Breast cancer and weight: What's the link?

The link between excess weight and increased cancer risk is well established. Being overweight or obese increases your risk for 12 cancers, including uterine, kidney, liver and colorectal cancer.

When it comes to breast cancer, the story isn't so clear. Being overweight does affect your breast cancer risk. But that effect is different before and after menopause.

Confusing? We talked to Karen Basen-Engquist, Ph.D., professor in Behavioral Science at MD Anderson. Here's what she had to say.

First, how do you know if you are a healthy weight, overweight or obese?

BMI, or body mass index, is a good way to get a general idea if you are a healthy weight. BMI is a tool to compare your height to your weight to see if you fall into one of four ranges: underweight, normal, overweight or obese.

Before menopause, how does being overweight or obese affect your breast cancer risk?

Before menopause, your breast cancer risk goes down by 18% for every five point increase on the BMI scale. That may sound like a great reason to pack on extra pounds. But that's not a good idea, says Basen-Engquist.

"Breast cancer is pretty rare in women before menopause. So those extra pounds would decrease your risk of something you are highly unlikely to get," she says.

That unlikely benefit is offset by the clear health risks of being overweight or obese: heart disease, diabetes and an increased lifetime risk for several other cancers.

Researchers aren't sure why extra weight reduces breast cancer risk before menopause, but the bottom line is, being overweight or obese is bad for your health.

"We would never advise anybody to gain weight in order to reduce their risk for cancer," says Basen-Engquist. "The competing risk outweighs any benefit."

After menopause, how does excess weight increase the risk of breast cancer?

After menopause, your breast cancer risk increases by about 12% for every five point increase on the BMI scale.

While BMI doesn't directly measure body fat, it does indicate that you probably have excess body fat. There are multiple ways in which body fatness can influence breast cancer risk. Body fat has an effect on hormones. And hormones have a big effect on your cancer risk. Here's why:

The hormone estrogen stimulates cell growth in the breasts and uterus. The more rapidly your cells grow and divide, the more likely something can go wrong, like cancer.  After menopause, women who are overweight or obese still have a supply of estrogen, when it should be decreasing.

"The idea is that the fat is prolonging your exposure to estrogen," says Basen-Engquist. "A number of breast cancer risk factors, including getting your period early, not having children or not breast feeding, and hormone replacement therapy, increase your lifetime exposure to estrogen."

Body fatness also is associated with increased levels of insulin and other growth factors that encourage cell growth, increasing your cancer risk.

Finally, obesity is associated with long-term inflammation. The body treats inflammation as an illness and tries to repair it, which means more cell growth and more risk for breast and other cancers.

But you can take steps to reduce your risk.

"It's never too late to lose weight," says Basen-Engquist. Studies show that losing weight after menopause can reduce your risk of both breast and uterine cancers.

What about body fat vs. weight?

Most studies use BMI when looking at the relationship between breast cancer and weight. But it's possible to have a BMI in the normal range and have a high percentage of body fat.

And emerging research shows that women with excess body fat have an increased risk of breast cancer, even if their BMI is normal.

To get an idea of how much body fat you have, measure your waist circumference. To get that number, run a flexible tape measure around the smallest part of your waist, just above your belly button. Exhale and measure. That number is your waist circumference.

There is no specific waist circumference that can tell you if you are at increased risk for breast cancer. But in general, if you are a woman with a waist measuring 31.5 inches or more, you are carrying excess body fat, and you are at higher-risk for cancer in general. For men, a waist measuring 37 inches or more indicates a high risk for cancer.

What is the best way to reduce cancer risk?

There are steps you can take to maintain a healthy weight and reduce your lifetime risk of breast cancer:

• Sit less, and aim for at least 150 minutes of moderate or 75 minutes of vigorous physical activity each week. Do strength-training exercises twice a week.
• Fill 2/3 of your plate with vegetables, fruits and whole grains. Fill the remaining 1/3 with lean animal protein or plant-based protein.
• Limit alcohol. For cancer prevention, it's best not to drink alcohol. If you do choose to drink alcohol, have no more than one drink if you are a woman and two a day if you are a man.
• Choose to breastfeed. Try to breastfeed exclusively for six months after giving birth, and continue even when other foods are introduced.

"Staying lean after menopause will reduce your breast cancer risk," says Basen-Engquist. "But staying lean over a lifetime is one of the most important things you can do to protect your overall health and reduce your risk for many cancers, including breast cancer."