What's the connection between obesity and breast cancer?

There are many different factors that can elevate your risk of developing breast cancer. Some of these are uncontrollable, such has having a mutation on the BRCA1 or BRCA2 gene or having dense breasts.

Others are considered controllable, such as how much you exercise and whether you eat a balanced, nutritious diet. Obesity, or being overweight, is one such "lifestyle factor" that many doctors consider to be a controllable risk factor for developing breast cancer that can also impact mortality rates.

The World Health Organization defines overweight and obesity as "abnormal or excessive fat accumulation that may impair health." Using the simple calculation of weight-for-height known as the body mass index, adults with a BMI greater than or equal to 25 are said to be overweight, while adults with a BMI greater than or equal to 30 are considered obese. (You can calculate your own BMI with this online calculator from the National Heart, Lung and Blood Institute.)

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According to WHO estimates, 1.9 billion adults age 18 and older – some 39 percent of the world's adult population – were overweight in 2016. Of them, 650 million people or about 13 percent of the adult population (11 percent of men and 15 percent of women) worldwide were obese in 2016.

Meanwhile, figures from the Centers for Disease Control and Prevention indicate that 70.7 percent of American adults over age 20 are overweight, and 37.9 percent of American adults over age 20 are obese.

Clearly, being too heavy is a global phenomenon, and being overweight or obese has been associated with a number of other chronic health problems including heart disease, diabetes and cancer.

The connection between a high BMI and breast cancer specifically is well-established. A high BMI has been isolated as a risk factor for developing the disease for many years, but more recently it's been noted as impacting mortality rates. A 2014 review study in Annals of Oncology indicated that "in breast cancer survivors, higher BMI is consistently associated with lower overall and breast cancer survival, regardless of when BMI is ascertained."

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The findings also indicated that when compared with normal-weight women, obese women had a 21 to 41 percent higher risk for total mortality and a 25 to 68 percent increased risk for breast cancer mortality, depending on how much time had passed since their breast cancer diagnosis. The study recommended that maintaining a BMI of between 18.5 and 24.9 is best for survival.

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But why does being overweight contribute to the development of cancer? Scientists are still figuring out the exact mechanism, but it seems there's a connection among fatty tissue, hormones and inflammation.

Part of the problem is likely due to the fact that adipose tissue, or fat, "secretes a lot of hormones and is part of the metabolism of the body. Adipose tissue is probably the largest endocrine organ in the body," says Dr. Sagar Sardesai, assistant professor of medicine at the Stefanie Spielman Comprehensive Breast Center at The Ohio State University Wexner Medical Center.

In a lean patient, this process is in balance, so it does not typically pose an issue. But in an overweight patient, an increase in circulating hormones can lead to inflammation, and this inflammation could lead to cancer developing. "As the [amount of] adipose tissue goes up and grows larger, it often gets what I call angry," or inflamed. Inflammation is associated with cancer, so the more inflammation in the body, the better the chances that cancer could develop.

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Dr. Neil Iyengar, a medical oncologist focusing on patients with breast cancer at Memorial Sloan Kettering Cancer Center in New York, explains the series of steps in which the growth of fatty tissue may lead to inflammation that could subsequently trigger cancer.

"The fatty tissue is a very active tissue, and it serves the purpose of maintaining homeostasis and ensuring that all of our metabolic processes are in order," he says. But it needs an adequate blood supply to conduct these activities, and as it expands, it can outgrow this supply.

"As soon as we put on too much fat or the fat tissue outgrows its blood supply and nutrition support, then that tissue becomes diseased and can no longer fulfill its multiple functions of maintaining metabolic homeostasis," he says. He says the fat cells die, and then the immune system tries to clear the dead cells out. But that's difficult to do because fat cells are among the largest cells in the body, "so it becomes an inefficient process."

This can lead to hormones being produced, including estrogen precursors that can directly stimulate the growth of breast cancer tumors. "But in addition to hormones, you have secretion of inflammatory molecules that can stimulate tumor growth. It's inflammation in a diseased fat pad that is leading to the building blocks, so to speak, that allows tumors to overtake that process and start to grow," Iyengar says.

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This process is similar to how a wound heals, Iyengar says. "The obese fat pad is a dysfunctional fat pad that is inflamed. On a molecular level, it resembles a healing wound because there are inflammatory molecules, there's collagen being laid down, fibrous tissue being laid down and there are new blood vessels that are trying to develop. A healing wound is good when you're trying to heal a trauma that happens in an acute setting, but chronically, that kind of situation can lead to tumor development," he says.

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No matter what the exact pathway is, the moral of the story is to try to keep your weight at a healthy level. More specifically, it's important to keep your fat levels in check all through life. Diet and exercise are the best tools with the fewest side effects currently available to control your weight, so speak with your doctor or a nutritional counselor about the best ways to maintain a healthy weight and activity level for the long term.

And keep in mind that maintaining that healthy weight becomes both more important and more difficult as we age. Gaining weight is a common phenomenon as the body changes with age. For women, menopause, which occurs on average at age 51, is a particular point of challenge that causes many to gain weight. The earliest stages of menopause, called perimenopause, can start when a woman is still in her 40s.

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"Weight gain in the years leading up to menopause is especially [problematic], particularly for estrogen-sensitive breast cancers," Sardesai says. It also gets a lot harder to shed excess pounds after menopause. So keep an eye on the scale as you go, and hopefully you can keep the pounds at bay now to help reduce your risk of developing breast cancer or improve your outlook later.

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