Describe the nutrients needed in a balanced human diet, the function and food sources of each, and the use of food tests to identify nutrients

What this dot point is asking

SEAB wants you to describe the components of a balanced human diet, the function and a food source of each nutrient, the effects of deficiency, and the standard food tests used to identify nutrients. The food tests (reagent and colour change) and the nutrient functions are very common exam marks.

The answer

A balanced diet

A balanced diet contains the right amounts of all the nutrients the body needs, in the right proportions. The components are:

• carbohydrates (energy),
• proteins (growth and repair),
• fats (energy store and insulation),
• vitamins (small amounts, various roles),
• mineral ions (small amounts, various roles),
• dietary fibre (helps the gut move food along),
• water (needed for all body processes).

Functions and sources

• Carbohydrates (bread, rice, potatoes) are the main source of energy, broken down to glucose for respiration.
• Proteins (meat, fish, eggs, beans) are needed for growth and repair of tissues and to make enzymes.
• Fats (oils, butter, nuts) are a concentrated energy store and provide insulation.
• Fibre (vegetables, wholegrains) is not digested but keeps food moving through the gut, preventing constipation.
• Water is the medium for reactions and transport, and replaces water lost in sweat and urine.

Vitamins and minerals

These are needed only in small amounts but are essential:

• Vitamin C (citrus fruit): keeps skin and gums healthy; a lack causes scurvy.
• Vitamin D (oily fish, sunlight): helps absorb calcium for bones; a lack causes rickets.
• Calcium (milk, cheese): for strong bones and teeth.
• Iron (red meat, leafy greens): to make haemoglobin; a lack causes anaemia.

Effects of an unbalanced diet

Too little of a nutrient causes a deficiency disease (such as scurvy or anaemia). Too much energy-rich food (carbohydrate and fat) without enough exercise leads to obesity, which raises the risk of heart disease and other conditions.

Food tests

Standard tests identify the main nutrients by a colour change:

• Starch: iodine solution turns from orange/brown to blue-black.
• Reducing sugars (e.g. glucose): warm with Benedict's solution; it turns from blue to brick-red (orange) precipitate.
• Protein: Biuret test (sodium hydroxide then copper(II) sulfate) turns from blue to violet/purple.
• Fats: the ethanol emulsion test gives a cloudy white emulsion (or a grease spot on paper).

Examples in context

Example 1. Designing a meal for a growing child. A child needs plenty of protein for growth, calcium for bones and enough energy from carbohydrates, so a balanced meal might include milk, lean meat or beans, wholegrains and vegetables. This applies the functions of each nutrient to a real dietary choice.

Example 2. Preventing deficiency diseases. Adding vitamin C to the diet of sailors on long voyages prevented scurvy, a historic example of linking a missing nutrient to a deficiency disease. Today, foods are often fortified with iron or vitamins for the same reason.

Try this

Q1. State the main function of carbohydrates and of proteins in the diet. [2 marks]

• Cue. Carbohydrates are the main source of energy; proteins are needed for growth and repair of tissues.

Q2. Name the reagent and the positive colour change for the test for starch. [2 marks]

• Cue. Iodine solution; it turns from orange/brown to blue-black.

Q3. Name one deficiency disease and the nutrient that is lacking. [2 marks]

• Cue. Scurvy (lack of vitamin C), or rickets (lack of vitamin D), or anaemia (lack of iron).