Information

34.1A: Digestive Systems - Biology


Animals use the organs of their digestive systems to extract important nutrients from food they consume, which can later be absorbed.

Learning Objectives

  • Summarize animal nutrition and the digestive system

Key Points

  • Animals obtain lipids, proteins, carbohydrates, essential vitamins, and minerals from the food they consume.
  • The digestive system is composed of a series of organs, each with a specific, yet related function, that work to extract nutrients from food.
  • Organs of the digestive system include the mouth, esophagus, stomach, small intestine, and the large intestine.
  • Accessory organs, such as the liver and pancreas, secrete digestive juices into the gastrointestinal tract to assist with food breakdown.

Key Terms

  • digestion: the process, in the gastrointestinal tract, by which food is converted into substances that can be utilized by the body
  • macromolecule: a very large molecule, especially used in reference to large biological polymers (e.g. nucleic acids and proteins)
  • alimentary canal: the organs of a human or an animal through which food passes; the digestive tract

Introduction to Animal Nutrition

All living organisms need nutrients to survive. While plants can obtain the molecules required for cellular function through the process of photosynthesis, most animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. The food consumed consists of protein, fat, and complex carbohydrates, but the requirements of each are different for each animal.

Animals must convert these macromolecules into the simple molecules required for maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food consumed to the nutrients required is a multi-step process involving digestion and absorption. During digestion, food particles are broken down to smaller components which will later be absorbed by the body.

Digestive System

The digestive system is one of the largest organ systems in the human body. It is responsible for processing ingested food and liquids. The cells of the human body all require a wide array of chemicals to support their metabolic activities, from organic nutrients used as fuel to the water that sustains life at the cellular level. The digestive system not only effectively chemically reduces the compounds in food into their fundamental building blocks, but also acts to retain water and excrete undigested materials. The functions of the digestive system can be summarized as follows: ingestion (eat food), digestion (breakdown of food), absorption (extraction of nutrients from the food), and defecation (removal of waste products).

The digestive system consists of a group of organs that form a closed tube-like structure called the gastrointestinal tract (GI tract) or the alimentary canal. For convenience, the GI tract is divided into upper GI tract and lower GI tract. The organs that make up the GI tract include the mouth, the esophagus, the stomach, the small intestine, and the large intestine. There are also several accessory organs that secrete various enzymes into the GI tract. These include the salivary glands, the liver, and the pancreas.

Challenges to Human Nutrition

One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Imbalances can have serious health consequences. For example, eating too much food while not expending much energy leads to obesity, which in turn will increase the risk of developing illnesses such as type-2 diabetes and cardiovascular disease. The recent rise in obesity and related diseases means that understanding the role of diet and nutrition in maintaining good health is more important than ever.


34.1A: Digestive Systems - Biology

Animals use the organs of their digestive systems to extract important nutrients from food they consume, which can later be absorbed.

Learning Objectives

Summarize animal nutrition and the digestive system

Key Takeaways

Key Points

  • Animals obtain lipids, proteins, carbohydrates, essential vitamins, and minerals from the food they consume.
  • The digestive system is composed of a series of organs, each with a specific, yet related function, that work to extract nutrients from food.
  • Organs of the digestive system include the mouth, esophagus, stomach, small intestine, and the large intestine.
  • Accessory organs, such as the liver and pancreas, secrete digestive juices into the gastrointestinal tract to assist with food breakdown.

Key Terms

  • digestion: the process, in the gastrointestinal tract, by which food is converted into substances that can be utilized by the body
  • macromolecule: a very large molecule, especially used in reference to large biological polymers (e.g. nucleic acids and proteins)
  • alimentary canal: the organs of a human or an animal through which food passes the digestive tract

Introduction to Animal Nutrition

All living organisms need nutrients to survive. While plants can obtain the molecules required for cellular function through the process of photosynthesis, most animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. The food consumed consists of protein, fat, and complex carbohydrates, but the requirements of each are different for each animal.

Balanced human diet: For humans, fruits and vegetables are important in maintaining a balanced diet. Both of these are an important source of vitamins and minerals, as well as carbohydrates, which are broken down through digestion for energy.

Animals must convert these macromolecules into the simple molecules required for maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food consumed to the nutrients required is a multi-step process involving digestion and absorption. During digestion, food particles are broken down to smaller components which will later be absorbed by the body.

