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While many factors may influence an individual's weight, overweight and obesity are due mainly to an imbalance of energy intake from the diet and energy expenditure (through physical activities and bodily functions). Genetic and environmental factors play a role, but attention to diet and physical activity is important not only for preventing weight gain, but also for weight loss and subsequent maintenance.

Energy intake

The total amount of food that your body needs depends on your age, sex, body size, level of physical activity and whether you are pregnant or breastfeeding. The body converts the protein, fat and carbohydrate in food to energy. Fat is the most concentrated source of energy.

Energy intake from food varies greatly between individuals, for example, the average intake for children ranges from about 6,000 kilojoules for children aged 2-3 to about 10,000 kilojoules for adolescents aged 14-16. The average energy intake for adults was about 10,000 kilojoules for men and about 7,000 kilojoules for women aged 18 and over.

Energy needs increase during periods of growth, during pregnancy and breastfeeding and with increasing physical activity.

Energy expenditure

The human body expends energy in three ways:

  • basal metabolism (the energy used to maintain vital body processes)
  • thermic processes (the energy taken to digest and absorb food)
  • physical activity (the energy used to move around)

Physical activity is the most variable component of energy expenditure, and the only component a person has any direct control over. For a normally active person, physical activity contributes about 20% to daily energy expenditure.

The balance

Healthy eating and physical activity are important for a healthy active life. Maintaining your weight means balancing the energy going into your body (as food and drink) and the energy being used for growth and repair, for physical activity, and to keep your bodily functions working. An excess energy intake, even a small amount over a long period, will cause weight gain. Children and adolescents need enough nutritious food to grow and develop normally. Older people need to keep physically active and eat nutritious foods to help maintain muscle strength and a healthy weight.

The Australian guide to healthy eating provides practical advice on the types and amounts of foods different groups should eat every day. Following these recommendations and limiting the number of energy-dense, nutrient-poor discretionary foods and drinks is the best way to maintain a healthy weight. Being physically active and eating healthily throughout life helps to promote health and wellbeing and prevent chronic disease.

Further information

The Healthy Weight website