What is obesity?
Obesity is a complex, chronic health condition characterised by an excess accumulation of body fat that can significantly affect health. It is not simply a matter of lifestyle choices — it is now recognised by leading medical organisations worldwide as a disease in its own right, with biological, genetic, environmental and social drivers.
In Australia, almost two thirds of adults live with overweight or obesity, making it one of the most pressing public health challenges of our time.
How is obesity defined?
Obesity is most commonly measured using body mass index (BMI) — a calculation based on height and weight. A BMI of 30 or above is classified as obese; between 25 and 30 is classified as overweight. While BMI is a useful population-level tool, it has limitations — it doesn't distinguish between fat and muscle mass, or account for where fat is stored in the body.
Waist circumference is increasingly recognised as an equally important measure, since fat stored around the abdomen (visceral fat) is particularly strongly linked to metabolic health risks, independent of overall body weight.
What causes obesity?
Obesity develops when energy intake consistently exceeds energy expenditure over time — but the factors that drive this are far more complex than simply eating too much or exercising too little.
Genetics play a substantial role, influencing appetite regulation, metabolic rate, fat storage and the body's response to food. The brain's appetite and reward systems — and the hormones that govern them — vary significantly between individuals. Sleep deprivation, chronic stress, certain medications (including some antidepressants, antipsychotics and steroids) and hormonal conditions like hypothyroidism can all promote weight gain.
The broader environment matters too. Access to affordable, nutritious food, opportunities for physical activity, socioeconomic factors and exposure to heavily marketed, energy-dense food all shape the conditions in which individual choices are made. Understanding obesity requires holding all of this complexity together.
Why does obesity affect health?
Excess body fat — particularly visceral fat around the organs — is metabolically active and drives chronic low-grade inflammation throughout the body. This contributes to a cascade of health effects, including insulin resistance (a key driver of type 2 diabetes), elevated blood pressure, abnormal cholesterol levels and increased cardiovascular risk.
Obesity significantly increases the risk of type 2 diabetes, heart disease, stroke, certain cancers, sleep apnoea, fatty liver disease, osteoarthritis and mental health conditions including depression and anxiety. The greater the degree of excess weight and the longer it is carried, the higher the cumulative risk.
It's also worth noting that not everyone with obesity will develop these complications — individual risk varies considerably — and that weight stigma itself is a significant barrier to people seeking care.
How is obesity treated?
Effective obesity management requires a personalised, sustained approach. There is no single solution that works for everyone.
Dietary change and increased physical activity remain central to treatment, but the evidence is clear that for most people these alone are insufficient to achieve and maintain significant weight loss long-term without additional support. Structured behavioural programs that address eating patterns, physical activity and psychological factors can make a meaningful difference.
Medication has an increasingly important role. There are many effective and widely used medications for obesity including GLP-1 receptor agonists such as semaglutide, dual GIP/GLP-1 receptor agonists such as tirzepatide have shown significant effectiveness in clinical trials, achieving levels of weight loss previously only seen with bariatric surgery.
These treatments are changing the landscape of obesity care.
For people with severe obesity, bariatric surgery remains the most effective long-term treatment, often resulting in substantial weight loss, remission of type 2 diabetes and significant reductions in cardiovascular risk.