Few studies have specifically
investigated underweight, but one from Malaysia (also mostly of
premenopausal women) reported that women with a BMI of less
than 18.5 kg/m2 had more breast cancer, as did those who were
overweight or obese. It is of course possible that the association we
see with underweight amongst this group of women with prevalent
breast cancers reflects weight loss associated with breast
cancer treatment or progression.