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Ovarian cancer and a low-fat diet

Wednesday 17 October 2007

The risk of ovarian cancer could be cut by 40% if the fat content of your diet is reduced by a third, reported the Daily Mail on October 11. A low fat diet “could almost halve the risk of ovarian cancer,” the newspaper said.

The story is based on a study that compared the ovarian cancer rates of post-menopausal women on a low-fat diet compared with those on a regular diet. Although the study was large, the link between the fat content of the diet and the risk of developing ovarian cancer remains unproven. A lower fat diet with a higher fruit and vegetable intake seems a sensible health choice, regardless of whether it reduces ovarian cancer risk.

Where did the story come from?

This research was carried out by Ross L. Prentice and colleagues of the Fred Hutchinson Cancer Research Centre, Seattle, and various other centres across the US. The study was funded by National Heart Lung and Blood Institute and was published in the peer-reviewed medical journal Journal of the National Cancer Institute.

What kind of scientific study was this?

This was a randomised controlled trial with the aim of investigating how the development of breast and colorectal cancer were affected by a low fat diet compared with a normal diet. The study also looked at other cancers that may be affected.

The researchers of the Women’s Health Initiative Dietary Modification trial randomly assigned 48,835 postmenopausal women between 1993 and 1998 to either a diet (dietary modification) group or a control group with a  normal diet. 40% of the women were assigned to the diet group and they received group behavioural modification sessions at regular intervals. These sessions trained them about nutrition and educated them about the diet, which aimed to reduce the fat content to 20% of the daily total energy intake and to increase the consumption of fruit and vegetables to more than five servings per day and grains to greater than six servings per day. The women were followed up for an average of 8.1 years.

The women in the diet group had to recall their diet over the past four days or past 24 hours at start of the study, after one year, and then about every three years thereafter. Twice yearly, the women were asked about the development of any cancer and any reports were verified using medical records or pathology reports. The researchers report that women in this trial could also choose to take part in other randomised trials at the same time, which were investigating hormone replacement therapy and calcium and vitamin D supplementation.

What were the results of the study?

The researchers found that there were no differences between the diet and control groups in the rates of any cancer, with the exception of ovarian cancer. However, although the rate of ovarian cancer in the diet group was less, further statistical calculations demonstrated that the risk for developing ovarian cancer was not significantly reduced by being in the diet group.

They then divided the analysis into two periods to see if this made a difference to the results. For the first four years of the study still found no significant difference in the risk of ovarian cancer in the diet group, but when they looked at the last 4.1 years only, they found that the risk of ovarian cancer was reduced by 40% by being in the diet group.

The researchers looked for a difference between the groups in the rates of invasive ovarian cancer and found none. They then looked at whether this was affected by analysing the dietary intake in more detail (e.g. looking at the number of servings of different food types individually) and found there only to be a reduced rate when comparing the groups according to fat intake.

What interpretations did the researchers draw from these results?

The researchers concluded that having a low fat diet may reduce the risk of ovarian cancer amongst post-menopausal women.

What does the NHS Knowledge Service make of this study?

Although this study has examined a very large number of women, it provides no convincing evidence that there is any link between ovarian cancer and fat intake.

  • The researchers were looking to find differences in the rates of any types of cancer between the diet and control group. Overall, no increase in the risk of developing cancer was found, there was also no increase in risk in the main types of cancer that the researchers were interested in (breast and colorectal cancer). The researchers then went on to examine the data on other types of cancer, and found a trend towards a difference in ovarian cancer. Researchers then investigated this finding using multiple tests. The use of multiple tests, the fact that the overall difference in risk of ovarian cancer with diet was not significant, and that this was not the main outcome the researchers intended to look at, reduces the reliability of these results.  
  • It is not certain that the women in the diet group followed a strict dietary plan: their food intakes were only monitored over a very brief period during the whole length of the study. Likewise, there is no way of knowing that the women in the “usual” diet group had an unhealthy higher fat diet throughout the study. 

A lower fat diet with higher fruit and vegetable intake is a sensible health choice, but it cannot be concluded from this study whether it has any effect on ovarian cancer risk.

Sir Muir Gray adds...

There are already enough reasons to eat a low fat and five a day diet.

Analysis by Bazian
Edited by NHS Website

Links to the headlines

Low-fat diet cuts ovarian cancer risk: study.

Reuters, 9 October 2007

Links to the science

Prentice RL, Thomson CA, Caan B, et al.

Low-Fat Dietary Pattern and Cancer Incidence in the Women's Health Initiative Dietary Modification Randomized Controlled Trial.

J Natl Cancer Inst 2007; Oct 9 [Epub ahead of print]