Metabolism (2024)

Most of the energy we use each day is used to keep all the systems in our body functioning properly. This is out of our control. However, we can make metabolism work for us when we exercise. When you are active, the body burns more energy (kilojoules).

Two processes of metabolism

Our metabolism is complex – put simply it has 2 parts, which are carefully regulated by the body to make sure they remain in balance. They are:

• Catabolism – the breakdown of food components (such as carbohydrates, proteins and dietary fats) into their simpler forms, which can then be used to provide energy and the basic building blocks needed for growth and repair.
• Anabolism – the part of metabolism in which our body is built or repaired. Anabolism requires energy that ultimately comes from our food. When we eat more than we need for daily anabolism, the excess nutrients are typically stored in our body as fat.

Your body’s metabolic rate (or total energy expenditure) can be divided into 3 components, which are:

• Basal metabolic rate (BMR) – even at rest, the body needs energy (kilojoules) to keep all its systems functioning correctly (such as breathing, keeping the heart beating to circulate blood, growing and repairing cells and adjusting hormone levels). The body’s BMR accounts for the largest amount of energy expended daily (50 to 80% of your daily energy use).
• Thermic effect of food (also known as thermogenesis) – your body uses energy to digest the foods and drinks you consume and also absorbs, transports and stores their nutrients. Thermogenesis accounts for about 5 to 10% of your energy use.
• Energy used during physical activity – this is the energy used by physical movement and it varies the most depending on how much energy you use each day. Physical activity includes planned exercise (like going for a run or playing sport) but also includes all incidental activity (such as hanging out the washing, playing with the dog or even fidgeting!).

Based on a moderately active person (30 to 45 minutes of moderate-intensity physical activity per day), this component contributes 20% of our daily energy use.

Basal metabolic rate (BMR)

This means combining exercise (particularly weight-bearing and resistance exercises to boost muscle mass) with changes towards healthier eating patterns, rather than dietary changes alone as eating too few kilojoules encourages the body to slow the metabolism to conserve energy.

Maintaining lean muscle mass also helps reduce the chance of injury when training, and exercise increases your daily energy expenditure.

An average man has a BMR of around 7,100 kJ per day, while an average woman has a BMR of around 5,900 kJ per day. Energy expenditure is continuous, but the rate varies throughout the day. The rate of energy expenditure is usually lowest in the early morning.

Factors that affect our BMR

Your BMR is influenced by multiple factors working in combination, including:

• Body size – larger adult bodies have more metabolising tissue and a larger BMR.
• Amount of lean muscle tissue – muscle burns kilojoules rapidly.
• Amount of body fat – fat cells are ‘sluggish’ and burn far fewer kilojoules than most other tissues and organs of the body.
• Crash dieting, starving or fasting – eating too few kilojoules encourages the body to slow the metabolism to conserve energy. BMR can drop by up to 15% and if lean muscle tissue is also lost, this further reduces BMR.
• Age – metabolism slows with age due to loss of muscle tissue, but also due to hormonal and neurological changes.
• Growth – infants and children have higher energy demands per unit of body weight due to the energy demands of growth and the extra energy needed to maintain their body temperature.
• Gender – generally, men have faster metabolisms because they tend to be larger.
• Genetic predisposition – your metabolic rate may be partly decided by your genes.
• Hormonal and nervous controls – BMR is controlled by the nervous and hormonal systems. Hormonal imbalances can influence how quickly or slowly the body burns kilojoules.
• Environmental temperature – if temperature is very low or very high, the body has to work harder to maintain its normal body temperature, which increases the BMR.
• Infection or illness – BMR increases because the body has to work harder to build new tissues and to create an immune response.
• Amount of physical activity – hard-working muscles need plenty of energy to burn. Regular exercise increases muscle mass and teaches the body to burn kilojoules at a faster rate, even when at rest.
• Drugs – like caffeine or nicotine, can increase the BMR.
• Dietary deficiencies – for example, a diet low in iodine reduces thyroid function and slows the metabolism.

Thermic effect of food

• Fats raise the BMR 0 to 5%.
• Carbohydrates raise the BMR 5 to 10%.
• Proteins raise the BMR 20 to 30%.
• Hot spicy foods (for example, foods containing chilli, horseradish and mustard) can have a significant thermic effect.

Energy used during physical activity

During strenuous or vigorous physical activity, our muscles may burn through as much as 3,000 kJ per hour. The energy expenditure of the muscles makes up only 20% or so of total energy expenditure at rest, but during strenuous exercise, it may increase 50-fold or more.

As a rough guide:

• Moderate exercise means you can talk while you’re exercising, but you can’t sing.
• Vigorous exercise means you can’t talk and exercise at the same time.

Metabolism and age-related weight gain

• Hypothyroidism (underactive thyroid) – the metabolism slows because the thyroid gland does not release enough hormones. A common cause is the autoimmune condition Hashimoto’s disease. Some of the symptoms of hypothyroidism include unusual weight gain, lethargy, depression and constipation.
• Hyperthyroidism (overactive thyroid) – the gland releases larger quantities of hormones than necessary and speeds the metabolism. The most common cause of this condition is Graves’ disease. Some of the symptoms of hyperthyroidism include increased appetite, weight loss, nervousness and diarrhoea.

Genetic disorders of metabolism

• Fructose intolerance – the inability to break down fructose, which is a type of sugar found in fruit, fruit juices, sugar (for example, cane sugar), honey and certain vegetables.
• Galactosaemia – the inability to convert the carbohydrate galactose into glucose. Galactose is not found by itself in nature. It is produced when lactose is broken down by the digestive system into glucose and galactose. Sources of lactose include milk and milk products, such as yoghurt and cheese.
• Phenylketonuria (PKU) – the inability to convert the amino acid phenylalanine into tyrosine. High levels of phenylalanine in the blood can cause brain damage. High-protein foods and those containing the artificial sweetener aspartame must be avoided.

Where to get help

• Your GP (doctor)
• Dietitians Australia Tel. 1800 812 942
• Australian guide to healthy eating