<p>Proteins are essential biomolecules that play indispensable roles in maintaining cellular homeostasis and regulating systemic metabolic processes. Protein deficiency is closely associated with age-related chronic diseases. Our purpose is to reveal the reason of protein deficiency in the elderly and potential treatments. A systematic literature search mainly on human studies addressing gastrointestinal aging and amino acid homeostasis was conducted. This review investigated the effects of aging on protein metabolism and summarized age-related physiological changes that collectively disrupt protein homeostasis. These changes include reduced secretion of gastric acid and digestive enzymes, which impair protein digestion in the stomach and small intestine. Weakened intestinal motility further delays the transit of digested proteins, reducing the efficiency of amino acid absorption. Additionally, decreased function of amino acid transporter proteins in the intestines and muscles limits the availability of amino acids for protein synthesis. Impaired liver and kidney function exacerbates these issues by affecting amino acid catabolism and clearance. Together, these alterations lead to an imbalance in amino acid homeostasis, contributing to protein deficiency, increasing the risk of sarcopenia in the elderly. Overall, we highlight changes in protein digestion, absorption, and amino acid metabolism in the elderly, and gives the possibility of improving amino acid utilization efficiency in the elderly, paving the way for new treatments for age-related protein deficiency in the future.</p>