Measurement of gastric and duodenal mucosal protein turnover in humans

Effect of severe undernutrition and subsequent refeeding on gut mucosal protein fractional synthesis in human subjects

OBJECTIVE

Undernutrition has been associated with gut mucosal atrophy, impaired absorption, and increased permeability. This study investigated the effect of severe undernutrition and subsequent refeeding on gastric and duodenal mucosal protein fractional syntheses (MPFS).

METHODS

MPFS was investigated in the gastric body, antrum, and duodenum of 23 severely undernourished patients by means of (14)C-leucine incorporation into tissue protein and repeated after a period of refeeding. Results were evaluated in comparison with a group of 22 healthy controls and presented as mean +/- standard error of the mean.

RESULTS

The mean body index of the undernourished patients was 13.29 +/- 0.33 kg/m(2) versus 22.67 +/- 0.63 kg/m(2) in the controls (P < 0.001). MPFS in the controls and undernourished patients were similar (gastric body, 36.49 +/- 2.88 versus 33.41 +/- 3.08%/d; gastric antrum, 25.51 +/- 2.20 versus 24.95 +/- 2.32%/d; duodenum, 25.90 +/- 2.58 versus 25.49 +/- 1.99%/d). After refeeding, the body mass index of the undernourished patients increased to 15.87 +/- 0.44 kg/m(2) (P < 0.001). The MPFS increased significantly (gastric body, 51.80 +/- 8.12%/d, P < 0.05; gastric antrum, 33.44 +/- 3.66%/d, P < 0.05; duodenum, 46.27 +/- 8.02%/d, P < 0.01), with the MPFS of the duodenum significantly greater than the control values (P = 0.01).

CONCLUSION

Despite severe undernutrition, MPFS of the gastric body, antrum, and duodenum remained similar to control values. Enteral feeding resulted in a significant increase in MPFS, indicating a trophic response.

The effects of undernutrition and refeeding on metabolism and digestive function

PURPOSE OF REVIEW

An intimate interrelationship exists between nutritional status and gut function. This review focuses on the consequences of a poor nutritional state on metabolism and digestive function, and evaluates the effects of refeeding.

RECENT FINDINGS

Severe undernutrition has been associated with increased fat and protein catabolism, reflected by a decreased respiratory quotient. Resting energy expenditure assessed in relationship to body weight was increased, probably as a consequence of changes in body composition. Protein synthesis, expressed per kg body weight, was decreased in undernourished patients with coexistent disease, but not in anorexia nervosa patients, indicating the detrimental effects of disease states. Severe undernutrition is associated with malabsorption, which improves following refeeding. Despite a high prevalence of villous atrophy in the duodenal mucosa in undernourished patients, mucosal protein fractional synthesis rates appeared normal. Refeeding resulted in a potent trophic response, and normalization of the mucosal morphology. Gastric and pancreatic secretion was significantly impaired by the undernourished state, with significant improvement following refeeding.

SUMMARY

Undernutrition is associated with impairment of digestive function, which is likely to further aggravate the nutritional state. Refeeding corrects this dysfunction, and results in disruption of this vicious circle.

Trypsin and splanchnic protein turnover during feeding and fasting in human subjects

Knowledge of the stimulatory effects of enteral and parenteral (intravenous) feeding on the synthesis and turnover of trypsin would help in the management of acute pancreatitis, because the disease is caused by the premature activation of trypsin. To investigate this, we labeled intravenous infusions with [1-(13)C]leucine and enterals with [(2)H]leucine and measured isotope enrichment of plasma, secreted trypsin, and duodenal mucosal proteins over 6 h by duodenal perfusion/aspiration and endoscopic biopsy. Thirty healthy volunteers were studied during fasting (n = 7), intravenous feeding (n = 6), or postpyloric enteral feeding [duodenal polymeric (n = 6), elemental duodenal (n = 6), and jejunal elemental (n = 5)]. All diets provided 1.5 g x kg(-1) x day(-1) protein and 40 kcal x kg(-1) x day(-1) energy. Results demonstrated that compared with fasting, enteral feeding increased the rate of appearance (71 +/- 4 vs. 91 +/- 5 min, P = 0.01) and secretion (546 +/- 80 vs. 219 +/- 37 U/h, P = 0.01) of newly labeled trypsin and expanded zymogen stores (1,660 +/- 237 vs. 749 +/- 133 units, P = 0.03). These differences persisted whether the feedings were polymeric or elemental, duodenal, or jejunal. In contrast, intravenous feeding had no effect on basal rates. Differential labeling of the plasma amino acid pool by enteral and intravenous isotope infusions suggested that 35% of absorbed amino acids were retained within the splanchnic bed during enteral feeding and that mucosal protein turnover increased from a fasting rate of 34 +/- 6 to 108 +/- 8%/day (P < 0.05) compared with no change after intravenous feeding. In conclusion, all common forms of enteral feeding stimulate the synthesis and secretion of pancreatic trypsin, and only parenteral nutrition avoids it.

The influence of intravenous infusions of glucose and amino acids of pancreatic enzyme and mucosal protein synthesis in human subjects

BACKGROUND

Animal studies have shown that the synthesis and secretion of pancreatic enzymes and the turnover of mucosal proteins is strongly influenced by diet.

METHODS

To determine whether the absorbed products of digestion are responsible for these changes, we investigated in groups of five healthy volunteers, the effects of i.v. infusions of amino acids (0.08 g/kg/h) and glucose (0.3 g/kg/h) on pancreatic enzyme and mucosal protein synthesis. Proteins were labeled in vivo by a 4-hours i.v. infusion of 14C-leucine and the enteric infusion of 3H-leucine tracer, during simultaneous cholecystokinin stimulation and duodenal collection of secreted pancreatic enzymes. Labeling of mucosal proteins was measured by endoscopic biopsy.

RESULTS

The amino acid infusions elevated plasma amino acid levels, and the glucose infusions increased both glucose and insulin concentrations. The rates for amylase and trypsin secretion were significantly lower during the first 2 hours of glucose infusion and the rate of synthesis of trypsin was delayed by i.v. amino acid infusions from 52.1 +/- 4.1 to 77.6 +/- 8.5 minutes. Mucosal protein turnover rates were unaffected. 3H-labeling via the enteral route showed similar enzyme synthesis rates but variable mucosal incorporation rates.

CONCLUSIONS

I.v. nutrients do not appear to stimulate the synthesis of pancreatic and mucosal proteins in human subjects.