Protein Digestion and Amino Acid Absorption in the Aging Gastrointestinal Tract

Worldwide, the proportion of older individuals in the population is increasing. The progressive loss of skeletal muscle mass with aging, or sarcopenia, has a major impact on our health care system and costs due to increased morbidity and a greater need for hospitalization and/or institutionalization. A sufficient protein intake is imperative to maintain skeletal muscle mass and overall health, and thereby stimulates active aging. Whereas the overall protein intake is generally lower in older as compared to young adults, findings indicate that older adults need more protein to increase muscle protein synthesis, known as anabolic resistance. The exact mechanisms underlying anabolic resistance are not fully discovered yet. Age-dependent changes in gastrointestinal tract physiology have been suggested as a potential cause.

Following protein ingestion, protein is cleaved into small peptides and amino acids by digestive enzymes secreted in the stomach (pepsin) and small intestinal lumen (pancreatic trypsin, chymotrypsin, elastase), as well as brush border enzymes (aminopeptidase, carboxypeptidase, dipeptidase). Brush border enzymes are responsible for the final digestion of small peptides into dipeptides, tripeptides and amino acids. Subsequently, these small peptides and amino acids are absorbed across the intestinal mucosa by various membrane-bound transporters. The majority of dietary-protein derived amino acids is released into the systemic circulation and transported and taken up by various peripheral tissues in the postprandial phase.

It has been shown that postprandial plasma amino acid availability is lower in older compared to young adults. This indicates that digestion and absorption of proteins are likely attenuated in older individuals, thereby reducing the systemic availability of protein-derived amino acids for muscle protein synthesis and other relevant physiological processes.

Limited studies have focused on changes in gastrointestinal tract physiology with aging. Human studies on the expression and activity of brush border enzymes and protein transporters (i.e. for dipeptides, tripeptides and amino acids) being crucial for the final steps of digestion and absorption, respectively, are lacking. Therefore, the aim of the present study is to assess the expression and activity of brush border enzymes and expression of protein transporters in small intestinal epithelial cells in older as compared to young adults.

Inclusion criteria

• Aged 18-35 years or 67+ years
• Body mass index (BMI) between 18.5 and 30 kg/m2
• Regular stool frequency ranging from 1-3 times/day

Exclusion criteria

• History of cardiovascular, respiratory, gastrointestinal, urogenital, neurological, psychiatric, dermatologic, musculoskeletal, metabolic, endocrine, haematological, immunologic disorders, allergy, major surgery and/or laboratory assessments which might limit participation in or completion of the study protocol, interfere with the execution of the experiment, or potential influence the study outcomes (to be decided by the principal investigator and responsible physician)
• Major abdominal surgery interfering with gastrointestinal function (upon judgement of the principal investigator and responsible physician)
• Use of medication which limit participation in or completion of the study protocol, interferes with the execution of the experiment, or potential influences the study outcomes (to be decided by the principal investigator and responsible physician)
• Use of supplementation (i.e. protein, vitamin, pre- and probiotic supplementation) within 14 days prior to testing
• Administration of investigational drugs or participation in any scientific intervention study prior or during the study, which may interfere with this study (upon judgement of the principal investigator and responsible physician) Specific diet (e.g. vegetarian, vegan, gluten free, no diary) within the study period
• Planning to lose weight during the study period
• Excessive alcohol consumption (defined as > 14 alcoholic consumptions per week)
• Smoking
• Drug use
• Pregnancy
• Lactation
• Donated blood three months prior to the test day
• No given permission to register participation in electronic patient file at MUMC+ and to add records of gastroduodenoscopy