ANNUAL REVIEWS
Annu.
Rev. Med. 1990. 4U41-48
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Annual
Reviews
Inc. All rights reserved
Further
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LACTOSE INTOLERANCE
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Hans A. Buller, M.D., and Richard J. Grand, M.D.
Divisions of Pediatric Gastroenterology and Nutrition, Departments of Pediatrics, Academic Medical Center, Amsterdam, The Netherlands; and The Floating Hospital, New England Medical Center, Tufts University School of Medicine, Boston, Massachusetts 02 1 1 1 KEY
WORDS:
lactase, phlorizin hydrolase, glycosylceramidase, genetics, development.
ABSTRACT Lactose intolerance is a prevalent clinical problem. Low lactase levels result either from intestinal injury, or as in the majority of the world's adult population, from alterations in the genetic expression of lactase phlorizin hydrolase. Progress is being made in the basic understanding of the molecular and ceIlular biology of this enzyme and of the scientific basis of clinical syndromes involving low lactase activity. GENERAL INTRODUCTION
Lactose (milk sugar) is a key nutrient in mammalian milk, comprising the major carbohydrate source during the neonatal period. Lactose is, from an evolutionary as wel1 as from a biological viewpoint, a unique sugar as only in milk does it exist as a free molecule. It is synthesized by lactose synthetase, exclusively in the mammary gland of virtuaIly all placental mammals [except the sea lion (1)], during late pregnancy and lactation. Lactose concentration in milk is inversely related to the content of fat and protein (2); human milk contains the highest concentration (7%) of lactose. Lactose is hydrolyzed to glucose and galactose by lactase, or more precisely, lactase-phlorizin hydrolase (LPH), an intrinsic microvillus mem brane glycoprotein with at least three characteristic enzyme activities: lactase (p-D-galactoside galactohydrolase, Ee 3.2.1.23), phlorizin hydro lase (phlorizin glucohydrolase, EC 3.2. 1.62) and glycosylceramidase (EC 14 1 0066--4219/90/040 1- 0 14 1$02.00
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
142
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&
GRAND
3.2.1.45-46) (3, 4). In addition, LPH is one of the three enterocyte enzymes with fJ-galactosidase activity; the others are lysosomal acid fJ-galactosidase (pH optimum 4.5, which probably does not contribute to dietary lactose hydrolysis) and a cytosolic p-galactosidase reportedly with no specificity for lactose (5). In contrast to the other disaccharidases, sucrase-isomaltase and maltase glucoamylase, lactase activity is rate limiting in the absorption of lactose (6). The location of the enzyme on the villus-crypt axis (with maximal expression at the upper viJlus) makes it particularly sensitive to viJlus injury (7). Neither prolonged ingestion of lactose in humans (1) nor exclusion of lactose from the diet (8) influences the capacity of the smaJl intestine to absorb lactose, which strongly suggests that the enzyme activity is not directly regulated by availability of substrate. Thus, human LPH differs from that in other mammals in which lactase-specific activity has been reported to increase in response to augmentation of the carbohydrate content of the diet (9). In such experiments, lactase-specific activity is thought to rise as a consequence of hyperphagia and reduced enzyme degradation. DEFINITION OF CLINICAL SYNDROMES
Many terms have been used to describe clinical symptoms induced by the ingestion of milk and/or milk products. Typical complaints include abdominal pain, cramps, or distention; nausea; flatulence; and diarrhea. In children and adolescents, vomiting may predominate. While patients commonly identify these symptoms as a consequence of milk intolerance, they can be based either on the inability to digest lactose or sensitivity to milk proteins (l0). Lactose intolerance is characterized by symptoms, as described above, after the ingestion of a test dose of lactose in water or milk. The term lactose malabsorption is reserved for those patients in whom the intestinal malabsorption of lactose has been confirmed using an appropriate test of lactose absorption (lactose absorption test) or malab sorption (lactose breath hydrogen test) (11). Lactase deficiency is defined only when low ( < 2 standard deviations below the mean), or very rarely no, level of lactase activity is found in a small intestinal biopsy sample appropriately assayed (5, 12). Lactase deficiency can be either a primary or secondary event. Primary lactase deficiency occurs as a developmental process in premature infants, or as a rare clinical syndrome (13, 14). It also appears as "late onset lactase deficiency" in the majority of the world's population around the age of five years. Secondary lactase deficiency is found following mucosal injury (13). The term late onset lactase deficiency is really a misnomer, as discussed below.
LACTOSE INTOLERANCE
143
DIAGNOSIS AND TREATMENT Diagnosis
The diagnosis of lactose malabsorption is based on a combination of clinical findings and the results of appropriate tests ( 1 1, 15-19). The use of screening tests for lactose malabsorption in the diagnosis of lactose intolerance has received wide attention ( 1 1).
