Friday, October 26, 2007

Puzzling news about weight and breast cancer risk

startled woman
A higher risk, linked to weight gain but not to weight in a very specially selected group of women, may not be a risk for women in general.

In a study on adiposity, adult weight change, and postmenopausal breast cancer risk, the following results have been found:

  • no link between weight at any age and breast cancer risk before menopause because these cases had been excluded from the study;
  • a lower breast cancer risk in women with a higher weight at age 18;
  • an equal breast cancer risk in women of any weight who have been weight-stable during their adult life;
  • an equal breast cancer risk in women who gained weight, even as much as 40 kg (6 stone) since age 18 and have taken hormones after menopause;
  • a higher, up to double breast cancer risk in women who gained the same weight but did not take hormones;
  • no risk change in women who lost weight in adult age.
Study details: More than three million questionnaires have been mailed to selected members of AARP, only about four percent of them responded, and every fifth of these has been excluded for various reasons. The remaining women, cancer-free at the beginning of the survey, have been asked questions about their weight and height at ages 18, 35, 50 and at the current age. The accuracy of this self-declaration has not been checked. The women have been followed up over the next five years in order to see if they developed breast cancer. This has been the case in more than two thousand women. The relative risks have been calculated comparing these cases with those not affected by cancer in different classes of weights and weight gains in different periods of life.

Nagging questions

The female hormone estrogen is considered one of the known risk factors for breast cancer. The longer a woman is exposed to it, for instance by an early beginning of puberty, the higher is her breast cancer risk in general. A number of studies show a higher breast cancer risk with hormone replacement therapy after menopause. Yet in this AARP study the results are opposite, indicating a lower risk for women taking hormones.

This is not easy to explain by biological mechanisms. Thus, it may be the result of bias in this study, caused by the specific selection of the cases and the controls or by the inaccurate recall of heights and weights long times ago. If, for instance, women with a lower education and a lower income would be less accurate in their weight and height estimates of early ages, higher weight differences may occur in this group. Low education and low income are known risk factors for many diseases, breast cancer included.

What if it were true?

It has not been proven without doubt, but let's be too generous and assume for the moment that an elderly woman may be at higher risk for breast cancer if she had been very thin when young, then has gained a lot of weight and does not take hormones. There is no use to tell her this bad news because weight loss will not lower her breast cancer risk, as the AARP study has shown. Best would be telling her not to worry about breast cancer but to be happy with her weight, and that a higher weight has been shown to prolong life in old age.

And what about warning young women from weight gain? Well, this is exactly what is happening these days in the world-wide war on obesity, with a number of unwanted side effects, see my earlier posts about distressed victims, dieting messages and youth suicide.

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