Study confirms that a higher body mass index in childhood is linked with lower risk of breast cancer

The study uses robust genetic epidemiology methods, including GWAS, meta-analysis, and Mendelian randomisation, although the authors themselves acknowledge methodological limitations, such as possible statistical artefacts (winner's curse, age selection bias) and the use of BMI as an imperfect marker of adiposity, even though it is the easiest and most commonly used in clinical practice, which requires cautious interpretation of the results.

The findings are consistent with previous evidence identifying prepubertal adiposity as a protective factor against breast cancer. The main contribution of the study is the specific analysis of the period between menarche and age 40 (previous studies include women at later stages, from age 18 onwards), where a protective effect of genetically determined BMI is initially observed. However, this effect is attenuated when adjusted for prepubertal adiposity, suggesting that early exposure to excess adipose tissue is the main determinant of risk, and that subsequent BMI largely reflects that biological history.

The study does not justify promoting or accepting excess weight in early life, as obesity carries multiple health risks. This is a study of population associations, not individual risk determinants, and recommendations should focus on healthy habits from a very early age, including nutritional education and physical exercise to maintain a normal weight throughout life.

This study reveals that a high BMI in nulliparous women between menarche and age 40 reduces the risk of breast cancer, with protective effects that diminish when adjusted for prepubertal adiposity. These results seem to support the idea that obesity at an early age does not have the same harmful effect as it does in adulthood and, above all, during menopause.

The large sample of 56,863 women and the methodology used (albeit with its limitations) confirm that there is a critical window at an early age when breast cancer prevention can be applied, highlighting the genetic influence of BMI in the early reproductive stages. However, these results must be taken into consideration due to genetic variability and possible biases in BMI.

Nevertheless, I believe that recommending “young women to gain weight” to avoid this tumour should be taken with great caution. In any case, it does tell us that breast cancer has multiple factors to consider and that fat as an endocrine organ does not seem to be the same depending on age and menopausal status, and that this effect seems to be somewhat more pronounced in RE+ [oestrogen receptor-positive] tumours.