Diet and Intestinal Disaccharidases1
Steve Hertzler
Yeonsoo Kim
Rubina Khan
Michelle Asp
Dennis Savaiano
1Abbreviations: LNP, lactase nonpersistence; LPH, lactase-phlorizin hydrolase; PBS, phosphate-buffered saline; PNG, pyridoxine-5′-β-D-glucoside; RER, rough endoplasmic reticulum; SI, sucrase-isomaltase; SNP, single nucleotide polymorphism.
Disaccharides are a significant source of carbohydrates in the diet. The major disaccharides include sucrose (O-α-D-glucopyranosyl-[1→2]-β-fructofuranoside), lactose (O-β-D-galactopyranosyl-[1→4]-β-glucopyranose), maltose (O-α-D-glucopyranosyl-[1→4]-β-glucopyranose), and trehalose (O-α-D-glucopyranosyl-[1→1]-α-glucopyranose). Lactose is the primary carbohydrate in human milk, which contains approximately 7% lactose by weight, among the highest of all mammalian milks (Table 76.1). Sucrose, lactose, and maltose comprise approximately 30%, 6%, and 1% to 2%, respectively, of the total carbohydrate in the diet (1). The majority of the maltose present in the intestine is derived from starch digestion, with only small amounts being contributed from grains and fermented beverages. The only significant dietary sources of trehalose are mushrooms and other fungi.
Because the small intestine is normally impermeable to disaccharides, intestinal disaccharidase activity is required for absorption of their component monosaccharides (2). In humans and other mammals, four known enzymes or enzyme complexes exist for disaccharide digestion: sucrase-isomaltase (SI), lactase-phlorizin hydrolase (LPH; lactase), maltase-glucoamylase, and trehalase (3). In contrast to the other enzymes that hydrolyze alpha-glucosidic bonds, lactase hydrolyzes beta-glucosidic bonds. Low levels of any of these enzymes in the intestinal mucosa results in carbohydrate malabsorption that also may be associated with clinical symptoms such as diarrhea, abdominal pain, and flatulence.
DEVELOPMENT OF BRUSH-BORDER DIGESTIVE ENZYMES
The carbohydrate-digesting enzymes of the small intestine are anchored in the brush border. The disaccharidases present include sucrase, lactase, glucoamylase, isomaltase, and trehalase. The substrates and products of each disaccharidase are shown in Table 76.2 (4). SI activity at 34 weeks of gestation reaches 70% of the adult level, rising to the adult level at birth (5). Glucoamylase and trehalase activities are detected at 13 weeks of gestation (6). Lactase activity develops later in gestation. The lactase activity is only 30% that of a full-term infant at 34 weeks of gestation and is still just 70% of the full-term level by 35 to 38 weeks (5).
The activity of disaccharidases in the brush border is recognized as the rate-limiting step in disaccharide digestion (7). Thus, congenital or acquired enzyme deficiencies cause poor absorption of disaccharides. In addition, loss of disaccharidase activity can occur secondary to damage to the intestinal mucosa because of certain diseases (e.g., alcoholism, celiac disease), infections, medications, surgery, or radiation exposure (8).
LACTASE-PHLORIZIN HYDROLASE
Location and Functions
The highest activity of LPH in humans is found in the jejunum, approximately 50 to 200 cm distal to the ligament of Treitz. Its activity is 25% lower at Treitz’s ligament and is minimal in the ileum (9). The gene for LPH is located on chromosome 2 and it directs the synthesis of a pre-pro form of LPH in the enterocytes. The pre-pro LPH is processed intracellularly (and possibly by pancreatic proteases) into the mature form that is anchored in the cell membrane at the brush border.
