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Role of the gastrointestinal tract in energy and protein metabolism


* Instituto de Nutricion y Tecnologia de los Alimentos (INTA), Casilla 138-11, Santiago, Chile.

1. Introduction
2. Cell and protein turnover
3. Nutrient absorption
4. Protein synthesis
5. Restriction of energy and protein intake
6. Fat absorption and exocytosis
7. Chronic environmental enteropathy

1. Introduction

The digestive tract constitutes an interface between the outer environment, represented by the lumen, and the rest of the body. It plays an important role in the metabolism of energy and protein because of its anatomical and functional characteristics. Among its main tasks are the digestion of the large molecules of foodstuffs into smaller, simpler ones that can be absorbed by the enterocytes. During this process, macromolecules lose their immunologic identity.

2. Cell and protein turnover

The anatomical substrates for these functions in the small intestine are a surface area that has been calculated in the adult to be approximately 40 m2 and an epithelium that has one of the highest turnover rates in the body (WILSON, 1962). In experimental animals, up to 40% of an intravenously administered dose of tritiated thymidine becomes incorporated in the epithelial lining of the small intestine, with an additional 20% incorporated by the gastric and the colonic mucosae; the whole rest of the organism incorporates the remaining 40% (HINRICHS, PETERSEN and BASERGA, 1964).

The cell mass of the adult intestinal epithelium has been estimated to be approximately 600 g; since it turns over in 5 days, 120 g of cells with a protein content of approximately 20% are shed into the lumen daily; this means that 25 g of high-quality protein are delivered to the lumen every day (NASSET, 1965; FREEMAN and KIM, 1978; MACDONALD, TRIER and EVERETT, 1964)). To this must be added the protein content of saliva (2-12 g of protein per day), gastric (8-18 g of protein per day) and pancreatic juice (12-30 g of protein per day) and bile (2-3 g of protein per day) (ALPERS, 1987; RINDERKNECHT, 1986; FREEMAN, SLEISENGER and KIM, 1983). As a result, daily endogenous protein secretion in the gastrointestinal tract may equal or even exceed the amounts provided by the diet (about 1 g/kg body weight/d).

This endogenous protein secretion is incremented by glycoproteins from the glycocalix, the mucus of goblet cells, and by the digestive enzymes of the brush border of the gastric epithelium, of the liver and of the pancreas (ALPERS, 1987; ITO, 1974; BENNET, LEBLOND and HADDAD, 1974; NEUTRA and FORSTNER 1987; NAIM et al., 1988). Furthermore, approximately 1% of the radioactivity associated with a close of intravenously administered labeled serum albumin appears in the gastrointestinal lumen; because the half-life of serum albumin is 10 days, one tenth of the turnover of serum albumin is explained by its intraluminal digestion (approximately 2 g/d) (FREEMAN and KIM, 1978; LaRUSSO, 1984).

3. Nutrient absorption

The intestinal absorption of glucose, galactose, amino acids and di- and tripeptides is accomplished by specific carriers which have an absolute requirement for Na+ in a stoichiometric proportion of two Na+ for each molecule of non-electrolyte (CRANE, 1962; LUECKE, HAASE and MURER, 1977; MATTHEWS et al., 1979). Absorbed Nat is transported to the intercellular space of the epithelium against a gradient by the Na+ - K+ dependent ATPase (sodium pump) which requires the expenditure of ATP (HARMS and WRIGHT, 1980).

Brush-border disaccharidases are turned over several times during the lifespan of enterocytes (JAMES et al., 1971; ALPERS and TEDESCO, 1975), which have the capacity to synthesize these complex glycoproteins at a high rate. Adaptation of the sucrase-isomaltase and the maltase activities to the presence of their specific substrates in the diet further increases the need for protein synthesis in the epithelium (ROSENSWEIG, HERMAN and STIFEL, 1971; ROSENSWEIG and HERMAN, 1969). Unabsorbed carbohydrates, including non-starch polysaccharides, are fermented in the large intestine by the resident bacterial flora (TOPPING, 1991). This results in the formation of short-chain fatty acids (SCFA) (acetic, propionic and butyric) whose transport by the colonic mucosa stimulates water and Na+ absorption.

Butyric acid is one of the main fuels for the colonic epithelium. This fermentative process, followed by absorption of the SCFA, is known as colonic salvage and may account for 5 to 6% of the energy intake when the diet provides 2000 kcal/d, including 30 to 40 g of non-starch polysaccharides (TOPPING, 1991). This salvage of energy and the salvage of nitrogen derived from the metabolism of the gut increase the economy and efficiency of gastrointestinal absorptive functions (LANGRAN et al., 1992).