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Fecal pH and Reducing Substances
The presence of low fecal pH and reducing substances indicates lactose malabsorption, but these tests are only valid when lactose has been ingested, intestinal transit time is rapid, stools are collected fresh and assayed immediately, and when bacterial metabolism of colonic carbo hydrate is incomplete ( 15). In general, lactose malabsorption is best con firmed using more specific tests. Lactose Absorption Test and Lactose Breath Hydrogen Test
The capacity for lactose absorption can be measured using the lactose absorption test (II). In adults, it has a sensitivity of 75% and a specificity of 96%. However, in children it is cumbersome, invasive, and time con suming and has largely been replaced by the lactose breath hydrogen test ( 17, 19). Although this latter test really measures lactose nonabsorption rather than lactose hydrolysis and monosaccharide uptake, its sensitivity (100%) and specificity (100%) (II) are supcrior to those for the lactose absorption test, and it is simple and noninvasive (II, 15-20). Lactase Activity in Intestinal Biopsy
The assay of lactase activity in small bowel biopsy samples establishes the presence of lactase deficiency and has becn used to define populations with "late onset lactase deficiency" (12). However, when lactase deficiency accompanies intestinal injury, the lesion may be focal or patchy; conse quently, intestinal biopsy samples may not yield an abnormal result. This test is invasive and time consuming, and assays are available only in a few centers. Normal values have been published ( 12). Comparison 0/ Tests
Studies are available comparing intestinal histology, lactase activity, and breath hydrogen test results in children with chronic diarrhea or abdominal pain ( 16, 20). In patients with abnormal intestinal histology, approximately 75% had abnormal lactose breath hydrogen values (sensitivity 75%). However, in patients with normal histology, the lactose breath hydrogen test was only 54% specific, a reflection of the presence of a patchy villus
144
BULLER & GRAND
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
lesion. Very similar values were found when lactase activities on biopsy were compared with results of lactose breath hydrogen tests (18, 20). Accordingly, an abnormal lactose breath hydrogen test is of value in identifying those patients who will be symptomatic after lactose ingestion. In children less than five years of age, an abnormal lactose breath hydrogen test always signifies abnormal intestinal mucosa, which usually needs fur ther definition with a small intestinal biopsy. In such cases, a normal lactose breath hydrogen test does not rule out an intestinal mucosal abnormality and should not be used to avoid an intestinal biopsy in the diagnosis of suspected mucosal disease [e.g. gluten-sensitive enteropathy (20)]. Treatment of Lactose Intolerance
The treatment of lactose intolerance includes four general principles: (a) reduction or restriction of dietary lactose, (b) substitution of alternative nutrient sources to avoid reduction in energy intake, (c) regulation of calcium intake, and (d) use of a commercially available enzyme substitute. When lactose restriction is necessary, the patient must be instructed to rcad labels of commercially prcpared foods, as hidden lactose may be difficult to identify. Calcium is supplemented in the form of calcium car bonate (Tums® and OsCal® are popular and effective). In infants, liquid calcium gluconate is readily tolerated and available (21). Commercially available "lactase" preparations are actually bacterial or yeast f3-galactosidases. When added to lactose-containing food or ingested with meals containing lactose, these are effective in reducing symptoms and breath hydrogen values in many lactose-intolerant subjects (22). However, they are not capable of completely hydrolyzing all dietary lactose, and the results achieved in individual patients are variable. Livecculture yogurt, which contains endogenous f3-galactosidase, is a useful alternative source of both calcium and calories and may be well tolerated by a number of lactose-intolerant patients (23). LACTASE DEVELOPMENT AND GENETICS LPH Development
The specific activity of LPH (expressed in units per mg protein) in the intestine of virtually all mammals exhibits a well-described developmental pattern ( 1, 24). Namely, there is a late gestational rise, with a peak in specific activity shortly after birth, and then a fall during weaning to the low levels that are seen in adulthood (I, 25). A somewhat different pattern is found in human intestine, one that depends upon genetic factors. In the majority of the world's population who develop "late onset lactase deficiency" in mid childhood, the pattern is similar to that in other mam-
LACTOSE INTOLERANCE
145
mals; a late gestational (27-32 weeks) rise, followed by persistence of lactase-specific activity until approximately age 5-7 years, with a fall there after to low adult levels (3). In the Caucasian population, especially those peoples from or derived from Northern Europe, and certain localized clusters of other racial origins, lactase-specific activity rises late in gestation and remains at, or slightly below, this level throughout adult life (3).
