Postmenopausal women who avoid becoming obese are cutting their risk of breast cancer by about a fifth, according to a new study by Cancer Research UK scientists.

Results published today in the JNCI1 also fuel the theory that sex hormones, particularly oestrogens, are the root cause behind how obesity increases breast cancer risk. A form of the hormone, called oestradiol, was especially associated with increased risk.

The researchers highlight obesity as an important risk factor that women can control through a sensible diet and exercise, unlike factors over which they have no control, such as a family history of the disease.

The study looked at eight separate investigations around the world and compared Body Mass Index (BMI) and sex hormone levels in 624 breast cancer patients and 1669 healthy women.

BMI indicates if a person has a healthy weight by expressing their weight in kilograms relative to their height per metre squared (kg/m2). The researchers split the women into five BMI groups – Less than 22.5 kg/m2, 22.5-24.9 kg/m2, 25.0-27.4 kg/m2, 27.5-29.9 kg/m2 and over 30 kg/m2. The healthy BMI range is between 18.5 and 25. A BMI between 25 and 30 is overweight and over 30 is obese.

Cancer Research UK scientists found that breast cancer risk is 18 per cent higher for obese women compared with those with a healthy weight. This increase was seen over the weight difference between the upper level of healthy weight (25 kg/m2) and lower level of obese weight (30 kg/m2).

Lead researcher Dr Tim Key of the Cancer Research UK Epidemiology Unit at Oxford University, says: “Women’s risk is affected by many fixed factors – a family history of the disease, the number of children they have, the age they have their children, when they start their periods and when they stop.

“But obesity is something that women have a level of control over. Put simply, maintaining a healthy weight avoids extra breast cancer risk for these women.”

Obesity causes an increase of oestrogens in the body. Earlier studies suggest that this oestrogen build-up may be linked to the increased breast cancer risk in obese postmenopausal women.

The research team found that as BMI climbed so did oestrogen levels and breast cancer risk. The results also showed that a type of oestrogen called oestradiol was particularly associated with increased risk.

Dr Key says: “We know that hormonal factors are central to the development of breast cancer. This study helps us to better understand the role obesity and certain hormones play in the mechanism that increases risk of the disease.

“Obesity may be the principal contributing factor for a substantial number of breast cancer cases. And with the obesity rate climbing it is imperative that we understand its dangers.”

Dr Lesley Walker, Director of Cancer Information for Cancer Research UK, adds: “Breast cancer risk increases with age, and obesity after the menopause further adds to this risk while causing a wide range of other health problems.

“As a mature woman and working mother I know how difficult it can be to eat healthily and avoid piling on the pounds, but this study implies that by maintaining a healthy weight through exercise and a balanced diet women can avoid extra breast cancer risk.”


  1. Journal of the National Cancer Institute95 (15)


To work out BMI, find a person’s weight in kilograms and height in metres. Multiply height by itself to square it. Then divide your weight by your height squared.

– Example: Weight: 75kg / Height 1.80m

1.80 x 1.80 = 3.24

75 divided by 3.24 = 23.1

BMI would be 23.1, which is within the healthy range.

The healthy BMI range is between 18.5 and 25. If BMI is:

  • Under 18.5, you are underweight
  • Between 25 and 30 you are overweight
  • Between 30 and 40 you are obese
  • Over 40 you are very obese

The eight studies included five in the US, and projects in Britain, Italy and Japan.

A woman’s absolute risk of breast cancer is related to age:

  • by age 30 : 1 in 1900;
  • by age 40 : 1 in 200
  • by age 50 : 1 in 50
  • by age 60 : 1 in 23
  • by age 70 : 1 in 15
  • Overall risk over a lifetime: 1 in 9

Other factors that effect breast cancer risk:

  • Having children reduces risk and being younger when having children also reduces risk.
  • The longer a woman breastfeeds, the lower her risk.
  • Starting periods at a young age and a late menopause increases risk.
  • The pill and HRT increase risk. Risk returns to normal once you stop taking them. More research on this is under way.
  • Drinking alcohol slightly increases risk. More research on this is under way.
  • A history of breast cancer in the family puts you at slightly increased risk. Most women with one or two affected relatives will never get breast cancer though. Your risk is greater if a close relative had breast cancer before the age of 50, or two or more close relatives have been affected. If you have a strong family history and are worried then discuss this with your GP. A GP may refer you to a breast care unit or genetics clinic.

Visit our website CancerHelp UK for clear, easy to understand information about cancer and cancer treatments.