Nutrition

The body needs energy for every process, from keeping the heart beating to running a cross-country race. In Australia food energy is measured in kilojoules (kJ). Only four food components supply energy: carbohydrate (about 17 kJ/g), protein (about 17 kJ/g), fat (about 37 kJ/g) and alcohol (about 29 kJ/g). Dietary fibre provides only a little, because it is not digested in the small intestine and is only partly fermented by gut bacteria.

Vitamins, minerals and water supply no energy, even though they are essential. Fat is the most energy-dense macronutrient, which is why small amounts of oil, butter or fried coatings add many kilojoules.

Total daily energy expenditure has three components. Basal Metabolic Rate (BMR) is the energy used to keep the body alive at complete rest: breathing, circulation, maintaining body temperature and cell repair. For most people it is the largest component. Physical activity is the most variable component and the one a person can change most easily. The thermic effect of food (about 5–10% of daily energy use) is the energy used to digest, absorb and process food.

Adolescents also need energy for growth, and pregnancy and breastfeeding raise requirements further.

Factors that raise BMR include greater body size and lean muscle mass, higher levels of thyroid hormones and pregnancy; growth in children and adolescents adds a further energy cost. BMR tends to fall with age as lean mass declines, and it is usually lower in females because, on average, they carry less lean tissue. Eating too few kilojoules also encourages the body to slow its metabolism, which works against crash dieting.

Energy balance compares energy intake with energy expenditure. When they are equal, body weight stays stable. A positive balance (intake greater than expenditure) leads to weight gain over time; a negative balance leads to weight loss. The Estimated Energy Requirement (EER) is the average energy intake predicted to keep a healthy person of a given age, sex, size and activity level in balance. It is often estimated by multiplying BMR by a physical activity level (PAL) factor.

The NHMRC’s acceptable macronutrient distribution ranges suggest about 15–25% of energy from protein, 20–35% from fat and 45–65% from carbohydrate, with saturated and trans fats limited.

Maintaining a healthy weight range means matching intake to the EER over weeks and months, not day by day. Adults are commonly screened with Body Mass Index (BMI = mass in kg ÷ height in m²; 18.5–24.9 is the healthy range) and waist circumference (risk increases above about 94 cm for men and 80 cm for women).

BMI does not separate muscle from fat, does not show where fat is stored, and needs age- and sex-specific charts for people under 18. The health-at-any-size perspective shifts the focus from weight loss to supporting health behaviours such as regular activity and varied eating for people of all sizes, and argues that weight stigma itself harms health by discouraging healthy behaviour and medical care.

A balanced response recognises both its value in reducing stigma and the evidence that excess body fat raises the risk of some conditions.