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The effect of continuous intraduodenal enteralw nutrition on gastric pH was compared with the effects of fasting and of parenteral and standard nutrition control regimens containing equal amounts of carbohydrate, protein, and lipid. Eleven healthy volunteers underwent four 24-hour intragastric pH-metry studies; serum glucose, calcium, immunoreactive insulin and gastrin levels were determined during fasting and enteral and parenteral regimens. Median 24-hour gastric pH during enteral nutrition (group median pH 1.4) was lower than during parenteral nutrition (pH 1.9; P= 0.0039 vs. enteral) but was not different from fasting (pH 1.4) or standard nutrition (pH 1.6) values. Median 24-hour serum glucose levels during enteral nutrition (group median, 4.8 mmoI/L) were higher than during fasting (4.0 mmol/L; P= 0.00098 vs. enteral) and lower than during parenteral nutrition (5.3 mmol/L; P= 0.0039 vs. enteral). Median 24-hour serum insulin levels during enteral nutrition (group median, 22.9 mU/L) were higher than during fasting (group median, 9.2 mU/L; P= 0.00098 vs. enteral) but similar to levels during parenteral nutrition (23.3 mU/L). Neither median 24-hour gastrin levels nor calcium levels were affected by any nutrition regimen. Thus, continuous enteral nutrition produces gastric pH values similar to those seen with fasting or standard nutrition, suggesting that, under most physiological conditions, gastric acidity is subject to close feedback control. Parenteral nutrition increases gastric pH, suggesting that systemic nutrients may influence this feedback mechanism.