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Genetics
of LPH
Extensive population studies have permitted an hypothesis for the genetic transmission of various lactose digestion phenotypes (3). Available data indicate that persistence of high levels of LPH enzyme is probably an autosomal-dominant trait. The concept of "late onset lactase deficiency" is based on the acceptance of this persistence of lactase activity as "normal," while the fall in lactase-specific activity has been considered "abnormal." However, this interpretation must be reevaluated because it is now clear that persistence of the capacity for lactose digestion in humans emerged over a span estimated to be a minimum of 10,000 years in at least three loci around the world (3), while the majority of the world's population, and virtually all placental mammals studied, show a reduction of LPH activity in adulthood. Examples of the prevalence of these genetic patterns are shown in Table 1.
Table 1
Prevalence of "late onset lactose malabsorption"
reported for various ethnic or racial groupsa Prevalence of lactose Group Orientals in
the US
American Indian� (Oklahoma) lbo, Yoruba (Nigeria) Black Americans Itali a ns
Aborigines (Aus tralia ) Mexican Americans Greeks White Americans
malabsorption
(%)
100 95 S9 81 71 67 56 53 24
Danes
3
Dutch
o
"Adapted from References 3 and 13. These values are derived
from
several dife f rent
Nevertheless , they are useful in assessing lactose intolerance in the clinical
selling. when lhe genetic background of the palient must be considered.
146
BULLER
&
GRAND
LPH Distribution, Enteric Flora, and Hormones
Lactase activity is not constant throughout the length of the small intestine, but maintains a characteristic proximal-to-distal gradient. In mature intes tine, maximal activity occurs in the proximal to mid jejunum, with lower activity in the duodenum and ileum (26). In view of the relatively low lactase levels in infants born prematurely (28 32 weeks), it is of interest that few develop clinical signs of carbohydrate intolerance. MacLean & Fink (27) demonstrated that lactose is malabsorbed in these infants and reaches the colon, where it is salvaged by colonic flora. In animal studies, hormonal and nutritional factors have been thought to control the devel opmental pattern of LPH at the time of weaning (9, 24, 28). Adrenal and thyroid hormones play a role in LPH regulation, but the precise contribution of each remains to be defined. Whether or not LPH is under hormonal control in humans is currently unknown.
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
-
CELLULAR AND MOLECULAR BIOLOGY OF LPH Despite a sizeable literature devoted to the measurements of LPH activity in various animal and human populations ( l , 3), and despite many studies oflactose intolerance in humans (3), there remain large gaps in our knowl edge of the genetic control, synthesis, glycosylation, and processing of LPH. Available data reveal that LPH is synthesized as a large single-chain precursor glycoprotein with a molecular mass of 220 kilodaltons in the rat and 215 kilodaltons in the human (29-31), and it is processed to a mic rovillus membrane form of 130 and 160 kilodaltons, respectively. Only recently has the primary structure of human and rabbit LPH been deduced from the sequence of the cloned cDNA (32). In humans, the gene may be localized on chromosome 2 (33). Computer analysis of the sequence indicated a four-fold internal homology, which suggests partial gene dupli cation. Although LPH is generally thought to hydrolyze only lactose and phlori zin, the enzyme is capable of cleaving both the f3-g1ucosyl and f3-galactosyl linkages as found in glycosylceramides (4, 34-36). These glycolipids are found in mammalian milk, eggs, meat, and green leafy vegetables. Inhi bition studies, as well as heat inactivation, suggest the existence of at least two active sites in LPH (4, 37). In addition, recent studies in rats demonstrate that total LPH activity persists in adult small intestine, and thus LPH may play a role (presumably glycolipid digestion) beyond the period when lactose ingestion pre dominates (4). Although the persistence of LPH would appear to be in conflict with the pattern of decreasing specific activity of the enzyme during
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
LACTOSE INTOLERANCE
147