The human enzyme has two catalytic sites, both on the luminal side of the enterocyte cell membrane. These active sites, β-galactosidase (EC 3.2.1.23) and phlorizin hydrolase (EC 3.2.1.62), comprise Glu1749 in domain IV and Glu1273 in domain III, respectively (10). The β-galactosidase portion is able to hydrolyze lactose, cellobiose, o-nitro-phenyl-β-glucopyranoside, and o-nitrophenyl-β-galactopyranoside (10). Phlorizin hydrolase is able to hydrolyze phlorizin, β-glycopyranosyl-ceramides, and m-nitro-phenyl-β-glucopyranoside (10).
The human enzyme has two catalytic sites, both on the luminal side of the enterocyte cell membrane. These active sites, β-galactosidase (EC 3.2.1.23) and phlorizin hydrolase (EC 3.2.1.62), comprise Glu1749 in domain IV and Glu1273 in domain III, respectively (10). The β-galactosidase portion is able to hydrolyze lactose, cellobiose, o-nitro-phenyl-β-glucopyranoside, and o-nitrophenyl-β-galactopyranoside (10). Phlorizin hydrolase is able to hydrolyze phlorizin, β-glycopyranosyl-ceramides, and m-nitro-phenyl-β-glucopyranoside (10).
TABLE 76.1 LACTOSE CONTENT OF SELECTED DAIRY FOODS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Beyond its well-recognized role in lactose digestion, evidence indicates that LPH could be involved in the hydrolysis of other nutritionally important β-glucosides. For example, although the glycosylated forms of isoflavones and flavonoids occur in nature, only the aglycone forms can be absorbed from the intestine. It was previously assumed that the colonic microflora was mainly responsible for this deconjugation. However, two studies (11, 12) demonstrated that the lactase catalytic site of LPH is able to hydrolyze glycosylated isoflavones and flavonoids, making them available for absorption in the small intestine. Similarly, the hydrolysis of a β-glucosidic linkage is necessary to release pyridoxine from pyridoxine-5’-β-D-glucoside (PNG), an important step in increasing the bioavailability of this form of vitamin B6 that provides roughly 15% of the total vitamin B6 in a mixed diet (13). Mackey et al (13) reported that LPH purified from rat small intestinal mucosa possesses the capability to hydrolyze PNG in vitro.
TABLE 76.2 ROLE OF BRUSH-BORDER ENZYMES IN DIGESTION OF DISACCHARIDES AND STARCH | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||||||
Types of Hypolactasia and Lactase Nonpersistence
Full-term infants, regardless of racial or ethnic background, generally possess high levels of lactase activity. Congenital lactase activity is the rare condition in which lactase is absent at birth. Even in Finland, where the condition is most common, only 42 cases were reported from 1966 to 1998 (14). In these infants, lactase activity in jejunal biopsy specimens is reduced to 0 to 10 IU/g protein, and severe diarrhea results from unabsorbed lactose (14). Treatment with a lactose-free formula eliminates the diarrhea and promotes normal growth and development. Congenital lactase deficiency is a separate clinical entity from congenital lactose intolerance. Congenital lactose intolerance is a rare and serious disease with vomiting, failure to thrive, dehydration, disacchariduria including lactosuria, renal tubular acidosis, aminoaciduria, liver damage, and cataracts as possible clinical sequelae (15, 16, 17, 18). The cause of this disorder is not lactase deficiency but rather the gastric absorption of intact lactose (16). Although this condition can be fatal in early infancy if not recognized, a milk-free diet leads to rapid recovery; and often, patients may be able to tolerate a normal diet (with milk) after 6 months of age (15).