Digestive System

The digestive system is one of the largest organ systems in the human body. It is responsible for processing ingested food and liquids. The cells of the human body all require a wide array of chemicals to support their metabolic activities, from organic nutrients used as fuel to the water that sustains life at the cellular level. The digestive system not only effectively chemically reduces the compounds in food into their fundamental building blocks, but also acts to retain water and excrete undigested materials. The functions of the digestive system can be summarized as follows: ingestion (eat food), digestion (breakdown of food), absorption (extraction of nutrients from the food), and defecation (removal of waste products).

Generalized animal digestive system: This diagram shows a generalized animal digestive system, detailing the different organs and their functions.

The digestive system consists of a group of organs that form a closed tube-like structure called the gastrointestinal tract (GI tract) or the alimentary canal. For convenience, the GI tract is divided into upper GI tract and lower GI tract. The organs that make up the GI tract include the mouth, the esophagus, the stomach, the small intestine, and the large intestine. There are also several accessory organs that secrete various enzymes into the GI tract. These include the salivary glands, the liver, and the pancreas.

Challenges to Human Nutrition

One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Imbalances can have serious health consequences. For example, eating too much food while not expending much energy leads to obesity, which in turn will increase the risk of developing illnesses such as type-2 diabetes and cardiovascular disease. The recent rise in obesity and related diseases means that understanding the role of diet and nutrition in maintaining good health is more important than ever.


34.4 Digestive System Regulation

By the end of this section, you will be able to do the following:

  • Discuss the role of neural regulation in digestive processes
  • Explain how hormones regulate digestion

The brain is the control center for the sensation of hunger and satiety. The functions of the digestive system are regulated through neural and hormonal responses.

Neural Responses to Food

In reaction to the smell, sight, or thought of food, like that shown in Figure 34.20, the first response is that of salivation. The salivary glands secrete more saliva in response to stimulation by the autonomic nervous system triggered by food in preparation for digestion. Simultaneously, the stomach begins to produce hydrochloric acid to digest the food. Recall that the peristaltic movements of the esophagus and other organs of the digestive tract are under the control of the brain. The brain prepares these muscles for movement as well. When the stomach is full, the part of the brain that detects satiety signals fullness. There are three overlapping phases of gastric control—the cephalic phase, the gastric phase, and the intestinal phase—each requires many enzymes and is under neural control as well.

Digestive Phases

The response to food begins even before food enters the mouth. The first phase of ingestion, called the cephalic phase , is controlled by the neural response to the stimulus provided by food. All aspects—such as sight, sense, and smell—trigger the neural responses resulting in salivation and secretion of gastric juices. The gastric and salivary secretion in the cephalic phase can also take place due to the thought of food. Right now, if you think about a piece of chocolate or a crispy potato chip, the increase in salivation is a cephalic phase response to the thought. The central nervous system prepares the stomach to receive food.

The gastric phase begins once the food arrives in the stomach. It builds on the stimulation provided during the cephalic phase. Gastric acids and enzymes process the ingested materials. The gastric phase is stimulated by (1) distension of the stomach, (2) a decrease in the pH of the gastric contents, and (3) the presence of undigested material. This phase consists of local, hormonal, and neural responses. These responses stimulate secretions and powerful contractions.

The intestinal phase begins when chyme enters the small intestine triggering digestive secretions. This phase controls the rate of gastric emptying. In addition to gastrin emptying, when chyme enters the small intestine, it triggers other hormonal and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder.

Hormonal Responses to Food

The endocrine system controls the response of the various glands in the body and the release of hormones at the appropriate times.

One of the important factors under hormonal control is the stomach acid environment. During the gastric phase, the hormone gastrin is secreted by G cells in the stomach in response to the presence of proteins. Gastrin stimulates the release of stomach acid, or hydrochloric acid (HCl) which aids in the digestion of the proteins. However, when the stomach is emptied, the acidic environment need not be maintained and a hormone called somatostatin stops the release of hydrochloric acid. This is controlled by a negative feedback mechanism.

In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum.

Link to Learning

Visit this website to learn more about the endocrine system. Review the text and watch the animation of how control is implemented in the endocrine system.

Another level of hormonal control occurs in response to the composition of food. Foods high in lipids take a long time to digest. A hormone called gastric inhibitory peptide is secreted by the small intestine to slow down the peristaltic movements of the intestine to allow fatty foods more time to be digested and absorbed.

