A recent study, reported in the BBC and the Guardian suggested that being obese protects against some types of prostate cancer but increases the risks of others. What should we make of this apparently conflicting information? Let’s take a look at the study, and the evidence linking body weight and prostate cancer.
The study compared 392 men with prostate cancer with an equal number of healthy men. It found that men had lower risks of prostate cancer if they were resistant to the hormone insulin – a common trait in overweight or obese people. (Insulin helps our bodies convert sugars into energy – obese people are less good at this because they don’t respond properly to insulin).
So far, so good. But the heavier men were also more likely to develop aggressive prostate tumours that were more likely to spread to other parts of their body.
These results may seem confusing but they tie in very well with what we already know. Much larger studies over the past few years have found similar results and it’s part of a shift in the way we think about the causes of prostate cancer.
Up till recently, most studies suggested that a man’s risk of prostate cancer wasn’t affected by their weight.
But recently, scientists have started to realise that not all prostate cancers are born equal. Aggressive tumours are very different to non-aggressive, slow-growing ones. They can almost be thought of as two different diseases with different causes.
It now seems that being overweight or obese onlyincreases the risk of aggressive tumours and may actually reduce the risk of slow-growing, non-aggressive tumours.
This explains why earlier studies found no overall link between body weight and prostate cancer – the opposite effects on the two types of tumours were cancelling each other out.
Some studies have also found that being obese increases the likelihood that a prostate tumour will progress to a more advanced stage, and spread to other parts of the body.
It’s not clear why this is, but the most likely theory pins the blame on two sex-related hormones – testosterone and oestrogen.
Obese men have a lower level of testosterone than men with a healthy weight, and this is linked to a higher risk of aggressive prostate cancer. They also have higher levels of the female sex hormone oestrogen, which has been linked to a lower risk of non-aggressive tumours (but not aggressive ones).
So in obese men, abnormally high oestrogen levels lower the risk of non-aggressive tumours, while unusually low testosterone levels crank up the risk of aggressive ones.
With these two effects tugging in opposite directions, where does the balance sit? Is it possible that obese men actually have an winning ticket in the cancer prevention stakes?
Unfortunately, the evidence says that they don’t. Several large studies have found that obese men are still 20-40% more likely to die from prostate cancer than those with a healthy weight, probably because they are more likely to get aggressive cancers that are harder to treat.
And of course, as we keep reminding people, obesity also causes several other types of cancer including bowel, oesophageal and kidney cancers, and probably many more.
For reducing the risk of cancer, a healthy body weight is still the best option.