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‘Super’ broccoli – no firm evidence it prevents cancer in humans

by Henry Scowcroft | Analysis

6 October 2011

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There's little hard evidence that eating more broccoli prevents cancer

A new type of ‘super’ broccoli, developed by scientists at the Institute of Food Research and the John Innes Centre, has been launched in the UK, leading once again to headlines about the vegetable’s ‘cancer-fighting’ abilities.

The researchers have specially bred the new broccoli, sold as ‘Beneforté’, to increase its levels of glucoraphanin, a chemical which our bodies convert to another, sulforaphane, whose health properties have been heavily scrutinised in the lab.

But how strong is the evidence that eating broccoli – either normal broccoli or this new souped-up variety – can actually prevent cancer? Let’s take a look at what the science says.

Studies in the laboratory

Sulforaphane is certainly an interesting chemical, and appears to be able to affect the inner workings of our cells in a number of ways.

Various intricate lab studies have shown that it can switch on enzymes inside cells that are involved in processing potentially harmful compounds, switch off genes involved in inflammation, cause cells to stop dividing, and even persuade them to commit suicide by apoptosis.

There’s also evidence that the chemical – and foods containing it – can affect how tumours grow in animals, or affect their levels of various cancer-associated genes and proteins.

All of these abilities could, in theory, mean that sulforaphane-containing foods might help prevent cancer in people. But they don’t constitute proof on their own.

Studies in people

There are two other lines of evidence that hint that sulforaphane, or foods that raise its levels in our bodies, might help prevent cancer. But again there’s nothing we consider a cast-iron link.

Firstly, large prospective population studies have shown that certain types of cancer are slightly less common among people who eat so-called ‘cruciferous’ vegetables like broccoli more regularly.

This may be because of chemicals like sulforaphane, but equally could be due to these studies not fully taking into account other lifestyle factors (after all, people who eat lots of broccoli do tend to pursue a generally healthy lifestyle – dissecting out the effect of these vegetables alone is scientifically a very tricky thing to do).

The second line of evidence comes from clinical trials, where people have eaten a controlled amount of broccoli, and the effects on their bodies carefully monitored. This evidence is far scarcer than the laboratory and animal studies we mentioned above.

In one 2008 study, led by the team behind Beneforté, men with a pre-cancerous prostate condition, who ate a daily portion of broccoli had altered gene behaviour in their prostates, compared to men who ate peas every day.

The study involved just 22 men, and only followed them for a year. It didn’t follow whether the men subsequently developed prostate cancer, so it was impossible to say whether this altered genetic behaviour had a lasting health effect. More research is needed to answer this question.

And it’s worth noting the conflicting results from the EPIC study, a huge Europe-wide study of diet and cancer which we’re helping fund. The EPIC investigators concluded that “intake of cruciferous vegetables was not associated with risk” of prostate cancer.

Health claims

Putting all of this evidence together, sulforaphane – or its precursor, glucoraphanin – is certainly capable of influencing our inner nuts and bolts in some way. But whether this affects cancer rates is still debatable. Large studies like EPIC have yet to convincingly show a protective effect of any one single vegetable or fruit against cancer risk.

And we certainly can’t say, based on the current evidence, that broccoli containing higher levels of a particular chemical is ‘better’ at preventing cancer than common-or-garden varieties.

All of this makes media claims that a “cancer-fighting super broccoli” is now on sale, somewhat perplexing – especially when they’re repeated by high-profile public figures.

To its credit, the Beneforté website itself goes out of its way not to make such claims (its section on ‘the science behind glucoraphanin’ doesn’t even mention the chemical itself), relying instead on oblique references to ‘increased levels of antioxidant enzymes’. We’ve discussed the ‘antioxidant fallacy‘ at length on this blog.

Likewise, the press release from the Institute of Food Research is also very careful to point out that glucoraphanin is “thought to help explain the link between eating broccoli and lower rates of heart disease and some forms of cancer,” rather than claiming that eating the broccoli would prevent cancer.

Reducing cancer risk

As we never tire of pointing out, there are sound, evidence-based, scientifically justified things that the UK population can do to reduce the risk of cancer.

If fewer of us smoked; if we upped our average exercise levels; if we kept to a healthy weight; if we took a little more care not to burn in the sun; if we all put a little more fruit and veg in our shopping baskets, and a little less red and processed meat; if we drank a little less booze, then cancer rates would ultimately be affected for the better.

Even then, healthy living is no guarantee: it stacks the odds in our favour, but even if we all lived extremely healthily, some people would still develop cancer – despite their best efforts.

But any claims of ‘superfoods’ outside of these general health guidelines – no matter how shiny the website, nor how compelling the headline – simply isn’t science.

It’s marketing.