Understanding the hormonal control of the digestive system is an important area of ongoing research. Scientists are exploring the role of each hormone in the digestive process and developing ways to target these hormones. Advances could lead to knowledge that may help to battle the obesity epidemic.

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    Invertebrate Digestive Systems

    Animals have evolved different types of digestive systems to aid in the digestion of the different foods they consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only one opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones) use this type of digestion. Gastrovascular cavities, as shown in [link]a, are typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”. Ingested material enters the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity.

    The alimentary canal , shown in [link]b, is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other. Earthworms are an example of an animal with an alimentary canal. Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus.



    Avian

    Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth and so their digestive system, shown in Figure 15.7, must be able to process un-masticated food. Birds have evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits and nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food and keep their body weight low. The stomach of birds has two chambers: the proventriculus, where gastric juices are produced to digest the food before it enters the stomach, and the gizzard, where the food is stored, soaked, and mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.

    Figure 15.7. The avian esophagus has a pouch, called a crop, which stores food. Food passes from the crop to the first of two stomachs, called the proventriculus, which contains digestive juices that break down food. From the proventriculus, the food enters the second stomach, called the gizzard, which grinds food. Some birds swallow stones or grit, which are stored in the gizzard, to aid the grinding process. Birds do not have separate openings to excrete urine and feces. Instead, uric acid from the kidneys is secreted into the large intestine and combined with waste from the digestive process. This waste is excreted through an opening called the cloaca.


    Label Digestive System

    This worksheet was designed for anatomy students to practice labeling the organs of the digestive system. It is a little more advanced than what is typically seen in health or basic biology classes because it includes the three sections of the small intestine (duodenum, jejunum, ileum) and the three sections of the colon.

    (Note: I excluded the section of the sigmoid colon is not on the labeling, but you could add it if your students learn it. Some textbooks omit.)

    There are two versions included in the Google doc (and pdf file), one has a word bank and the other does not. This can be used for differentiated instruction or even as a way for students to practice with a word bank and then move onto something more challenging.

    I usually do not assign grades for this type of practice. It is fairly easy for students to just look up the answers. I spend a lot of time in anatomy encouraging students to look at these types of worksheets as a way to study, practice, and become prepared for chapter assessments.

    I usually give students about 10 minutes to do the worksheet and then project the image on the whiteboard and have volunteers write down the answers so students in class can check their work. Plus, kids love writing on the whiteboard!

    Guided notes and Google slides also available for the digestive system.

    Students can practice with this flashcards on Quizlet.

    Download PDF Version with form fields for distance learning.
    Download PDF version with drop-down list for distance learning.
    *You will need to download them to use the forms.

    HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms


    Digestive System: Games

    Online games are a fun way to learn more about science topics. Here you will find links to an assortment of interactive games and activities for use at home or in the classroom. These games are designed for a variety of skill levels and interests. They can be used in a computer lab, on an interactive whiteboard, or on individual devices. Flash, Java, Shockwave, QuickTime or other interactive plug-ins may be required.

    How much do you know about the digestive system? Try this interactive quiz and find out! This tutorial may help you.

    Name the Organs Can you name the organs of the digestive system? This interactive lets you drag the names of the organs to their corresponding parts. Good practice.


    Digestion Interactive For Kids Choose from a table of food and explore the digestion process with this interactive.

    Build-a-Body Brainpop has a digestive interactive for you to construct the entire digestive system from start to finish.

    Print out a Digestive System coloring page and word search from Kids Health.


    Interactions between Naturally Occurring Toxins and Digestive Physiology

    Secondary metabolites (SMs) are compounds produced and/or sequestered by plants and animals that do not appear to play a major role in their primary nutritional or regulatory metabolism. Their functions include communication, attraction, or in defense against herbivores, predators, pathogens, and competitors (202). SMs are so pervasive that it is almost a certainty that any thorough analysis of a plant food, and maybe even many animal foods, will identify some SMs. Some are thought to play an important role in human health, variously acting as antioxidants or antimicrobials, modifying hormone titers, and interfering with DNA synthesis. Other SMs directly damage GIT mucosa, such as lectins (451), proanthocyanidins (2), and hydrolysable tannins (251). Protease inhibitors can permeabilize the peritrophic membrane of caterpillars (326). In the following sections, we highlight numerous examples of key digestive processes being influenced by compounds from many of the major groups of SMs ( Table 4 ). But, also, considering the structural and functional diversity of digestive tracts among animals, it should not surprise that impacts of SMs are not necessarily general but depend on digestive features and perhaps even adaptive counterresponses of consumers.

