Nutrition & Digestion in Humans

Balanced Diet

• A balanced diet includes an adequate amount of nutrients needed to maintain good health.
• Five main groups of nutrients:
  • Carbohydrates: Provide energy and dietary fibre.
  • Fats (lipids): Provide energy and essential fatty acids.
  • Proteins: Provide essential amino acids for growth and repair of tissues.
  • Vitamins and Minerals: Required for healthy growth and development. Minerals, such as calcium, are also important constituents of body tissue such as bone and teeth.
  • Water and dietary fibre are also essential components of a balanced diet, but are not considered nutrients because they are not digested by the body.
    • Water provides the solvent for various metabolic activities. Cytoplasm and blood plasma mainly made up of water and is thus important in transport as well.
    • Dietary fibre (roughage) helps to move food through the alimentary canal, thus preventing bowel diseases such as constipation and colon cancer.
  • A balanced diet provides energy for the growth and repair of all cells and the proper functioning of all our vital organs.

Carbohydrates

• Organic molecules made up of carbon (C), hydrogen (H) and oxygen (O).
• General formula: CnH{2n}O_n, where hydrogen and oxygen atoms are in a 2:1 ratio.
• Glucose (simple carbohydrate): C6H{12}O_6
• Three main groups:
  • Monosaccharides: Simplest carbohydrates, made up of one sugar molecule (e.g., glucose, fructose, galactose).
  • Disaccharides: Formed when two monosaccharides combine via a condensation reaction where a water molecule is removed.
    • Can be broken down into monosaccharides by hydrolysis (addition of water).
    • Condensation of Monosaccharides:
      • Glucose + Glucose = Maltose + Water
      • Glucose + Fructose = Sucrose + Water
      • Glucose + Galactose = Lactose + Water
    • Hydrolysis of Disaccharides:
      • Maltose + Water = Glucose + Glucose
      • Sucrose + Water = Glucose + Fructose
      • Lactose + Water = Glucose + Galactose
  • Polysaccharides: Consist of many monosaccharide molecules joined by condensation reactions.
    • Starch, glycogen, and cellulose are polysaccharides made of long chains of glucose arranged differently.
    • Hydrolyzed into monosaccharides or disaccharides.
    • Hydrolysis requires enzymes to speed up the reaction.
• Functions:
  • Energy source: Oxidized during respiration to produce energy.
  • Energy storage: Excess glucose stored as fat or glycogen in animals, starch in plants.
  • Dietary fibre source: Cellulose (plant cell walls) provides dietary fibre.
• Food sources:
  • Sugars: Fruits, honey, jams, biscuits, milk, grains, chocolate, sugar beet, sugar cane, carrots.
  • Starches: Potatoes, cereals, bread, pasta.

Fats (Lipids)

• Organic molecules made of carbon, hydrogen, and oxygen.
• Lipids include triglycerides, phospholipids, waxes, steroids, and cholesterol.
• Fats are solid at room temperature; oils are liquid.
• Fat molecules consist of glycerol and fatty acids.
• Hydrolysis breaks fats into fatty acids and glycerol, involving the addition of water and an enzyme.
• Functions:
  • Alternative energy source and storage.
  • Essential component of cell membranes.
  • Solvent for synthesis, transport, and absorption of some vitamins and hormones.
  • Insulating material to reduce heat loss.
• Most fatty acids can be synthesized by the body.
• Food sources: Cooking oils, butter, nuts, cheese, fatty meat, and fish (herring, salmon).
• Animal fats are mostly saturated and may cause coronary heart disease.
• Unsaturated fats (nuts, vegetable oils, fish) are healthier.

