This prospective, observational study evaluates whether serum bicarbonate HCO3 levels can predict enteral nutrition intolerance (EFI) in adult ICU patients.Objective: To determine if lower HCO3 levels correlate with the development of EFI.Design: A 12-month study involving 70 participants at Sultangazi Haseki Training and Research Hospital.Methodology: Patients starting enteral nutrition within 48 hours of admission will be monitored for 72 hours. Daily HCo3 pH, and lactate levels will be recorded from routine morning blood draws.Hypothesis: Lower bicarbonate levels are an independent predictor of EFI development in critically ill patients.

Inclusion Criteria:

• Adult patients aged 18 and older.
• Patients started on enteral nutrition (EN) within the first 48 hours of their intensive care unit admission.
• Patients predicted to remain in the intensive care unit for at least 72 hours.
• Patients (or their legal representatives) who have provided consent to participate in the study.

Exclusion Criteria:

• Patients under the age of 18.Individuals who are currently pregnant or breastfeeding.
• Patients with gastrointestinal (GI) perforation, active GI bleeding, or mechanical bowel obstruction.
• Patients who have undergone major abdominal surgery (e.g., esophagectomy, gastrectomy) within the last month.
• Patients receiving total parenteral nutrition (TPN) or those who are only fed orally.
• Patients requiring specialized enteral formulas (e.g., diabetic, renal, immunomodulatory, elemental, or semi-elemental) due to clinical necessity.
• Patients with a known diagnosis of diabetes or those with HbA1c >6.5 requiring a diabetic enteral formula.
• Patients with terminal-stage malignancy or those requiring specific enteral nutrition protocols.
• Patients with advanced (Child-Pugh C) liver failure.
• Patients with chronic kidney failure requiring dialysis.
• Patients with diabetic ketoacidosis, severe metabolic asidozis, or those requiring initial bicarbonate replacement.
• Patients who are brain dead or in the palliative care stage.

The study population consists of adult patients aged 18 and older who are admitted to the intensive care unit (ICU). This group specifically includes those who begin enteral nutrition (EN) within the first 48 hours of their ICU admission and are expected to remain in the unit for at least 72 hours. The target sample size is 70 participants, factoring in potential drop-outs to maintain statistical power. Certain clinical conditions are excluded to ensure data consistency, such as pregnancy, 18 years of age or younger, active gastrointestinal bleeding or perforation, and recent major abdominal surgery. Additionally, patients requiring specialized enteral formulas or those with advanced organ failure (e.g., Child-Pugh C liver failure or dialysis-dependent kidney failure) are not included in the study population.