However, obesity – which affects roughly one-third of American adults – is believed to contribute to 20 percent of all deaths from cancer in the United States. And now, a new study from Michigan State University reveals that, when it comes to fighting fat-related cancer in overweight (and even some normal-weight ) folks, belly fat is enemy No. 1.

Abdominal Fat and Cancer: The Surprising Connection

The fat on your frame is all but the same, and it exists in two distinct layers. The top, pinchable layer, which sits directly above your skin, is called subcutaneous fat. The deeper layer of fat, which takes up residence in and around your organs and makes for a rounder belly, is termed visceral fat, abdominal adiposity or belly fat.

While some subcutaneous and visceral fat is normal, mounting research shows it’s excess visceral fat, rather than subcutaneous fat, that has the biggest impact on health. For example, one review published in the Journal of Cardiovascular Disease Research concluded that abdominal obesity, regardless of body mass index (a ratio of height and weight), is strongly linked to an increased risk of cardiovascular disease. Meanwhile, a 2016 study published in the International Journal of Obesity found that 30 percent of Americans with so-called “healthy” weights actually have poor cardiometabolic health, with symptoms including insulin resistance, excessive inflammation and other conditions long associated with obesity. In other words, slim-looking folks with bellies can be far more unhealthy than people who are bigger all around.

Now, researchers have found that excess belly fat's risks go beyond heart health. To investigate how belly fat – not just fat in general – affects the risk of cancer, Michigan State University researchers examined mice and found that their visceral fat tissue produced large amounts of a protein, called fibroblast growth factor-2 or FGF2, that can cause a formerly healthy cell to turn cancerous.

“We found that the largest contribution of this protein in the circulation came from the visceral adipose tissue, because when we surgically removed the visceral fat, the blood levels of this protein were almost undetectable,” says lead study author Jamie Bernard, assistant professor of pharmacology and toxicology at Michigan State University.

What’s more, Bernard collected samples of visceral fat tissue samples from women undergoing hysterectomies and evaluated their levels of the cancer-promoting protein. Upon transfer into mice, the samples containing higher levels of FGF2 triggered more cancer tumors, linking the protein, produced from visceral fat, to cancer formation in humans. (For ethical reasons, scientists can’t try to give humans cancer.)

Still, visceral fat’s production of FGF2 isn’t the only way it may increase the risk of cancer.

“Fat doesn’t just sit there like we once previously thought,” says Martha Belury, professor of endocrinology, diabetes and metabolism at Ohio State University. “It is an active endocrine organ. It secretes different types of proteins, inflammatory factors and hormones that directly affect the organs around it.”

Bernard adds that substances released by visceral fat travel to the liver, where they could influence the production of blood lipids that have been linked to metabolic syndrome.

That explains why cancers of the visceral organs – including the liver, pancreas, intestines, uterus, gallbladder and kidneys – appear to be the most strongly influenced by excessive visceral fat levels. After controlling for other factors, researchers have found that high visceral fat levels increase the risk of colon cancer three-fold.

Meanwhile, visceral fat’s secretion of estrogen is believed to at least partially explain visceral adiposity’s link to both pre- and post-menopausal breast cancer. Interestingly, though, visceral fat may have the strongest influence on the development hormone-receptor-negative breast cancers, which don’t contain estrogen receptors and do not respond to hormonal treatments.

For instance, in one Harvard School of Public Health study of 620 women with breast cancer, those who had the largest waists were nearly three times more likely to be diagnosed with hormone-receptor-negative breast cancer compared to those with the smallest waists. And women with the greatest waist-to-hip ratios were nearly twice as likely to be diagnosed with hormone-receptor-negative breast cancer compared to those with the smallest ratios.

What’s Your Waistline?

While imperfect, waist circumference and waist-to-hip ratio (the two measures used in the Harvard study) are easy methods of estimating your visceral fat levels – and determining if they need adjusting.

To measure your waist circumference, simply wrap a flexible measuring tape or string around your waist, at the top of your hipbones. A waist circumference of more than 35 inches in women and 40 in men is a marker of abdominal obesity and chronic health risk, according to the American Heart Association, National Heart, Lung and Blood Institute.

Then, to determine your waist-to-hips ratio, divide the above number by the circumference of the largest part of your hips. A waist-to-hip ratio of greater than 0.85 in women and 0.9 in men is another indicator of abdominal obesity and health risk, according to the World Health Organization.

If one or both of these tests suggest that you have less-than-healthy levels of abdominal fat, talk to your primary care provider about evaluating your overall health and disease risk – and how reducing your levels of abdominal fat can help improve both.