Nutrient functions and energy balance: the functions and sources of proteins, fats, carbohydrates, vitamins, minerals, fibre and water; energy balance, basal metabolic rate, physical activity level and total energy expenditure; the consequences of positive and negative energy balance.

What this key area is asking

The SQA wants you to explain what each nutrient does in the body and where it is found, then bring those nutrients together through the idea of energy balance: how the energy taken in from food compares with the energy the body uses, why basal metabolic rate and physical activity level set a person's total needs, and what happens to health when intake and expenditure do not match.

The energy-yielding macronutrients

Carbohydrate is the body's main and preferred fuel. Starchy carbohydrates (bread, pasta, rice, potatoes) release glucose steadily for respiration; sugars give a faster rise in blood glucose. The brain and red blood cells rely heavily on glucose.

Fat (lipids) is the most concentrated energy source and an energy store in adipose tissue. It insulates the body, protects organs, and carries the fat-soluble vitamins A, D, E and K. Essential fatty acids (such as omega-3) must come from the diet because the body cannot make them.

Protein supplies amino acids for growth, repair and the building of enzymes, antibodies and hormones. The body uses protein for energy only when carbohydrate and fat are short, because its first job is structural. Animal proteins and soya are "high biological value" because they supply all the essential amino acids.

Micronutrients, fibre and water

Fibre (non-starch polysaccharide) is not digested but adds bulk to faeces, speeds transit through the gut, prevents constipation and is linked with a lower risk of bowel cancer and lower blood cholesterol. Water makes up about two thirds of the body, is the medium for chemical reactions and transport, carries waste to the kidneys, and regulates temperature through sweating.

Energy balance

Energy out has three parts: basal metabolic rate (BMR), the energy used at complete rest to stay alive; physical activity, the energy used in movement; and a smaller amount used to digest and process food. BMR is the largest part for most people.

BMR is raised by greater lean muscle mass (muscle is active tissue), by being male, by growth, pregnancy and lactation, by illness or fever, and by the hormone thyroxine. It falls with age and with loss of muscle.

Positive and negative energy balance

Examples in context

Example 1. An athlete in training. A rower has a high lean muscle mass, which raises BMR, and trains for hours a day, giving a high PAL. Their total energy requirement may be double that of a sedentary person of the same age, so they must eat far more carbohydrate to refuel glycogen and enough protein to repair muscle.

Example 2. An older, less active adult. With age, muscle mass falls and activity often drops, lowering both BMR and PAL. If the person keeps eating the same amount they ate when younger, they move into positive energy balance and gain weight, which is why energy needs should be reviewed across the life stages.

Try this

Q1. State the energy value, in kJ per gram, of fat. [1 mark]

• Cue. About 37 kJ per gram (more than twice that of carbohydrate or protein).

Q2. Explain why a person with more lean muscle has a higher basal metabolic rate. [2 marks]

• Cue. Muscle is metabolically active tissue that uses energy even at rest, so more muscle means more resting energy use.