Résumé
Being overweight appears to be associated with a higher risk of post-menopausal breast cancer in most studies. Although the relative risk of breast cancer related to Quetelet's index is generally weak (range 1.1–1.9 in the major cohort studies), some studies have found that timing of weight gain and body fat distribution could be more significant factors of an increased risk. Conversely, obesity appears to be slightly correlated with a decreased risk of breast cancer in pre-menopausal women. These contrasting effects of excess weight on breast cancer incidence according to menopausal status, and the lack of a strong association between obesity and breast cancer in some studies, could be due to a number of confounding factors. Among these factors, age, country of origin, family history, alcohol consumption, nutrition, and hormonal treatment could account for the differences observed, and are reviewed in the present study. Obesity and central fat distribution are believed to act through endocrine intermediates such as hyperinsulinaemia and steroid hormones. Since obesity is one of the few breast cancer risk factors that can be modified, the influence of weight loss, particularly in women at high risk, deserves to be further investigated.