Human Nutrition


Balanced Diet

A balanced human diet contains all essential ingredients in the correct proportions. There are certain factors that affect diet such as:

  • Age  Children require more protein per kg of body weight than adults 
  • Gender  Males generally use more energy than females
  • Activity  Higher levels of physical activity will increase demand for nutrients 
  • Pregnancy  Higher demands for nutrients in order to supply foetus with energy for development 
  • Breast feeding mother – Higher requirements for vitamin and water 


Malnutrition is the lack of a balanced diet such as deficiencies, excesses or imbalances in a person’s intake of nutrients. 

Malnutrition can lead to consequences such as: 

  • Obesity (excessive nutrients) – more likely to get diseases such as heart disease, diabetes and strokes; pain on joints due to excess weight
  • Starvation (insufficient nutrients) – more likely to die as body doesn’t get required nutrients and therefore, can’t function
  • Heart Disease (excessive saturated fat and cholesterol) – fat deposits build inside arteries making them stiffer and narrower leading to lower blood (and oxygen) getting through; muscles can’t work properly; blood clot forms resulting in a heart attack
  • Kwashiorkor (too much carbohydrates, too little protein) – underweight for age but look like they’re overweight due to high carbs in diet
  • Constipation (lack of fibre) 
  • Marasmus (shortage of energy in diet) – undernourishment causing a child’s weight to be significantly low for their age.

Essential Nutrients

Nutrient Function Sources Deficiency
CarbohydratesStorage and Provision of Energy; Regulation of Blood Glucose; Sparing the Use of Proteins for Energy; Breakdown of Fatty Acids; Used as Dietary Fibre; etc.Rice; Potatoes; Bread; etc.Hypoglycaemia
(Low glucose or blood sugar level)
Fats & OilsStorage and Provision of Energy; Insulation; Myelin Sheath Formation; Membrane Formation; Hormone Regulation; Cholesterol Formation; etc.Butter; Milk; Animal Fat; etc.Essential Fatty Acid Deficiency (EFA)
ProteinsGrowth; Tissue Repair; Enzymes; Hormones Regulation; Antibody Production; etc.Meat; Fish; Soya; Eggs; etc.Kwashiorkor
Vitamin CMaintains healthy skin and gumsCitrus FruitScurvy
Vitamin DMaintains hard bones; helps calcium absorptionMilk; Cheese; Egg Yolk, etc.Rickets
CalciumHealthy bones and teeth; blood clottingMilk; Cheese; FishRickets + Slow Blood Clotting
FibreMaintains Bulk of Undigested Food Passing Through the Digestive System; Allows Digestive System to Constantly “Push” the Food ThroughVegetables; Fruits; etc.Constipation
IronMakes Haemoglobin in BloodRed Meat; Liver; Eggs; etc.Anaemia
WaterTransport; Blood Formation; Cytoplasm; Allows Enzymes to Work and Metabolic Reactions to Take PlaceDrinks; Fruits; Vegetables; etc.Dehydration

Alimentary Canal

Useful Definitions

  • Ingestion – Taking in of substances e.g. food and drink into the body through the mouth
  • Egestion – Passing out of food (as faeces) that has not been digested or absorbed via the anus
  • Mechanical digestion – Break down of food into smaller pieces without chemical change to food molecules
  • Chemical digestion – Break down of large insoluble molecules into small, soluble molecules
  • Absorption – Movement of chemically digested food molecules through the small intestine walls into the blood
  • Assimilation – Movement of digested food molecules into the cells of the body where they are used and become a part of the cells

Structure and Function

Mouth – This is the location of ingestion. Mechanical digestion of food occurs as we chew on it with our teeth. Chemical digestion occurs due to amylase enzymes in our saliva which break down starch into maltose. 

Oesophagus – Round clumps of food (boluses) are passed down the oesophagus via peristalsis from the mouth to the stomach. Peristalsis is the contraction and relaxation of the oesophagus wall muscles which creates a wave-like motion that pushes the food down the canal. (Epiglottis covers trachea meanwhile).

Stomach – Mechanical digestion occurs as the stomach walls squeeze the food to liquefy it and mix it with enzymes and mucus (from goblet cells) – mixture is called chyme. Gastric juices contain pepsin (a protease) which chemically digests proteins. It also contains hydrochloric acid which kill bacteria, but also maintains an optimum acidic pH for pepsin. After one or two hours, sphincter muscle allows chyme into the duodenum.

Pancreas – The pancreas secretes pancreatic juice into the duodenum. The juice contains a large variety of different enzymes involved in the chemical digestion of food. 

  • Trypsin breaks down proteins into polypeptides
  • Lipase breaks down lipids into fatty acids and glycerol
  • Amylases break down starch to maltose
  • Sodium hydrogen carbonate to partially neutralize the pH as most enzymes in the pancreatic juice have an optimum pH of around 7.

