toe a hydrogen nonproducer, regardless of criterion. Onhe contrary, in 1 patient ingestion of the electrolyteolution resulted in a positive reading.

The difference between peak and fasting values ofreath hydrogen excretion was significantly correlatedith the absolute value of breath hydrogen excretion at

he 6th hour (r � .88) and with the sum of breath-ydrogen excretion at the 5th, 6th, and 7th hours (r �85). Patients who were positive for malabsorption ac-ording to the conventional criterion were also positivehen assessed with the use of the new criteria (Figs 1

nd 2). Among the subjects who proved negative foralabsorption on the basis of the old criterion and

ositive on the basis of the 6th-hour criterion (Fig 1),0 of 21 were found to be lactose intolerant. Of theubjects who proved negative for malabsorption ac-

able I. Positive findings after LactoseMalabsorber Patients), Lactulose (positiveontrol), and Colyte (negative control) according

o the different criteria used in 84 patients

Criterion Positive findings

Hydrogen

Hours aftercarbohydrate

exposure Lactose Lactulose Colyte

�20 ppm 1–4 43/84 77/84 0/846 ppm 6 64/84 84/84 1/8415 ppm 5–7 68/84 84/84 1/84

� change from baseline.

ording to the old criterion and positive according to i

he criterion of the sum of readings at hours 5, 6, and 7Fig 2), intolerance was demonstrated in 11 of 25. Weetected no significant correlation between severity ofymptoms and the level of breath hydrogen excretion.

ISCUSSION

The first description of the presence of hydrogen gasn expired air dates back to the early 1960s, in a reporty Nielsen and coworkers.18 Toward the end of thatecade, Calloway suggested the measurement of breathydrogen excretion in expired air as a means of mon-toring intestinal gas production.19 It was, however, thereat work of Bond and Levitt20,21 that made it possibleo introduce the hydrogen breath test into clinical prac-ice. Although some 30 years have passed since itsntroduction, many aspects of the test still require clar-

ig 1. Comparison of the conventional criterion and the new criterionf a breath hydrogen excretion value � 6 ppm 6 hours after lactulosexposure. � value represents the increase in breath hydrogen excre-ion over the fasting level within 4 hours of carbohydrate ingestion.he conventional criterion is based on an increase over the fastingalue greater than 20 ppm. The vertical and horizontal dotted linesndicate the cutoff points of the conventional and new criterion,espectively.

fication. One of the most important topics is undoubt-

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dly the criterion of positivity. Previous reports fromur group3,13 demonstrate that the diagnostic accuracyf the positivity criterion may be affected on one handy the cutoff value and on the other by the duration ofhe test. In fact, as expected, the reduction of theositivity cutoff value from 20 to 10 ppm leads to anncrease in sensitivity but a reduction in specificity.hese data confirm the results of previous papers,hich describe a similar pattern for diagnostic accu-

acy.7,22 Moreover, prolonging the duration of the testeyond the widely used duration of 4 hours after thedministration of lactulose, a nonabsorbable disaccha-ide, to test for hydrogen production makes it possibleo observe a progressively lower prevalence of hydro-

ig 2. Comparison of the conventional criterion and the new criterionased on a value greater than 15 ppm resulting from the addition ofreath hydrogen excretion values taken 5, 6, and 7 hours afteractulose exposure. � value represents the increase in breath hydro-en excretion over the fasting level within 4 hours of carbohydratengestion. The conventional criterion is based on an increase over theasting value greater than 20 ppm. The vertical and horizontal dottedines indicate the cutoff points of the conventional and new criterion,espectively.

en nonproducers. Two new diagnostic criteria for lac- s

ose malabsorption were therefore formulated,3 withhe experimental carbohydrate malabsorption modelepresented by the administration of lactulose.3 Theccuracy of these two criteria proved greater than thatf the conventional criterion: unexpected improve-ents in both sensitivity and specificity were evident,

ccompanied by simplification of the method used. Inact, the adoption of these criteria does not require strictetermination of fasting levels; these are based onbsolute values and not on the difference between peaknd fasting values. This provides at least two advan-ages: first, a lower number of samples may be col-ected, namely those at hours 5, 6, and 7; second, bothhorter hospitalizations and shorter duration of involve-ent of health-care workers may be achieved.These new criteria, although generally accepted,4

ave still not been adopted as standard clinical practice;n this study we therefore sought to further validatehese criteria in the diagnosis of lactose malabsorption,till on the basis of the increase in breath hydrogenxcretion over the baseline value greater than 20 ppmithin 4 hours of lactose ingestion.2,20