The loss of intestinal lactase activity (hypolactasia) is either congenital or acquired, and lactase is the only digestive enzyme for which greatly reduced activity in adulthood is common. Acquired hypolactasia is subdefined as primary or secondary. Primary hypolactasia (also referred
to as lactase nonpersistence [LNP]) is a genetically programmed, irreversible loss of the majority (90% to 95%) of intestinal lactase activity that occurs sometime after weaning, probably between 3 and 5 years of age (19, 20). LNP affects approximately 75% of the world’s population (Table 76.3). Interestingly, most Northern Europeans and a few pastoral tribes in African and the Middle East maintain high lactase levels throughout life (21). Because the loss of lactase is the normal pattern in mammalian physiology (humans are the only known mammalian species to have a subpopulation that retains lactase activity) and is not pathogenic, the use of the term “lactase deficiency” to describe the primary loss of lactase is incorrect. Last, the terms lactase nonpersistence and lactose intolerance should not be used interchangeably. The former term simply describes the loss of lactase activity, whereas the latter pertains to the development of clinical symptoms resulting from lactose maldigestion. Two major hypotheses exist to explain the pattern of LNP distribution in the world population. The first—the geographic hypothesis—was proposed by Simoons in 1978 (22). According to this hypothesis, mutations for lactose persistence occurred several thousand years ago, during the origin of dairying. In those geographic regions where dairy farming was practiced, individuals with the mutation coding for lactose digestion had improved tolerance to milk and gained a selective advantage over their counterparts, especially when living under marginal nutritional conditions.
to as lactase nonpersistence [LNP]) is a genetically programmed, irreversible loss of the majority (90% to 95%) of intestinal lactase activity that occurs sometime after weaning, probably between 3 and 5 years of age (19, 20). LNP affects approximately 75% of the world’s population (Table 76.3). Interestingly, most Northern Europeans and a few pastoral tribes in African and the Middle East maintain high lactase levels throughout life (21). Because the loss of lactase is the normal pattern in mammalian physiology (humans are the only known mammalian species to have a subpopulation that retains lactase activity) and is not pathogenic, the use of the term “lactase deficiency” to describe the primary loss of lactase is incorrect. Last, the terms lactase nonpersistence and lactose intolerance should not be used interchangeably. The former term simply describes the loss of lactase activity, whereas the latter pertains to the development of clinical symptoms resulting from lactose maldigestion. Two major hypotheses exist to explain the pattern of LNP distribution in the world population. The first—the geographic hypothesis—was proposed by Simoons in 1978 (22). According to this hypothesis, mutations for lactose persistence occurred several thousand years ago, during the origin of dairying. In those geographic regions where dairy farming was practiced, individuals with the mutation coding for lactose digestion had improved tolerance to milk and gained a selective advantage over their counterparts, especially when living under marginal nutritional conditions.
More recently, the malaria hypothesis has been proposed by Anderson and Vullo (23). The authors suggest that malaria selected for LNP. Noting that LNP is common in geographic areas of world that are endemic for malaria, the authors posit that the genetic tendency for LNP would cause lactose maldigestion and intolerance symptoms leading to a corresponding decline in milk intake in affected individuals. Because milk products are excellent sources of riboflavin, it is further proposed that many of these individuals may have had marginal riboflavin deficiency. A state of marginal riboflavin deficiency that could be tolerated by the person and yet still lead to localized flavin deficiency in the erythrocytes, is theorized to inhibit the multiplication of malaria parasites and, thus, reduce mortality from malaria. Although this hypothesis is interesting, a study conducted in Northern Sardinia showed no differences in the prevalence of LNP in villages with a past history of low, intermediate, or high malaria morbidity and mortality (24). A further commentary on this study (25) points out that, in contrast to the data on LNP, the frequencies of glucose-6-phosphate dehydrogenase deficiency and β-thalassemia trait (two disorders that are known to be selected for by malaria) were strikingly higher in the areas with a high past malarial endemicity versus the area with low malarial endemicity. Thus, the limited evidence presented so far does not support the malaria hypothesis.
TABLE 76.3 PREVALANCE OF LACTASE NONPERSISTENCE IN VARIOUS ETHNIC GROUPS | |||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||||||||||||||||||||||||||||||
In LNP individuals, lactase activity in the jejunal enterocytes is found in a mosaic-type pattern, meaning that some jejunal enterocytes produce high levels of lactase, whereas others, even those sharing the same villus, do not produce lactase (26, 27). Thus, rather than a uniform reduction in lactase production among all enterocytes, LNP individuals may have a patchy distribution of lactase-producing enterocytes that are low in number relative to the non-lactase-producing enterocytes. However, in lactase-persistent individuals, all enterocytes may produce lactase.
