Animal Physiology 3. Digestion and metabolism

Nutrition involves taking food in the form of meals and converting them into nutrients that can be of sort across the intestinal wall. This is done through a sequence of different processes, ranging from physical rendering of the food, to acid digestion in the stomach, to absorption of amino acids and sugars in the small intestine, to emulsification of fats, and then to the recovery of water, salts, and other nutrients in the colon.

KW: digestion; food; nutrients; mouth; salivary gland; amylase; saliva; stomach; acid; pepsin; protein digestion; small intestine; exocrine pancreas; bicarbonate; bile salts; liver; emulsification; proteins; amino acids; fat; fatty acids; micelle; complete digestive tract; incomplete digestive tract; deuterostome; protostome; chime; trypsin; gut flora; reabsorption;

Absorption of nutrients across intestinal wall uses energy to actively transport sugars and amino acids across the intestinal epithelium. Central to this is the standing gradient mechanism, in which a cell uses energy to establish a potential energy gradient in one form that can be used to actively transport other materials. The standing gradient mechanism is illustrated by the absorption of water across the intestinal epithelium, in which ATP energy is used to transport sodium and establish a potential energy gradient and osmotic potential, which drives water absorption across the intestinal epithelium.

KW: digestion; standing gradient; belly patch; water reabsorption; sodium transport; intestinal epithelium; aquaporin; sodium channel; water balance; colon; crypt; active transport;

Absorption of sugars in amino acids across the intestinal wall uses a standing gradient mechanism in which glucose and amino acids are piggybacked onto a sodium carrier, which translocates both sodium and the sugar or amino acid into the epithelial cell. The standing potential energy gradient is maintained by sodium transport out of the cell which keeps the sodium concentration within the cell low. In addition to the standing gradient mechanism, absorption of sugars involves a series of complex enzymatic reactions that ultimately convert various disaccharides and stereoisomers of glucose into glucose.

KW: digestion; sugar absorption; amino acid absorption; standing gradient; sodium transport; fructose transport; galactose; lactose; hydrolase; sucrose; isomerization; brush border; facilitated diffusion; lactose intolerance; cultural evolution; dairy husbandry;

Absorption of fats involves the physical translocation of the micelles formed in the small intestine across the intestinal wall, and into the lacteal vessels. In the process, the lipid micelles are turned into a lipoprotein known as a chylomicron, which is transported to the liver for further modification. Fat absorption and metabolism continues through the use of lipoproteins acting as shuttles for lipids between cells, adipose tissue, and the liver. Specifically, fat transport involves the conversion between two common classes of lipoproteins, the very low density lipoproteins, or VLDLs and the intermediate density lipoproteins, or IDLs.

KW: digestion; micelles; endocytosis; exocytosis; chylomicron; lacteal; liver; adipose tissue; cells; very low density lipoprotein; VLDL; intermediate density lipoprotein; IDL; dietary fat; shuttle; bile salt; protein; phospholipid; VLDL to IDL ratio;

Cholesterol metabolism is a special case of lipid metabolism, in part because cholesterol plays a vital role in a number of cell functions. Cholesterol is absorbed in the diet, and is transported around the body on a lipoprotein shuttle system, similar to that for dietary fats. The lipoproteins involved in the cholesterol metabolism are in a different class of lipoproteins called the low density lipoproteins or LDLs, and the high density lipoproteins, or HDLs. These form part of a larger regulatory system in which cholesterol levels within the cells and blood are regulated.

KW: digestion; cholesterol; HDL; IDL; high density lipoprotein; low density lipoprotein; shuttle; liver; cells; cardiovascular disease; LDL to HDL ratio; bile salts; cholesterol export; dietary cholesterol;

Optimizing gut function can be modeled by using diminishing returns theory. Specifically, diminishing returns theory seeks to ask the question “For how long should a meal be retained within the digestive system to optimize the obtaining of nutrients from it?” Optimum gut function can sometimes mean expelling a meal even if there are considerable undigested nutrients left in it.

KW: digestion; sea anemone; incomplete digestive tract; diminishing returns theory; chemical kinetics; nutrient extraction rate;

In the complete digestive tract, diminishing returns theory accounts for all the costs of obtaining the meal and having a digestive tract ready to process it, as well as nutrient and energy that can be extracted from the meal. Guts can be optimized for two different functions: for increasing the maximum yield of energy, or maximizing the net energy yield rate. Each propose an optimum retention time for digesting food, which sets, among other things, and optimum that links.

KW: digestion; complete digestive tract; diminishing returns theory; chemical kinetics; net energy yield; net energy yield rate; optimum retention time;

Diminishing returns theory lets us calculate how diet quality affects optimum retention time, and from there, optimum gut length. Optimum gut length is set by a combination of a foods energy density, and metabolic demand. Low-quality foods generally require high-quality foods.