This is not an easy task, given the absence of a test orarameter that could represent the gold standard for theiagnosis of lactose malabsorption. The determinationf lactase activity with the use of intestinal biopsy maye misleading. Previous studies involving the measure-ent of lactase activity have yielded conflicting re-

ults,23,24 probably as a result of the marked effect ofmall differences in biopsy site.25 However, the meth-dology described in a previous paper by our group3

ay be considered a useful and easy tool with which toefine the accuracy of the criterion used.In this study, we adopted the following procedure: a

ydrogen producer subject responds to carbohydratealabsorption with an increase in intestinal hydrogen

roduction and, therefore, an increase in breath hydro-en excretion. False-positive results could be caused byhe washout of previously ingested carbohydrates fromhe small bowel14 or enhanced colonic stirring that inurn liberates hydrogen entrapped in the feces.12 Weherefore administered a quantity of nonabsorbable,onfermentable electrolyte solution, equivalent to thatf lactose solution, to all patients as a means of rulingut this possibility.3,15 On the contrary, false-negativeesults, the product of hydrogen nonproducer status,ere excluded with the administration of a fermentable

olution. In this way, it can reasonably be presumedhat in each of our patients, a negative finding on breathesting after oral lactose administration is really indic-tive of the nonarrival of the carbohydrate in the colonie, normal absorption) and that a positive reading iseally the result of the fermentation of the carbohydrate

olution administered (ie, malabsorption). With these

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J Lab Clin MedVolume 144, Number 6 Di Stefano et al 317

reliminary remarks, we can calculate with certaintyhe specificity of the new criteria, which proved similaro that of the conventional criteria.

As far as sensitivity is concerned, we can now shiftur attention to symptom occurrence. The improvediagnostic accuracy of breath tests conducted in accor-ance with the new criteria was achieved in concertith a reduction in the oral carbohydrate load.3 Previ-us studies were performed with the use of an oral dosef 50 g of lactose—that is, the amount of lactoseontained in about a liter of milk. The ingestion of sucharge amounts of carbohydrate provokes intoleranceymptoms in most people with lactose malabsorption,26

nd therefore the presence of intolerance symptomsas considered an important diagnostic tool.27,28 It haseen shown that a reduction in the carbohydrate load ton amount more frequently introduced by people allowsmore physiologic evaluation and is accompanied by a

ower incidence of symptoms.29,30 As a consequence,fter exposure to a low amount of lactose, the positiveredictive value of a symptom is high; on the contrary,he negative predictive value of a symptom must beonsidered low. In this light, intolerant subjects whoroved positive according to the new criteria but neg-tive according to the old ones should be considered toave been affected by lactose malabsorption and intol-rance. The adoption of the new criteria therefore im-roves the diagnostic accuracy of the test. The lactose-alabsorber status of asymptomatic subjects who are

egative with the old criteria and positive with the newnes can only be clarified if an effective gold standards available.

Finally, the prevalence of lactose malabsorption inur series of patients was high, reaching 80%. How-ver, this result is not surprising, for at least two rea-ons: first, it was shown that the prevalence of lactosealabsorption in the Italian general population may be

igher than 70%31 and therefore particularly high; sec-nd, our series of patients is not representative of thetalian general population because it included severalatients with intestinal organic diseases responsible forhe presence of a secondary lactase deficiency. Our aimas to evaluate the accuracy of new criteria, so we

nrolled patients with miscellaneous gastrointestinalisorders, who were at greater risk of lactose malab-orption.

In conclusion, the adoption of the 6th-hour and theriterion based on the sum of hydrogen breath excretiont the 5th 6th and 7th hours makes it possible toiagnose lactose malabsorption in 24% and 27% ofatients, respectively, who were negative according tohe old criterion. On the basis of the improved sensi-ivity achieved with these criteria, it is possible to

uggest that the old one be abandoned.

The adoption of new criteria seems to be useful inlinical practice, for at least two reasons: first, becausehey do not require the determination of fasting levels,ewer samples may be collected and a shorter durationf hospitalization is required, necessitating a shortereriod of involvement of health-care workers. Second,he new criteria are more accurate, and improvementsn both sensitivity and specificity are accompanied byimplification of the method used.

EFERENCES

1. Levitt MD, Donaldson RM. Use of respiratory hydrogen (H2)excretion to detect carbohydrate malabsorption. J Lab Clin Med1970;75:937–45.

2. Metz G, Jenkins DJA, Peters JJ, Newman A, Blendis LM. Breathhydrogen as a diagnostic methods for hypolactasia. Lancet 1976;I:1155–8.

3. Strocchi A, Corazza GR, Ellis CJ, Gasbarri G, Levitt MD.Detection of malabsorption of low doses of carbohydrate: accu-racy of various breath H2 criteria. Gastroenterology 1993;105:1404–10.

4. Fernandez-Banares F, Gassull MA. Accuracy of breath H2 cri-teria to detect carbohydrate malabsorption. Gastroenterology1994;107:323–4.

5. Strocchi A, Corazza GR, and the Tenue Study Group. Qualitycontrol study of H2 breath testing for the diagnosis of carbohy-drate malabsorption in Italy. Ital J Gastroenterol Hepatol 1997;29:122–7.

6. Di Stefano M, Veneto G, Malservisi S, Cecchetti L, Minguzzi L,Strocchi A, et al. Lactose malabsorption and intolerance andpeak bone mass. Gastroenterology 2002;122:1793–9.

7. Kotler OP, Holt PR, Rosensweig NS. Modification of the breathhydrogen test increased sensitivity for the detection of carbohy-drate malabsorption. J Lab Clin Med 1982;100:798–805.