development, the findings merely indicate that the quantity of LPH per enterocyte decreases at a time when the number of enterocytes increases dramatically. This concept is corroborated by the observation that LPH synthesis falls in parallel with the decline in lactase-specific activity in rats during weaning (38). A similar decline in LPH synthesis has been found in humans considered to be lactase deficient (39, 40). It is now clear that significant levels of mRNA for LPH also persist in adult rat intestine, and that the quantity of mRNA during postnatal development and the quantity of LPH protein follow a similar pattern (4 1). In view of these observations, knowledge of the molecular mechanisms involved in the developmental changes in LPH in animals will provide the basis for future studies of the regulation of LPH gene expression in human intestine. Literature Cited 1. Kretchmer, N. 1971. Gastroenterology 61: 805-13 2. Palmiter, R. D. 1969. Nature 221: 91214 3. Platz, G. 1987. Advances in Human Gen etics 16: 1-77. New York: Plenum 4. Buller, H. A., van Wassenaer, A. G., Raghavan, S., Montgomery, R. K., Sybicki, M. A., Grand, R. 1. 1989. Am. J. Physiol. 257: G616-23 5. Asp, N. G., Dahlqvist, A. 1972. Anal. Biochem. 47: 527-38 6. Alpers, D. H. 1986. Physiology of the Gastrointestinal Tract II, pp. 1469-88. New York: Raven. 1780 pp. 7. Boyle, 1. T., Celano, P., Ko1d ovsky, O. 1980. Gastroenterology 79: 503-7 8. Kogut, M. D. , Donnell, G. N., Shaw, K. N. F. 1967. J. Pedialr. 71: 75-81 9. Goda, T. , Bustamante, S. , Koldovsky, O. 1985. J. Pedialr. Gastroenterol. Nutr. 4: 998-1008 10. Grand, R. 1., Sutphen, J. L., Dietz, W. H., eds. 1987. Pediatric Nutrition. The ory and Practice, pp. 615-25. Boston, London: Butterworth. 829 pp. 1 I. Newconler, A. D., McGill, D. B., Thomas, P. J., Hofmann, A. F. 1975. N. Engl. J. Med. 293: 1232-36 12. Welsh, J. D., Poley, J. R., Bhatia, M. , Stevenson, D. E. 1978. Gastroenterology 75: 847-55 13. Mobassaleh, M., Montgomery, R. K., Biller, J. A., Grand, R. J. 1985. Pedi atrics 75: 160-66 14. Savilahti, E., Launiala, K., Kuitunen, P. 1983. Arch. Dis. Child. 58: 246-52 15. Newcomer, A. D. 1984. J. Pediatr. Gastroenterol. Nutr. 3: 6-8
1 6. Barr, R. G. , Perman, 1. A., Schoeller, D. A., Watkins, J. B. 1978. Pediatrics 62: 393-401 17. Douwes, A. c., Fernandes, J., Degen hardt, H. J. 1978. Arch. Dis. Child. 53: 939 42 18. Barr, R. G., Levine, M. D., Watkins, J. B. 1979. N. Engl. l Med. 300: 1449-52 19. Ostrander, C. R . , Cohen, R. S., Hopper, A. 0., Stevenson, D. K. 1983. J. Pediatr. Gastroenterol. Nutr. 2: 525-33 20. Hyams, J. S., Stafford, R. J., Grand, R. 1., Watkins, J. B. 1980. J. Pediatr. 97: 609-12 21. Grand, R. J., Sutphen, J. L., Dietz, W. H., eds. 1987. See Ref. 10, pp. 329, 344 22. Scrimshaw, N. S., Murray, E. B. 1988. Am. J. Clin. Nutr. 48(Supp1.): 1129-36 23. Newcomer, A. D., McGill, D. B. 1984. N. Engl. J. Med. 310: 42-43 24. Henning, S. J. 1981. Am. J. Physiol. 241: G199-214 25. Doell, R. G. , Kretchmer, N. 1962. Biochim. Biophys. Acta 62: 353-62 26. Newcomer, A. D., McGill, D. B. 1966. Gastroenterology. 51: 481-88 27. MacLean, W. c., Fink, B. B. 1980. J. Pediatr. 97: 383-88 28. Yeh, K. Y. , Moog, F. 1974. Science 182: 77-79 29. Buller, H. A., Montgomery, R. K., Sasak, W. V., Grand, R. J. 1987. J. Bioi. Chern. 262: 17206-11 30. Buller, H. A., Rings, E. H. H. M., Montgomery, R. K., Sasak, W. V., Grand, R. J. 1989. Biochern. J. 263: 249-54 .
.
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Naim, H. Y., Sterchi, E. E., Lentze, M. .J. 1987. Biochern. J. 241: 427-34 32. Mantei, N., Villa, M., Enzler, T., Wacker, H., Bull, W., et al. 1988. EM BO
37.
Birkenmeier, E., Alpers, D. H. 1974 . Biochirn. Biophys. Acta 350: 100-12
38.
Junas, M. M., Montgomery, R. K., Grand, R. J. 1985. Pediatr. Res. 19: 956-
33.
Skovbjerg, H., Gudmand-Hoyer, E., Fenger, H. J. 1980. Gut 21: 360-64 40. Witte, J., Lluyd, M., Korsmu, H., Lorenzsonn, V., Olsen, W. 1989. Gastro
31.
J. 7: 2705-13 Kruse, T. A., Bolund, L., Grzeschik, H.,
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Ropers, H. H., Sjostrom, H., et al. 1988. FEBS Lett. 240: 123-26 34. Brady, R. 0., Gal, A. E., Kanfer, J. N., Bradley, R. M. 1965. J. Bioi. Chern. 240: 3766-70 35. Kobayashi, T., Suzuki, K. 1981. J. Bioi. Chern. 256: 7768-73 36. Leese, H. J., Semenza, G. 1973. J. Bioi. Chern. 248: 8170-73
62 39.
41.
enterology 96: A549
Goldman, D. A., Buller, H. A., McLe more, B. R., Montgomery, R. K., Mat sudaira, P. T., Grand, R. J. 1989. Gastro enterology 96: A175
Annu.