Proteins

• Organic molecules of long chains of amino acids made of carbon, hydrogen, oxygen, and nitrogen; may contain sulphur.
• Twenty different amino acids combine to form millions of protein molecules.
• Green plants manufacture all necessary amino acids.
• Humans need to obtain eight essential amino acids from food.
• The body can make the other twelve non-essential amino acids.
• Amino acids link via condensation reactions to form polypeptides (long chains of amino acids).
• Peptide bond: The bond formed between amino acids.
• Proteins form when polypeptides link and fold into specific 3D shapes.
• Hydrolysis breaks protein molecules into polypeptides and amino acids.
• Enzymes speed up hydrolysis.
• Proteins can be an energy source when carbohydrate and fat reserves are depleted.
• Main functions:
  • Synthesis of new proteins for growth and repair of cells.
  • Synthesis of enzymes, some hormones, and antibodies.
• Food sources: Meat, seafood, eggs, milk, beans, nuts, grains, and vegetables (e.g., French beans).
• Protein deficiency diseases (kwashiorkor and marasmus):
  • Rapid weight loss and stunted growth.
  • Very little muscle and fat, visible bones.
  • Swollen abdomen and body tissues (water retention).
  • Increased susceptibility to infections.
  • Cracked, scaly skin and dry, brittle hair.
  • Irritability, fatigue, and disinterest.

Water

• Molecule: 2 hydrogen atoms and 1 oxygen atom (H_2O).
• Water molecules form hydrogen bonds with each other.
• Functions:
  • Metabolism: Breaks down complex molecules by hydrolysis.
  • Solvent: Many chemical reactions occur in an aqueous medium.
  • Transport: Dissolves substances for nutrient, hormone, and waste transport.
  • Key component: Protoplasm, lubricants, digestive juices, blood, and tissue fluid.
  • Temperature control: Component of sweat; cools the body upon evaporation.

Vitamins and Minerals

• Vitamins: Organic substances needed in small amounts for normal growth and metabolism.
• Minerals: Inorganic substances also needed in small amounts.
• Roles: Ensure proper functioning of bodily processes.
• Deficiency: Lack of any vitamin or mineral results in a deficiency disease.

Excess Vitamins and Minerals

Minerals

Calcium (more than 1500 mg)

• Nausea, vomiting, loss of appetite, increased urination, kidney toxicity
• Mental confusion and irregular heart rhythm.

Iron (more than 20 mg)

• Cell damage and toxicity, stomach upset, constipation, blackened stools.

Magnesium

• Heart problems, inability to breathe.

Protein

• Excess urea excretion can lead to increased calcium excretion.
• Some specific excess proteins can lead to allergic reactions in some individuals.
• More than 2.5 grams per kilogram body weight for years can lead to reduced kidney function.
• Increased toxic residues lead to liver dysfunction.
• Vitamin B6 deficiency since a high protein diet requires increased levels of Vitamin B6.

Minerals

Carbohydrate

• Generalized vascular disease, pancreas produces lots of insulin, which over the years, the insulin receptors may become insensitive to insulin.
• Excess insulin causes increased blood pressure which may lead to hardening of arterial walls (arteriosclerosis).

Fats

• Damage to the immune system, obesity.

Fiber

• Gassiness, bloating, frequent bowel movements.

Water

• Sodium levels in body fluids drop which cause cells to swell and burst, leading to more problems and even death.

Energy Needs of Individuals

• Energy and nutrient requirements vary with age, gender, activity, and occupation.
• Basal metabolism: Energy required at rest for breathing, heart function, temperature maintenance, and chemical reactions.
• Basal metabolic rate (BMR): Energy used for basal metabolism.
• Energy from food is also used to keep the body warm (thermogenesis) and for physical activity.
• BMR is higher in males, those with larger body mass, and infants/young children; decreases with age and slightly in very hot climates.

Malnutrition

• Imbalance due to over-nutrition or under-nutrition.
• Over-nutrition: Consuming more energy than required.
  • Excess carbohydrates and fats are stored, increasing body mass.
  • Overweight/obesity: Body mass exceeds 20% of ideal weight.
  • Problems with high salt, sugar, cholesterol, and alcohol intake.
• Obesity is increasing, especially in developed and developing countries, due to:
  • Lack of physical exercise.
  • Well-heated homes/workplaces (reduced energy loss).
  • Better transportation (less stair climbing).
  • Convenience/fast foods (high calories).
  • Increased stress, using convenience foods to save time and "comfort eating".
• Overweight/obese individuals have a higher risk of diabetes, hypertension, coronary heart disease, cancer, arthritis, and stroke.
• Under-nutrition: Consuming insufficient food to meet energy requirements.
  • Inadequate protein and vitamins for growth, development, and metabolic processes.
  • Prolonged lack of nutrients leads to starvation.

