Red meat and breast cancer – still no solid evidence

The possible link between eating lots of red or processed meat and breast cancer has been in the news again today.

But has a new study published last night in the BMJ – looking at whether eating red meat in early adulthood is linked to the disease – pushed the body of evidence towards a yes?

The short answer is no. Despite this latest study, there’s still no clear link between eating red meat and breast cancer.

So what do we know about red meat and cancer, and more specifically, breast cancer? And where does this new study fit in?

A meaty topic

First, let’s consider red meat and cancer in general. Based on research over many years there’s now a large body of evidence supporting a link between people who eat a lot of red or processed meat and their chances of developing bowel cancer. And there’s growing evidence for a possible link to both stomach and pancreatic cancers.

But for other types of cancer, things are much less certain. And when it comes to breast cancer, despite several large studies, good evidence of a link just isn’t there.

For example, in 2009, researchers looked across a number of previous studies on red meat and breast cancer in premenopausal women, and found that, when the data came from studies monitoring the diet and health of women over time – something called a ‘cohort study’ – there’s no convincing evidence of a link.

Importantly, only in studies which involved grouping women with and without breast cancer and looking back on what might separate them in their food history – in this case eating red meat – did a weak link pop up. But these studies, called case-control studies, are far less reliable than cohort studies.

A year later, in 2010, another similar analysis found no convincing link between red meat and increased risk of the disease.

It’s fair to say that, looking at it all together, there hasn’t been enough evidence to say breast cancer’s more common among women who eat large amounts of red or processed meat.

So how does this new research stack up? Is it enough to overturn the evidence to date?

What did they show?

This latest study included 88,803 premenopausal women who had been taking part in the US Nurses’ Health Study II – a large cohort study (the more-reliable type that follows people over time) that started in 1989. These women initially completed a food questionnaire in 1991 about their food intake in the past year.

The questionnaire covered a broad shopping basket of items, including:

• Unprocessed red meat (such as beef, pork, lamb and hamburgers).
• Processed red meat (such as hot dogs, bacon, sausages and salami).
• Poultry (chicken and turkey).
• Fish (such as tuna, mackerel, salmon, sardines).
• Legumes (such as tofu or soybeans, beans, lentils and peas) and nuts.

The women reported how often they ate these foods, ranging from “never or less than once a month” to “six or more per day”. The researchers then asked them to complete more food questionnaires – including an estimate of how much alcohol they drank – in 1995, 1999, 2003 and 2007.

This allowed the researchers to split the women into five different groups based on the amount of each food they ate.

And every two years the team collected information on whether new cases of breast cancer had been diagnosed. They also used these questionnaires to collect information on important risk factors linked to breast cancer – including age, weight, family history, smoking and more.

During the next 20 years, 2,830 of the women developed breast cancer.

When the team analysed the data – taking into account some of the key risk factors for breast cancer – they estimated that the women in the group who regularly ate the largest amount of red meat, calculated as around one and half portions per day – or the equivalent of three rashers of bacon (that’s a lot of meat to eat every day) – had a 22 per cent increased risk of breast cancer, compared to women in the lowest meat-eating group.

As with many of these studies, this is a ‘relative risk’ – and you can read this post for more info about why this can be a confusing way to present statistics.

They then estimated how incremental increases in eating red meat might affect this relative risk. Looking at the middle value – or ‘median’ – for food intake in each of the five groups across the duration of the study, they found that each additional ‘serving per day’ of red meat gave a predicted 13 per cent increased risk of breast cancer across all the women – again a ‘relative’ risk that makes it hard to draw real-life information from.

It’s important to point out this is very different to saying that eating a single extra sausage on any given day will increase a woman’s risk of breast cancer by 13 per cent – the sort of confusion we often see in the wake of media reporting of these stories.

Finally, in their statistical analysis, the researchers showed that substituting a portion of red meat per day with either poultry or legumes predicted a 17 and 14 per cent lower risk of breast cancer respectively across all the women.

So why can’t we say that this study pushes red meat into the ‘yes’ category for risks linked to breast cancer?

Breast cancer – it’s complicated

Firstly, when it comes to diet and cancer, no single study will suddenly push the weight of evidence one way or another – we need to survey the complete landscape rather than a selected few lumps and bumps.

It’s important to point out what was good about this study:

• It involved a large number of women (88,803).
• There was a long period of follow up during the study – plus regular questionnaires on eating food and drinking alcohol.
• The researchers made a good attempt at accommodating certain risk factors for breast cancer.

Despite these strengths, the study only showed a weak link between red meat and breast cancer.

Now for the study’s weaknesses. Breast cancer is complicated – and some risk factors, like exercise levels and certain genetic links weren’t taken into account.

The genetic links are particularly difficult to accommodate in a study like this. But being aware of their omission is important, because family history linked to genetics is one of the biggest factors in influencing a woman’s breast cancer risk.

So, based on this latest study, and all the other available evidence, there is still no clear evidence of a link between the amount of red meat a woman eats and her chances of developing breast cancer.

But it’s important to mention there are other things that women can do to help lower their risk – including maintaining a healthy weight, drinking less alcohol and being physically active.

And, of course, given the link to bowel and possibly stomach cancers, it’s not a bad idea to swap some red meat for white meat, beans or fish.

When it comes to breast cancer and red meat, the best we can say for now is that the jury’s still out. And we know it won’t mute the attention that dietary links to breast cancer receive – only more research can do that.

Nick

Reference

• Farvid M.S, et al. (2014). Dietary protein sources in early adulthood and breast cancer incidence: prospective cohort study, BMJ, (348:g3437) DOI: 10.1136/bmj.g3437

Beef fajita image from Flickr