What is metabolism?

Metabolism is the process where your body converts the food you eat and drink into energy for your body to use.

The food and drink that you eat contains:

• carbohydrates
• fats
• proteins

Your body breaks these nutrients down into simpler forms. These are then converted into energy by your body.

Your body uses this energy to allow for:

• your organs to function
• growth and repair of body tissues
• digestion of your food
• energy used when moving muscles, such as physical activity

How is metabolism measured?

In Australia, the amount of energy your body uses and that you consume through food and drink is measured in kilojoules (kj).

Many people are familiar with the term calorie. A calorie is another measurement of energy. One calorie is 4.18 kilojoules.

Calories are also sometimes called kilocalories.

How does my metabolism work?

Your metabolism involves two processes, catabolism and anabolism. These two processes are carefully balanced to help you stay healthy.

• Catabolism is the process of breaking down food into simpler forms, releasing energy.
• Anabolism is the process of using this energy to build and repair cells in your body.

Sometimes, your body takes in more energy than it needs. When this happens, your body stores the excess energy as fat.

What affects my metabolism?

Your metabolism at rest (basal metabolic rate or BMR) can be affected by different factors including your:

• body size and composition — larger people burn more kilojoules, even when resting. This includes people with more muscle.
• sex — males usually burn more kilojoules than females of the same age and weight. This is partly because males have more muscle.
• age — as you get older, you lose muscle, which slows your metabolism
• genes
• infections and illnesses

When you use your muscles, you burn kilojoules. This is why athletes with more muscle mass have a higher basal metabolic rate. Fat does not burn energy.

If you are living with overweight or obesity, you may have a faster basal metabolism, because larger bodies need more energy to maintain them.

Together, these factors make up your basal metabolic rate (BMR). That is the minimum amount of energy your body needs to survive and maintain basic body functions such as:

• growing and repairing your cells
• using your brain
• pumping blood around your body
• moving your muscles
• keeping your body temperature constant
• making hormones and enzymes
• breaking down food

How does my health affect my metabolism?

Health issues can affect your metabolism. For example, hyperthyroidism can increase your metabolism and cause sudden weight loss. Hypothyroidism can disrupt your metabolism and lead to weight gain and obesity.

Your metabolism can affect your health. People have different metabolic rates, which can influence their weight. Living with overweight or underweight conditions can, in turn, affect your health.

When you're sick, your body has a higher metabolic rate and needs more energy to help fight off the infection or illness.

When should I see my doctor?

See your doctor if you:

• are worried about your weight
• want to learn more about your metabolism
• are interested in trying a weight loss product

Your doctor may recommend seeing a dietitian who can educate and guide you to improve your health.

Can I increase my metabolism?

Everyone has a different metabolic rate. This is because everyone's basal metabolic rate (BMR) and level of physical activity varies.

You may be tempted to try to 'boost your metabolism' to burn more kilojoules and lose weight. There is little scientific evidence to show that any special foods or supplements can 'boost' your metabolism.

Although it's hard to change your metabolism, the more physical activity you do, the more kilojoules (energy) you will use.

Some ways to burn kilojoules include:

• aerobic activity such as walking, cycling and swimming
• strength training
• moving around more each day

Strength training can increase your metabolism by increasing your muscle mass.

If you practice healthy eating by maintaining a balanced diet, you can influence the amount of energy your body is taking in.

How does dieting affect my metabolism?

If you follow sporadic or extreme diets, fast or restrict the amount of food you eat, you trigger your body to save energy for later. This is a survival instinct. As a result, your metabolic rate slows down since your body is trying to save energy.

If you lose too much weight too quickly, you will lose both fat and muscle. Muscle burns kilojoules, but fat does not. This means that when you stop your diet, you will have less muscle mass and your BMR will be slower.

Read more about weight loss and dieting.

Resources and support

• Visit Your Move to find more ways to get active and improve your health.
• Visit Nutrition Australia's Inspiring Healthy Eating webpage to learn more about eating well.
• If you are concerned about your health or your metabolism, you should see your doctor or call healthdirect on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available to speak to, 24 hours, 7 days a week.

Do you prefer to read in languages other than English?

• The Life! Program, a free healthy lifestyle program website can be translated into some community languages.
• The NSW Government Multicultural Health Communication Services has resources including videos about healthy eating in many community languages.

Looking for information for Aboriginal and/or Torres Strait Islander people?

Learn about the Aboriginal and Torres Strait Islander Guide to Healthy Eating.