KW: digestion; food quality; energy density; bulk flow rate; optimum retention time; optimum gut length; metabolic rate;

Many diets contain a mixture of high-quality foods and low-quality foods. It’s therefore difficult to provide an optimum gut length for such mixed quality diets. The high-quality component require short retention times, while the low-quality component requires long retention times. Animals balance these conflicting demands by essentially building two digestive tract into one, providing diversion chambers for the low-quality components to digest longer. In animals, this adaptation arises in three different ways:-hindgut fermenters, foregut fermenters, and coprophagy.

KW: digestion; mixed diet quality; hindgut fermenters; fore gut fermenters; cecum; ruminant; stomach; esophagus; fundus; cardia; pylorus; rabbit; coprophagy;

Digestion involves a coordinated and sequential processing of a meal as it passes through the gut. There are three basic phases to digestion: a cephalic phase, a gastric phase, and an intestinal phase. Digestion is characterized by various levels of hormonal control and interaction between these phases. The endocrine glands behind this control are the tissues of the digestive tract itself.

KW: digestion; cephalic phase; gastric phase; intestinal phase; gastrin; cholecystokinin; CCK; pancreas; gallbladder; bile; stomach acid; gastric motility; duodenum; secretin; gastrointestinal peptide; GIP;

Control of appetite is tied in with the bodys system of pleasure, reward, and learning. Appetite is regulated through an interaction between the brain’s limbic system, and systems of appetite regulation located in the medulla of the spinal cord.

KW: digestion; cephalic phase; pleasure reward system; limbic system; medulla; ventral tegmentum; nucleus accumbens; appetite regulatory network; orexigenesis; anorexigenesis; anorexia; neuropeptide Y; melanocyte stimulating hormone;POMC; MSH; pro-opiomelanocortin;

Control of appetite is also tied in with feedbacks onto the cephalic phase of digestion from the stomach and intestines. There are various levels of hormonal control, with hormones secreted from adipose tissue, the stomach, and the large intestine that feedback onto the appetite control centers of the brain.

KW: digestion; cephalic phase; appetite regulatory network; orexigenesis; anorexigenesis; anorexia; neuropeptide Y; melanocyte stimulating hormone; POMC; MSH; pro-opiomelanocortin; adipose tissue; leptin; fundus; ghrelin; colon; peptide YY; pancreas; insulin; glucagon;

The metabolic rate is fundamentally a rate of energy consumption, energy used to do the order producing work that underlies all of life. Thermodynamically, all this energy is ultimately released as heat. This enables one to measure and animals total metabolic rate by measuring its total rate of heat production, a method known as direct calorimetry.

KW: energetics; metabolic rate; free energy; second law of thermodynamics; entropy; heat production; direct calorimetry; ice calorimeter; Antoine Lavoisier;

The thermodynamic aspect of metabolic rate is inevitably coupled to chemical metabolism, that is the oxidation of metabolic fuels such as carbohydrates, lipids, and proteins, to provide free energy to do order-producing work. Because the oxidation of metabolic fuels is a chemical process, chemical metabolism has a stoichiometric relationship to energy metabolism. One can measure metabolic rate, therefore, by measuring the consumption rate of metabolic fuels, and their oxidants, mostly oxygen, or by measuring the production rate of chemical wastes, such as carbon dioxide. This is a method known as indirect calorimetry.

KW: energetics; metabolic rate; chemical metabolism; carbohydrates; lipids; indirect calorimetry; stoichiometry;

Methods of measuring metabolic rate through heat production rate, or oxygen consumption rate, always involve restraint of the animal, either in a metabolic chamber, or tied to an instrument via a respiratory mask. These methods do not allow the measurement of metabolic rates of animals in their natural environments, or engaging in their normal behaviors. There is a method of indirect calorimetry, known as the doubly-labeled water technique, which allows the measurement of metabolic rates of free ranging animals in their natural environments. This method uses clearance rates of isotopes of oxygen and hydrogen from the body. Here, we explain how tracer isotopes can be used to estimate the water turnover rate in an animal’s body.

KW: energetics; metabolic rate; isotope; protium; tritium; oxygen 18; turnover rate; water balance; evaporation; respiration; defecation; urination; indirect calorimetry; doubly labeled water;

The method of doubly-labeled water involves measuring the turnover rates of two isotopes contained within water, tritium for hydrogen, and oxygen 18 for oxygen. By measuring the clearance rate of these two isotopes, one can estimate both the water turnover rate of the body, and the carbon dioxide turnover rate of the body. The latter is related stoichiometrically to the metabolic rate. The technique of doubly labeled water, therefore, allows one to measure the metabolic rates of animals in their natural environments and engaging in their natural behaviors.

KW: energetics; doubly labeled water; isotopes; tritium; oxygen 18; turnover rate; carbon dioxide turnover rate; stoichiometry; indirect calorimetry;

Respiratory quotient is the ratio of carbon dioxide production over oxygen consumption. This value differs between metabolic fuels. Carbohydrates, for example, have respiratory quotients of 1. Lipids, on the other hand, have respiratory quotients that are considerably less, about 0.7. By measuring the respiratory quotient one can estimate several important things about an animal’s chemical metabolism. For example, one can estimate the mix of metabolic fuels, that is, the proportions of carbohydrates and lipids being consumed. The respiratory quotient also enables one to estimate the energy equivalence between moles of oxygen consumed, and joules of energy produced. This quantity also varies depending upon the mix of metabolic fuels.