Rev. Med. 1990. 4U41-48
Copyr(qhl © 1990 by
Annual
Reviews
Inc. All rights reserved
Further
Quick links to online content
LACTOSE INTOLERANCE
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Hans A. Buller, M.D., and Richard J. Grand, M.D.
Divisions of Pediatric Gastroenterology and Nutrition, Departments of Pediatrics, Academic Medical Center, Amsterdam, The Netherlands; and The Floating Hospital, New England Medical Center, Tufts University School of Medicine, Boston, Massachusetts 02 1 1 1 KEY
WORDS:
lactase, phlorizin hydrolase, glycosylceramidase, genetics, development.
ABSTRACT Lactose intolerance is a prevalent clinical problem. Low lactase levels result either from intestinal injury, or as in the majority of the world's adult population, from alterations in the genetic expression of lactase phlorizin hydrolase. Progress is being made in the basic understanding of the molecular and ceIlular biology of this enzyme and of the scientific basis of clinical syndromes involving low lactase activity. GENERAL INTRODUCTION
Lactose (milk sugar) is a key nutrient in mammalian milk, comprising the major carbohydrate source during the neonatal period. Lactose is, from an evolutionary as wel1 as from a biological viewpoint, a unique sugar as only in milk does it exist as a free molecule. It is synthesized by lactose synthetase, exclusively in the mammary gland of virtuaIly all placental mammals [except the sea lion (1)], during late pregnancy and lactation. Lactose concentration in milk is inversely related to the content of fat and protein (2); human milk contains the highest concentration (7%) of lactose. Lactose is hydrolyzed to glucose and galactose by lactase, or more precisely, lactase-phlorizin hydrolase (LPH), an intrinsic microvillus mem brane glycoprotein with at least three characteristic enzyme activities: lactase (p-D-galactoside galactohydrolase, Ee 3.2.1.23), phlorizin hydro lase (phlorizin glucohydrolase, EC 3.2. 1.62) and glycosylceramidase (EC 14 1 0066--4219/90/040 1- 0 14 1$02.00
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
142
BULLER
&
GRAND
3.2.1.45-46) (3, 4). In addition, LPH is one of the three enterocyte enzymes with fJ-galactosidase activity; the others are lysosomal acid fJ-galactosidase (pH optimum 4.5, which probably does not contribute to dietary lactose hydrolysis) and a cytosolic p-galactosidase reportedly with no specificity for lactose (5). In contrast to the other disaccharidases, sucrase-isomaltase and maltase glucoamylase, lactase activity is rate limiting in the absorption of lactose (6). The location of the enzyme on the villus-crypt axis (with maximal expression at the upper viJlus) makes it particularly sensitive to viJlus injury (7). Neither prolonged ingestion of lactose in humans (1) nor exclusion of lactose from the diet (8) influences the capacity of the smaJl intestine to absorb lactose, which strongly suggests that the enzyme activity is not directly regulated by availability of substrate. Thus, human LPH differs from that in other mammals in which lactase-specific activity has been reported to increase in response to augmentation of the carbohydrate content of the diet (9). In such experiments, lactase-specific activity is thought to rise as a consequence of hyperphagia and reduced enzyme degradation. DEFINITION OF CLINICAL SYNDROMES
Many terms have been used to describe clinical symptoms induced by the ingestion of milk and/or milk products. Typical complaints include abdominal pain, cramps, or distention; nausea; flatulence; and diarrhea. In children and adolescents, vomiting may predominate. While patients commonly identify these symptoms as a consequence of milk intolerance, they can be based either on the inability to digest lactose or sensitivity to milk proteins (l0). Lactose intolerance is characterized by symptoms, as described above, after the ingestion of a test dose of lactose in water or milk. The term lactose malabsorption is reserved for those patients in whom the intestinal malabsorption of lactose has been confirmed using an appropriate test of lactose absorption (lactose absorption test) or malab sorption (lactose breath hydrogen test) (11). Lactase deficiency is defined only when low ( < 2 standard deviations below the mean), or very rarely no, level of lactase activity is found in a small intestinal biopsy sample appropriately assayed (5, 12). Lactase deficiency can be either a primary or secondary event. Primary lactase deficiency occurs as a developmental process in premature infants, or as a rare clinical syndrome (13, 14). It also appears as "late onset lactase deficiency" in the majority of the world's population around the age of five years. Secondary lactase deficiency is found following mucosal injury (13). The term late onset lactase deficiency is really a misnomer, as discussed below.
LACTOSE INTOLERANCE
143
DIAGNOSIS AND TREATMENT Diagnosis
The diagnosis of lactose malabsorption is based on a combination of clinical findings and the results of appropriate tests ( 1 1, 15-19). The use of screening tests for lactose malabsorption in the diagnosis of lactose intolerance has received wide attention ( 1 1).
