Technical Abstract: The splanchnic bed is comprised of the liver and the portal drained viscera (PDV). The PDV, which include the stomach, intestines, pancreas, and spleen, represent 4-6% of body weight, yet they account for 20-35% of whole-body protein turnover and energy expenditure. The high nutrient needs of the gut are met first as a result of first-pass metabolism. Consequently, the first-pass metabolism of dietary nutrients by the gut, especially amino acids, has a critical influence on their availability to peripheral tissues and whole body requirements. Moreover, the systemic availability of dietary amino acids is key determinant of lean body growth rate. A complicating factor in the measurement of gut nutrient utilization is that the intestinal mucosa receives nutrients from two sources, the diet and the arterial circulation. However, combining measurements of the net portal balance with enteral and intravenous infusions of stable isotopically labeled amino acids provides an in vivo model that can distinguish the proportion of amino acids that are derived from the diet and arterial input. Using this technique in fed infant pigs, we found that 30-40% of the total amino acid intake is used by gastrointestinal tissues. The relative PDV utilization of individual amino acids from the diet and arterial inputs varies widely and dietary amino acids are the preferred fuel over dietary glucose. Stable isotopically labeled amino acids also enable the determination of the metabolic fate of individual amino acids. These studies have shown that insufficient protein supply or mode of feeding affects PDV amino acid utilization and consequently has a bearing on whole-body growth. Key Words: Intestine, Swine, Nutrition