Energy metabolism and BMR (SACE Stage 2 Nutrition)

What this dot point is asking

You need to explain what basal metabolic rate is, how it combines with activity and digestion to give total energy expenditure, and the factors that raise or lower a person's energy requirements.

Basal metabolic rate

Basal metabolic rate (BMR) is the rate at which the body uses energy at complete rest, simply to stay alive. This energy keeps the heart beating, lungs breathing, brain working, body temperature stable and cells maintained. For most people, BMR is the largest single part of daily energy use, often more than half.

BMR varies between people because the organs and tissues that use energy differ in size and activity.

The components of total energy expenditure

A person's total energy expenditure is made up of three parts:

• Basal metabolic rate: the resting energy described above, the biggest share.
• Physical activity: the energy used for all movement, from walking to sport. This is the part people can change most easily, and it varies hugely between an inactive and a very active person.
• Thermic effect of food: the energy used to digest, absorb and process food, a smaller share.

Adding these gives the kilojoules a person needs each day to stay in energy balance. Eating more than this total leads to weight gain; eating less leads to weight loss.

Factors that affect BMR and energy needs

Body size and composition

Larger bodies use more energy. Muscle is more metabolically active than fat, so a person with more muscle has a higher BMR even at the same weight. This is why building muscle through activity raises resting energy use.

Age

BMR is high during growth, peaks in youth, and tends to fall with age as muscle mass declines, which links to lower energy needs in older adults.

Sex

Males on average have more muscle and less fat than females, giving a somewhat higher BMR.

Hormones

The thyroid hormone thyroxine controls metabolic rate; an overactive thyroid raises BMR and an underactive thyroid lowers it.

Other factors

Fever, stress, cold environments, pregnancy and growth all raise energy use. Strict dieting can lower BMR as the body conserves energy.

Linking to energy balance and health

Understanding energy expenditure explains energy balance from the macronutrient content. If intake matches total expenditure, weight is stable; a sustained surplus is stored as body fat, raising the risk of overweight and the diet-related diseases in Topic 2. It also explains why two people of the same weight can need very different amounts of food.

In short, basal metabolic rate is the resting energy the body needs to stay alive, and together with physical activity and the thermic effect of food it sets total energy expenditure, which varies with body size, composition, age, sex and hormones.