Reacción a "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.