They found that an increase of 5 kg (11 pounds) in body fat mass was associated with an increased risk of 35% of this type of breast cancer. A 5-kilogram increase in fat mass in the trunk was associated with a 56% increase in risk.
The fat in the trunk is "defined by the fat contained in the trunk, in addition to the head and limbs," according to the study.
"The main conclusion is that having excess body fat, even when you have a normal body mass index, is associated with an increased risk of breast cancer," said Dannenberg.
A person's BMI is calculated by a formula involving its height and weight; a "normal" BMI is considered between 18.5 and 24.9, according to the study.
The researchers also analyzed blood data obtained at the beginning of the Women's Health Initiative for other factors that are known to play a role in the development of breast cancer, such as the elevation of insulin molecules.
Editorial authors also point out that other researchers have examined the subject with different results and note that "these observations suggest that components of metabolic health, rather than the presence of the complete metabolic syndrome, may contribute to the risk of breast cancer."
"I think it's a good step forward that leads us to look at BMI as an indicator of obesity to actually look at the specific location of fat concentration in the body," said Anton-Culver, who was not involved in the research.
Scientists knew there was an association between obesity and cancer, but Anton-Culver says the new study takes the research beyond that general association.
"They say correctly in the summary, that obesity is associated with breast cancer, but more specifically, obesity around the abdomen is more specific to this association," she said.
Although Anton-Culver thinks the research is strong, she pointed out that she only looked at a specific cancer.
"I do not know if we look at the same problems with other cancers as the result, what will it be, is specific for breast cancer?" she said. "We need to ask that question next, because obesity is a risk factor for other cancers."