4. Protein synthesis

Protein synthesis is very active in the intestinal epithelium. Proteins with higher molecular weight are metabolized at faster rates than smaller molecules (ALPERS, 1972a; 1977). Incorporation of amino acids into cell proteins depends to a large extent on the route of administration. Intravenously administered labeled leucine is incorporated into cells of the crypt-villus junction whereas intraluminal administration results in heavy labeling of cells near the villus tip. This suggests that villus cells synthesize protein at rates that exceed those of the crypts (ALPERS, 1972b). The recycling of amino acids in the intestinal epithelium must be very high to meet the rates of protein synthesis. Since protein concentration in the epithelium remains constant, the synthesis of new protein must be equilibrated by export and breakdown.

The hyperplasia of the mucosa following extensive resection is also indicative of a considerable capacity for protein synthesis. For hyperplasia or recovery of intestinal mass to occur, adequate amounts of monosaccharides, amino acids and long-chain fatty acids are necessary (TOPPING, 1991; WESER, VANDEVENTER and TAWIL, 1982; SAKATA, 1986).

5. Restriction of energy and protein intake

Starvation is followed by atrophy of the intestinal wall, especially of the mucosa, and similar changes occur during exclusive feeding by the parenteral route (ALTMAN, 1972; JOHNSON et al., 1975; FORD et al., 1984; ROEDIGER, 1986). Reversal of this atrophy is accomplished only by nutrients administered by the enteral route (NASSET, 1965; JOHNSON et al., 1975).

Enterocytes use a variety of fuels, some provided by the diet during absorption; however, the largest proportion is endogenous. In studies in vivo in rats it has been shown that glutamine is the main source of the energy used by the intestinal mucosa in the postabsorptive state (WINDMUELLER and SPAETH, 1975; 1978; 1980). Part of the ammonia generated by the metabolism of glutamine in the intestinal epithelium is released to the lumen, from where it is utilized by bacteria or salvaged for synthesis of non-essential amino acids in the colon (WIND-MUELLER and SPAETH, 1980; MORAN and JACKSON, 1990). Furthermore, glutamine and other polyamines stimulate mitotic activity in the crypts (LUK and BAYLIN, 1983).

Restriction of protein and energy intake in marasmic malnutrition, induces a slowing of cell renewal as evidenced by the decrease in mitotic activity in the crypt cells both in humans and in experimental animals (BRUNSER et al., 1966; 1968; BROWN, LEVINE and LIPKIN, 1963). In infantile protein and energy malnutrition leading to kwashiorkor, the mitotic index in the crypts of Lieberkühn is only slightly decreased; this is similar to findings in protein-malnourished rats in whom the mitotic index remains very close to normal despite prolonged periods of decreased protein intake (BRUNSER et al., 1966; 1968).

In marasmic infants the architecture of the intestinal mucosa is close to normal, while histological alterations are more severe in kwashiorkor (BRUNSER et al., 1968). The reason for these differences is not known; in marasmus, the preservation of mucosal histology may be due in part to the capacity of the enterocytes to recycle some of the molecules of their cytoplasm. In marasmic infants, enterocytes frequently have large autophagosomes in which the sequestered organelles are degraded by Iysosomal enzymes until only undigested structures remain as residual bodies (BRUNSER, CASTILLO and ARAYA, 1976). Molecules released by this process may be recycled by the enterocytes and contribute to maintain a more normal mucosal function and structure. Furthermore, kwashiorkor is an acute condition, while marasmus is chronic and allows adaptations of metabolic activities to low levels of protein and energy intake (SUSKIND, MURTHY and SUSKIND, 1990).

6. Fat absorption and exocytosis

The absorption of dietary fat also involves expenditure of energy and synthesis of protein. Absorption of fatty acids and their transport to the endoplasmic reticulum in the enterocytes requires the synthesis of Z protein and of intestinal 'fatty acid-binding' protein (GORDON et al., 1982), while their activation to acyl-CoA for intracellular resynthesis of triglycerides or phospholipids requires expenditure of ATP (SENIOR, 1964). The glycerol backbone for resynthesis of neutral fat may originate from 2-monoglycerides liberated by the lumenal hydrolysis of triglycerides, or it may be produced by the anaerobic metabolism of glucose during which ATP is synthesized (SENIOR, 1964).

