"Women over 50 should lose weight to cut breast cancer risk," reports The Daily Telegraph.
Previous research has shown that the chances of getting breast cancer are higher for women who are overweight or obese. This may be because overweight women produce more oestrogen. Oestrogen can cause breast cells to grow, which could lead to cancer if the cells grow uncontrollably. However, there is not much evidence about whether losing weight, especially in later life, can lower this increased chance of getting breast cancer.
This study looked at more than 180,000 women aged 50 and over. It found that women who lost weight and kept it off for a period of about 10 years were less likely to be diagnosed with breast cancer, compared to women whose weight remained stable. Women who were overweight or obese before they lost weight saw their chances fall most. Greater weight loss was also linked to a bigger reduction in the chances of getting breast cancer.
The results only applied to women not taking hormone replacement therapy (HRT) to treat the symptoms of menopause.
The results suggest that women over 50 who are overweight or obese could reduce their chance of getting breast cancer by losing weight and keeping it off.
Weight is not the only thing that can increase the chance of getting breast cancer. Other things you can do to that may help prevent breast cancer include:
- regular exercise
- eating a healthy diet
- not drinking alcohol
Where did the story come from?
The researchers who carried out the study were from 18 universities or research centres across the US, Australia and Japan. The study was funded by the Breast Cancer Research Foundation and the National Cancer Institute in the US. It was published in the peer-reviewed Journal of the National Cancer Institute.
The Daily Telegraph and The Times both covered the study, with headlines suggesting that most or all women over 50 should lose weight. Neither made it clear that women of a healthy weight do not need to lose weight. What kind of research was this?
This study was an observational study that combined findings from 10 cohort studies in the US, Australia and Japan. Cohort studies are good ways to spot links between things that can increase the chance of getting breast cancer (called risk factors), such as weight gain or loss, and outcomes, such as breast cancer diagnosis. But they cannot tell us for definite whether a risk factor causes an outcome. Other things could be involved.
What did the research involve?
Researchers combined information from 10 big studies of women and their chance of getting breast cancer, which included women aged 50 and over. The researchers used studies that had recorded women's weight and height at least 3 times and gave information about breast cancer diagnosis in the years following the weight and height recordings.
Women were excluded if they had not reported their weight or height, or if the reported weight or height measurements were extremely unlikely. The final sample included 180,885 women. The researchers tracked any breast cancer diagnoses for these women from medical records or cancer registries.
Researchers looked at weight change between the first and second recording (an average 5 years apart). They categorised weight loss as:
- less than 2kg
- 2kg to 4.5kg
- 4.5kg to 9kg
- 9kg or more
They then looked at weight change between the second and third recording (again, an average 5 years apart). They categorised weight loss as:
- sustained – the weight stayed off
- partly sustained – some but not all the weight was regained
- not sustained – all the weight was regained
Researchers compared the chance of women with sustained, partly sustained or not sustained weight loss getting breast cancer with the chance of women who had not changed weight by at least 2kg over the 3 recording periods getting breast cancer. They adjusted the results to take account of a range of other things known to be linked to an increased chance of getting breast cancer, including:
- body mass index (BMI) at the start of the study
- physical activity levels at the start of the study
- use of HRT
What were the basic results?
The average BMI of women at study start was 25, which is the top end of the healthy BMI category. This increased to an average of 25.6 after 5 years, so just going into the overweight category. The researchers used the 20% of women who maintained a stable weight over the study period as the comparison group.
During the study follow-up period, 6,930 breast cancers were diagnosed. This represents 3.8% of the women in the study.
Women who lost weight and kept it off had a reduced chance of breast cancer compared to women of stable weight:
- 13% lower chance for women who lost 2kg to 4.5kg (hazard ratio (HR) 0.87, 95% confidence interval (CI) 0.77 to 0.99)
- 16% lower chance for women who lost 4.5kg to 9kg (HR 0.84, 95% CI 0.73 to 0.96)
- 26% lower chance for women who lost 9kg or more (HR 0.74, 95% CI 0.58 to 0.94)
Women who gained weight, and women who regained all the weight they had lost, had the same chance of getting breast cancer as women of stable weight. However, women who lost 9kg or more and regained some but not all of it still had a 23% lower chance of getting breast cancer, compared to women of stable weight (HR 0.77, 95% CI 0.62 to 0.97).
When the researchers looked more closely at the figures, they found the results were strongest for women who were overweight or obese at the start of the study. Unsurprisingly, few women who were a healthy weight at the start of the study lost more than 4.5kg. Those who did lose more than 4.5kg did not show a change in their chance of getting breast cancer.
How did the researchers interpret the results?
The researchers said: "We found that losing weight – and keeping it off – was associated with lower breast cancer risk for women aged 50 years and over." They added: "It is not too late to lower your risk of breast cancer if you have gained weight after age 50."
Many things affect a woman's chance of getting breast cancer. These include family history and genetics, hormone use, when or if she has children, alcohol use and being overweight or obese. This study supports current understanding and suggests that losing weight may help to reduce your chance of getting breast cancer (as well as many other types of cancer) if you are overweight or obese. However, losing weight does not affect the other risk factors for breast cancer, so it will not totally remove your chance.
Because this was an observational study, it is not possible to fully remove the influence of all these other health and lifestyle factors that may differ between women who lost weight or not. This means it still cannot tell us for definite whether weight loss caused the reduction in the chance of getting breast cancer, or why there was a lower chance. Other factors affecting both weight and the chance of getting breast cancer could be involved.
There are some other limitations. Although the numbers of women in the study are quite high, results for some of the individual groups are small. For example, only 19 women lost 2kg to 4.5kg and regained some of the lost weight. That's too small to assess these women's chance of getting breast cancer.
8 of the 10 studies providing data used women's self-reported weight and height measurements, rather than independently measured results. This could possibly introduce some inaccuracy, which might have affected the results.
Nevertheless, the study is overall a reminder that being overweight or obese raises the risk of breast cancer, along with other illnesses such as diabetes and cardiovascular disease. Find out more about how to lose weight if you need to do so.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Times, 18 December 2019
Daily Telegraph, 17 December 2019
Links to the science
Journal of the National Cancer Institute. Published online 17 December 2019