The assessment of body composition and energy requirements is important for the nutrition of healthy and pathological states such as gastrointestinal disorders, which are known to be associated with abnormalities in body composition among persons with malnutrition and malabsorption. Careful monitoring of body composition is thus recommended for assessing body water compartments, predicting caloric needs and physical performance, and evaluating the development of muscle mass in persons with malabsorption. In the present review we briefly describe the basic body composition models and various techniques used for their assessment and discuss the utility of measuring caloric requirements in persons with gastrointestinal disorders. Assessing body composition may improve the prognosis of malnutrition caused by gastrointestinal disorders and may be useful in monitoring diet treatment.