KW: energetics; respiratory quotient;, RQ; oxygen consumption rate; carbon dioxide consumption rate; energy equivalence; proteins; amino acids;

Compared to the remarkable diversity of form amongst animals, variation of metabolic rate among animals is reducible to just two dimensions: body size, and metabolic lifestyle. Body sizes of animals range over nearly 18 orders of magnitude of mass. This requires us to think about variation in a different way. In this lecture, we introduce you to some important mathematical and conceptual tools you will need to be able to analyze any variation that is due to body size, metabolic rate included.

KW: body size; scaling; allometry; power equation; logarithm; body mass; body length;

How should metabolic rate scale with body size? There are many likely possibilities, which we explore here. For example, for many years, variation of metabolic rate was assumed to be limited by the surface area of the skin through which an animal can lose metabolic heat. This relationship, in which metabolic rate scales to the two thirds power of body mass, was known as the surface law. As it turns out, metabolic rate scales by unusual exponent that is close to three fourths. This stands out as one of the major unresolved problems in comparative physiology.

KW: scaling; surface law; two thirds power; three fourths power; metabolic rate; body size; power equation; logarithm; Kleiber;

One prominent explanation for the three fourths power law is that it is a statistical error. Specifically, it has been argued that variation of metabolic rate within animal types follows the surface law, namely two thirds power scaling. This explanation does not account for the three fourths power scaling of metabolic rate that seems to characterize variation across different animal types.

KW: surface law; Heusner; statistical artefact; regression analysis; metabolic rate; variance; scaling;

An ingenious explanation for the three fourths power law can be derived from mechanical considerations. Animals and trees are elastically similar, that is their risk of elastic failure appears to be the same across all body sizes and animal types. Applying elastic similarity criteria to the muscles, which are the major source of the metabolic consumption of energy, yields a scaling exponent of three fourths.

KW: elastic similarity; three fourths power law; muscle mechanics; buckling failure; animal shape; metabolic rate; scaling;

Fractal scaling represents a peculiar kind of geometry in which shape can vary with size, but form is held constant. The exchange surfaces of animals for respiratory gasses, nutrients and wastes are fractal surfaces. This may put a peculiar scaling limitation on metabolic exchange rates, which may explain a three fourths power scaling in a physiologically meaningful way. It seems not to be a complete explanation, however.

KW: fractal geometry; Koch snowflake; similarity; lung; exchange; metabolic rate; scaling;

Another major source of variation of metabolic rate is the so-called metabolic lifestyle. Animals metabolic rates parse into two dramatically different energy use patterns. On the one hand are mammals and birds, which generate considerable amounts of metabolic heat to maintain a warm and steady body temperature. On the other hand are nearly every other animal of the animal kingdom, which use metabolic energy sparingly, and do not expend considerable amounts of energy on regulation of the body temperature. These two very different energy lifestyles correlate with significant differences of mitochondrial density.

KW: metabolic rate; energy misers; energy wastrels; mitochondria; body size;

The existence of energy wastrels poses an interesting evolutionary question. What are the selective advantages of expending 90% of your metabolic energy budget to the wasteful production of heat rather than the production of offspring? There is a plausible explanation for this, but only if the high metabolic rate of energy wastrels enables them to command enough energy to outcompete the more energetically efficient energy misers.

KW: metabolic rate; energy misers; energy wastrels; reproduction; fitness; selection; growth; energy budget; homeostasis;

The selective advantage of being an energy wastrel is predicated on the ability it confers to command a large metabolic stream of resources and energy. How this ability comes about is probably due to the effects of high metabolic rate on efficiency of locomotion. Specifically, animals that are energy wastrels should be able to run faster more efficiently than energy misers can. The evolutionary question turns, therefore, on how metabolic lifestyle affects the locomotory capacity of animals.

KW: metabolic rate; energy misers; energy wastrels; fitness; running speed; aerobic scope; cost of transport; momentum;

Animals with high metabolic lifestyles have the same aerobic scopes as do animals with low metabolic lifestyles. However, because of the height resting metabolism of energy wastrels, their ability to run a robust we at higher speeds is much greater than that for energy misers. This difference is the key to understanding how animals with a high-energy metabolic lifestyle can outcompete those animals that are more efficient and sparing in their use of metabolic energy.

KW: metabolic rate; energy misers; energy wastrels; fitness; running speed; symmorphosis;

The selective advantage of a high metabolic rate lifestyle is predicated on the ability of energy wastrels to cover large amounts of territory. This ecological dimension of metabolic lifestyle can be quantified by estimating the effects of metabolic rate and body size on the daily movement distance and home range of sizes of different kinds of animals.

KW: metabolic rate; energy misers; energy wastrels; fitness; daily movement distance; daily energy expenditure; snakes; colubrid; Brown racer; reproduction; carnivore; non-carnivore;