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Today's study was about weight loss, not breast cancer

The link between diet and cancer is in the news again

Today, a story appeared in the press claiming that following a “strict diet” for two days a week could “cut breast cancer risk by 40%”.

We think this headline is misleading. In fact, when you actually look at the research the story is based on there simply isn’t the evidence for such a statement.

The research itself was published in the Journal of Obesity, and it wasn’t even about breast cancer – instead it focused on weight loss.

But a press release accompanying the research did choose to make an intriguing leap of logic and heavily link a decent piece of research about diet to breast cancer (during breast cancer awareness month. Strange that.)

Let’s have a look at the study – and the press release – in more detail.

What was the study actually about?

In the study, researchers gave 100 women one of two different diets to follow for six months, and at the end of the six month period, they measured how effective the diets had been for weight loss.

One diet involved eating 1,500 calories each day, and the other was an ‘intermittent’ diet of 650 calories for two days, followed by a ‘normal’ diet for the rest of the week.

The main finding of the study was that the two diets were roughly equivalent in terms of how much weight the women lost.

But it didn’t look at breast cancer

As a study looking into effective methods of weight loss, this is a solid, well-designed bit of research. But it’s very difficult to see how the specific claims about reducing breast cancer risk can be made, based on the paper alone: the researchers did not look at how many women developed breast cancer during the study.

However, over the course of the study, the researchers did measure the levels of various hormones in the women’s blood, and found that the levels of some of them did vary slightly between the two groups..And it’s true that some of the hormones they measured have been linked to breast cancer risk by other studies.

But these links are complicated and still not fully understood, so noticing any changes in these hormones over a six month period doesn’t mean a real-world effect on cancer risk.

And it’s a huge leap to say that small changes in hormone levels during six months of a particular diet can affect a woman’s risk of the disease.

In any case, it would actually be impossible to measure any effect on breast cancer risk over the course of this study – for two reasons:

  • The study was much too small to look at cancer risk. Usually, for a prospective study into a cancer as common as breast cancer, you’d expect many thousands of women to participate, not just one hundred.
  • The researchers only followed up the women for six months. Again, in a good study looking at cancer risk, the follow-up would need to be several years at least.

So the study can’t be used to draw any conclusions about whether these diets affected breast cancer risk.

What did the press release say?

The press release begins:

INTERMITTENT DIETING CAN HELP WOMEN AT HIGH RISK OF BREAST CANCER

Research […] published today in the International Journal of Obesity – has suggested that women at increased risk of breast cancer who follow a two-day intermittent diet can successfully lose significant weight.

The findings are seen as significant as there is strong evidence to suggest that excess body fat is an important risk factor for breast cancer.

None of this is untrue. But as we saw above, the research did not focus on whether the women on the study – some of whom did have a higher than average risk of breast cancer – went on to develop the disease. So it seems singularly unhelpful to frame a diet study in this way – and guaranteed to lead to the sort of reporting we saw in the mainstream media.

As we said, the research showed that there was no difference between the two types of diet in terms of weight loss; and it’s impossible to make any inferences about breast cancer risk from a six-month study of a hundred women.

A quote in the press release says:

A previous study … suggested that losing weight can reduce breast cancer risk by up to 40 per cent, which is why this new study could have implications for breast cancer.

Again, this is quite a leap to make, given the actual data in the study.

Weight and breast cancer

There is concrete evidence that being overweight after the menopause increases breast cancer risk. Women who are overweight after the menopause have about a 10-20 per cent higher breast cancer risk than women with a healthy weight, and obese women have a 30 per cent higher risk. This evidence has come from large studies that have been designed to measure effects on cancer risk, like the Million Women Study, which Cancer Research UK helps to fund.

But this is a far cry from saying that following an ‘intermittent’ diet could reduce breast cancer risk more than a normal one.

Living healthily

We do know that lifestyle has quite a big impact on breast cancer risk. Keeping a healthy weight, being physically active and drinking less alcohol are all great ways to reduce the risk of the disease. And things like how many children a woman has and whether she breastfeeds also have an impact.

And there’s much more to learn about these links – which is why we, and other charities (including those that supported this study) are keen to know more about the links between lifestyle and breast cancer.

But the way this study has been promoted, and subsequently reported, has been been misleading and inaccurate. In short, this was a study about dieting and weight loss, and not about breast cancer at all. And it can’t be used to conclude anything about breast cancer risk, nor about how women should or shouldn’t diet.

Jess and Henry


Reference

Harvie, M. et al (2010). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women International Journal of Obesity DOI: 10.1038/ijo.2010.171

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