Bile – Yellowish-green substance, produced by the liver, stored in the gallbladder. Bile has the function of emulsifying fat into droplets to increase the surface area for lipases to come and digest them. Bile is also basic which assists in neutralizing the acidity of the food coming from the stomach.

Villi – Covers the walls of the small intestine. Cells covering villi make enzymes that complete the process of digestion. Maltase breaks down maltose to glucose; proteases break down polypeptides to amino acids; and lipases break down lipids to fatty acids and glycerol.

Duodenum – This is the first part of the small intestine. It receives pancreatic juice which contains enzymes for the chemical digestion of food. 

Ileum – This is the second part of the small intestine. The inner walls have finger-like extensions called villi which massively increases the surface area for nutrient absorption. 

Colon – This is the first part of the large intestine. The main function of the colon is to reabsorb water from undigested food – less compared to small intestine – and also bile salts to return back to the liver. 

Rectum – The second part of the large intestine. The rectum stores faeces – indigestible food such as roughage or fibre; bacteria; dead cells; bile pigments – until it is egested.

Anus – Muscles control egestion of faeces.

Cholera Infection

Cholera bacteria releases toxins which causes chloride ions to be secreted into the small intestine. This increases the concentration of the fluids in the small intestine. This causes the osmotic movement of water into the lumen of the small intestine from blood, and leads to diarrhoea. Diarrhoea means that large amounts of water from the body is lost due to the difference in water potential between the lumen of the small intestine and blood in the body. Diarrhoea can be treated using oral rehydration therapy. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium. 

NOTE: Diarrhoea is defined as the loss of watery faeces

Key Terms

Chemical Digestion – Chemical digestion involves breaking down large, insoluble food into smaller soluble nutrients that can be absorbed and used by the cells. Chemical digestion is carried out by enzymes. A lot about chemical digestion has already been covered above in regards to the alimentary canal, and what enzymes are found where. Summary of enzymes that you should be aware of:

Enzyme Site of ActionOptimum pH ConditionsSubstrate DigestedEnd Products
AmylaseMouth; DuodenumSlightly AlkalineStarchMaltose (broken down by Maltase, located on the epithelial lining of small intestine, to Glucose)
ProteaseStomach (Pepsin); Duodenum (Trypsin)Acid in Stomach; Alkaline in DuodenumProteinAmino Acids
LipaseDuodenumAlkalineFatFatty Acids and Glycerol

Remember, the acidic pH of the stomach is maintained by hydrochloric acid which also kills bacteria via denaturing their enzymes. The alkaline conditions of the small intestine are maintained by pancreatic juice and bile. The bile also has the function of emulsifying fats into droplets to increase the surface area for digestion

Absorption – After large food molecules get digested by enzymes, the small soluble nutrients diffuse into the small intestine walls and then into the blood. Sometimes the concentration of nutrients i.e. glucose may be higher in the blood than in the small intestine. In such cases diffusion cannot be relied upon. Instead, active transport is used for absorption. Absorption is defined as the movement of small food molecules and ions through the wall of the intestine into the blood. It can either be done through diffusion or active transport. The inner walls of the small intestine have finger-like structures called villi which greatly increases the surface area of absorption. 

Structure of a Villus –

  • Blood vessels/ capillaries
    • Absorbs glucose and amino acids by diffusion
  • Lacteal
    • Absorbs fatty acids and glycerol
  • Epithelial lining
    • One cell thick to increase diffusion rate 
    • Microvilli increases surface area even further

Assimilation – After absorption, nutrients are taken to the liver in the hepatic portal vein where the liver processes them before they go any further. Some are broken down, some converted, some stored and some remain unchanged. Nutrients dissolved in blood plasma are then taken to other parts of the body to become assimilated with a cell.

Mechanical Digestion

Types of Human Teeth

Mechanical digestion involves physically breaking food into smaller pieces, and our teeth serve exactly that function.

There are four types of teeth: 

  • Molars
    • Located at back of mouth 
    • 4 or 5 cusps
    • 2 or 3 roots 
    • Used for chewing and grinding food 
    • Used for chewing and grinding food 
  • Pre-molars
    • Behind canines 
    • 2 cusps 
    • 1 or 2 roots
    • Used to tear and grind food 
  • Canines
    • On either side of incisors 
    • More pointed than incisors 
    • Used to bite pieces of food 
  • Incisors
    • In front of the mouth 
    • Chisel shaped 
    • Used to bite off food pieces

Structure of Human Teeth

Dental Decay

  • Dental decay is caused by bacteria which is present on the surface of our teeth.
  • The bacteria and food deposits form a layer called plaque. 
  • Gums swell and become inflamed.
  • Bacteria in plaque feed on sugars which produce acid that dissolves the enamel, resulting in a hole.
  • As the hole deepens it may eventually reach the nerves which result in pain i.e. tooth ache. 

Proper Dental Care

  • Avoid sugary food so that bacteria cannot make acid 
  • Regular cleaning to remove plaque 
  • Use floss to remove trapped food 
  • Visit the dentist regularly for treating early decay and removal of thick plaque 


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