    Table 4

    Examples of Impacts of Plant Secondary Metabolites on Digestive Processes

    ProcessCompound(s)SourceEffectSpeciesReferences(s)
    Digesta transitRicinoleic acidCastor plant (Ricinus communis)Faster digesta transitHuman(27)
    Digesta transitSenna anthraquinone glycosidesGenus SennaFaster digesta transitHuman(414)
    Digesta transitTrihydroxyanthraquinone (emodin/cascara)Buckthorn Rhamnus alaternusIncreased time between defecations (slower transit?)Yellow-vented Bulbul (Pycnonotus xanthopygos)(440)
    Digesta transitPyridine alkaloids—nicotine, anabasineTree Tobacco (Nicotiana glauca)Faster digesta transitPalestine Sunbird (Nectarinia osea)(426)
    Carbohydrate HydrolysisAlkaloids (sugar mimics in latex sap)Mulberry (Mortis spp.)Inhibition of sucrase and trehalaseSilkworm S. ricini(212)
    Carbohydrate Hydrolysis27 kDa proteinMungbean (Vigna radiata) seedsInhibition of α-amylaseCallosobruchus maculates (insect)(473)
    Carbohydrate HydrolysisExtractsMorus albaInhibition of sucrase, maltaseHumans, rat(350)
    ProteolysisNonprotein compoundCastor bean (Ricninus communis)Trypsin inhibitionInsect-coffee leaf miner (L. coffeella)(383)
    ProteolysisProteinase inhibitorSoybeanTrypsin inhibitionPig(293)
    ProteolysisOvorubin (protein)Snail (Pomacea canaliculata) eggTrypsin inhibitionBovine(129)
    ProteolysisProteinase inhibitorN. alataChymotrypsin InhibitionHelicoverpa larvae(132)
    LipolysisDiterpene carnosic acidSage (Salvia officinalis)Lipase inhibitionMice(344)
    LipolysisPhenolics or other extractsTeaInhibition of lipid emulsificationHumans(320,401)
    LipolysisPhenols, tannin-strictininTeaInhibition of lipasePigs(192)
    LipolysisTriterpenoid saponin(s)Acanthopanax senticosus fruitInhibition of lipasePig(291)
    Fiber digestionPolyphenolics (anthraquinone, proanthocyanidins and other tannins InhibitionRuminants(271,314)
    Fiber digestionTerpenoids (essential oils, saponins) InhibitionRuminants(2,357)

    Secondary metabolites and transit time

    There is a long history of use by humans of natural products as laxatives (31). Some SMs that alter digesta transit in humans and wild animals are listed in Table 4 . An important consequence of rapid digesta transit can be malabsorption, as occurs even for animals with rapid transit time ingesting passively absorbed compounds. For example, digestion time (and glucose absorption) was reduced when sunbirds ingested nectar from tobacco plants that contain particular alkaloids (426). In contrast, the anthraquinone, emodin, which tends to speed digesta through the gut of humans (137), appears to have the opposite effect on the frugivorous bird the Yellow-vented bulblul, and increases the bird’s apparent digestive efficiency on emodin-containing fruit (440). The few examples in Table 4 show how the compounds that influence transit time are chemically heterogeneous, and they also could act through a variety of mechanisms. These might include osmotically based mechanisms, which might draw water into the lumen by acting as introduced osmolytes or by receptor-mediated increase in secretion of ions, or by a nonosmotic mechanism such as direct action on motility patterns via receptor-mediated changes in neuromuscular activity [e.g., reference (27)].

    Secondary metabolites and endogenous enzyme activity

    Chemicals from many of the major groupings of SMs (e.g., alkaloids, phenolics, and terpenoids) inhibit animals’ intrinsic mechanisms of breakdown of carbohydrates, fats, and proteins ( Table 4 ). In many cases, the compounds have been shown to inhibit enzymatic breakdown in vitro, and effects are also manifest at the whole animal level in reduced nutrient digestibility and/or growth rate [e.g., references (212, 344, 473)]. Mechanisms vary, including competitive (350) and noncompetitive (473) enzyme inhibition as well as disruptions of the emulsification process important in digestion of fat (401).

