A new US study suggests that weight gain throughout adulthood is linked to an increased risk of developing breast cancer after the menopause for women who do not undergo menopausal hormone therapy (MHT).

The prospective study is published in the 22 October issue of the Archives of Internal Medicine and is the work of researchers at the National Cancer Institute (NCI), part of the National Institutes of Health, based in Bethesda, Maryland, and the AARP (American Association of Retired Persons) based in Washington, DC.

Scientists already knew that being obese or overweight increased the chance of a woman having breast cancer after the menopause. But the relationship between the timing and the amount of weight gain in adulthood had not been investigated before.

Dr Jiyoung Ahn, researcher at the Division of Cancer Epidemiology and Genetics at the NCI, and colleagues reviewed data on 99,039 women who took part in the National Institutes of Health – AARP Diet and Health Study. Body measurements and other personal history data in the study were obtained through self reports in 1996, while breast cancer incidence was assessed from cancer registers, which showed 2111 cases up to the year 2000.

The results showed that:

  • BMI at the age of 50 and 35 years, and waist-hip ratios were linked with greater breast cancer risk, particularly for women not undergoing menopausal hormone therapy (MHT).
  • Weight gained from the age of 18 to the current age, between 18 and 35, between 35 and 50, and between age 50 and the current age was consistently linked with raised breast cancer risk among non MHT users, but not in current MHT users.
  • The link between adult weight change and breast cancer risk was stronger in women who started their periods later than it was for those who started them earlier in life.
  • Among MHT nonusers, the links between BMI and weight change in adulthood were stronger for advanced breast cancers than for nonadvanced breast cancers.

BMI stands for Body Mass Index, which is a person’s weight in kilograms divided by the square of their height in metres.

It would seem that the timing of when the weight gain occurred during adulthood was important too, especially if the weight gain occurred after 35. For instance women who put on more than 66 pounds (30 kilos) between the ages of 18 and 35 had a 65 per cent higher risk, but those who increased their weight by the same amount between the ages of 35 to 50 had a 223 per cent higher risk of developing breast cancer after the menopause.

It is important to realise that the study is not suggesting that only non users of MHT are at an absolute higher risk of developing postmenopausal breast cancer. What it is saying is that the sensitivity to weight gain is more visible among MHT non users. MHT brings its own risks to developing breast cancer and unless you look underneath the “hood” you don’t see how MHT hides the effect of weight gain.

Speculating on their findings the researchers said it could be because adipose tissue (the fat tissue) produces estrogen, the same hormone that is produced in ovaries in premenopausal women. But as the ovaries begin to shut down, the effect of estrogen in the fat tissue becomes relatively stronger, so the more fat there is, the more likely that estrogen levels will be maintained later in adulthood and through the menopause. Estrogen is the hormone that helps certain breast cancers to develop.

The results appear to confirm that putting more estrogen into the body increases the risk of breast cancer for older women, whether that estrogen comes from hormone therapy or from having too much adipose tissue, i.e. being overweight. The postmenopausal women at lowest risk seem to be the ones with the lowest levels of estrogen, i.e. those who stay lean and do not use MHT.

“Adiposity, Adult Weight Change, and Postmenopausal Breast Cancer Risk.”
Jiyoung Ahn; Arthur Schatzkin; James V. Lacey Jr; Demetrius Albanes; Rachel Ballard-Barbash; Kenneth F. Adams; Victor Kipnis; Traci Mouw; Albert R. Hollenbeck; Michael F. Leitzmann.
Arch Intern Med, Oct 2007; 167 (19): pp 2091 – 2102.

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Written by: Catharine Paddock