Nutrients & Digestion

The Seven Classes of Nutrients

Your body needs a variety of nutrients to grow, repair itself, produce energy, and regulate its processes. There are seven classes of nutrients that every human body requires:

Carbohydrates

Carbohydrates are the body's primary energy source. They are broken down into glucose, which fuels cellular respiration. There are two types:

• Simple carbohydrates (sugars): glucose, fructose, sucrose — found in fruit, sweets, and soft drinks. Digested and absorbed quickly.
• Complex carbohydrates (starch): chains of glucose — found in bread, rice, pasta, and potatoes. Digested slowly, providing sustained energy.

Proteins

Proteins are the building and repair nutrients. They are made up of chains of amino acids. There are 20 different amino acids; 8 are essential — the body cannot make them and they must be consumed in food.

• Animal sources: meat, fish, eggs, dairy (contain all essential amino acids — "complete proteins")
• Plant sources: beans, lentils, soy, nuts (usually lack one or more essential amino acids — "incomplete proteins")

Fats (Lipids)

Fats store energy (more than double the energy per gram compared to carbohydrates), provide insulation, protect organs, and form cell membranes. They also dissolve and transport fat-soluble vitamins (A, D, E, K).

• Saturated fats: solid at room temperature; found in butter, cheese, fatty meat — linked to heart disease if eaten in excess.
• Unsaturated fats: liquid at room temperature; found in olive oil, avocado, oily fish — generally healthier.

Vitamins

Vitamins are organic compounds needed in small amounts for metabolic processes. They are grouped by solubility:

Minerals

Minerals are inorganic elements required for structural and regulatory functions:

• Calcium (Ca): Builds bones and teeth; needed for muscle contraction and nerve function. Sources: dairy, leafy greens. Deficiency → osteoporosis.
• Iron (Fe): Component of haemoglobin in red blood cells — carries oxygen. Sources: red meat, spinach, legumes. Deficiency → iron-deficiency anaemia.
• Iodine (I): Required for thyroid hormone production, which regulates metabolism. Sources: iodised salt, seafood. Deficiency → goitre (enlarged thyroid).
• Phosphorus (P): Bone structure, ATP energy molecules. Sources: meat, dairy, nuts.

Water

Water makes up about 60% of the human body by mass. It is the medium in which all chemical reactions take place. Key roles:

• Transports dissolved substances (nutrients, hormones, waste) in blood and lymph
• Regulates body temperature through sweating and respiration
• Lubricates joints and cushions organs
• Participates in hydrolysis reactions during digestion

Dietary Fibre

Dietary fibre (roughage) consists of plant cell wall material — mainly cellulose — that the human digestive system cannot break down. It is not absorbed but plays a vital role:

• Adds bulk to food, stimulating peristalsis (muscular contractions moving food through the gut)
• Prevents constipation and reduces the risk of bowel cancer
• Slows glucose absorption, helping to stabilise blood sugar levels

Sources: whole grains, vegetables, fruit, legumes, nuts.

Balanced Diet & Malnutrition

A balanced diet provides all seven nutrients in the correct proportions needed for health. The EatWell plate (food plate concept) shows that meals should consist largely of starchy carbohydrates and fruit & vegetables, with moderate protein and dairy, and minimal fat and sugar.

The Digestive System

Digestion is the process of breaking large, insoluble food molecules into small, soluble ones that can be absorbed into the bloodstream. The alimentary canal runs from mouth to anus:

The Mouth

Mechanical digestion: teeth physically break food into smaller pieces, increasing the surface area for enzymes. The tongue mixes food with saliva to form a bolus.

Chemical digestion: salivary glands produce amylase, which begins to break down starch into maltose (a reducing sugar). The optimum pH for salivary amylase is around 7 (neutral).

The Oesophagus

The bolus is swallowed and travels down the oesophagus by peristalsis — coordinated waves of muscular contraction and relaxation in the wall of the tube. No digestion occurs here; the oesophagus is purely a transport tube.

The Stomach

• Muscular walls churn food, continuing mechanical digestion.
• Gastric glands secrete gastric juice containing hydrochloric acid (HCl, pH ~2) and the enzyme pepsin.
• HCl kills most ingested bacteria and provides the acidic pH needed for pepsin activity.
• Pepsin begins protein digestion, breaking proteins into shorter polypeptide chains.
• The semi-liquid mixture (chyme) passes slowly into the small intestine through the pyloric sphincter.

The Small Intestine

This is where most chemical digestion and absorption take place. It is 6–7 m long and divided into the duodenum and ileum.

• Bile (produced by the liver, stored in the gall bladder) is released into the duodenum. It does not contain enzymes — it emulsifies fats, breaking fat droplets into tiny globules to increase surface area for lipase.
• Pancreatic juice (from the pancreas) contains amylase, lipase, and protease — completing digestion of all three macronutrients.
• The intestinal wall itself secretes additional enzymes (maltase, lactase, etc.).
• Absorption: the inner wall is lined with millions of tiny finger-like projections called villi (singular: villus), which are themselves covered in microvilli (the "brush border"). This increases the surface area enormously. Dissolved nutrients pass into the blood capillaries and lacteals (for fat) within each villus.

The Large Intestine

Undigested material (mainly fibre) passes into the large intestine (colon). The colon reabsorbs water and mineral salts from the remaining material. Billions of gut bacteria (the microbiome) break down some fibre. The remaining material forms faeces, which is stored in the rectum and eliminated through the anus.

Food Tests Summary