Diet
A balanced diet contains the right amounts of carbohydrates (energy), fats (energy store, insulation), proteins (growth and repair), vitamins (C for healthy skin and gums; D for absorbing calcium), minerals (calcium for bones and teeth; iron for haemoglobin), fibre (helps food move through the gut) and water. Lack of a nutrient causes deficiency diseases: scurvy (vitamin C), rickets (vitamin D or calcium), anaemia (iron). Energy needs depend on age, sex, activity and pregnancy.
≈ 35 minDigestion and absorption
Digestion breaks large insoluble molecules into small soluble ones that can be absorbed. Physical digestion (teeth chewing, stomach churning) increases surface area. Chemical digestion uses enzymes: amylase (mouth, pancreas) turns starch into maltose; protease (pepsin in stomach, trypsin from pancreas) turns proteins into amino acids; lipase (pancreas) turns fats into fatty acids and glycerol. Bile from the liver neutralises stomach acid and emulsifies fats. Products are absorbed in the small intestine, whose villi give a huge surface area; water is absorbed mainly in the small intestine and colon.
≈ 45 min