Food Tests

Reducing Sugars (e.g., glucose)

• Benedict's test: Add 2 cm³ of sample and 2 cm³ of Benedict's solution to a test tube, shake, and heat in a boiling water bath for 2-3 minutes.
  • Clear blue solution: No reducing sugars.
  • Green or yellow precipitate: Small to moderate amount.
  • Brown or red precipitate: Large amount.

Non-Reducing Sugars (e.g., sucrose)

• If initial Benedict's test is negative, add 1 cm³ of sample and 1 cm³ of dilute hydrochloric acid to a test tube, shake, and heat in a boiling water bath for 2-3 minutes.
• Add 1 cm³ of dilute sodium hydroxide solution and 2 cm³ of Benedict's solution, shake, and heat in a boiling water bath for 2-3 minutes.
• A green, yellow, brown, or red color indicates the presence of non-reducing sugars.

Starch

• Add 2 cm³ of food sample to a test tube, followed by a few drops of iodine solution.
  • Yellowish brown: Starch absent.
  • Blue black: Starch present.

Protein (Biuret Test)

• Add 2 cm³ of sample and 2 cm³ of dilute sodium hydroxide solution to a test tube, shake, and add 2 drops of copper(II) sulphate solution. Shake gently.
  • Blue solution: Protein absent.
  • Purple or violet: Protein present.

Fats (Ethanol-Emulsion Test)

• Add 2 cm³ of sample and 2 cm³ of ethanol to a test tube, shake vigorously for 1 minute, then add 3 cm³ of water.
  • Clear solution: Fat absent.
  • Cloudy white suspension: Fat present.

Grease Spot Test

• Rub a drop of sample onto a piece of paper.
  • A translucent spot indicates the presence of fat.

Digestion in Humans

• Heterotrophic nutrition: Obtaining nutrition by consuming and breaking down complex organic molecules.
• Autotrophic nutrition: Manufacturing complex organic molecules from simple inorganic molecules using energy from light or chemical reactions.
• Heterotrophic nutrition processes:
  • Ingestion: Taking food into the body.
  • Digestion: Breaking down large, complex, insoluble food molecules into smaller, soluble molecules.
  • Absorption: Absorbing small, soluble food molecules into the blood and body cells.
  • Assimilation: Storing or using food molecules for respiration, growth, and development.
  • Egestion: Removing undigested food from the body.

Mammalian Digestive System

Mouth or Oral Cavity

• Food enters through the mouth.
• Mechanical digestion: Chewing breaks down food into smaller particles, increasing the surface area to volume ratio.
• Chemical digestion: Digestive enzymes break down large insoluble food molecules into small soluble molecules.
• Salivary glands secrete saliva (neutral pH) containing water, mucus (for softening and lubrication), and salivary amylase (breaks down starch into maltose).
• Tongue rolls food into boli which are swallowed into the oesophagus via the pharynx.

Pharynx

• Passageway for food and air.
• Branches into the oesophagus (to the stomach) and trachea (to the lungs).
• Epiglottis covers the trachea during swallowing to prevent food entry.

Oesophagus

• Food travels via peristalsis: Rhythmic, wave-like contractions and relaxations of gut walls.
• Peristalsis moves food from mouth to stomach.
• Gut wall has longitudinal and circular muscle layers that are antagonistic.

Stomach

• Muscular and folded walls with gastric pits leading to gastric glands.
• Gastric juice contains mucus, hydrochloric acid (HCl), and pepsin.
• HCl provides an acidic medium (pH 1-3) for enzyme activity and kills microorganisms.
• Mucus protects stomach walls.
• Pepsin digests proteins into polypeptides; produced as inactive pepsinogen, converted to pepsin by HCl.
• Peristalsis churns food, mixing it with gastric juice to form chyme.
• Chyme is released into the small intestine through the pyloric sphincter.

