Nutrient Transport Across Vertebrate Intestine

Digestive adaptations of aerial lifestyles

Flying vertebrates (birds and bats) are under selective pressure to reduce the size of the gut and the mass of the digesta it carries. Compared with similar-sized nonflying mammals, birds and bats have smaller intestines and shorter retention times. We review evidence that birds and bats have lower spare digestive capacity and partially compensate for smaller intestines with increased paracellular nutrient absorption.

Alligators and Crocodiles Have High Paracellular Absorption of Nutrients, But Differ in Digestive Morphology and Physiology

Much of what is known about crocodilian nutrition and growth has come from animals propagated in captivity, but captive animals from the families Crocodilidae and Alligatoridae respond differently to similar diets. Since there are few comparative studies of crocodilian digestive physiology to help explain these differences, we investigated young Alligator mississippiensis and Crocodylus porosus in terms of (1) gross and microscopic morphology of the intestine, (2) activity of the membrane-bound digestive enzymes aminopeptidase-N, maltase, and sucrase, and (3) nutrient absorption by carrier-mediated and paracellular pathways. We also measured gut morphology of animals over a larger range of body sizes. The two species showed different allometry of length and mass of the gut, with A. mississippiensis having a steeper increase in intestinal mass with body size, and C. porosus having a steeper increase in intestinal length with body size. Both species showed similar patterns of magnification of the intestinal surface area, with decreasing magnification from the proximal to distal ends of the intestine. Although A. mississippiensis had significantly greater surface-area magnification overall, a compensating significant difference in gut length between species meant that total surface area of the intestine was not significantly different from that of C. porosus. The species differed in enzyme activities, with A. mississippiensis having significantly greater ability to digest carbohydrates relative to protein than did C. porosus. These differences in enzyme activity may help explain the differences in performance between the crocodilian families when on artificial diets. Both A. mississippiensis and C. porosus showed high absorption of 3-O methyl d-glucose (absorbed via both carrier-mediated and paracellular transport), as expected. Both species also showed surprisingly high levels of l-glucose-uptake (absorbed paracellularly), with fractional absorptions as high as those previously seen only in small birds and bats. Analyses of absorption rates suggested a relatively high proportional contribution of paracellular (i.e., non-mediated) uptake to total uptake of nutrients in both species. Because we measured juveniles, and most paracellular studies to date have been on adults, it is unclear whether high paracellular absorption is generally high within crocodilians or whether these high values are specific to juveniles.

Tannic acid inhibition of amino acid and sugar absorption by mouse and vole intestine: Tests following acute and subchronic exposure

The acute effects of tannin (tannic acid; TA) on nutrient absorption were studied by measuring sugar and amino acid uptake across the brash border (luminal membrane) of intact intestine in the presence and absence of TA. Incubation of tissue for 4-9 min in TA solution (1 mg/ml) caused a reduction in passive influx ofL-glucose in voles and mice and a reduction in carrier-mediated influx ofD-glucose and total influx ofL-proline in mice, but not voles. In subchronic experiments, mice and voles were fed for 7-14 days a diet with 4% TA, but there was no significant effect on intestinal brush border uptake ofL-glucose,D-glucose, orL-proline (or three other amino acids tested in voles). In a synthesis of our study with others in the literature, three inferences are made from the patterns of effects across solutes, time scales of exposure, and species. First, the transport inhibitory effects following acute exposure are probably mediated by two processes: increased resistance to passive flux across an effective unstirred layer juxtaposed to the brush border membrane, perhaps due to tannin-mucin binding, and reduced Na(+)-coupled nutrient uptake across the intestinal brush border. Second, there is a species sensitivity difference in TA's effect on the second process. Third, the negative effects observed at the acute time scale in vitro do not necessarily occur in animals eating TA subchronically because little TA reaches the luminal membrane, or if it does its effects are quickly reversed when the tissue is removed and washed with solution free of TA.

Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance

Birds during migration must satisfy the high energy and nutrient demands associated with repeated, intensive flight while often experiencing unpredictable variation in food supply and food quality. Solutions to such different challenges may often be physiologically incompatible. For example, increased food intake and gut size are primarily responsible for satisfying the high energy and nutrient demands associated with migration in birds. However, short-term fasting or food restriction during flight may cause partial atrophy of the gut that may limit utilization of ingested food energy and nutrients. We review the evidence available on the effects of long- and short-term changes in food quality and quantity on digestive performance in migratory birds, and the importance of digestive constraints in limiting the tempo of migration in birds. Another important physiological consequence of feeding in birds is the effect of diet on body composition dynamics during migration. Recent evidence suggests that birds utilize and replenish both protein and fat reserves during migration, and diet quality influences the rate of replenishment of both these reserves. We conclude that diet and phenotypic flexibility in both body composition and the digestive system of migratory birds are important in allowing birds to successfully overcome the often-conflicting physiological challenges of migration.

Seasonal Fruit Preferences for Lipids and Sugars by American Robins

Fruit preference by birds is a complex process based upon the morphology and spatial arrangement of fruits and on the physiological needs and capabilities of birds. In North America, most fruits can be divided into two groups based on nutritional content: those rich in sugars relative to lipids, and those rich in lipids relative to sugars. To investigate how fruit preference may change seasonally and to determine if it is correlated with physiological state, we designed a simple laboratory experiment using American Robins (Turdus migratorius) and artificial fruits. During summer and autumn, we offered eight robins a choice between synthetic sugar-rich and lipid-rich fruits of equal caloric value and then measured food intake and assimilation efficiency for each fruit type. Overall, robins preferred sugar-rich to lipid-rich fruits during both seasons. Robins had a higher assimilation efficiency for sugars than for lipids during both seasons, although assimilation efficiency of lipids increased significantly from summer to autumn. During experiments, robins consumed significantly more sugar-rich than lipid-rich fruits in summer but not in autumn. Coupling fruit intake with assimilation efficiency indicates that in summer, robins had a higher rate of energy gain from sugars than from lipids, but by autumn the rate of energy gain from lipids increased to nearly the same level as that from sugars. Our results suggest that robins prefer sugar-rich fruits because of their simple and fast rate of digestion, enabling higher rates of energy gain, but that lipid-rich fruits become important with the onset of autumn.

Hyperglycemia in hummingbirds and its consequences for hemoglobin glycation

We measured levels of glucose and glycated hemoglobin in the blood of three of the world's smallest nectarivorous birds, the Anna's (Calypte anna), Costa's (Calypte costae), and ruby-throated hummingbirds (Archilochus colubris). Plasma glucose levels of hummingbirds that were fasted overnight (17 mM) were higher than those in any mammal and are among the highest ever measured in a fasting vertebrate. Glucose levels in hummingbirds just after feeding were extreme, rising as high as 42 mM. The surprisingly high blood glucose concentrations in hummingbirds were accompanied by glycated hemoglobin levels that are the highest ever measured in birds but are lower than those of non-diabetic humans. How hummingbirds tolerate blood glucose levels that cause serious neurological and microvascular pathologies in diabetic humans and animals remains unknown.