Malnutrition results from the imbalance of nutrients and energy provided to the body (too low), relative to its needs (too high). These needs increase dramatically with illness. This is certainly the case for patients with gastrointestinal diseases. Sub-optimal dietary intake, metabolic stress, malabsorption and increased nutrient demands, put a patient with gastrointestinal disease, at high-risk for malnutrition. The causes, consequences and assessment and monitoring indicators of malnutrition are reviewed herein.