They found that an increase of 5 pounds (11 pounds) in the fat mass of the whole body was associated with a 35% increase in the risk of this type of breast cancer. An increase of 5 kg of fat mass of the trunk was associated with an increase in risk of 56%.
The fat of the trunk is "defined by the fat contained in the bust apart from the head and the limbs", according to the study.
"The main takeaway is that having excess body fat, even when you have a normal body mass index, is associated with an increased risk of breast cancer," Dannenberg said.
A person's BMI is calculated through a formula that implies their height and weight; a "normal" BMI is considered between 18.5 and 24.9, according to the study.
The researchers also examined the blood data taken at the start 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 increase in insulin molecules.
The editorial authors also point out that other researchers have looked at the topic with different results and have noted that "these observations suggest that the components of metabolic health, rather than the presence of a complete metabolic syndrome, may contribute to the risk of cancer otherwise".
"I think it's a good step forward that takes us from looking at the body mass index as an indicator of obesity to really looking at the particular site of fat concentration in the body," said Anton-Culver, who he was not involved in the research.
The scientists knew that there was an association between obesity and cancer, but Anton-Culver states that the new study shifts 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 that association," he said.
Although Anton-Culver thinks the research is strong, he emphasized that it looked like only a specific cancer.
"I do not know if we look at the same problems with other tumors because the result, what will be, is specific for breast cancer?" she said. "We need to ask that question later, because obesity is a risk factor for other cancers."