More on broccoli...

A couple of months ago, we published a blog post criticising the way the launch of a new ‘super broccoli’ was reported in the media.

The researchers at the Institute of Food Research, who carried out the science underpinning this, contacted us to clarify the strength of the evidence of a link between broccoli and cancer – specifically prostate cancer. We agreed that we’d post their response, and to make clear that they do not endorse any media headlines or other coverage that are not based upon scientific evidence, and disassociate themselves from many of the headlines. Here’s what they had to say:

In their blog post, Cancer Research UK referred to a paper published in 2004 as part of the EPIC study, that reported no link between how much fruit and vegetables a man ate, and his risk of prostate cancer.

This paper also reported no association between ‘cruciferous’ vegetables – including broccoli – and risk of prostate cancer.

However, a later paper published in 2009, also as part of the EPIC study, reported a ‘statistically significant’ association between cruciferous vegetable consumption and risk of prostate cancer, based upon a study of a smaller group of men known as the EPIC-Heidelberg cohort.

The association between relative risk and cruciferous vegetable consumption for both studies are shown together in Figure 1.

A graph showing the association between relative risk of prostate cancer and intake of cruciferous vegetables

Figure 1. The association between relative risk of prostate cancer and intake of cruciferous vegetables. A is from the 2004 EPIC study, which reported no association. B is from the 2009 Heidelberg cohort study, which reported a significant inverse association. Both studies used men in their early fifties and so at very low risk of prostate cancer (Figure 2). The main difference in between the studies in that the EPIC study followed men for 4.8 years, while the Heidelberg study had a follow up of 9.4 years.

Both studies were similar in many ways, including the range of vegetables they ate. So why do they give different results?

The answer is in the finer details of the study.

The 2004 EPIC study followed 130,544 men for an average of 4.8 years. 1,104 of them developed prostate cancer (0.85 per cent) over this period. Their average age was 52.

In contrast, the Heidelberg cohort followed a smaller number of similar men, but for much longer – nearly ten years. In this subgroup, there were 11,405 men, 328 of whom developed prostate cancer – that’s 2.9 per cent. Their average age at the start – 52 – was the same.

In this second diagram, you can see how age affects a man’s chances of prostate cancer.

Age-related incidence of prostate cancer

Figure 2. Age-related incidence of prostate cancer. Note the relative low risk of men in their mid fifties.

As you can see, prostate cancer risk for men in their mid fifties – the average age at the end of the 2004 EPIC study – is very low, which is why in this study only 0.8 per cent of the men developed prostate cancer.

Rates of the disease increase as men enter their late fifties and early sixties (the endpoint of the 2009 Heidelberg cohort) which is why in this study there is a higher incidence, 2.9 per cent, (although this is still relatively low).

This means that conclusion of the 2004 EPIC paper – ‘no link’ – isn’t necessarily very helpful. It was assessing a group of men for whom prostate cancer is unusual, and thus it is not surprising that it found no significant association with diet.

There are two other similar studies worth highlighting.

Firstly, in 2007 a Canadian group found a reduced risk of aggressive prostate cancer in men who ate large amounts of cruciferous vegetables. Again, this study contrasts with the 2004 report from EPIC in two important respects.

Firstly, the men were older at the start of the study, and secondly the follow up period was longer. Both factors contribute to the increased risk of prostate cancer. Here are the numbers:

  • Men in study: 29,362 (all without prostate cancer)
  • Number of cases of prostate cancer: 1,338
  • Percent incidence: 4.6 per cent
  • Mean age of men at start: 63
  • Average follow up: 8 years

Secondly, earlier this year, a US team found that men who already had early-stage prostate cancer (that hadn’t spread) who ate higher amounts of cruciferous vegetables had a lower chance of their prostate cancer spreading. Again, here are the details:

  • Men in study: 1,560 (all with early prostate cancer)
  • Number of cases of progression of prostate cancer: 134
  • Percent progression: 8.6 per cent
  • Age of men at start: <60 – >70
  • Average follow up: 5 years

So, when Cancer Research UK’s bloggers’ highlight the 2004 EPIC paper to imply that the link between broccoli and prostate cancer is still debatable, they’re not reflecting the whole picture – it’s a study of men who would not normally be at risk of prostate cancer, and it is therefore not surprising that no association with diet is reported.

Other studies that have used more informative cohorts have reported statistically valid links between prostate cancer risk and cruciferous vegetable intake. So that’s why we’re continuing our research efforts to understand the science behind this phenomenon, which will hopefully end up benefitting men in the long run.

– Professor Richard Mithen, Institute of Food Research

So there are indeed some studies that appear to show that prostate cancer is less common, or spreads less readily, amongst people who eat large amounts of cruciferous vegetables. But we need to be clear – none of this is conclusive, and this is an area where more research is needed.

And there’s certainly no conclusive evidence – yet – if there’s a particular chemical in broccoli that’s behind these associations. Future studies, such as ones we’re funding, and those carried out by our colleagues at the Institute of Food Research, will help answer these questions.

The link between diet and cancer is complex, and unravelling the effects of different foods and nutrients on cancer risk is very difficult. In terms of diet, the strongest scientific evidence for cancer risk reduction is a healthy balanced diet high in fibre, fruit and vegetables and low in red and processed meat, saturated fat and salt.

Yinka Ebo, Cancer Research UK