    One of the best studied chemical groups are protease or proteinase inhibitors (PIs), which bind to digestive proteins and reduce digestive efficiency and hence growth rate (237, 385). Many insects, mammals, and birds respond by increasing secretion of proteolytic enzymes and, in the vertebrates, by increasing the size of the pancreas, which synthesizes many of the enzymes, often with the net effect of restoring digestive efficiency and growth rate. A competing hypothesis about the animals’ response is that overproduction of digestive proteins is to the detriment of other essential proteins in the body, and that growth rate thus does not recover (237). Research suggests antagonistic coevolution between plants and herbivores in which the plants produce a variety of PIs with specific action against different kinds of proteases and the animals produce digestive enzyme variants that are fairly insensitive to the PIs (237). Trypsin inhibitor in castor bean leaf extract inhibited trypsin-like activity in the coffee leaf miner (Leucoptera coffeella Table 4 ) but not bovine trypsin (383). Helicoverpa larvae have been identified whose chymotrypsin activity is resistant to a serine PI from Nicotiana alata, whereas other Helicoverpa larvae have an enzyme variant that is susceptible (132). Their respective cDNAs were isolated and critical residues that conferred resistance were identified. Helicoverpa larvae were also found to produce midgut proteases (85) or trypsin isoforms (313) that were either sensitive or insensitive to inhibition by soybean trypsin inhibitor (STI). The STI-senstive trypsim isoforms were produced constitutively, but production of the induced STI-insensitive forms was regulated transcriptionally following ingestion of STI (313).

    A somewhat analogous scenario is emerging from studies of inhibitors of carbohydrases. Mulberry leaves produce sugar-mimic alkaloids that inhibit sucrase and trehalase activity ( Table 4 ). However, activities in domesticated silkworms (Bombyx mori), which are mulberry specialists, are not affected whereas activities in Eri silkworms (Samia ricini), which are generalist insect herbivores, were inhibited by very low concentrations of the alkaloids (212). In another example, when larvae of bean weavils (Zabrotes subfasciatus) were fed seeds of Phaseolus vulgaris they secreted inducible isoforms of alpha-amylases that were insensitive to the alpha-amylase inhibitor that is found in the plant, whereas their constitutively produced alpha-amylase was inhibited by SMs in the plant [reference (29) see also references (29, 403)]. The entire topic of coevolution of digestive enzymes and plants SMs is not only interesting but also very important, because plant biologists are now experimentally manipulating in crop plants the genes that regulate inhibitory SMs to enhance resistance to crop pests.

    Tannins are water-soluble polyphenolic compounds with a molecular weight between 300 and 3000 Da, and have the putative function as possible digestibility reducers (248). They can interact with proteins and other macromolecules in vitro through hydrogen bonding and hydrophobic bonds, and thus bind enzymes and their nutrient substrates. Several studies document their inhibition of many enzyme activities in vitro: proteases, lipase, alpha-amylase, maltase, sucrase, and lactase [e.g., references (69, 304)]. These data lead to an expectation that they will reduce diet digestibility (26).

    Even if digestive enzymes are inhibited in vitro, the effects can, in principle, be prevented or reversed in vivo by change in pH or by surfactants (detergents) such as bile acids or other tannin-binding material in the gut such as mucus (26). Some mammals that commonly consume tannins secrete proline-rich (20%�% proline) proteins in their saliva that are thought to preferentially bind tannins (197). The complexed tannins may escape both enzymatic and microbial degradation, and may be excreted in the feces, thus protecting the animal from either damage to the gut epithelium, true digestibility reduction, or toxicity (11). But, this response leads to increased fecal loss of the energy and nitrogen in the tannin-protein complex and thus to a decline in apparent digestive efficiency, though not true digestive efficiency per se (409). In some species, the relationship between dietary tannin content and reduction in apparent digestibility can be used to increase the accuracy of predictive equations of food digestibility based on food chemical composition (201). Thus, with tannins, the effects on animals are not general but depend on the particular tannin structure, concentration, and on particularities of the consumer. Because of this, it has been argued that they are not typically disruptors of intrinsic breakdown processes in either insects (26) or monogastric mammals (409).

    Intestinal enzymes can activate certain toxins. Some SMs are synthesized and stored in plants as glycosides, that is, essentially bound to a glucose molecule, which can provide the plant a measure of self-protection from the more toxic aglycone (202). These SMs are thus stored in an inactive form until activated by a glycohydrolase enzyme (e.g., β-glucosidase). The enzyme may be stored in the plant, in which case maceration by a consumer causes release of the aglycone toxin, or the enzyme might be a component of the consumer’s physiological processes such as intrinsic digestive enzymes or microbial enzymes (202).

