Surgical trauma can lead to an intense inflammation and impairment of the capacity to fight against infections. Fish oil is composed by substances called omega-3 fatty acids which have anti-inflammatory effects. When infused through the vein as part of a nutritional treatment of some surgical patients, fish oil have been shown to preserve some defense capabilities against infections and to decrease postoperative release of inflammatory substances, contributing to the shortening of length of hospital and intensive care unity (ICU) stay and complications, mainly infectious complications. These reported benefits have been encouraging the vein infusion of fish oil as a kind of medicine to contribute for the treatment of surgical patients. In current study the investigators evaluated the effect of preoperative infusion of pure fish oil in patients with gastrointestinal cancer (stomach and colon) on their clinical outcomes. The hypothesis considered that preoperative infusion of fish oil, not as part of nutritional therapy but as a medicine agent, can attenuates inflammation and improves the capacity of surgical patients to fight against infections, improving their treatment.