Diet and Prostate cancer
Transcript of segment within “The Health Report” radio broadcast by
Dr Norman Swan on 6 September 2010
This transcript was typed from a recording of the program. The ABC cannot guarantee its complete accuracy because of the possibility of mishearing and occasional difficulty in identifying speakers.
If you're a regular listener then you'll know that a recurrent
theme is that what we eat definitely helps to define what we are, especially
when it comes to cancer. Michael Pollak is Professor of Medicine and Oncology
at McGill University in Montreal. He has a special interest in eating and cancer,
especially prostate cancer. But, as you'll hear, what he's found probably
applies to other malignancies as well.
Most of the research concerning diet and prostate cancer over
the past decade has emphasised micronutrients. These are constituents of the
diet that are consumed in very small quantities like minerals and vitamins.
Antioxidants, selenium, things like that.
Selenium, licopenes, vitamins.and there have been
associations in population surveys that have implied that certain deficiencies
could increase the risk of prostate cancer, and naturally that led to some
enthusiasm to try to lower prostate cancer risk by providing micronutrient
supplements. And that led to a large clinical trial in North America called the
Select Trial which involved thousands of men, many years of follow-up, and
the results are disappointing. The supplementation simply did not alter the risk
of prostate cancer.
So now some of the research direction concerning nutrition and cancer has shifted away from micronutrients to macronutrients, and by macronutrients we mean the major food groups.
Protein, fat, carbohydrate and also its macro effects on the
body such as obesity.
Exactly, and here we find that eating too much in terms of
prostate cancer, like cardiovascular disease, is not good for you.
It increases your risk, or increases bad outcomes if you get
Excellent question. The effect on risk is measurable but small,
the effect on outcome is very significant. Obese men are somewhat more likely
to get prostate cancer but prostate cancer patients who are obese do considerably worse.
Which is more than just it's hard to operate on them?
Yes, it's not just due to surgical complications. The studies
are quite definitive in showing that there's a biological difference. The question
is why do obese people with prostate cancer do worse?
To what extent do they do worse?
Death rates can be three to four times higher in obese men
than in normal-weight men all other factors being equal; so just the presence
of obesity can have an enormous effect on outcome. Our best treatments have
relatively little effect on outcome compared to just being overweight.
So it overwhelms prostate treatment?
Yes. The initial research suggested a simplistic idea that you
eat too much your body has somehow more energy than it should and the
cancer uses this energy, this excess nutrition, to grow better. But we now
know that it's not the case, it's not that there's an excess of glucose to feed
the cancer, for example, because cancers are very good at extracting nutrients
They'll take it anyway?
Yes, so you're not going to fertilise the cancer more by
providing more nutrients, they can get all the nutrients they need even if
you're thin. So how then does obesity affect the cancer? The latest research
suggests that obesity changes the hormonal environment of the man and in
turn it's the obesity-related hormones that worsen the prognosis. We're not
talking about the regular hormones that people think about in the context of
It's not testosterone?
It's not testosterone.
So is it insulin, is it things to do with the risk of diabetes?
Right, it's insulin, and insulin-like growth factors. The
prostate is not just sensitive to androgens like testosterone but also appears to
be responsive to insulin, which is newer information.
Insulin tells cells, especially in our muscles, to import sugar,
glucose from the blood to use as energy. Insulin-like growth factors are
another family of hormones, chemical messengers, which aren't well
understood but one of their messages is to tell cells to go forth and multiply.
Both these hormones do this by docking with a receptor on the surface of a cell like a lock and key.
Research has found that prostate cancer cells have receptors for both insulin and insulin-like growth factors, IGFs, and one thing that certainly goes along with obesity are high insulin levels.
This has blindsided many prostate cancer researchers until relatively recently because for years they'd been focused on the male hormone testosterone, and had left insulin to the researchers looking at metabolic diseases like diabetes. Michael Pollak again.
If we take mice that have experimental prostate cancer and
we simply feed these mice too much glucose, too much sugar, a junk food
diet, we can show that not surprisingly that makes their insulin levels go up.
But what's intriguing is it makes their prostate cancers grow faster and when
we examine the insulin receptors on those tumours they get more activated.
So that we really have fairly direct experimental evidence that higher levels of
insulin can stimulate prostate cancer growth.
Insulin receptors detect the levels of insulin and the levels of insulin are fluctuating hour to hour according to how much you eat. Then there's the separate but closely related insulin-like growth factor receptors they detect the amount of IGF in the blood. The levels of IGF vary in a less acute manner with how much you eat. IGF 1 levels tend to vary less meal to meal but they're lower in people who are eating less.
So you've got a story that's confirmed in animals, or it's
suggested it's confirmed in animals, that the more calories you take the higher
your insulin and the faster your tumour grows. That could be a worry for
people with Type 2 diabetes who have been prescribed insulin. I mean what's
the story here?
Well there's a very interesting convergence of research in this
area, there have actually been recent reports of excess cancer amongst insulin
users. These reports are highly controversial and of course people who are
insulin dependent diabetics do not have much choice. Even if it were to be
associated with a somewhat increased cancer risk.
Because if you've got Type 1 diabetes you'll die without it?
Exactly. On the other hand.
Just finishing off on Type 1 -- in Type 1 diabetes if you're
taking insulin, well you probably aren't taking excess insulin in your body,
you're probably taking the right amount.
Right, the physiological amount, and Type 2 diabetics actually
their problem is insulin resistance.
And the result is you're actually even without insulin
treatment you've got very high levels of insulin.
And so these people, these Type 2 diabetics have high blood
sugars and high insulins and the high insulin is not working in the classic
insulin target tissues but may actually be stimulating cancers. Now an
interesting drug is metformin because metformin is used in the treatment of
Type 2 diabetes.
It's a very old drug, a very reliable drug, in fact it makes you
lose weight as well.
Yes, we are a big fan of metformin and it's now the subject of
cancer research because we are testing the hypothesis that metformin use by
lowering the glucose and especially lowering the insulin might be beneficial for
prostate cancer patients who have high insulin levels. And in Sydney,
Australia, one of the first clinical trials of metformin for prostate cancer is
beginning I think later this year.
In obese men?
Men who have high insulin levels and these are mainly obese
Has anybody done an intervention where they've got obese
men to lose weight and found whether or not that makes a difference to
prostate cancer outcomes because that's the other acid test here?
Of course that would be highly desirable simply to forget the
drugs and just lose weight. We predicted that that would be as effective.
However we also know that it's very hard to lose weight and to maintain the
weight loss. So the immediate message for prostate cancer patients is you
have another reason, perhaps even a special reason to aim for your ideal body
weight but this drug metformin and other drugs under development may
provide a special help.
And what about other cancers?
There are similar lines of research for colon cancer and for
breast cancer, high levels of insulin are also implicated in those settings and
either lifestyle or drug treatments that might lower the insulin levels are under
study as ways to provide new kinds of hormonal management for these
Michael Pollak is Professor of Medicine and Oncology at McGill
University in Montreal.
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