The molecular basis for LNP has received much attention. LNP is an autosomal recessive trait and the gene for human LPH is located on chromosome 2q21 (28). Initial studies suggested that alterations in the posttranslational modifications of LPH were responsible for the low lactase activity in hypolactasia (29, 30). Rossi et al (26) found that intestinal biopsies of hypolactasic individuals in Southern Italy had substantial levels of lactose mRNA. Thus, it was thought that hypolactasic persons do synthesize the lactase protein, but that posttranslational modifications cause it to be either malfolded and enzymatically inactive or result in its intracellular degradation (31). Sebastio et al (32) studied individuals with the hypolactasic phenotype and the lactase-persistent phenotype. There was no clear difference in the lactase mRNA levels in the intestinal biopsies of individuals with either phenotype. The authors concluded that expression of lactase is controlled at the posttranscriptional level.
Despite this evidence, the current opinion is that lactase regulation is primarily at the level of transcription. Numerous studies (33, 34, 35) have demonstrated the importance of the presence of an adequate lactase mRNA level to have expression of LPH activity. Krasinski et al (36) found that LPH mRNA levels in rats were abundant before weaning but declined twofold to fourfold during weaning. The LPH activity observed in the rats corresponded with the protein and mRNA levels at the different life stages. Thus, transcriptional mechanisms
were thought to be responsible for regulation of lactase biosynthesis. Escher et al (37) studied lactase specific activity and lactase mRNA levels in Asian, black, and white patients. They observed that lactase activity always corresponded with the lactase mRNA levels, thereby suggesting that transcriptional regulation is responsible for variable lactase activity. Further, studies of the porcine LPH gene have identified a sequence CE-LPH1 in the promoter region, which binds a trans-acting nuclear factor NF-LPH1. High levels of NF-LPH1 were found in enterocytes of newborn pigs that had high lactase activity, whereas the levels were lower in adult pigs that had low lactase activity. It was suggested that the nuclear factor NF-LPH1 could be implicated in the lowering of lactase activity at weaning and could provide an explanation for the molecular regulation of hypolactasia (38). Subsequent studies have shown that other nuclear factors also may interact with the CE-LPH1 promoter region (39).
were thought to be responsible for regulation of lactase biosynthesis. Escher et al (37) studied lactase specific activity and lactase mRNA levels in Asian, black, and white patients. They observed that lactase activity always corresponded with the lactase mRNA levels, thereby suggesting that transcriptional regulation is responsible for variable lactase activity. Further, studies of the porcine LPH gene have identified a sequence CE-LPH1 in the promoter region, which binds a trans-acting nuclear factor NF-LPH1. High levels of NF-LPH1 were found in enterocytes of newborn pigs that had high lactase activity, whereas the levels were lower in adult pigs that had low lactase activity. It was suggested that the nuclear factor NF-LPH1 could be implicated in the lowering of lactase activity at weaning and could provide an explanation for the molecular regulation of hypolactasia (38). Subsequent studies have shown that other nuclear factors also may interact with the CE-LPH1 promoter region (39).
The newer finding in the area of the genetics of lactose intolerance is the discovery of single nucleotide polymorphisms (SNP) that appear to define individuals who will maintain or lose lactase activity after weaning. The first of these to be discovered is C/T-13910, in European populations (40). A one base-pair polymorphism 13.9 kb upstream from the lactase gene on chromosome 2 appears to be responsible for determining lactose digestion status in many European populations. The location of the SNP appears to be the binding site for the transcription factor Oct-1. The expression of the lactase gene is severalfold higher for the T-13910 allele. Both T/T-13910 and C/T-13910 allow enough transcription binding that nonpersistence is seen only with the homozygous C/C-13910 allele. This is a plausible molecular explanation for the dominance of tolerance long observed in lactose genetics. A second essential finding is that SNPs vary by racial group around the world. Initial thought was that maybe three different SNPs existed for Europeans, Middle Eastern populations, and Africans. However, to date, at least eight unique SNPs have been identified (41). It is possible that much of the variation around age at onset and degree of intolerance could be related to the specific SNPs.