Exocytosis of fat resynthesized in the endoplasmic reticulum of enterocytes requires the synthesis of apolipoproteins, especially of apolipoprotein AI and AIV (GREEN et al., 1980). The rate of synthesis of apoAIV during fat transport is probably greater than for apoAI, suggesting that this is a specific response and meets functional requirements of the absorptive cells. The fact that apoCIII is not secreted is further indication of a highly selective and regulated process (BLAUFUSS et al., 1984). ApoAI and apoAIV increase considerably in the plasma during neutral fat absorption. Despite this, their concentration in the cytoplasm of the enterocytes remains almost unchanged, suggesting that considerable amounts of these polypeptides are secreted into the lymph (ALPERS, LANCASTER and SCHONFELD, 1982). Intestinal alkaline phosphatase is also secreted during a fatty meal although, in absolute terms, the quantities are small (YOUNG, YEDLIN and ALPERS, 1981).

7. Chronic environmental enteropathy

Disturbances of digestive and absorptive function may induce considerable losses of nutrients and result in variable degrees of malnutrition, both in children and adults. Their discussion, however, is beyond the scope of this commentary. We will refer instead to a condition that affects a large segment of the world population living in the less developed countries, where environmental sanitation is unsatisfactory. This condition was first described in morphological studies of the small intestine of apparently healthy individuals living in the tropics (LINDENBAUM, KENT and SPRINZ, 1966; LINDENBAUM, 1968; KEUSCH, 1972).

The histological changes consisted of some blunting of the villi, a more basophilic staining of the cells at the tip of the villi, a mild-to-moderate increase of crypt depth, and an increase of lymphocytes and plasma cells in the lamina propria (BAKER, 1973; BRUNSER, EIDELMAN and KLIPSTEIN, 1970). Because of the geographic area where this condition was first described, it was originally called tropical enteropathy. This was an acquired condition, because the histologic changes were not seen in fetuses that miscarried shortly before term. Furthermore, in people moving to these areas from developed countries, the same morphological changes of the intestinal mucosa appeared after a few months, if they lived in conditions similar to those of the local residents; these changes regressed when they returned to well-sanitated environments (LINDENBAUM, KENT and SPRINZ, 1966; LINDENBAUM, GERSON and KENT, 1971).

The morphologic changes also improved when inhabitants from tropical areas moved to well-sanitated environments. The malabsorption associated with this condition affected all nutrients in variable degrees. Some individuals went on to develop a more severe course, with chronic diarrhea, severe malabsorption and macrocytic anemia (tropical sprue) (KLIPSTEIN, 1968a; b).

Individuals with tropical enteropathy were found to have an abnormal flora in the upper small intestine, mainly aerobes or microaerophilic bacteria (GORBACH et al., 1970; 1971). It is thought that the main cause of all these alterations is the passage of bacteria, viruses, and parasites along the gastrointestinal tract with contaminated foodstuffs and fluids. Most likely it is not necessary for the bacteria to have enteropathogenic capabilities to induce these effects.

It was later shown that people living in temperate areas may develop similar histological and functional changes if they live in environments with high levels of microbiological contamination (THOMAS, CLAIN and WICKS, 1976; PEREA et al., 1978; BRUNSER et al., 1987). For these reasons we feel that the expression 'chronic environmental enteropathy' is a more accurate description of this condition than 'tropical enteropathy'.

In the studies carried out in Chile we demonstrated that affected individuals have mild histologic changes and minor degrees of malabsorption. Along with increased fecal losses of nitrogen and fat, a decrease of the transport capacity of the small intestine for glucose was demonstrated, with a lower Vmax and saturation of its transport capacity occurring at lower concentrations. Fecal losses of nitrogen increased disproportionately when the diet provided moderate amounts of fiber, to levels far above those reported for individuals living in well-sanitated environments on similar levels of fiber intake (BRUNSER et al., 1987).

It was also shown that when the diet provided protein at maintenance levels, any change in intestinal absorptive capacity that resulted in greater fecal losses of nitrogen, such as those produced by increased dietary fiber intake, resulted in a compensatory decrease of urinary nitrogen losses (ESPINOZA et al., 1984). Microaerophilic and anaerobic bacteria were detected in increased numbers in the first loops of the jejunum. Three months of living in the well-sanitated environment of a metabolic ward resulted in the complete reversal of the morphologic changes (BRUNSER et al., 1987).

Chronic environmental enteropathy has to be taken into account when calculating the protein and energy requirements of people who live in areas where microbiological contamination is highly prevalent and where the fiber content of the diet is high.


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