Obesity and Respiratory Health: Beyond Sleep Apnoea

The relationship between obesity and respiratory health is well established for obstructive sleep apnoea, but the effects of excess weight on the respiratory system are considerably broader. Obesity affects lung mechanics, airway inflammation, and respiratory muscle function in ways that impair breathing during both sleep and wakefulness — and that interact with common respiratory conditions including asthma and chronic obstructive pulmonary disease (COPD).

How does obesity affect lung mechanics?

Excess adipose tissue — particularly abdominal fat — exerts mechanical pressure on the diaphragm and chest wall, reducing the space available for lung expansion. This reduces functional residual capacity (the volume of air remaining in the lungs after a normal exhalation) and expiratory reserve volume. In people with severe obesity, these reductions can be substantial: a BMI of 40 is associated with a 25 to 30 per cent reduction in functional residual capacity compared to a healthy-weight individual.

Reduced lung volumes cause small airways to close during normal breathing — a phenomenon called airway closure — which leads to ventilation-perfusion mismatch and reduced blood oxygen levels. This is exacerbated in the supine position, which is why people with obesity often experience breathlessness when lying flat.

Obesity hypoventilation syndrome

Obesity hypoventilation syndrome (OHS) — defined as a combination of obesity (BMI of 30 or above), daytime hypercapnia (elevated blood CO2), and sleep-disordered breathing — affects approximately 10 to 20 per cent of people with a BMI above 40. It carries a substantially higher mortality risk than obstructive sleep apnoea alone and is associated with pulmonary hypertension, right heart failure, and reduced life expectancy. Many cases go undiagnosed until a late stage.

Obesity and asthma

Obesity is both a risk factor for developing asthma and a modifier of asthma severity. People with obesity are 1.5 to 2 times more likely to have asthma than lean individuals, and their asthma tends to be harder to control with standard treatment. The mechanisms include airway inflammation driven by adipokines, mechanical effects on airway calibre, and gastro-oesophageal reflux — which is more common in obesity and can trigger airway hyperreactivity.

A 2013 randomised controlled trial published in the British Medical Journal found that a structured weight loss programme significantly improved asthma control, quality of life, and lung function in people with obesity and asthma, with effects comparable to adding a second controller medication.

What does weight loss do for respiratory health?

Weight loss consistently improves respiratory function. Studies show that a 10% reduction in body weight increases functional residual capacity, reduces airway resistance, and improves exercise tolerance. In people with OHS, weight loss can resolve daytime hypercapnia and reduce dependence on ventilatory support.

If breathlessness or respiratory symptoms are affecting your quality of life, speak to the Weight Loss Wales team about a doctor-led weight management programme.