Gradual Versus Immediate Goal-dose Enteral Nutrition in Abdominal Surgery Patients

Inclusion Criteria: 1. Informed consent of patients or their legal representatives to participate in this study. 2. patients undergoing selective operation without trauma 3. patients following medium or major abdominal surgery 4. NRS 2002≥ 3 Exclusion Criteria: 1. Psychiatric disorders 2. Pregnancy or breast-feeding women 3. Malnutrition 1. Weight loss \>10%-15% in 6 months 2. BMI\<18.5 3. SGA score with stage C 4. Albumin \< 30g/L 4. Unstable vital signs or unstable hemodynamics (such as systolic blood pressure \< 90 mmHg or mean arterial pressure \< 70 mmHg after rapid 500 ml crystal or 200 ml gel infusion, or the 50% increase of vascular active drug infusion rate in an hour, etc) 5. Refuse to participate in the study 6. Mortality rates expected to more than 50% in 6 months with malignant or irreversibility diseases 1. Cancer in terminal stage or 2. HIV positive at end-stage or CD4 \< 50/mm3 3. Cardiopulmonary resuscitation (CPR) before cardiac arrest and nervous system function not fully recovery 4. Four levels of physical activity of the patients defined by New York heart association 5. Rely on breathing machine because of chronic diseases 7. Life expectancy less than 24 hours of dying patients 8. Refractory shock to meet any of the following article 1. The infusion rate of dopamine \> 15 ug/kg/min 2. The infusion rate of dobutamine \> 15 ug/kg/min 3. The infusion rate of epinephrine and norepinephrine \> 30 ug/min 4. The infusion rate of phenylephrine \> 50 ug/min 5. The infusion rate of milrinone \> 0.5 ug/kg/min 6. The infusion rate of vasopressin \> 0.04 U/min 7. Inter aortic ballon pump (IABP) 9. Hepatic insufficiency (alanine/aspartate transaminase/bilirubin 200% above normal range) 10. Renal insufficiency(creatinine 200% above normal range) 11. Metabolic diseases（hyperthyroidism/ hypothyroidism, adrenal cortex disorders) 12. EN can not reach 30% of target energy in 48 hours after surgery 13. Burn area exceeding 20% of the patient's body surface 14. Autoimmune diseases or immune dysfunction or history of organ transplantation 15. International standardization ratio (INR) more than 3.0 or platelet count \< 30000 cells/mm3 or other hemorrhagic diathesis 16. Intracranial hemorrhage one month before enrolment 17. General contraindications to infusion therapy or history of severe allergy against ingredients of enteral and parenteral nutrition 18. Has already participated in another clinical trial 19. Has started to nutritional support therapy before enrolment 20. Diabetes mellitus (anamnestic and/or under medical treatment)