Increasing Reactive oxygen and nitrogen production occurred simultaneously with decreasing serum and intracellular level of antioxidants and enzymes in critical ill patients, which result in increasing ventilator dependency and length of stay in intensive care unit and it also accelerate organ failures in patients. In this double blind clinical trial, the investigators examine effect of alfa-lipoic acid on those patients who admitted to intensive care unit that the investigators expect to stay for more than 7 days in this ward and who have tube feeding and don't have severe liver and kidney failure, AIDS and hepatitis. After randomization of included patients by block randomization the investigators will give 900mg/day alfa-lipoic acid for ten days to treatment group and identical placebo to control group by naso-gastric tube. The purposes of this study are decreasing ventilator dependency period, length of stay in ICU, mortality and decelerate of organ failures.

Inclusion Criteria:

• age ≥ 18;
• expected length of stay more than 7 days;
• using enteral feeding method during admitted to ICU

Exclusion Criteria:

• having the history of autoimmune disease
• severe renal or liver failure
• AIDS,
• hepatitis;
• having severe malnutrition at the admission time;
• having TPN at the admission time in ICU;
• extreme intolerance to enteral feeding.