Topic 7: Human nutrition

Nutrition is how humans take in food and use it to grow, repair tissues, and release energy. But food as we eat it is far too large to enter our cells — a sandwich cannot simply slip into your bloodstream. The digestive system solves this problem with a clever combination of physical breakdown, chemical enzymes, and a beautifully adapted absorbing surface. In this topic you’ll follow food on its journey from mouth to anus and discover exactly how it becomes part of you.

A balanced diet

A balanced diet contains all the nutrients the body needs in the correct proportions. The seven components are carbohydrates, proteins, fats, vitamins, minerals, fibre (roughage) and water.

• Carbohydrates provide energy quickly — found in bread, rice and pasta.
• Proteins are needed for growth and repair — found in meat, fish, eggs and beans.
• Fats store energy and insulate the body — found in butter and oils.
• Vitamin C prevents scurvy; vitamin D helps absorb calcium for bones.
• Calcium strengthens bones and teeth; iron makes haemoglobin.
• Fibre provides bulk so muscles can push food along (peristalsis), preventing constipation.

Needs vary with age, sex, activity and pregnancy — a manual labourer needs more energy than an office worker.

Physical and chemical digestion

Digestion has two parts. Physical (mechanical) digestion breaks food into smaller pieces — the teeth chew and the stomach churns. This increases the surface area for enzymes to work on. Chemical digestion uses enzymes to break large insoluble molecules into small soluble ones.

• Amylase (made in the salivary glands and pancreas) breaks starch into maltose.
• Proteases such as pepsin break proteins into amino acids.
• Lipase breaks fats into fatty acids and glycerol.

Bile, made in the liver and stored in the gall bladder, is not an enzyme. It emulsifies fats — breaking large fat droplets into tiny ones to increase surface area — and neutralises the acidic food leaving the stomach so enzymes in the small intestine can work.

The journey through the alimentary canal

Food passes through one long tube called the alimentary canal, moved along by waves of muscle contraction called peristalsis.

• Mouth: teeth chew; salivary amylase begins digesting starch.
• Stomach: churns food, makes pepsin, and produces hydrochloric acid that kills bacteria and gives the right pH for pepsin.
• Small intestine (duodenum and ileum): receives bile and pancreatic enzymes; digestion is completed and most absorption happens here.
• Large intestine (colon): absorbs water, leaving solid faeces.
• Rectum and anus: store and remove faeces (egestion).

Absorption and the villi

The small intestine is superbly adapted for absorption — taking digested food into the blood. Its inner wall is folded and covered in millions of tiny finger-like projections called villi. These provide a huge surface area. Each villus has:

• a wall only one cell thick, giving a short diffusion distance;
• a rich network of blood capillaries to carry away glucose and amino acids;
• a lacteal to absorb fatty acids and glycerol into the lymph;
• a good blood supply that keeps a steep concentration gradient.

The glucose and amino acids travel in the blood to the liver via the hepatic portal vein. The liver regulates glucose, deals with excess amino acids (deamination) and breaks down toxins such as alcohol.