8. Thompson DG, O’Brien JD, Hardie JM. lnfluence of the oropha-ryngeal microflora on the measurements of exhaled breath hy-drogen. Gastroenterology 1986;191:853–60.

9. Rosenthal A, Solomons NW. Time-course of cigarette smokecontamination of clinical hydrogen breath-analysis tests. ClinChem 1983;29:1980–1.

0. Payne DL, Welsh JD, Claypool PL. Breath hydrogen (H2) re-sponse to carbohydrate malabsorption after exercise. J Lab ClinMed 1983;102:147–50.

1. Strocchi A, Ellis C, Levitt MD. Reproducibility of measurementsof trace gas concentrations in expired air. Gastroenterology 1991;101:175–9.

2. Strocchi A, Levitt MD. Factors affecting hydrogen productionand consumption by human fecal flora. The critical roles ofhydrogen tension and methanogenesis. J Clin Invest 1992;89:1304–11.

3. Corazza GR, Strocchi A, Sorge M, Benati G, Gasbarrini G.Prevalence and consistency of low breath H2 excretion followinglactulose ingestion. Possible implications for the clinical use ofthe H2 breath test. Dig Dis Sci 1993;38:2010–6.

4. Kerlin P, Zinsmeister A, Phillips S. Motor response to food of theileum, proximal colon, and distal colon in healthy humans.Gastroenterology 1983;84:762–70.

5. Di Stefano M, Miceli E, Malservisi S, Missanelli A, Strocchi A,Corazza GR. Mixing of the intestinal content and variations offermentation capacity do not affect the results of hydrogen breath

test. Am J Gastroenterol 2003;98:1584–7.

1

1

1

1

2

2

2

2

2

2

2

2

2

2

3

3

J Lab Clin Med318 Di Stefano et al December 2004

6. Suarez FL, Savaiano DA, Levitt MD. A comparison of symp-toms after the consumption of milk or lactose-hydrolized milk bypeople with self-reported severe lactose intolerance. N EnglJ Med 1995;333:1–4.

7. Briet F, Pochart P, Marteau P, Flourie B, Arrigoni E, RambaudJC. Improved clinical tolerance to chronic lactose ingestion insubjects with lactose intolerance: a placebo effect? Gut 1997;41:632–5.

8. Nielsen JP. Trace constituents in breath as related to flatulence.Proceedings of the Fifth Annual Dry Bean Research Conference;1961; Denver, CO. Western Regional Research Laboratory,United States Department of Agriculture; 1961. p. 49.

9. Calloway DH. Respiratory hydrogen and methane as affected byconsumption of gas-forming foods. Gastroenterology 1966;51:383–9.

0. Levitt MD. Production and excretion of hydrogen gas in man.N Engl J Med 1969;281:122–7.

1. Levitt MD, Bond JH. Volume, composition, and source of in-testinal gas. Gastroenterology 1970;59:921–9.

2. Rosado JL, Solomons NW. Sensitivity and specificity of thehydrogen breath-analysis test for detecting malabsorption ofphysiological doses of lactose. Clin Chem 1983;29:545–8.

3. Welsh JD, Poley JR, Bhatia M, Stevenson DE. Intestinal disac-charidase activities in relation to age, race, and mucosal damage.Gastroenterology 1978;75:847–55.

4. Wallis JL, Lipski PS, Mathers JC, James OF, Hirst BH. Duode-

nal brush-border mucosal glucose transport and enzyme activi-

ties in aging man and effect of bacterial contamination of thesmall intestine. Dig Dis Sci 1993;38:403–9.

5. Triadou N, Bataille J, Schmitz J. Longitudinal study of thehuman intestinal brush border membrane proteins. Distributionof the main disaccharidases and peptidases. Gastroenterology1983;85:1326–32.

6. Newcomer AD, McGill DB, Thomas PJ, Hofmann AF. Toler-ance to lactose among lactase deficient American Indians. Gas-troenterology 1978;74:44–6.

7. Solomons NW, Garcia-Ibanez R, Viteri FE. Hydrogen breath testof lactose absorption in adults: the application of physiologicaldoses and whole cow’s milk sources. Am J Clin Nutr 1980;33:545–54.

8. Veligati LN, Treem WR, Sullivan B, Burke J, Hyams JS.�10ppm versus �20ppm: a reappraisal of diagnostic criteria forbreath hydrogen testing in children. Am J Gastroenterology1994;89:758–61.

9. Suarez FL, Savaiano DA, Levitt MD. A comparison of symp-toms after the consumption of milk or lactose-hydrolyzed milkby people with self-reported severe lactose intolerance. N EnglJ Med 1995;333:1–4.

0. Suarez FL, Savaiano DA, Arbisi P, Levitt MD. Tolerance to thedaily ingestion of two cups of milk by individuals claiminglactose intolerance. Am J Clin Nutr 1997;65:1502–6.

1. Burgio GR, Flatz G, Barbera C, Patane R, Boner A, Cajozzo C,et al. Prevalence of primary adult lactose malabsorption andawareness of milk intolerance in Italy. Am J Clin Nutr 1984;39:

100–4.