Does maintaining a healthy weight still help after a cancer diagnosis?

Obesity after a cancer diagnosis is associated with a higher risk of death in most cancer types. In an analysis of more than 6.3 million cancer patients, the likelihood of dying from cancer was 17% higher in people with obesity, and overall mortality was 14% higher. The risk of cancer recurrence was also 13% higher. These are associations, not proven cause and effect, but the scale and consistency of the findings are impressive.

There are, however, exceptions. In lung cancer, kidney cancer and melanoma, patients with obesity actually survive better than patients without. This is known as the 'obesity paradox'. Researchers do not yet know why this is the case. For these three cancer types, the usual advice about weight does not apply without qualification.

Importantly, weight loss after a diagnosis is not automatically good news. In women with breast cancer who lost 10% or more of their body weight in the 18 months following diagnosis, the risk of death was more than twice as high as in women without such weight loss. Weight gain, on the other hand, was not associated with worse survival. Unintentional weight loss is therefore more of a warning signal than a favourable sign.

Muscle loss is at least as important as the number on the scale. In cancer patients with severe muscle breakdown, median survival was 8.4 months, compared with 28.4 months in patients with good muscle mass. That difference held regardless of whether a person was overweight or underweight. BMI alone therefore tells only part of the story.

Protein-rich nutritional drinks containing omega-3 fatty acids can modestly slow weight loss and muscle loss during chemotherapy (on average nearly 2 kg more weight retained). Whether this also improves survival has not yet been demonstrated. Standard dietary counselling or regular nutritional drinks had no measurable effect on weight in these studies.