Extra weight may not be good for your health in general, but heavy men appear more likely to survive a particular form of immune system cancer, according to a U.S. study.

The study, published in the Journal of Clinical Oncology, concerned an aggressive form of non-Hodgkin lymphoma called diffuse large B-cell lymphoma (DLBCL).

Non-Hodgkin lymphomas include a large group of cancers that affect the lymphatic system, a part of the immune system, and DLBCL is the most common form.

Some research has linked obesity to a greater risk of developing DLBCL. In certain other cancers - like breast and colon cancers - obese patients might also have a worse prognosis than those who are thinner when they are diagnosed.

"We expected something similar when we started this project," said Kenneth Carson of Washington University School of Medicine in St. Louis, the lead researcher.

What they actually found came as a surprise.

Of more than 2,500 U.S. veterans with the cancer, men who were obese at diagnosis had only about two-thirds the risk of dying during the study period that normal-weight men had, after considering other factors like age and overall health.

Out of 625 obese men, 294 - or 47 percent - died over the next five years. That compared with 64 percent of 849 men who were normal weight when diagnoses.

Men who were not obese, but overweight, also had a better prognosis than their normal-weight counterparts, a 27 percent lower death risk during the study period.

It's not clear exactly why heavy men with the lymphoma would survive longer, and Carson stressed that the findings don't mean that men should allow themselves to gain or hang on to excess weight.

But finding out could lead to a better understanding of the cancer, and better treatment for all, Carson said.

"In my mind, there are two main factors that might explain why overweight and obese patients have improved survival: differences in disease biology, or differences in patient responses to chemotherapy."

It's possible that bigger patients are able to tolerate more intense chemotherapy before having severe side effects. But, Carson's team added, other research suggests that obese chemotherapy patients may tend to get lower doses relative to their body size.