    β-glucosidases are an important group of glucohydrolases found in the small intestine tissue of mammals, with apical membrane-bound lactase phlorizin hydrolase and broad-specificity cytosolic β-glucosidase being the most widely studied, including in humans, rats, and guinea pigs (95, 113, 342). β-glucosidase activity has also been measured in guts of numerous invertebrates (5, 143, 151, 157, 183, 374, 391). β-glucosidase activity is reduced in some insects that have either been selected for tolerance to plant glycosides (114) or habituated to diets with higher levels of glycosides (152, 355). It is not known whether such genetic or phenotypic adaptive response to dietary glycosides occurs in a vertebrate species. Although birds may have a homolog of the lactase gene (162), it is uncertain whether birds are capable of hydrolyzing plant glycosides, which might make them relatively immune to these plant toxins compared to other animals. Levels of lactase activity are trace or immeasurably low in chickens (84) and in house sparrows (P. domesticus) and common bulbuls (Pycnonotus bartatus) (K. M. Lessner andW. H. Karasov, unpublished data). Also, in a study with cedar waxwings (Bombycilla cedrorum), the birds were not affected by the toxic glycoside, amygdalin, when administered orally, excreting it intact (422).

    Secondary metabolites and the microbiota

    SMs from major groups such as phenolics and terpenoids are known to have antimicrobial activity (460). Terpenoid compounds, including essential oils and saponins (glycosides of terpenes and steroids), appear to have the largest negative effects, based on a meta-analysis of 185 treatments in ruminants in 36 studies (357). For example, the magnitude of inhibition of plant cell-wall digestibility was 23% for essential oils, 11% for saponins, and 3% for tannins (all relative to controls). Scores of specific essential oils have been tested and found to be inhibitory against many bacterial genera (2), and in the meta-analysis, they and saponins also appeared to inhibit protozoal growth (357). Besides inhibiting fermentation, essential oils can decrease the rate of bacterial deamination of protein in the lumen (2).

    The complexing ability of proanthocyanidins and other tannins makes them reactive with bacterial cell walls and extracellular enzymes (311, 314). This could be the basis for how they can reduce microbial fermentation (39, 181, 300, 432) and growth, alter microflora populations, and reduce attachment of fungi and bacteria to substrates (2). Another phenolic SM, usnic acid found in some lichens, had a potent antimicrobial effect against 25 of 26 anaerobic rumen bacterial isolates from reindeer (Rangifer tarandus) (424), but one isolate was resistant.

    The usnic acid-resistant microbe is one of at least three fairly well-documented examples of ruminal microorganisms that can apparently tolerate some SMs. Sundset et al. (423, 424) showed that usnic acid was apparently degraded in the rumen, and characterized a resistant bacterium that they proposed be named Eubacterium rangiferina. Their findings help explain earlier findings that rumenal microbiota from reindeer performed better at in vitro digestion when usnic acid was added, whereas addition of usnic acid to sheep rumenal microbiota depressed digestion (355). Another famous example is the bacterium Synergistes jonesii, which is capable of degrading mimosine metabolites and imparts mimosine resistance in the host ruminant, allowing it to eat Leucaena spp. (248). Finally, some GI microorganisms can apparently tolerate high concentrations of tannins, and tannin-tolerant or tannin-degrading bacterial species (189, 388) have been isolated from a variety of wild mammals worldwide, especially those that consume diets high in tannin content (314). Some of the features that may impart microbial tolerance to tannins are secretion of extracellular polysaccharide that separates the microbial cell wall from the tannin (314) and microbial enzymes such as gallate decarboxylase and tannin acyl hydrolase (2).

    Secondary metabolites and absorption

    Most reports of impacts of SMs on absorption refer to polyphenolic compounds, of which there are at least ten classes of compounds characterized by possessing several hydroxyl groups on aromatic rings. Martel et al. (307) provide a recent review of impacts of polyphenolics on intestinal absorption of organic cations, thiamin, folic acid, and glucose.

