When it comes to cancer, carrying extra weight has long been considered dangerous.

The two in five American adults with obesity are at an increased risk of thirteen types of cancer — like breast, colon, and pancreatic cancers — which make up 40% of all cancers diagnosed in the country every year.

But thanks to new research, we’re learning more about a twist in the relationship between body weight and cancer survival that seems to defy conventional wisdom.

Understanding what’s called “the obesity paradox” could help us change the way we treat cancer and give patients a better shot at survival.

What is the obesity paradox?

Because being overweight or obese increases your risk of cancer, you’d think it would also decrease your cancer survival. That’s a common perception some researchers are saying is in fact a misconception.

A number of studies have shown that obesity is associated with improved cancer survival: something referred to as the “obesity paradox.”

The idea has been investigated for well over a decade. In 2016, researchers found that those classified as overweight had the best survival outcomes of colorectal cancer, while those who were underweight face particularly high mortality.

Research has continued to show obesity is associated with survival in pancreatic cancer, breast cancer, non-small cell lung cancer, melanoma, and renal cell carcinoma.

That said, there’s been mixed results in other studies, and many experts don’t agree the “obesity paradox” exists at all.

In 2021, a massive study looking at more than 6 million cancer patients showed the opposite — that obesity was generally associated with worse survival, with some exceptions, like lung and kidney cancer, as well as melanoma.

A landmark 2016 study suggested there could be a number of confounding factors in research showing a false association between obesity and cancer survival.

For example, BMI can’t tell fat from muscle — and it could be muscle, not necessarily extra fat, that may help some cancer patients live longer.

Other studies suggest that factors beyond weight at diagnosis — including blood sugar regulation, inflammation and weight loss in the years leading up to diagnosis — may better explain why some cancer patients live longer than others.

How this could change cancer treatment

Researchers have put forth several theories about why obesity might help cancer patients survive longer.

First, obesity could help immunotherapy work better. One study showed that obese patients on the medication were at a 56% less risk of cancer progression or death.

Another idea is that extra fat provides energy reserves during treatment — aka the hibernation hypothesis.

“Excess adipose tissue serves as a nutrient reserve and confers a survival advantage in times of stress, such as anti-cancer treatment,” one research team writes. “This is akin to the hibernation theories in evolutional biology whereby species store up energy in anticipation of harsh times ahead.”

New research from McGill shows another factor associated with obesity, diet, could help cancer patients survive.

They found that diets often considered “unhealthy” — high in added sugars, refined grains, saturated fats — were associated with better responses to a type of cancer immunotherapy.

The secret is in the gut bacteria: these diets encouraged growth of gut bacteria like Lactobacillus johnsonii, which produce chemicals that helped the body’s immune system fight cancer.

Researchers hope to learn more about how to boost these healthy microbes and improve cancer treatment in the future.