We know that obesity causes health problems — from diabetes and heart disease to high cholesterol. Now, after analyzing 1,000 studies, scientists have pinpointed a link between obesity and 13 different types of cancer.
According to a study published in the New England Journal of Medicine, an “absence of body fatness” lowers the risk for the following cancers: gastric cardia (stomach cancer), liver, gallbladder, pancreas, ovary, thyroid, myeloma (cancer of the white blood cells) and meningioma (brain tumor).
The study authors, who work at the World Health Organization’s International Agency for Research on Cancer (IARC), had previously linked obesity to five other cancers: breast (in postmenopausal women), kidney, uterine, esophageal and colorectal. Their new findings bring the grand total to 13.
The results were similar for men and women in North America, Europe and the Middle East, and study subjects were deemed overweight or obese by their BMI (body mass index), a measure of body fat based on height and weight.
How Are Extra Pounds Linked to Cancer?
Does excess body fat cause cancer, or is it merely a correlation? Although we can't be sure, research suggests that obesity itself may be a cause.
“When we looked at both animal models and human data, we found evidence for a cause-and-effect relationship for cancer and obesity,” says Graham Colditz, M.D., M.P.H., of Washington University School of Medicine and chair of the IARC Working Group.
Our bodies are comprised of millions of cells that support the internal organs and otherwise keep the body functioning. In a healthy body, when cells become damaged and die off, new cells grow and divide to take their place.
Cancer occurs when this process doesn’t work as it should. The damaged cells don’t die off. When new cells form and divide without stopping, the result is cancer, according to the National Cancer Institute.
The Effect of Obesity on Hormones and Inflammation
There are several reasons carrying extra weight affects one’s cancer risk. Hormones and inflammation are two of the leading explanations.
Women’s bodies naturally produce estrogen, a sex hormone that helps maintain sexual functioning and bone health. But fatty tissue creates too much estrogen, which at these high levels are associated with cancer.
Another contributing hormone is insulin. While the purpose of insulin is to help the body convert sugar (glucose) from carbohydrates into energy, “Obesity increases the risk of developing insulin resistance,” says Colditz. “Because of this, the body needs to produce greater amounts of insulin, which can lead to carcinogenesis.”
Obesity can also cause chronic inflammation. Ordinarily, inflammation is how the body fights back against wounds, infection and disease, according to the National Cancer Institute.
However, heavier people often produce inflammatory reactions even with no injury, which over time causes cell damage. “The reasons why obesity can cause chronic inflammation are unclear,” adds Colditz.
Losing Weight May Prevent Cancer, But It’s Not Proven
The study did have some limitations. For example, Colditz and his team don’t know how obesity affects cancer risk at different ages.
“If you’re obese at 80, does that present a different risk than being obese at 60?” he says. “We aren’t sure, but there’s a suggestion for some cancers that obesity earlier in life presents [more of] an increased risk than obesity later on.”
Another unanswered question: Does losing weight reduce cancer risk? “The problem there is that most people who lose weight don’t keep it off long enough to assess their cancer risk,” he says.
When Colditz’ team looked at bariatric surgery patients [who tend to maintain weight loss], they observed a reduced risk of breast and endometrial cancer.
How to Avoid Weight-Related Cancer
Karen Collins, nutrition adviser to the American Institute for Cancer Research (AICR), stresses that maintaining a proper weight is vital for your health. “Twenty percent of cancer risk is related to obesity,” she says — although exactly how or why isn’t proven.
“The problem is, most people think of fat as that plastic blob we see at the doctor’s office — the stuff that just prevents us from bending over. It’s not just about fitting into a pair of jeans,” Collins adds.
The study also found that obesity itself has its own impact on cancer risk, regardless of nutrition quality. In other words, you’re not off the hook just because your diet is clean.
“There’s a misconception that people can eat whatever they want [and not gain weight] as long as the food is healthy,” says Collins. “But if the food is adding extra calories [beyond] what you burn off, you’ll store it as extra fat,” says Collins.
The main message of his study, Colditz says, is to avoid weight gain and to shed extra pounds if you’re carrying them. Exercise to blast fat and regulate hormone levels, watch your portion sizes (check out the AICR’s handy serving-size table) and incorporate these cancer-fighting foods into your diet.
What Do YOU Think?
Were you surprised to learn about the connection between obesity and cancer risk? Will this research encourage you to change your habits? Let us know your thoughts in the comments section.