Animal Nutrition

Overview: The Need to Feed

Animal nutrition is a fundamental aspect of biology, focusing on how animals obtain and process food to meet their energy and biosynthetic needs. Animals are classified based on their dietary habits:

• Herbivores: Eat mainly autotrophs (plants and algae).

• Carnivores: Consume other animals.

• Omnivores: Regularly eat both plants/algae and animals.

An adequate diet must satisfy three main needs:

• Fuel for cellular work (energy).

• Organic raw materials for biosynthesis.

• Essential nutrients that cannot be synthesized by the animal.

Animals employ various feeding mechanisms:

• Suspension feeding: Filtering small food particles from water.

• Substrate feeding: Living in or on their food source.

• Fluid feeding: Sucking nutrient-rich fluids from a host.

• Bulk feeding: Eating large pieces of food.

Homeostatic Mechanisms and Energy Budget

ATP Generation and Energy Storage

Animals generate ATP primarily through the oxidation of carbohydrates, proteins, and fats. ATP powers basal metabolism, activity, and thermoregulation. Excess calories are stored as glycogen (in liver and muscles) and fat.

Glucose Regulation

Glucose is a major cellular fuel. Hormones regulate glucose metabolism, maintaining homeostasis. When intake is less than expenditure, stored fuel is mobilized and oxidized.

Caloric Imbalance

• Undernourishment: Chronic deficiency in calories leads to depletion of glycogen and fat, breakdown of proteins, muscle loss, and brain impairment.

• Overnourishment (Obesity): Excess intake results in fat storage and associated health risks.

Obesity and Weight Regulation

Obesity is a global health problem, contributing to diabetes, cardiovascular disease, and certain cancers. Body weight is regulated by homeostatic feedback circuits and hormones affecting appetite and fat metabolism.

Obesity and Evolution

Fat hoarding was historically advantageous for survival. Some species, such as petrels, become obese as chicks to meet high energy demands.

Essential Nutrients and Diet

Carbon Skeletons and Essential Nutrients

Animals require carbon skeletons for biosynthesis and must obtain essential nutrients in preassembled form. Malnutrition results from missing essential nutrients, even if caloric intake is sufficient.

Classes of Essential Nutrients

• Essential amino acids

• Essential fatty acids

• Vitamins

• Minerals

Essential Amino Acids

Animals require 20 amino acids; about half can be synthesized, while the rest must be obtained from food. Eight amino acids (nine in infants) are essential. Protein deficiency leads to malnutrition, such as kwashiorkor.

Animal products (meat, eggs, cheese) are reliable sources. Plant proteins are often incomplete, so vegetarians must combine sources (e.g., beans and corn) to obtain all essential amino acids.

Some animals, like penguins, use muscle protein for amino acids during periods of high demand (e.g., molting).

Essential Fatty Acids

Most fatty acids can be synthesized, but certain unsaturated fatty acids (e.g., linoleic acid) must be obtained from the diet. Deficiencies are rare.

Vitamins

Vitamins are organic molecules required in small amounts. Humans need 13 essential vitamins, classified as water-soluble or fat-soluble. Water-soluble vitamins are excreted in urine if consumed in excess, while fat-soluble vitamins can be toxic if overconsumed.

Minerals

Minerals are inorganic nutrients required in small amounts. Excessive intake, such as sodium, can be harmful. Examples include calcium, iron, potassium, and phosphorus.

Malnutrition and Disease

Malnutrition is more common than undernutrition. Neural tube defects, such as failure to enclose the developing brain and spinal cord, are linked to vitamin deficiencies (e.g., folic acid). Supplementation has reduced such defects.

Food Processing: Ingestion, Digestion, Absorption, and Elimination

Main Stages of Food Processing

Food processing in animals involves four main stages:

• Ingestion: Act of eating.

• Digestion: Breaking down food into absorbable molecules (enzymatic hydrolysis).

• Absorption: Uptake of nutrients by body cells.

• Elimination: Removal of undigested material.

Digestive Compartments

Specialized compartments prevent self-digestion. Intracellular digestion occurs within food vacuoles (e.g., sponges), while extracellular digestion takes place in compartments continuous with the outside (e.g., stomach).

Mammalian Digestive System

Structure and Function

The mammalian digestive system consists of an alimentary canal and accessory glands (salivary glands, pancreas, liver, gallbladder). Food is moved by peristalsis, and sphincters regulate passage between chambers.

Oral Cavity, Pharynx, and Esophagus

Food is lubricated and digestion begins in the oral cavity. Saliva protects, lubricates, buffers, and contains antibacterial agents. Teeth chew food, exposing it to salivary amylase. The tongue shapes food into a bolus. The pharynx is a junction for the esophagus and trachea, with the epiglottis preventing food from entering the windpipe. Peristalsis moves food down the esophagus.

The Stomach

The stomach stores food and secretes gastric juice (hydrochloric acid and pepsin). Pepsin is activated by acid and begins protein hydrolysis. Mucus protects the stomach lining. Gastric ulcers are caused by Helicobacter pylori, which destroys mucus and causes inflammation.

The Small Intestine

The small intestine is the major site of digestion and absorption. The duodenum receives chyme from the stomach and mixes it with digestive juices from the pancreas, liver, gallbladder, and its own lining. The pancreas produces proteases (trypsin, chymotrypsin) and alkaline solution. The liver produces bile, stored in the gallbladder, which aids fat digestion. The brush border produces additional enzymes. Jejunum and ileum are sites of nutrient absorption.

Hormonal Regulation

Hormones coordinate digestive juice secretion:

• CCK: Stimulates release of digestive enzymes and bile.

• Enterogastrone: Inhibits peristalsis and acid secretion.

• Gastrin: Stimulates gastric juice production.

• Secretin: Stimulates pancreas to release bicarbonate.

Absorption of Nutrients

The small intestine's surface area is increased by villi and microvilli, enhancing absorption. Each villus contains blood vessels and a lacteal. Amino acids and sugars enter the bloodstream via active transport. Fats are recombined into chylomicrons and transported into lacteals.

The Large Intestine

The colon recovers water from the alimentary canal. Feces become more solid as they move through the colon. The colon houses Escherichia coli, which produce vitamins (biotin, folic acid, vitamin K, B vitamins). Feces contain bacteria, cellulose, and undigested material. The anus has voluntary and involuntary sphincters.

Critical Thinking Questions

• How does food swallowed by an astronaut reach the stomach in zero gravity?

• Explain how people can become obese even if their intake of dietary fat is relatively low compared with carbohydrate intake.