    Many studies indicate that a variety of polyphenolics (mainly flavonoids) inhibit mediated glucose uptake by SGLT1 and/or GLUT2, based on experiments using intestine in situ, isolated tissue and cells, brush border membrane vesicles, and Xenopus laevis oocytes expressing the transporter proteins (307), and one study found that polyphenols depressed SGLT1 gene expression (351). These compounds occur in plant foods typically as glycosides. Phloretin (an aglycone) and phloridzin (its glycoside), members of the flavonoid subclass chalcones, are used as inhibitors of GLUT-2 and SGLT-1 respectively, in glucose absorption studies. But, studies have shown that a variety of flavonoids from multiple subclasses inhibit glucose transport (82, 255, 267, 274, 307, 408, 411). Proposed mechanisms for flavonoids inhibiting glucose absorption include competitive and noncompetitive inhibition. Only the mechanism for phloridzin’s inhibition of SGLT-1 has been rigorously proven to be competitive inhibition by phloridzin binding to SGLT-1 directly (346, 477, 478).

    Skopec and Karasov (408) predicted that phloridzin would inhibit glucose absorption at the whole animal level when administered at ecological concentrations (they used 10 mmol/L), and that the effects would be more pronounced in nonflying mammals that rely on mediated pathway(s) for glucose absorption than birds that rely more on a nonmediated, paracellular pathway. They found that phloridzin inhibited whole-animal glucose absorption efficiency by more than 36% in laboratory rats, whereas it did not significantly decrease glucose absorption in American robins (408). Another flavonoid, isoquercetrin, also significantly decreased glucose absorption in rats but not in robins. They did not ascribe the difference to any major difference between rat and robin in the types of intestinal glucose transporters, because birds and mammals appear to share the similar suite of intestinal sugar transporters (292, 332). Instead, they ascribed the difference in the inhibition by these plant SMs of glucose absorption to the rats’ much greater reliance on glucose transporters for intestinal glucose absorption than is the case for robins. Because plant toxins mediate so many interactions between mammals and birds and their plant resources (e.g., leaf, fruit and seed diet selection, and seed and pollen dispersal), physiological differences between mammals and birds in their responses to toxins should have many ecological ramifications (86).

    There is evidence that some flavonoid glycosides may be transported by SGLT-1 (10, 82, 274, 459), which could potentially lead to competitive inhibition of glucose transport. However, Kottra and Daniel (267) used Xenopus oocytes expressing SGLT1 in a two-electrode voltage clamp technique to test 27 flavonoids carrying glucose residues at different positions as well as their aglycones. None of them generated significant transport currents, which seems to be good direct evidence for lack of Na + -coupled transport via SGLT1. But, as has been demonstrated many times, some glycosylated and nonglycosylated flavonoids did show structure-dependent inhibition of glucose transport. Competitive inhibition by flavonoid transport does not seem to be the mechanism.

    Reports of impacts of SMs on absorption of other substrates are scanty. The phenolic, tannic acid, nonspecifically inhibited D-glucose and L-proline uptake by isolated mouse intestine, possibly by reduction in the Na + gradient for Na + -coupled nutrient uptake across the apical membrane (251). Another set of phenolics, catechins, which are monomeric flavanols, are reported to inhibit cholesterol absorption, perhaps by reducing micellar solubility and precipitating cholesterol (222), and they are reported to interact with lipid bilayers (336), which could lead to alterations in transport.

    It is to be expected that water-soluble toxins that are not too large in molecular size will also have access to the paracellular pathway (238b). Some of the major classes of naturally occurring toxins in plants, such as alkaloids and phenolics (202), include many water-soluble compounds in the molecular size range that could access the paracellular space (243). Nicotine, for example, has a MW of 162 Da, its cationic forms are water soluble, and it was found to be absorbed by the paracellular pathway in cell culture (TR146 cells) (343).

    Lipophilic toxins are also anticipated to permeate membranes passively at rates positively related to their octanol or oil:water partition coefficients, which was found to be the case in a survey of 36 flavonoids using Caco-2 cell monolayers (431). In this experimental model, rates can be decreased by the presence of salivary proteins that form complexes with polyphenols (60, 61). Other physical barriers proposed to limit passive diffusions of SMs are the peritrophic envelope of insects and surfactants (14, 15, 284). The discovery of efflux transporters over the past 2 to 3 decades across many animal phyla revealed another process by which passive absorption of lipophillic SMs might be limited. These include the ABC transporters such as multidrug resistance proteins and permeability glycoprotein, or P-glycoprotein. Several reviews are available regarding their interactions with SMs (299, 331, 412).

