Mammalian Nutrition and Metabolism

• Metabolism converts food into energy.

Study Topics

• Nutrition
• Digestion
• Enzymes
• Absorption
• Metabolism

Nutrition

• This is the process of taking in food and using it for growth, metabolism, and tissue repair.

Nutritional Requirements of Mammals

• Major components of food include:
  • Carbohydrates, proteins, fats, vitamins, minerals, and water.
  • Carbohydrates, proteins, fats, and vitamins are organic nutrients.
  • Minerals and water are inorganic nutrients.

Carbohydrates

• These are a major source of energy for the body.
• They fuel the brain and improve concentration and mood.
• They provide fiber, promoting a healthy digestive system.
• Pentoses, ribose, and deoxyribose are important constituents of nucleic acids.
• Carbohydrates are classified as simple or complex.
• Simple carbs include monosaccharides (one sugar molecule) like glucose, fructose, and galactose.
• Simple carbs include disaccharides (two sugar molecules) like maltose, lactose, and sucrose.
• Complex carbs include polysaccharides (more than 6 sugar molecules) like starches, fibers, and glycogen.
• Simple carbohydrates lead to a quick spike in blood sugar.
• Complex carbohydrates lead to a slow, gradual increase in blood sugar.

Lipids

• These are biomolecules soluble in organic solvents but insoluble in water.
• Lipids have four major functions:
  • They form the structural component of biological membranes.
  • They provide energy reserves in the form of triglycerides.
  • They are precursors of vitamins and hormones.
  • They are precursors of bile acids, which aid in lipid absorption during digestion.
• Lipids types:
  • Simple or storage lipids, conjugated or compound lipids, and derived lipids.
• Simple lipids include neutral fats and waxes.
• Compound lipids include phospholipids and glycolipids.
• Derived lipids including lipoproteins.

Simple Lipids (Fats and Oils)

• Both are esters of fatty acids with various alcohols.
• Fats are solid at room temperature and formed from saturated fatty acids.
• Saturated fats have no double bonds in their chemical structure and are saturated with hydrogen atoms, giving them a solid consistency at room temperature.
• Oils are liquid at room temperature and formed from unsaturated fatty acids.
• Unsaturated fats have one (monounsaturated, e.g., olive oil) or more (polyunsaturated, e.g., sunflower and corn oil) double bonds.

Compound Lipids

• These are esters of fatty acids and alcohol with additional groups, e.g., phospholipids and glycolipids.
• Phospholipids are major constituents of the cell membrane.

Derived Lipids

• These are substances derived from simple or compound lipids by hydrolysis.
• Examples include fatty acids, glycerol, steroids, and ketone bodies.

Proteins

• Proteins provide basic building blocks.
• Proteins repair and replace tissues.
• Proteins are involved in transportation, formation, clotting, and defense.
• Proteins are macromolecules composed of amino acids.
• Proteins make up approximately 20% of the human body and are present in every single cell.
• Amino acids are commonly called protein's building blocks.
• Amino acids are linked together by a peptide bond in a protein molecule.
• Dipeptides are 2 amino acids.
• Tripeptides are 3 amino acids.
• Polypeptides are more than 10 amino acids.

Amino Acids

• Twenty different amino acids forms many different proteins found throughout the body.
• Amino acids are classified based on nutritional aspects:
  • Non-essential amino acids: the body can synthesize them (eleven amino acids).
  • Essential amino acids: the body cannot synthesize them, and they must be obtained from the diet (nine amino acids).
  • Semi-essential amino acids: the body can synthesize them, but not enough during infancy, growth, and in diseased states, so more are required in the diet (two amino acids).

Vitamins

• Vitamins are organic compounds, essential in small quantities, for the operation of normal bodily metabolism.
• Vitamins cannot be manufactured in the cells of the body.
• Vitamins do not provide energy but act as coenzymes for many metabolic enzymes.
• Vitamins are classified as water-soluble or fat-soluble.
  • Water-soluble vitamins include the vitamin B group and vitamin C.
  • Fat-soluble vitamins include vitamins A, D, E, and K.

Nutritional Content of a Well-Balanced Diet

• Carbohydrates (330g daily): main source of energy; fiber confers many health benefits.
• Protein (100g daily): major structural blocks.
• Fat (75g daily): energy storage; synthesis and repair of cell parts.
• Water (2000g daily): solvent; lubricant; medium for transport and temperature regulation.
• Vitamins (<300mg daily): enable chemical reactions in the body.
• Minerals (5-10g daily): aid enzymes function; electrical balance; generate nerve impulse; bone structure.

Four Stages of Food Processing

• Ingestion: taking in/eating food.
• Digestion: breaking down food (mechanically/chemically) into nutrients.
• Absorption: taking in nutrients by cells; transport of products into the circulatory system and to the rest of the body.
• Elimination: removal of undigested solid wastes.

