Nutrition in Humans — Diet & Digestion

Why we need food

Every cell in your body needs raw materials: fuel for respiration, building blocks for growth, and substances that keep chemical reactions running smoothly. We get all of these from our diet — the food and drink we take in.

A meal you eat, though, is useless to your cells until it has been broken down. This chapter follows food on its journey: from the plate, through the gut, into the blood, and finally into the cells themselves.

Key terms

Diet — everything an organism eats and drinks.

Balanced diet — one that contains all the required nutrients in the correct proportions and amounts for that individual.

Digestion — the breakdown of large, insoluble food molecules into small, soluble ones that can be absorbed.

A balanced diet and its nutrients

A balanced diet supplies seven things: carbohydrates, proteins, lipids (fats), vitamins, minerals, water and dietary fibre. Get the proportions wrong and you develop a deficiency or store excess.

The exact amounts depend on the person. A growing teenager, a manual labourer and a pregnant woman all need more energy or specific nutrients than an inactive office worker. Three factors that affect requirements are age, activity level and pregnancy.

Watch out

Fibre and water are not "proper" nutrients in the sense that they aren't digested or used to build tissue, but they are still essential parts of a balanced diet. Don't leave them out of an exam answer.

Exam tip

Learn each row as a set of role + source + deficiency. A common question gives you a deficiency disease (e.g. scurvy or rickets) and asks which nutrient is missing — vitamin C and vitamin D respectively.

The alimentary canal

Food travels down one long muscular tube — the alimentary canal (gut) — helped by associated organs that add juices but that food never passes through (pancreas, liver, gall bladder, salivary glands).

In order, food passes through:

1. Mouth — teeth chop and grind food; salivary glands add saliva containing amylase.
2. Oesophagus — muscular tube carrying food to the stomach by peristalsis.
3. Stomach — a muscular bag that churns food and adds acid and protease.
4. Small intestine — the duodenum (first part, where pancreatic juice and bile arrive) and the ileum (long second part, where absorption happens).
5. Large intestine — absorbs water from the remaining material.
6. Rectum — stores faeces.
7. Anus — faeces leave the body here.

The associated organs: the pancreas makes enzymes; the liver makes bile; the gall bladder stores bile until it is needed.

Five processes

The whole system carries out five jobs. Learn them in order:

Ingestion — taking food into the mouth.
Digestion — breaking food down. This is partly mechanical (teeth chewing, the stomach churning — increasing surface area) and partly chemical (enzymes breaking bonds).
Absorption — small soluble molecules pass through the gut wall into the blood.
Assimilation — the absorbed molecules are taken into cells and used (e.g. amino acids built into new proteins).
Egestion — undigested material (fibre, dead cells) leaves the body as faeces through the anus.

Watch out

Egestion is the removal of food that was never absorbed. Don't confuse it with excretion, which is the removal of waste made by the body's own reactions (e.g. urea, carbon dioxide).

Peristalsis

Food does not just fall down the gut — it is squeezed along by peristalsis. Behind the food, circular muscles in the gut wall contract; in front of it they relax. This wave of muscle contraction pushes the ball of food (the bolus) forwards. It is why you can swallow even when upside down.

Digestive enzymes

Large food molecules are too big to cross the gut wall. Enzymes break them into small, soluble units. Each enzyme is specific to one type of food.

Key terms

Enzyme — a biological catalyst that speeds up a reaction without being used up.

Carbohydrates are digested by amylase, proteins by protease, lipids by lipase.

The stomach is acidic (around pH 2). This kills many microbes and gives the stomach protease its ideal working pH. By contrast, enzymes in the small intestine work best in alkaline conditions.

The role of bile

Bile is made by the liver, stored in the gall bladder, and released into the duodenum. It is not an enzyme, but it has two important jobs:

It emulsifies fats — breaking large fat droplets into many tiny ones. This gives a much larger surface area for lipase to work on, so fat is digested faster.
It is alkaline, so it neutralises the acid arriving from the stomach. This raises the pH to the alkaline level that the small intestine's enzymes need.

Exam tip

Emulsification is physical, not chemical — bile does not chemically break fat down. It just increases the surface area for lipase. Saying "bile digests fat" loses marks.

Absorption in the small intestine

Once food is fully digested, the small soluble molecules (glucose, amino acids, fatty acids, glycerol) are absorbed in the ileum. The ileum is superbly adapted for this.

Its adaptations are:

Very large surface area — it is long (several metres) and its inner wall is folded into millions of finger-like villi. Each villus cell carries even tinier microvilli, multiplying the surface area further.
Thin wall — each villus is only one cell thick, so molecules have a very short distance to diffuse.
Good blood supply — a dense network of capillaries quickly carries absorbed glucose and amino acids away, keeping the concentration gradient steep.
A lacteal — a tiny lymph vessel in the centre of each villus that absorbs the products of fat digestion (fatty acids and glycerol).

Worked example

Explain why a damaged ileum with flattened villi causes weight loss and tiredness.

Flattened villi mean a much smaller surface area for absorption.

Fewer nutrients — glucose, amino acids, fats, iron — are absorbed into the blood.

Less glucose for respiration causes tiredness; poor iron absorption causes anaemia; the lost nutrients cause weight loss.

The end of the journey

Whatever is left — mostly fibre, water and dead cells — passes into the large intestine, where most of the remaining water is absorbed. The solid faeces are stored in the rectum and finally removed by egestion through the anus.

Key terms

Recap the order: ingestion → digestion (mechanical + chemical) → absorption → assimilation → egestion. If you can place each organ against one of these five processes, you understand the chapter.