    Conclusion and future directions

    This review has uncovered numerous areas for future research focused on important gaps in the comparative physiology of the GI tract. Thus, we end with a short list of some of the potential areas for future research.

    Technical advances. Understanding of the physiology of the GI tract is being transformed by the advent of increasingly sophisticated molecular and postgenomic tools to study the expression of digestive enzymes and transporters. We can anticipate tremendous strides in our understanding of the mechanistic basis of digestive function and absorption. Application of these tools has already advanced knowledge about molecular steps in dietary and developmental regulation of gene expression of carbohydrases in rodents (Section 𠇏lexible adjustment of digestive enzymes to diet change”) and chickens (Section “Patterns in birds”). The new tools will also illuminate mechanistic details about differences between species, as has been done for differences between human populations for carbohydrases related to starch and lactose digestion (Section “Molecular mechanisms for differences in enzyme activities between populations/species”).

    The importance of research on invertebrates. The literature on the function of the animal GI tract is dominated by research on vertebrates, especially mammals. Although this is not surprising, given the biomedical importance of much of this work, the field of comparative digestive physiology is constrained by our ignorance of most invertebrate groups. Molecular and physiological research on sugar and amino acid transport in a few invertebrates, for example, has revealed that it may occur on familiar transporters, such as SGLT1 for glucose, but with unfamiliar counterions (K + , not Na + ), or even on transporters unfamiliar to vertebrate biologists (Sections �sorption of carbohydrates” and “Pathways for amino acid and peptide absorption”). Furthermore, the digestive physiology of the key invertebrate model species, Drosophila melanogaster and C. elegans, is very little studied. Although this aspect of the biology of these species includes many features adapted to their specific diet (both species are predominantly microbivores), there is tremendous opportunity to use both these model species to investigate the fundamentals of GI tract function. As an illustration, recent work on Drosophila has revealed the central importance of Janus kinase/signal transducers and activators of transcription signaling and the resident microbiota as determinants of the turnover of midgut epithelial cells and gut homeostasis in this insect (47).

    Integration of mechanisms with whole organism processes and performance. There is opportunity to integrate molecular events with whole organism processes, including response to diet composition and the nutritional requirements of the animal (that can change with age, developmental stage, environmental circumstance, etc.). House sparrows that have the ability as nestlings to upregulate carbohydrase activity on high-carbohydrate diet (Section 𠇏lexible adjustments of transporters to diet change”) lose the ability in adulthood (73), and the molecular details of such regulatory differences during development remain to be described in this and other species. The finding of apparent homeostatic regulation of digestive enzymes in Locusta (Section 𠇏lexible adjustment of digestive enzymes to diet change”) is novel and should be further investigated in this and other species. There is an increasing appreciation of the need to study responses to complex diets and diets in which two or several different classes of nutrients may not vary independently. Emphasizing this point, it is becoming increasingly evident that the physiological response of animals to diets can be complex, and may not necessarily serve to maximize energy gain or even fitness under some circumstances. Disentangling these complexities represents a major challenge for the coming years.

    The significance of the microbiota. The taxonomic diversity of gut microbiota remains to be described in many major animal taxa and compared across metabolic groupings (e.g., vertebrate homeotherms vs. poikilotherms). Besides differences among animal species, microbial biodiversity can vary among populations within species and even among individuals within populations. The genetic, developmental, and environmental (e.g., diet) determinants of all this microbial biodiversity in guts remain to be determined as well as its functional significance to the host [but see reference (334)]. Our understanding of how hosts recover important nutrients from their microbiota is incomplete, especially for those that do not engage in coprophagy such as pigs, humans, herbivorous birds, and fish. Questions include (i) the location and magnitude of lysozyme and other digestive enzyme capacity and the pathways for absorption of microbially produced essential nutrients and (ii) whether the associated fluxes are great enough to affect nutritional requirements.

    Importance of an evolutionary perspective. Overlaying these important questions is the central role of evolutionary processes in any explanation of the diversity of digestive physiology across the animal kingdom. Although phylogenetically informed methods have been used in biology for many years, there remain great opportunities to apply these approaches to comparative physiology of digestion and absorption. These methods will lead to new hypotheses that will require testing by the full range of molecular and organismal physiological techniques and will encourage much-needed research on the physiology of various animal groups. Without doubt, there is much functional diversity and mechanistic novelty in the digestive systems of animals still awaiting discovery.