Digestion

• This is the process where food is broken down by the digestive system into small nutrients to be absorbed by the body.

Organs Involved in Digestion

• Digestive tract organs.
• Accessory organs.

Digestion Occurs In

• The mouth
• The stomach
• The small intestine.

Accessory Organs

• Salivary glands.
• Liver.
• Gallbladder.
• Pancreas.

Mechanical and Chemical Digestion

• Mechanical digestion: involves chewing the food into smaller pieces.
• Chemical digestion: enzymes and acids help break down the food.

Enzymes

• These are specific protein catalysts act on substrate at their active site to form products.
• Substrate: the reactant that enzyme acts on.
• Active site: a pocket in the enzyme where a substrate binds that catalytic center converts substrate to product.

Localization of Enzymes

• Enzymes exert their function both inside and outside the cell.
• Enzymes are synthesized by ribosomes and modified by the Golgi Apparatus.
• Extracellular enzymes act outside a cell.
• Intracellular enzymes act inside a cell.

Enzyme Activity

• It refers to the catalytic ability of an enzyme to increase the rate of the reaction.

Factors Affecting Enzymes Activity

• Concentration of enzyme and substrate (affinity of enzyme to substrate).
• pH and temperature.
• Activators and inhibitors.
• Each enzyme has an optimal temperature of 37-40° C. If temperature is too high, the bonds are disrupted and the protein denatures.
• Each enzyme has an optimal pH; most enzymes have a pH between 5-9. Except digestive enzymes in the stomach work best at about pH 2.

Enzyme Activators

• These are non-protein molecules needed for the activity of specific enzymes.
• Enzyme activators may be:
  • Cofactors: non-protein, small inorganic compounds and metal ions such as Ca2+, Mg2+, Zn2+, Cu2+.
  • Coenzymes: non-protein, organic molecules derived from water-soluble vitamins.
• Some enzymes don't need an activator and are called simple enzymes.
• Those that need an activator are called complex (conjugated) enzymes.
• Apo enzyme is an inactive enzyme lacking it's non-protein part.
• Holoenzyme refers to the active enzyme with its non-protein component.

Enzyme Inhibitors

• These are substances that inhibit the activity of an enzyme.
• Enzyme inhibitors are reversible or irreversible.
  • Reversible inhibitors: these prevent enzymes from combining with a substrate. The activity of the enzyme is restored when the inhibitor is removed, and they occur normally inside the body to regulate the activity of enzymes.
  • Irreversible inhibitors such as organo-phosphorus compounds (insecticides): the activity of the enzyme is permanently stopped.

Digestion in the Mouth

• Two types of digestion occur:
  • Mechanical, and chemical.
• Mechanical digestion (mastication): the teeth cut and grind the solid material into small pieces. Mixing with saliva secreted by salivary glands forms a round mass called "bolus".
• Chemical digestion (enzymatic digestion): carbohydrates are digested by the salivary amylase enzyme. Saliva is secreted by 3 pairs of salivary glands.
• Saliva consists mainly of water, Cl and HCO3- of Na, K and Ca and the digestive enzyme amylase pH = 6-7.4.
• Salivary amylase: it hydrolyzes the glycosidic bonds of polysaccharides, producing smaller molecules as oligosaccharides and disaccharides. It is deactivated once food reaches the acidic pH environment of the stomach.

Digestion in the Stomach

• In the stomach, the food mass is converted into a liquid called chyme after mixing with gastric juice.
• Gastric glands contain several types of cells that secrete different products, forming gastric juice, which is highly acidic (pH 1.0 – 3.5).
• Gastric juice is secreted in response to:
  • Nervous stimulation is initiated by the taste, smell, and sight or even thinking of food.
  • Hormonal stimulation upon contact of food with the pyloric antrum mucosa.
  • Gastrin hormone is released and passes into the bloodstream, circulates with the blood, and then returns to the wall of the stomach stimulating the gastric glands to produce gastric juice.

Gastric Enzymes

• Pepsin (pH 1.8-3.5): A proteolytic enzyme secreted from chief cells as pepsinogen (inactive) and changed to the active form (pepsin) by HCl. Converts proteins into polypeptides +oligopeptides+ amino acids
• Function of HCl:
  • Kills most of the bacteria taken in with food.
  • Converts the inactive pepsinogen into active pepsin.
  • Provides a favorable medium for pepsin action.
• Rennin (milk-curdling enzyme):
  • A proteolytic enzyme works at pH 5-6.
  • In infants, it digests milk protein (casein).
  • Casein is converted to soluble para-casein and then to insoluble paracaseinate.