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Fecal pH and Reducing Substances
The presence of low fecal pH and reducing substances indicates lactose malabsorption, but these tests are only valid when lactose has been ingested, intestinal transit time is rapid, stools are collected fresh and assayed immediately, and when bacterial metabolism of colonic carbo hydrate is incomplete ( 15). In general, lactose malabsorption is best con firmed using more specific tests. Lactose Absorption Test and Lactose Breath Hydrogen Test
The capacity for lactose absorption can be measured using the lactose absorption test (II). In adults, it has a sensitivity of 75% and a specificity of 96%. However, in children it is cumbersome, invasive, and time con suming and has largely been replaced by the lactose breath hydrogen test ( 17, 19). Although this latter test really measures lactose nonabsorption rather than lactose hydrolysis and monosaccharide uptake, its sensitivity (100%) and specificity (100%) (II) are supcrior to those for the lactose absorption test, and it is simple and noninvasive (II, 15-20). Lactase Activity in Intestinal Biopsy
The assay of lactase activity in small bowel biopsy samples establishes the presence of lactase deficiency and has becn used to define populations with "late onset lactase deficiency" (12). However, when lactase deficiency accompanies intestinal injury, the lesion may be focal or patchy; conse quently, intestinal biopsy samples may not yield an abnormal result. This test is invasive and time consuming, and assays are available only in a few centers. Normal values have been published ( 12). Comparison 0/ Tests
Studies are available comparing intestinal histology, lactase activity, and breath hydrogen test results in children with chronic diarrhea or abdominal pain ( 16, 20). In patients with abnormal intestinal histology, approximately 75% had abnormal lactose breath hydrogen values (sensitivity 75%). However, in patients with normal histology, the lactose breath hydrogen test was only 54% specific, a reflection of the presence of a patchy villus
144
BULLER & GRAND
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
lesion. Very similar values were found when lactase activities on biopsy were compared with results of lactose breath hydrogen tests (18, 20). Accordingly, an abnormal lactose breath hydrogen test is of value in identifying those patients who will be symptomatic after lactose ingestion. In children less than five years of age, an abnormal lactose breath hydrogen test always signifies abnormal intestinal mucosa, which usually needs fur ther definition with a small intestinal biopsy. In such cases, a normal lactose breath hydrogen test does not rule out an intestinal mucosal abnormality and should not be used to avoid an intestinal biopsy in the diagnosis of suspected mucosal disease [e.g. gluten-sensitive enteropathy (20)]. Treatment of Lactose Intolerance
The treatment of lactose intolerance includes four general principles: (a) reduction or restriction of dietary lactose, (b) substitution of alternative nutrient sources to avoid reduction in energy intake, (c) regulation of calcium intake, and (d) use of a commercially available enzyme substitute. When lactose restriction is necessary, the patient must be instructed to rcad labels of commercially prcpared foods, as hidden lactose may be difficult to identify. Calcium is supplemented in the form of calcium car bonate (Tums® and OsCal® are popular and effective). In infants, liquid calcium gluconate is readily tolerated and available (21). Commercially available "lactase" preparations are actually bacterial or yeast f3-galactosidases. When added to lactose-containing food or ingested with meals containing lactose, these are effective in reducing symptoms and breath hydrogen values in many lactose-intolerant subjects (22). However, they are not capable of completely hydrolyzing all dietary lactose, and the results achieved in individual patients are variable. Livecculture yogurt, which contains endogenous f3-galactosidase, is a useful alternative source of both calcium and calories and may be well tolerated by a number of lactose-intolerant patients (23). LACTASE DEVELOPMENT AND GENETICS LPH Development
The specific activity of LPH (expressed in units per mg protein) in the intestine of virtually all mammals exhibits a well-described developmental pattern ( 1, 24). Namely, there is a late gestational rise, with a peak in specific activity shortly after birth, and then a fall during weaning to the low levels that are seen in adulthood (I, 25). A somewhat different pattern is found in human intestine, one that depends upon genetic factors. In the majority of the world's population who develop "late onset lactase deficiency" in mid childhood, the pattern is similar to that in other mam-
LACTOSE INTOLERANCE
145
mals; a late gestational (27-32 weeks) rise, followed by persistence of lactase-specific activity until approximately age 5-7 years, with a fall there after to low adult levels (3). In the Caucasian population, especially those peoples from or derived from Northern Europe, and certain localized clusters of other racial origins, lactase-specific activity rises late in gestation and remains at, or slightly below, this level throughout adult life (3).
