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Could 5:2 diet play a role in preventing breast cancer?

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“Women who follow the 5:2 diet ‘could reduce their risk of breast cancer’,’’ the Mail Online reports.

“Women who follow the 5:2 diet ‘could reduce their risk of breast cancer’,’’ the Mail Online reports.

A small study found some women who followed the diet experienced breast cell changes thought to be protective againstbreast cancer. But the study was too small and too short to prove this is definitely the case.

The 5:2 diet is based on the idea that you eat a normal healthy diet for five days of the week and a fasting diet – recommendations are usually around 500 calories for women and 600 for men – for the other two days.

The study involved 24 women who were overweight or obese, aged 35 to 45, free of cancer or diabetes, and with a higher than average breast cancer risk.

The women were told to drop their calorie intake by 75% on two consecutive days a week and follow a Mediterranean diet for the remaining five.

The women lost weight and body fat – about 5% for both – and registered positive changes in the way their bodies were handling energy, fat and insulin.

Around half the women showed biochemical changes in their breast tissue that was interpreted as potentially related to breast cancer risk.

These changes fall a long way from proving that a 5:2 diet would reduce breast cancer risk in all women, although sustained weight loss is known to reduce breast cancer risk.

For more information, read the Behind the Headlines special report on the 5:2 diet.

Where did the story come from?

The study was led by researchers from the Genesis Breast Cancer Prevention Centre, University Hospital of South Manchester NHS Foundation Trust.

It was funded by Prevent Breast Cancer and Breast Cancer Now, both charities. The authors declared no conflicts of interest.

Published in the peer-reviewed Breast Cancer Research, the study isopen-access, so it is free to view online and download.

The Mail Online covered the study facts accurately, but did not emphasise its many limitations – for example, the perils of generalising findings from around 20 women to all women with breast cancer. As such, its headline is potentially misleading.

What kind of research was this?

This small cohort study investigated the effects of an intermittent calorie-restricted diet on breast cancer risk.

Breast cancer is the most common type of cancer in the UK. But if it’s treated early enough, it can be prevented from spreading to other parts of the body and the chances of survival are high.

Studies show that losing weight and restricting your energy intake are linked to lower breast cancer risk, but the specific effects of periodic or intermittent calorie restriction are not known.

This study wanted to test whether women on an intermittent diet would show any biochemical signs of a reduction in breast cancer risk.

A large study measuring calorie restriction over the long term, looking for links to diagnosed cases of breast cancer, would be a more reliable way of investigating this topic.

These sorts of studies can be time-consuming and expensive to run, though, so small studies like this also have their place and aim to make early inroads into the area.  

What did the research involve?

A small group of women followed a two-day-a-week calorie-restricted diet to see how it affected biological processes potentially related to breast cancer risk.

More than 800 women were invited to participate. Most ignored the invite and others were later excluded as ineligible, leaving a small select group of 24 who took part from start to finish.

The 24 recruits were obese or overweight women aged 35 to 45 with a higher than average risk of breast cancer (more than 17% lifetime risk) who were under surveillance by a Manchester genetic counselling clinic.

Only women who reported having low activity levels (less than 40 minutes of moderate activity a week), who had not had a breast scan in the last year, and who had a pre-specified breast density were allowed to take part. Women with conditions like diabetes or cancer were excluded.

The diet resembled a 5:2 diet, where calories are restricted on two consecutive days a week.

The researchers worked out how many calories each woman needed each day, and asked them to reduce them by 75% on two consecutive diet days a week over the period of one menstrual cycle – an average of 29 days in this group.

On calorie-restricted days the women had to get their 5 A DAY from 80g of vegetables and one 80g portion of fruit, as well as six portions of low-fat dairy produce, such as two pints of semi-skimmed milk.

For the other five days they followed a Mediterranean-style diet– 45% energy coming from low glycaemic index (GI) carbohydrates, 30% from fat, and 25% from protein. 

GI shows how quickly each food affects your blood sugar (glucose) level when carbohydrates are eaten.

A dietitian checked the women’s compliance to the diet by reading food diaries they kept to log what they ate and drank.

Blood, urine, body fat and breast tissue samples were analysed before, during and at the end of the diet to monitor changes in body composition and breast cancer risk, including changes at a genetic level.

Four of the 24 women didn’t have any genetic data available, so these findings relate to just 20 women.

Women were told to remain inactive, the logic being to keep physical activity constant so that any breast cancer risk changes might be attributed to changes in diet only. 

What were the basic results?

Women followed the diet with good compliance for an average of 29 days, achieving the 75% calorie reductions on two consecutive fast days as planned.

However, this influenced what happened on the next five days. There was a carryover effect whereby the women continued to reduce their calorie intake on five days, where it should have bounced back up to 100%.

They averaged 38% lower than this, meaning that over the seven-day week they were actually reducing their calories by around 45%, much more than the target.

Unsurprisingly, the women lost weight and body fat – averaging around 5% reductions in both. On the two low-calorie days, their bodies were much better able to deal with blood sugar efficiently. This continued on the other five days, although to a lesser extent.

The blood analyses showed 527 biochemical molecules significantly changed during the two calorie-restricted days – and the vast majority remained changed after five days of normal eating.

About half the women (11, 55%) showed signs of down-regulation to biochemical pathways involved in cell metabolism, making fats, and the way the body makes and metabolises energy sources.

Three women showed signs of changes to breast cancer-related genes involved in breast cell differentiation – the process a cell goes through to become specialised in a function or tissue – and collagen. Most women didn’t have these changes.

How did the researchers interpret the results?

The authors’ conclusions were unspectacular and accurate: “The transcriptional response to IER [intermittent energy restriction] is variable in breast tissue, which was not reflected in the systemic response, which occurred in all subjects.”

They went on to say: “The mechanisms of breast responsiveness/non-responsiveness require further investigation.” 


This study shows that intermittent calorie restriction has an immediate and fluctuating effect on our bodies that varies from person to person.

For the 24 inactive, overweight or obese, middle-aged women in this study, about half showed signs of genetic and biochemical changes to processes that might loosely be linked to breast cancer risk.

A very small number (three) had changes more directly associated with breast cells processes, but, again, loosely linked to breast cancer risk.

These links weren’t consistent, clear or assessed over a long enough period to really know how the 5:2 diet or similar might affect breast cancer risk.

This means the study’s findings don’t support the Mail Online’s headline that, “Women who follow the 5:2 diet ‘could reduce their risk of breast cancer’.”

Today’s media coverage also implies that the tentative findings of between 3 and 11 women – those with the least vague chance of being linked to breast cancer – applied to most women with breast cancer.

If you pluck three people from a crowd of, say, 50,000 (the number of new invasive breast cancer cases each year in the UK in 2013) and try to make generalisations about specific parts of these people’s lives, most people would clearly see you’re more likely to get it wrong than right.

The same applies here. The study sample was small, and definitely not large enough to be able to make solid statements about breast cancer in general.

As the causes of breast cancer aren’t fully understood, it’s not known if it can be prevented altogether. 

Regular exercise and a healthy, balanced diet are recommended for all women because they can help prevent many conditions, includingheart diseasediabetes and many forms of cancer.

Studies have looked at the link between breast cancer and diet, and although there are no definite conclusions, there are benefits for women who maintain a healthy weight, exercise regularly, and who have a low intake of saturated fat and alcohol.


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