Obesity as a medical condition: why it isn't about willpower

How common is it in South Africa?

The South African Demographic and Health Survey (2016) found that about two in three women and one in three men aged 15 and older were living with overweight or obesity. Worldwide, the WHO estimates more than one billion people are living with obesity.

Why "a chronic disease" and not a lifestyle choice?

• Biology pushes back. After weight loss, appetite hormones shift and the body burns slightly less energy, which makes regain likely without ongoing support.
• Genes matter. Body weight is strongly influenced by inheritance.
• Environment matters. Cheap, energy-dense food, long commutes, shift work, stress and short sleep all push weight up.
• Medicines and conditions can contribute, including some antidepressants, steroids, diabetes medicines, an underactive thyroid and PMOS (previously called PCOS).

Recognising this isn't an excuse. It is the basis for treating weight with the same long-term approach used for blood pressure or diabetes, and without blame.

How is it measured?

• BMI (weight in kg divided by height in metres squared). A BMI of 25 to 29.9 is classed as overweight and 30 or more as obesity. For people of Asian descent, lower thresholds are often used.
• Waist circumference. A waist of 94 cm or more (men) or 80 cm or more (women) is linked to higher metabolic risk.
• Health effects. Increasingly, doctors also look at whether weight is affecting your health: blood sugar, blood pressure, liver, joints, sleep, mood and mobility.

BMI is a useful screening tool but doesn't measure body fat directly, so it is interpreted alongside other information.

A newer way of defining obesity

In January 2025, an international commission published in The Lancet Diabetes & Endocrinology proposed moving beyond BMI alone. It recommends confirming excess body fat with at least one other measure (such as waist circumference, waist-to-hip ratio or waist-to-height ratio) or a direct body fat measurement, and then separating two situations:

• Clinical obesity: excess body fat is already affecting how the body's organs or the whole person function, for example through breathlessness, joint problems, raised blood sugar or reduced mobility. This is treated as an illness that needs active treatment.
• Preclinical obesity: excess body fat with organs still working normally, but a higher risk of future problems. The focus is on health advice, monitoring and prevention.

This approach is being discussed and adopted gradually, so you may still see BMI-based terms in most settings.

How much weight loss makes a difference?

You don't need to reach an "ideal" weight to benefit.

• About 5% to 7% weight loss reduced the development of type 2 diabetes by 58% in people at high risk in the Diabetes Prevention Program.
• About 5% reduces liver fat, with larger losses improving inflammation and scarring.
• Modest weight loss also lowers blood pressure and can ease knee pain and sleep apnoea.

The main treatment options

Treatment is personal and usually combines several approaches:

1. Nutrition you can sustain: enough protein, plenty of vegetables and fibre, fewer sugary drinks and ultra-processed foods. No single "best diet" suits everyone.
2. Physical activity: 150 to 300 minutes of moderate activity a week, plus strength training to protect muscle.
3. Sleep and stress: short sleep and chronic stress increase appetite and cravings.
4. Behavioural support: structured programmes and regular follow-up improve results.
5. Reviewing medicines that may cause weight gain.
6. Anti-obesity medicines: for some adults, alongside lifestyle support. See our guide to questions to ask about GLP-1 medicines.
7. Metabolic (bariatric) surgery: for some people with higher BMIs or weight-related conditions.