Tests in mice show a possible mechanism for how it happens. The findings, published in the journal Cancer Research, support studies that suggest people who consume more sugar have a higher risk of cancer— especially breast cancer.
"A lot of patients are told it doesn't matter what you eat after you are diagnosed with cancer. This preliminary animal research suggests that it does matter," said Lorenzo Cohen of the University of Texas M.D. Anderson Cancer Center, who worked on the study.
Related: Cutting processed sugar for just 9 days improves health
The findings add one more piece of evidence to a growing body of science that is showing a Western style diet is a major risk factor for many types of cancer. Other research has shown that at least two-thirds of all cases of cancer come down to lifestyle choices: tobacco use, an unhealthful diet and a lack of exercise.
Research has also pointed to refined sugar as one of the culprits. But this factor is harder to pin down, since "sugar" is a very broadly used term, some sugars are vital nutrients and the body uses a form of sugar called glucose to generate energy.
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Cohen's team found that fructose in particular affects a metabolic process, or pathway, called 12-LOX. It helps cells metastasize, or spread.
"The majority of cancer patients don't die of their primary tumor. They die of metastatic disease," Cohen told NBC news.
These findings help explain what other researchers have seen looking at cancer patients in general — those who eat more sugary foods are more likely to have advanced cancer.
Related: 5 things you may not know about sugar
Cohen's team used mice for their study but say they took many steps to make sure the process was as close as possible to what happens in people. They fed the mice sugar in doses very similar to what Americans eat every day and they used mice that are genetically predisposed to breast cancer in much the same way that many people are.
They fed mice four different diets that were either heavy in starch or heavy in different types of sugar.
"A human study reported that dietary sucrose/fructose/glucose but not starch is associated with increased risk of breast cancer," they wrote in their report.
When the mice were six months old, 30 percent of those fed a starch-dominant diet had breast cancer. But half the mice fed extra sucrose had breast tumors. And the more sugar they were fed, the bigger the tumors grew.
Fructose and inflammation
Sucrose or table sugar is actually composed of two sugars— glucose and fructose. Cohen's team wanted to see if one or the other made a difference, because the body processes them differently.
"Fructose is processed more by the liver, glucose by the pancreas," Cohen said.
They studied where the sugar went in the bodies of the mice. When the mice got more fructose, they grew larger tumors and faster.
This supports other findings that have shown pancreatic tumors also thrive on fructose.
"It seems that fructose is driving this inflammatory process more than glucose," Cohen said. "It seems from these series of experiments that it really fructose that within the sucrose that is the driver of the tumorigenic process."
Any sugar helped make the tumors grow faster, but fructose did it significantly more.
It's still not quite clear just how this happens and it's not clear how the LOX-12 pathway affects cancer, Cohen and colleague Peiying Yang said. But it appears fructose makes LOX-12 more active.
The implications for people are clear. Cohen notes that fructose consumption in the U.S. surged from about half a pound a person a year in 1970 to more than 62 pounds a year in 1997. That's mainly due to the broad use of high fructose corn syrup.
Other experts and several trade groups representing the food and beverage industry argue that fructose and sugar in general are safe ingredients and say there's really no evidence that fructose is any worse than any other sugars. They point out that fructose is found in natural fruit, for example.
Samantha Okazaki / TODAYFruit does provide fructose, but it’s mixed with fiber and other nutrients.
Cohen says that like oxygen, a little is vital for life but too much is toxic.
"We need glucose. We need sugar. It is an energy source and we need it to live," he said.
"We refine sugar that extracted from its source and consume in extremely high quantities."
Fruit does provide fructose, but it's mixed with fiber and other nutrients. This study didn't look at whether that affects the growth of tumors, but other experts point out that sweetened soft drinks— the largest single source of sugar in the western diet — provide only sugar and no other nutrients.
The World Health Organization says people should try to get no more than 5 percent of calories from sugar.
Health officials routinely advise Americans to eat less processed sugar.
"USDA, much to the anger of the sugar industry, said the maximum amount of sugar one should consumer in one's diet is 10 percent of calories from sugar," Cohen said. That's around 6 teaspoons a day for women and 9 teaspoons a day for men."
That was the lowest dose of sugar that Cohen's team fed their mice— and even that amount fed tumor growth, he said.
An average 12 ounce can of soda has 10 teaspoons of sugar.
There are other reasons to minimize sugar. Other studies show sugar-heavy diets can fuel heart disease, diabetes and Alzheimer's disease. But cutting sugar can lower blood pressure and blood sugar levels after only a few days.
A study published in June estimated that eating too much sugar killed 184,000 people a year.
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