Obese people are known to have a higher risk of colon cancer. Now, a new study suggests they may have poorer long-term survival odds than their thinner counterparts if they do develop the disease.

The latest findings, reported in the journal Clinical Cancer Research, suggest that excess weight may particularly affect male survivors’ long-term prognosis.

In a study of nearly 4,400 U.S. adults treated for colon cancer, researchers at the Mayo Clinic in Rochester found that obese patients were one-quarter to one-third more likely to die over the next eight years than their normal-weight counterparts.

The relationship between obesity and survival appeared stronger among men - possibly, the researchers speculate, because men are more likely than women to have their excess body fat concentrated in the belly. Abdominal obesity is particularly linked to hormonal effects that, in theory, could contribute to colon cancer development or the cancer’s aggressiveness.

However, whether and how obesity, per se, affects colon cancer survival remains unclear. The current study points to a relationship between obesity and long-term survival, but does not prove that excess body fat directly affects a patient’s prognosis.

Still, the researchers say the findings suggest that people treated for colon cancer should try to maintain a body mass index lower than 30, the cutoff for obesity. Body mass index, or BMI, is a measure of weight in relation to height.

“People may think, ‘I already have cancer. What difference does my weight make?’ But this study suggests the cancer may behave more aggressively if you’re obese,” lead researcher Dr. Frank A. Sinicrope said in an interview.

Obese adults who’ve been treated for colon cancer can talk with their doctors about the best ways to safely lose weight, according to Sinicrope - though, he noted, it is unclear how receptive patients dealing with cancer may be to the prospect of losing weight.

Sinicrope and his colleagues based their findings on 4,381 U.S. adults who took part in any of seven clinical trials testing a chemotherapy regimen for colon cancer. All had stage II or III cancer, meaning the cancer had spread deeper into the colon wall or to nearby lymph nodes.

After eight years, the researchers found, 42 percent of the patients had died, and 36 percent had seen their cancer recur.

Among the 787 men who were normal-weight at the start of the study, 53 percent were alive eight years later. That compared with 42 percent of men who were very obese - having a BMI of 35 or higher.

When the researchers considered several other factors, including the patients’ age and stage of cancer, very obese men were 35 percent more likely than normal-weight men to die during the study period.

Among women, 61 percent of normal-weight patients were still alive after eight years, versus 55 percent of women who were moderately obese - having a BMI between 30 and 35. Fifty-nine percent of very obese women were still alive after eight years; when other factors were considered, very obese women did not have a significantly higher risk of death than normal-weight women.

Milder obesity, however, was linked to a 24 percent higher risk of death.

In theory, excess body fat could affect colon cancer aggressiveness, according to the researchers. Obesity, particularly abdominal obesity, is associated with higher levels of the hormones insulin and insulin-like growth-factor-1 (IGF-1), which have been shown in lab research to promote the growth and spread of colon cancer cells. Studies have also found that men and women with relatively high IGF-1 levels have a higher risk of developing colon cancer than those with lower levels of the hormone.

It’s not clear why more-severe obesity was not related to survival among women in this study. One possibility, Sinicrope said, is that the relationship is more complex in women owing to the effects of estrogen - which some research suggests is protective against colon cancer.

It’s possible, for instance, that very obese women were more likely to have been on hormone replacement therapy, Sinicrope noted. But the study did not have data on that.

The study also lacked information on patients’ diets and exercise habits - factors that could affect colon cancer prognosis or a person’s risk of death from other causes, like heart disease.

Further studies, the researchers conclude, are needed to show whether and why obesity affects colon cancer prognosis.

SOURCE: Clinical Cancer Research, online March 9, 2010.