The study, presented at the European Association of Urology Congress, EAU21, followed more than 1500 patients over three years. Patients classed as obese - with a BMI over 30 - had a ten percent higher survival rate than thinner patients over 36 months.
Although obesity is usually associated with an increased risk of death from many cancers and some other chronic diseases, there is some evidence in a few cancers of a survival advantage for patients with a high body mass index. This phenomenon is known as the 'obesity paradox.
Nicola Fossati, Alberto Martini, and colleagues at San Raffaele University in Italy wanted to test whether the 'obesity paradox' held true for patients with metastatic castration-resistant prostate cancer - an advanced form of the disease that no longer responds to testosterone lowering treatments.
They looked at survival rates in 1,577 patients involved in three different clinical trials, with an average age of 69 and an average BMI of 28. They found that BMI was a protective factor in both overall and cancer-specific survival, with a 4% higher overall survival probability and 29% cancer-specific survival probability. Even when they adjusted for higher doses of chemotherapy given to larger patients, the team found the protective effect remained. Over 36 months, around 30% of obese patients survived compared to 20% of overweight and normal-weight individuals.
Dr. Nicola Fossati, a urologist at San Raffaele University says: "Looking at patients with metastasis of prostate cancer, we found that obese patients are living longer. This means that BMI could be used to predict survival in these patients. "This obesity paradox has been seen in some other cancers, possibly due to the relationship between tissue fat and cancer genomes, and more research is needed in this area. It's also possible that improved survival may be due to the interaction of chemotherapy with other drugs. Obese patients in this older age group tend to be taking medication for other conditions and we do not fully understand how these medicines interconnect.
"Nevertheless, we would not recommend weight gain to anyone with this or another disease. Obesity is a risk factor for many cancers and other diseases and patients should always aim for a healthy BMI of 18 to 24."
Professor Peter Albers, from Düsseldorf University, who chairs the EAU Scientific Congress Office, said: "There are many possible explanations for the association of body weight with a positive outcome in metastatic cancers. It might be that patients with higher BMI are able to tolerate the toxicity of the treatments and their side effects better; in prostate cancer, it might be due to the protective impact of hormones found in tissue fat; it is known that healthy men with slightly higher BMI have a higher overall life expectancy compared to very slim ones.
"However, at the moment, these are just hypotheses. Further research is needed to identify the biological mechanism behind these different outcomes. Until that mechanism is proven, we can't recommend any change to treatment for patients with advanced prostate cancer."
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