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Human Nutrition

  • All living organisms need food.
  • An important difference between plants and animals is that green plants can make food in their leaves but animals have to take it in ‘ready-made’ by eating plants or the bodies of other animals.
  • The regular course of eating and drinking adopted by an individual is called diet
  • Most animals need 7 types of nutrients in their diet. These include:
    • carbohydrates,
    • proteins,
    • fats,
    • water,
    • fiber,
    • vitamins, and
    • minerals
  • The amount of energy needed is provided mainly by our carbohydrate and fat intake.
  • Dietary requirements of animals depend on age, sex, and activity.
    • Age: The energy demand increases until we stop growing. While children are growing, they need more protein per kg of body weight than adults do.
    • Sex: Generally, males use up more energy than females. Pregnant women need extra nutrients for the development of the fetus

Balanced Diet

  • A balanced diet is a diet that contains all the main nutrients in the correct amounts and proportions  to maintain good health.
  • Malnutrition is the result of not eating a balanced diet.
  • This may be due to:
    • wrong amount of food; too little or too much
    • incorrect proportion of main nutrients
    • lacking in one or more key nutrients

Effects of Malnutrition

  • Obesity: Too much food (carbohydrate, fat, or protein)
  • Coronary Heart Disease: Too much saturated/animal fat in the diet results in high cholesterol levels. Cholesterol can stick to the walls of the arteries, gradually blocking them. If the coronary arteries become blocked, the results can be angina and coronary heart disease.
  • Starvation: Too little food can result in starvation. Extreme slimming diets, such as those that avoid carbohydrate foods, can result in the disease anorexia nervosa
  • Childhood Protein Energy Malnutrition (Kwashiorkor): Wrong proportion of nutrients, e.g. too much carbohydrates (starchy foods) and a lack of protein can lead to kwashiorkor in young children
Food Nutrients

Causes and Effects of Mineral and Vitamin Deficiencies

  • Vitamin Deficiency
    • Vitamin D helps in the absorption of calcium and phosphorus through the gut wall.
    • Bones are made of the mineral calcium phosphate
    • A lack of the vitamin, therefore, results in poor calcium and phosphorus deposition in bones, leading to softening
    • The weight of the body can deform bones in the legs, causing the condition called rickets in children
    • Adults deficient in vitamin D can suffer from osteo-malacia; they are very vulnerable to fracturing bones if they fall
  • Iron Deficiency
    • Lack of iron in the diet can lead to iron-deficiency anemia, which is a decrease in the number of red blood cells.
    • The symptoms of anemia are feeling weak, tired and irritable
  • The alimentary canal is a long tube which starts at the mouth, runs through the stomach and intestines and finishes at the anus
  • It is part of the digestive system.
  • Food is broken down with the help of digestive juices, which contain special chemicals called enzymes
  • Digestion of food macromolecules involves 6 basic processes:
    • Ingestion: The taking in of food substances
    • Digestion:
      • Mechanical Digestion: The breakdown of food into smaller pieces without chemical change to the food molecules
      • Chemical Digestion: The breakdown of large, insoluble molecules into small, soluble molecules
    • Absorption: The movement of small food molecules and ions through the wall of the intestine into the blood
    • Assimilation: The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
    • Egestion: The passing out of food that has not been digested or absorbed, as feces, through the anus
  • The digestive system includes the accessory organs:
    • teeth,
    • tongue,
    • gallbladder,
    • salivary glands,
    • liver,
    • pancreas
  • The main regions of the alimentary canal and associated organs are:
    • mouth,
    • oesophagus,
    • stomach,
    • small intestine (duodenum + ileum),
    • large intestine (colon + rectum),
    • anus


  • Carbohydrate digestion starts in the mouth
  • It has the salivary glands which secret saliva
  • The saliva contains the enzyme amylase which breaks down complex carbohydrates into simple sugar (chemical digestion)
  • It bears the teeth for biting off and chewing food (homogenization)


  • The food from the mouth moves through the oesophagus into the stomach through a process known as peristalsis