TABLE 76.4 POTENTIAL CAUSES OF SECONDARY HYPOLACTASIA | |||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||||||||||||||||||||||||||||||||
Secondary acquired hypolactasia occurs because of enterocyte damage resulting from disease, medication, surgery, or radiation (42). Table 76.4 lists some of the causes of secondary hypolactasia. In one study of malnourished patients, lactase was reduced to a greater degree than other disaccharidases and was the last of the disaccharidases to recover (43). A possible explanation is that the activity of lactase is only about 50% as high as the other disaccharidases, even in lactase persistent individuals (44). A key issue in the management of secondary hypolactasia is lactose restriction in the diet. Although removing lactose-containing dairy foods may improve clinical tolerance, it also may deprive a malnourished patient of the nutritional value of these foods. Secondary hypolactasia can be reversed once the underlying problem has been resolved, but the process is slow and can take 6 months or longer (42).
Clinical Assessment of Lactase Activity
Lactase activity is assessed by either direct or indirect methods. The direct measurement of the lactase activity obtained by a small intestinal mucosal biopsy or intestinal perfusion is the most accurate, but these methods are invasive and carry risk for complications, such as intestinal bleeding (45). Thus, direct methods are rarely performed clinically.
Indirect methods for assessing the digestion of a dose of lactose include breath tests (hydrogen, 13CO2, 14CO2), blood tests (glucose and galactose), urine tests (galactose, lactose/lactulose ratio), fecal tests (pH, reducing substances) and intolerance symptoms. Of these methods, the breath hydrogen test is currently the most widely used. The test is based on the principle that lactose, which escapes digestion in the small intestine, is fermented by the colonic bacteria, resulting in the generation of hydrogen gas (the only known source of molecular hydrogen in the body). A portion of this hydrogen gas diffuses into the blood and is excreted via the lungs. The method has excellent sensitivity and specificity, but careful attention to the test protocol is required (44).
Lactose Maldigestion and Symptoms of Lactose Intolerance
The well-documented high prevalence of LNP in much of the world’s population has unfortunately misled many to believe that lactose intolerance is equally common. However, a large body of evidence now exists demonstrating that symptoms of lactose intolerance in response to physiological amounts of lactose (8 to 16 fl oz of milk) affect only a small fraction of lactose maldigesters (46). An example is a study by Carroccio et al (47). In this study, 323 Sicilian adults (72 children aged 5 to 16 years, 141 adults aged 17 to 64 years, and 110 elderly adults aged 65 to 85 years) underwent breath hydrogen testing with a 25-g lactose dose (1 g/kg for children) to determine lactose digestion status and were queried for the presence of lactose intolerance in the ensuing 24-hour period. Of the 323 individuals, 117 (36%) were classified as lactose maldigesters. Just 13 of the lactose maldigesters experienced symptoms of lactose intolerance, which was 4% of the total study group and 11% of the lactose maldigesters.
Another concern is that many individuals may selfdiagnose lactose intolerance when they may not be lactose maldigesters. Two studies by Suarez et al and another by Johnson et al (48, 49, 50) demonstrated that 30% to 33% of subjects who claim to have lactose intolerance are, in fact, lactose digesters. Of the 49 subjects in the Carroccio et al study (47) who had self-reported milk intolerance at entry into the study, just five were both lactose maldigesters and lactose intolerant. It is likely that some individuals who self-diagnose with lactose intolerance may have an underlying bowel. The findings indicate that diagnosing lactose intolerance solely on the basis of reported symptoms after milk ingestion is unreliable. Objective tests of lactose maldigestion, or the evaluation of symptoms in a doublemasked, placebo-controlled study are necessary (51).
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
