Low-fat diet reduces risk of breast cancer death

Friday May 25 2018

"A low-fat diet full of fruits and vegetables could reduce risk of breast cancer death," the Mail Online reports.

This headline is based on a long-term follow-up of a US trial conducted in the 1990s, which included nearly 50,000 postmenopausal women.

The women were assigned to continue their usual diet or to follow a diet low in fat and high in fruit and vegetables for 8 years.

During the trial, 1,764 women developed breast cancer. The low-fat diet didn't have a significant effect on the risk of breast cancer developing, but the researchers then followed the women who developed breast cancer for a further 10 years.

They found that the number of women who lived at least 10 years after receiving a diagnosis was better for women who'd followed the low-fat diet – 82%, compared with 78% on the usual diet.

Admittedly, this is only a small difference. But this well-conducted trial generally supports what's already understood about breast cancer.

A diet high in saturated fat is already an established risk factor for breast cancer, as well as other types of cancer, as is being overweight or obese.

Fruit and vegetables may have influenced the results – though it could be that they're just part of a healthy overall lifestyle.

This study adds to the overwhelming amount of evidence that a balanced diet and regular physical activity will reduce your risk of cancer, as well as many other long-term conditions.

Where does the study come from?

The study was conducted by researchers at City of Hope National Medical Center in Duarte, California, and various other institutions in the US.

Funding was provided by the National Heart, Lung, and Blood Institute, the National Institutes of Health, the Department of Health and Human Services, and the American Institute for Cancer Research.

It was published in the peer-reviewed journal JAMA Oncology.

The Mail's reporting was broadly accurate.

What kind of research was this?

This was a follow-up of a randomised controlled trial conducted in the 1990s, where postmenopausal women were assigned to a low-fat diet.

The original trial aimed to see if the low-fat diet reduced the risk of breast cancer.

This analysis looked at whether women who developed breast cancer lived longer if they followed the low-fat diet compared with those who didn't.

The trial benefitted from a randomised design, which is unusual for a dietary study involving so many women.

Such studies normally have to be observational, as you can't normally randomise thousands of people to a follow a particular diet.

Randomising the women to the different groups meant that any confounding factors that could have potentially influenced the results would at least be balanced between the 2 groups.

What did the researchers do?

The Women's Health Initiative (WHI) Dietary Modification (DM) trial randomised 48,835 postmenopausal women to either a low-fat or normal diet.

The women were matched for underlying cancer risk factors and received mammography screening every 2 years, or more often if they were on hormone replacement.

The trial was conducted across 40 US centres between 1993 and 1998.

The low-fat diet aimed to reduce fat to 20% of total energy intake. Participants were also encouraged to increase their intake of fruit, vegetables and grains.

The diets were guided by nutritionists, who led 18 group sessions a year combined with an individual session.

Each participant was set her own fat intake goal and completed food frequency questionnaires during the study.

Women who developed breast cancer during the trial continued with the low-fat diet, with closer nutritional guidance. The whole dietary intervention lasted 8 years.

A total of 1,764 women developed breast cancer during the trial, but the low-fat diet did not have a significant effect on the risk of breast cancer developing.

Women with breast cancer were then followed up for an average of 11 further years to see if the diet had an effect on cancer survival.

What were the basic results?

There were 516 deaths among the 1,764 women with breast cancer. 82% of women in the low-fat diet group survived to 10 years compared with 78% of women in the usual-diet group.

This meant that the low-fat diet reduced risk of death by 22% (hazard ratio 0.78, 95% confidence interval 0.65 to 0.94).

Most deaths resulted from breast cancer, as would be expected, and 91 deaths were from cardiovascular disease (CVD). There were fewer deaths from both breast cancer and CVD in the low-fat group.

What did the researchers conclude?

The researchers concluded that, "In women who received a diagnosis of breast cancer during the dietary intervention period, those in the dietary group had increased overall survival.

"The increase is due, in part, to better survival from several causes of death."

Conclusion

This study has many strengths. It's unusual to find a dietary trial that's included thousands of participants, provided them with a carefully guided dietary intervention for 8 years, and followed them up closely.

In general, the findings support what we already know about breast cancer risk factors. A diet high in saturated fats is already a recognised risk factor for breast cancer.

Unfortunately, this study didn't find such convincing evidence that the low-fat diet was protective against the cancer developing in the first place, as the difference fell short of statistical significance.

But women who continued with the low-fat diet after they developed cancer saw a significant difference to their overall survival – although it must be acknowledged that at only 4% absolute difference in survival, this was fairly small.

The low-fat diet also included more fruit and vegetables and wholegrains.

But we don't know whether fruit and veg have a direct effect on risk of breast cancer or survival, or whether the lower risk is more down to eating a generally healthier diet lower in saturated fats.

It's also worth bearing in mind that women following low-fat diets may be less likely to be overweight or obese – another established risk factor for breast cancer.

Many risk factors for cancer are unfortunately out of women's control, such as hereditary and hormonal factors.

But leading a healthy lifestyle can help reduce the risk of breast cancer and many other chronic diseases.

This means:

  • aiming for a healthy weight
  • eating a diet low in saturated fats, salt and sugar, and high in fruit, vegetables and fibre
  • taking regular exercise
  • not smoking
  • moderating how much alcohol you drink

Analysis by Bazian
Edited by NHS Choices