  • The stomach lining produces strong digestive juices, stores, and lubricates food
  • It secretes gastric juice which contains water, HCl, salts, mucus, and pepsin
  • HCl destroys foreign materials and bacteria, provides optimum pH for pepsin activity and denatures proteins
  • Mechanical and chemical digestion of protein begins in the stomach
  • Pepsin breaks down proteins into amino acid chains (chemical digestion) while the stomach mixes and churns food around by more waves of peristalsis into a mash


  • It is divided into the duodenum and ileum
  • The small intestine contains villi for absorption
  • Villi are finger-like structures. In the center of each villus, is a lymphatic duct (sinus or lacteal) where fats are absorbed
  • Food from the stomach is released slowly into the duodenum.
  • The duodenum DOES NOT produce any digestive juices, but has two pancreatic juices and bile poured into it from the pancreas and liver respectively
  • Bile contains bile salts and bicarbonate and solubilises lipids.
  • Bicarbonate neutralizes HCl from the stomach and provides alkaline pH for pancreatic enzymes
  • Food goes through jejunum, where no digestion takes place, to the ileum
  • The ileum is very long to allow for digestion and releases intestinal juice and mucus
  • The intestinal juice contains carbohydrates, e.g. maltase.
  • Maltose is broken down by maltase to glucose on the membranes of the epithelium lining the small intestine
  • The ileum passes digested food into the blood
  • It has a very large surface area due to the villi to which enzymes are attached
  • The villi allows final digestion and rapid diffusion of digestive products
  • The absorbed products are taken to the liver and other body parts for processing, storage or distribution
  • Water is absorbed in both the small intestine and the colon, but most absorption of water happens in the small intestine


  • The pancreas makes digestive juices called pancreatic juice released into the duodenum through the pancreatic ducts


  • It stores a fluid called bile, made in the liver
  • The bile flows from the gallbladder along the bile duct into the duodenum in response to food
  • Its not an enzyme but helps to digest fatty foods and neutralizes stomach acid
  • It emulsifies lipids


  • The blood from the intestine enters the liver, carrying nutrients, vitamins and minerals and other products from digestion.
  • The liver has a lot of functions; stores some nutrients, changes them from one form to another, and releases them into the blood according to the activities and needs of the body


  • It is divided into colon and rectum
  • The unabsorbed and undigested material moves to the large intestine, where water and salt are absorbed.
  • The useful substances that are not absorbed in the small intestine, e.g water and minerals, are absorbed through the walls of the colon into the blood
  • The removal of fluid forms solid-like feces
  • The rectum stores the undigested food as fecal matter till it can be passed out
  • The teeth is a hard structure made of bone-like material designed for tearing and grinding food (mechanical digestion).
  • There are different types with specific functions including:
    • Incisors: chisel-shaped for cutting or nipping
    • Canines: conical or fang-like for tearing or piercing
    • Premolars and molars: Broad crowns with rounded tips for grinding or crushing
Structure of the Human Teeth

  • Crown: The part of a tooth that is visible above the gum line
  • Gum is tissue that overlays the jaws
  • Root: The rest of the tissue embedded in the jaw bone
  • Enamel: The surface of the crown covered by a very hard layer. This layer is replaced by cement in the root, which enables the tooth to grip to its bony socket in the jaw
  • Dentine: Below the enamel is a layer of dentine. It is softer than enamel
  • Pulp Cavity: Inside the dentine is the pulp cavity containing nerves and blood vessels. These enter the tooth through a small hole at the base of the root
Causes of Dental Decay
  • Bacteria are present on the surface of our teeth
  • Food deposits and bacteria form a layer called plaque
  • Bacteria on the plaque feed on sugars, producing acid
  • This acid dissolves enamel, forming a hole
  • Dentine underneath the enamel is softer – it dissolves more rapidly
  • If the hole reaches the pulp cavity, bacterial infection can get to the nerve
  • This results in toothache and possibly, an abscess (an infection in the jaw)
Proper Care of Teeth
  • Avoid sugary food, especially between meals, so bacteria cannot make acid, and clean teeth regularly to remove plaque
  • Use dental floss or a toothpick to remove pieces of food and plaque trapped between them
  • Use fluoride toothpaste (or drink fluoridated water) – fluoride hardens tooth enamel
  • Visit a dentist regularly to make sure tooth decay is spotted early and any stubborn plaque (called calculus) is removed