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
Genetics
of LPH
Extensive population studies have permitted an hypothesis for the genetic transmission of various lactose digestion phenotypes (3). Available data indicate that persistence of high levels of LPH enzyme is probably an autosomal-dominant trait. The concept of "late onset lactase deficiency" is based on the acceptance of this persistence of lactase activity as "normal," while the fall in lactase-specific activity has been considered "abnormal." However, this interpretation must be reevaluated because it is now clear that persistence of the capacity for lactose digestion in humans emerged over a span estimated to be a minimum of 10,000 years in at least three loci around the world (3), while the majority of the world's population, and virtually all placental mammals studied, show a reduction of LPH activity in adulthood. Examples of the prevalence of these genetic patterns are shown in Table 1.
Table 1
Prevalence of "late onset lactose malabsorption"
reported for various ethnic or racial groupsa Prevalence of lactose Group Orientals in
the US
American Indian� (Oklahoma) lbo, Yoruba (Nigeria) Black Americans Itali a ns
Aborigines (Aus tralia ) Mexican Americans Greeks White Americans
malabsorption
(%)
100 95 S9 81 71 67 56 53 24
Danes
3
Dutch
o
"Adapted from References 3 and 13. These values are derived
from
several dife f rent
Nevertheless , they are useful in assessing lactose intolerance in the clinical
selling. when lhe genetic background of the palient must be considered.
146
BULLER
&
GRAND
LPH Distribution, Enteric Flora, and Hormones
Lactase activity is not constant throughout the length of the small intestine, but maintains a characteristic proximal-to-distal gradient. In mature intes tine, maximal activity occurs in the proximal to mid jejunum, with lower activity in the duodenum and ileum (26). In view of the relatively low lactase levels in infants born prematurely (28 32 weeks), it is of interest that few develop clinical signs of carbohydrate intolerance. MacLean & Fink (27) demonstrated that lactose is malabsorbed in these infants and reaches the colon, where it is salvaged by colonic flora. In animal studies, hormonal and nutritional factors have been thought to control the devel opmental pattern of LPH at the time of weaning (9, 24, 28). Adrenal and thyroid hormones play a role in LPH regulation, but the precise contribution of each remains to be defined. Whether or not LPH is under hormonal control in humans is currently unknown.
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CELLULAR AND MOLECULAR BIOLOGY OF LPH Despite a sizeable literature devoted to the measurements of LPH activity in various animal and human populations ( l , 3), and despite many studies oflactose intolerance in humans (3), there remain large gaps in our knowl edge of the genetic control, synthesis, glycosylation, and processing of LPH. Available data reveal that LPH is synthesized as a large single-chain precursor glycoprotein with a molecular mass of 220 kilodaltons in the rat and 215 kilodaltons in the human (29-31), and it is processed to a mic rovillus membrane form of 130 and 160 kilodaltons, respectively. Only recently has the primary structure of human and rabbit LPH been deduced from the sequence of the cloned cDNA (32). In humans, the gene may be localized on chromosome 2 (33). Computer analysis of the sequence indicated a four-fold internal homology, which suggests partial gene dupli cation. Although LPH is generally thought to hydrolyze only lactose and phlori zin, the enzyme is capable of cleaving both the f3-g1ucosyl and f3-galactosyl linkages as found in glycosylceramides (4, 34-36). These glycolipids are found in mammalian milk, eggs, meat, and green leafy vegetables. Inhi bition studies, as well as heat inactivation, suggest the existence of at least two active sites in LPH (4, 37). In addition, recent studies in rats demonstrate that total LPH activity persists in adult small intestine, and thus LPH may play a role (presumably glycolipid digestion) beyond the period when lactose ingestion pre dominates (4). Although the persistence of LPH would appear to be in conflict with the pattern of decreasing specific activity of the enzyme during
Annu. Rev. Med. 1990.41:141-148. Downloaded from www.annualreviews.org by University of Wyoming on 09/19/13. For personal use only.