Small Intestine

• About six metres long; walls contain glands that secrete intestinal juices.
• Receives bile (from the gall bladder) and pancreatic juice (from the pancreas).
• Alkaline fluids neutralize acidic chyme and provide a suitable alkaline medium for enzyme activity.
• Pancreatic juice contains pancreatic amylase (starch to maltose), trypsin (proteins to polypeptides), and lipase (fats to glycerol and fatty acids).
• Intestinal juice contains maltase (maltose to glucose) and peptidases (polypeptides to amino acids).

Liver

• Produces bile (stored in the gall bladder).
• Bile contains bile salts and bile pigments but no enzymes.
• Bile salts emulsify fats into smaller droplets, increasing surface area.
• Bile pigments are waste products excreted in faeces.

Small Intestine (Duodenum and Jejunum)

• Complete most of the digestion.
• Products of digestion: Glucose, amino acids, fatty acids, and glycerol.
• These are absorbed through the walls of the small intestine into the bloodstream.
• Water, vitamins, and minerals are also absorbed.

Small Intestine (Ileum)

• Adapted for absorption with features:
  • Very long (about 6 metres) for sufficient time.
  • Numerous folds and villi (covered with epithelial cells with microvilli) increase surface area.
  • Single-layer epithelium for easy nutrient absorption.
  • Each villus contains blood vessels and a lacteal (lymph vessel).
    • Blood vessels transport sugars and amino acids.
    • Lacteal transports fats.
  • Continual transport maintains a concentration gradient for absorption.
• Glucose and amino acids are absorbed by diffusion (high to low concentration) or active transport (against concentration gradient).
• Fatty acids and glycerol enter villi by diffusion, combine to form fat droplets, and enter the lacteal for transport via the lymphatic system.

Large Intestine (Colon)

• Unabsorbed, indigestible material enters as faeces.
• Function: Absorb water and minerals from faeces to prevent excessive water loss.

Large Intestine (Rectum and Anus)

• Faeces are stored in the rectum.
• Defaecation (egestion): Faeces are passed out through the anus.
• Egestion: Removal of undigested material.

Assimilation

Sugars

• Hepatic portal vein transports sugars to the liver.
  • Glucose transported to cells for respiration.
  • Excess glucose converted to glycogen and stored in the liver and muscle tissue.
  • Excess glucose converted to fats and stored in adipose tissues.

Amino Acids

• Hepatic portal vein transports amino acids to the liver.
  • Amino acids transported to cells for growth and repair or synthesis of enzymes and hormones.
  • Excess amino acids deaminated in the liver and converted to urea (excreted by the kidneys).
  • Excess amino acids converted to glucose or glycogen in the liver.

Fatty Acids

• Fats absorbed into the lymphatic system.
  • Fats transported to the blood to form new cell membranes.
  • Fats used by cells in respiration when glucose is insufficient.
  • Excess fats stored in adipose tissues.

Liver Functions:

• Maintenance of blood glucose concentration
• Production of bile
• Storage of minerals and vitamins
• Synthesis of proteins
• Deamination of amino acids
• Detoxification of blood
• Breakdown of red blood cells
• Production of cholesterol

Digestive Enzymes

• Biological catalysts made of protein that alter the rate of chemical reactions without being chemically changed.
• Can be reused; small amount needed.
• Specific in action.
• Substrate: The substance the enzyme acts on.
• Classified according to reactions they catalyze; in digestion:
  • Carbohydrases: Carbohydrates to disaccharides/monosaccharides (e.g., amylase).
  • Proteases: Proteins to amino acids (e.g., pepsin).
  • Lipases: Fats to fatty acids + glycerol.
• Affected by temperature and pH.
• Optimum temperature and pH: Highest enzyme activity.
• Denaturation: Irreversible destruction of a protein above optimum temperature.
• Denatured enzymes can no longer act as catalysts.
• Some enzymes work best in acidic conditions, others in alkaline.
• At extreme changes in pH from the optimum pH, the enzyme becomes denatured.