Weight gain has already been associated with several illnesses, including diabetes, heart disease and even insomnia. Now, according to new research published Monday in the journal Archives of Internal Medicine, it has been linked to another health problem: breast cancer.
Researchers at the National Cancer Institute in Bethesda, Md., analyzed data from 99,039 women who were part of a diet and health study of the National Institutes of Health. The women reported their body measurements, as well as their height and weight at ages 18, 35 and 50.
The researchers then calculated each woman's body mass index (BMI), which put them into the categories of underweight, normal weight or overweight. They also tracked which of the women developed breast cancer.
For women not taking hormonal therapy after menopause, gaining significant weight — between 44 and 66 pounds — at any point after age 18 can lead to an increased risk of breast cancer.
In addition, women who were considered normal weight at 18, but fell into the overweight or obese categories later in life had a 50 percent larger risk of developing breast cancer, compared to women whose weight stayed steady throughout their lives.
"Our study is the first to suggest that weight gain at any stage of life was associated with breast cancer risk," said Jiyoung Ahn, a researcher in the division of cancer epidemiology and genetics at the National Cancer Institute and one of the study's authors.
"Although obesity is a well-known risk factor of breast cancer among postmenopausal women, it was unclear whether weight gain during a particular stage of life is important or weight gain during throughout adulthood is important in developing breast cancer."
Doctors have been looking with closer scrutiny at the effects of weight on risk factors in recent years.
"Weight and weight gain have been associated with breast and other cancers in prior studies," said Dr. Clifford Hudis, chief of the breast cancer medicine service at Memorial Sloan-Kettering Cancer Center in New York. "This adds important information because it is a large study and is not limited to a narrow age range."
Other experts agree that exploring this topic is becoming more important to the study of cancer in general.
"There is increasing interest in lifestyles and their impact of cancer risk," said Dr. Anne Rosenberg, a breast surgeon at Thomas Jefferson University Hospital in Philadelphia.
"Obesity has been related to breast cancer, and present research efforts are directed toward elucidating how," said Rosenberg. "This study addresses the issue of when the weight change is a factor and if it is reversible."
The study used BMI as a way to track peoples' weight, but it only showed a glimpse of a woman's size at different points in her life — 18, 35 and 50. According to some experts, this method could be problematic.
"It is like a snapshot rather than a movie," said Hudis. "In this study it was weight change that mattered, independent of current BMI."
Other researchers point out alternative methods to determine someone's body structure. "There are better, more precise indicators [than BMI]," said Eva Surmacz, director of the obesity and cancer program at Temple University in Philadelphia.
"Hip-to-waist ratio (HWR) can approximate the amount of upper body fat [which is believed to be more connected with breast cancer than fat accumulated in hips and buttocks]," she said. "However, practical reasons, HWR would not be that easy for patients to measure and report."
Because weight is something that people can control to some extent, the researchers believe this study may help women get on a healthy track.
"We plan to examine potential genetic factors that may play a role in the relationship between weight gain and breast cancer risk," said study author Ahn.
Doctors agree that there are enough facts to encourage women to take health steps for maintaining a healthy body weight.
"Clearly, there is a growing body of evidence that women should be encouraged to maintain a weight close to the ideal body weight throughout their lives, and while this may help reduce risk it is not a guarantee of prevention," said Rosenberg. "Women should use diet and exercise to maintain appropriate body weights."
Dr. Marisa Weiss of breastcancer.org says the findings may offer women some hope for taking a proactive step to control their risk of developing breast cancer.
"Maintaining a healthy weight throughout your life is an important way for women to reduce their risk," she said. "Small and big weight gains and losses can make a difference. It's never too late to start."