A tale of gastric layering and sieving: Gastric emptying of a liquid meal with water blended in or consumed separately

Background: The process of gastric emptying determines how fast gastric content is delivered to the small intestine. It has been shown that solids empty slower than liquids and that a blended soup empties slower than the same soup as broth and chunks, due to the liquid fraction emptying more quickly. This process of 'gastric sieving' has not been investigated for liquid foods. Objective: To determine whether gastric sieving of water can also occur for liquid foods. Method: Two groups of men participated in a parallel design (n = 15, age 22.6 +/- 2.4 y, BMI 22.6 +/- 1.8 kg/ m(2), and n = 19, age 22.2 +/- 2.5 y, BMI 21.8 +/- 1.5 kg/m(2)) and consumed an isocaloric shake (2093 kJ, CARBOHYDRATES: 71 g, FAT: 18 g, PROTEIN: 34 g), either in a 500-mL version (MIXED) or as a 150-mL shake followed by 350 mL water (SEPARATE). Participants provided appetite ratings and were scanned using MRI to determine gastric emptying rate and volume at three time-points within 35 min post ingestion. Results: Gastric emptying the percentage emptied in 35 min was significantly smaller for MIXED (29 +/- 19%) than for SEPARATE (57 +/- 11%, p < 0.001). Conclusion: In the present study we show that gastric sieving can occur for liquid foods; water is able to drain from the stomach while a layer of nutrient rich liquid is retained. In indirect gastric emptying measurements, the behavior of labelling agents may be affected by the layering and confound emptying measurements.