LACTOSE INTOLERANCE
147
development, the findings merely indicate that the quantity of LPH per enterocyte decreases at a time when the number of enterocytes increases dramatically. This concept is corroborated by the observation that LPH synthesis falls in parallel with the decline in lactase-specific activity in rats during weaning (38). A similar decline in LPH synthesis has been found in humans considered to be lactase deficient (39, 40). It is now clear that significant levels of mRNA for LPH also persist in adult rat intestine, and that the quantity of mRNA during postnatal development and the quantity of LPH protein follow a similar pattern (4 1). In view of these observations, knowledge of the molecular mechanisms involved in the developmental changes in LPH in animals will provide the basis for future studies of the regulation of LPH gene expression in human intestine. Literature Cited 1. Kretchmer, N. 1971. Gastroenterology 61: 805-13 2. Palmiter, R. D. 1969. Nature 221: 91214 3. Platz, G. 1987. Advances in Human Gen etics 16: 1-77. New York: Plenum 4. Buller, H. A., van Wassenaer, A. G., Raghavan, S., Montgomery, R. K., Sybicki, M. A., Grand, R. 1. 1989. Am. J. Physiol. 257: G616-23 5. Asp, N. G., Dahlqvist, A. 1972. Anal. Biochem. 47: 527-38 6. Alpers, D. H. 1986. Physiology of the Gastrointestinal Tract II, pp. 1469-88. New York: Raven. 1780 pp. 7. Boyle, 1. T., Celano, P., Ko1d ovsky, O. 1980. Gastroenterology 79: 503-7 8. Kogut, M. D. , Donnell, G. N., Shaw, K. N. F. 1967. J. Pedialr. 71: 75-81 9. Goda, T. , Bustamante, S. , Koldovsky, O. 1985. J. Pedialr. Gastroenterol. Nutr. 4: 998-1008 10. Grand, R. 1., Sutphen, J. L., Dietz, W. H., eds. 1987. Pediatric Nutrition. The ory and Practice, pp. 615-25. Boston, London: Butterworth. 829 pp. 1 I. Newconler, A. D., McGill, D. B., Thomas, P. J., Hofmann, A. F. 1975. N. Engl. J. Med. 293: 1232-36 12. Welsh, J. D., Poley, J. R., Bhatia, M. , Stevenson, D. E. 1978. Gastroenterology 75: 847-55 13. Mobassaleh, M., Montgomery, R. K., Biller, J. A., Grand, R. J. 1985. Pedi atrics 75: 160-66 14. Savilahti, E., Launiala, K., Kuitunen, P. 1983. Arch. Dis. Child. 58: 246-52 15. Newcomer, A. D. 1984. J. Pediatr. Gastroenterol. Nutr. 3: 6-8
1 6. Barr, R. G. , Perman, 1. A., Schoeller, D. A., Watkins, J. B. 1978. Pediatrics 62: 393-401 17. Douwes, A. c., Fernandes, J., Degen hardt, H. J. 1978. Arch. Dis. Child. 53: 939 42 18. Barr, R. G., Levine, M. D., Watkins, J. B. 1979. N. Engl. l Med. 300: 1449-52 19. Ostrander, C. R . , Cohen, R. S., Hopper, A. 0., Stevenson, D. K. 1983. J. Pediatr. Gastroenterol. Nutr. 2: 525-33 20. Hyams, J. S., Stafford, R. J., Grand, R. 1., Watkins, J. B. 1980. J. Pediatr. 97: 609-12 21. Grand, R. J., Sutphen, J. L., Dietz, W. H., eds. 1987. See Ref. 10, pp. 329, 344 22. Scrimshaw, N. S., Murray, E. B. 1988. Am. J. Clin. Nutr. 48(Supp1.): 1129-36 23. Newcomer, A. D., McGill, D. B. 1984. N. Engl. J. Med. 310: 42-43 24. Henning, S. J. 1981. Am. J. Physiol. 241: G199-214 25. Doell, R. G. , Kretchmer, N. 1962. Biochim. Biophys. Acta 62: 353-62 26. Newcomer, A. D., McGill, D. B. 1966. Gastroenterology. 51: 481-88 27. MacLean, W. c., Fink, B. B. 1980. J. Pediatr. 97: 383-88 28. Yeh, K. Y. , Moog, F. 1974. Science 182: 77-79 29. Buller, H. A., Montgomery, R. K., Sasak, W. V., Grand, R. J. 1987. J. Bioi. Chern. 262: 17206-11 30. Buller, H. A., Rings, E. H. H. M., Montgomery, R. K., Sasak, W. V., Grand, R. J. 1989. Biochern. J. 263: 249-54 .
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Naim, H. Y., Sterchi, E. E., Lentze, M. .J. 1987. Biochern. J. 241: 427-34 32. Mantei, N., Villa, M., Enzler, T., Wacker, H., Bull, W., et al. 1988. EM BO
37.
Birkenmeier, E., Alpers, D. H. 1974 . Biochirn. Biophys. Acta 350: 100-12
38.
Junas, M. M., Montgomery, R. K., Grand, R. J. 1985. Pediatr. Res. 19: 956-
33.
Skovbjerg, H., Gudmand-Hoyer, E., Fenger, H. J. 1980. Gut 21: 360-64 40. Witte, J., Lluyd, M., Korsmu, H., Lorenzsonn, V., Olsen, W. 1989. Gastro
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J. 7: 2705-13 Kruse, T. A., Bolund, L., Grzeschik, H.,
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Ropers, H. H., Sjostrom, H., et al. 1988. FEBS Lett. 240: 123-26 34. Brady, R. 0., Gal, A. E., Kanfer, J. N., Bradley, R. M. 1965. J. Bioi. Chern. 240: 3766-70 35. Kobayashi, T., Suzuki, K. 1981. J. Bioi. Chern. 256: 7768-73 36. Leese, H. J., Semenza, G. 1973. J. Bioi. Chern. 248: 8170-73
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