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1School of Sport & Exercise Sciences, Loughborough University, Loughborough LE11 3TU, UK2Department of Biomedical Sciences, University Medical School, Foresterhill, Aberdeen AB25 2ZD, UK
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(Received 17 September 2003;
; first published online 24 October 2003)
Corresponding author R. J. Maughan: School of Sport & Exercise Sciences, Loughborough University, Loughborough LE11 3TU, UK. Email: r.maughan{at}lboro.ac.uk
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Introduction |
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The rate of gastric emptying of carbohydrates in various forms has been more extensively studied than that of other macronutrient solutions or suspensions, and it is recognized that energy density has a much stronger effect than osmolality, at least as far as carbohydrate solutions are concerned (Vist & Maughan, 1994, 1995). The effect of protein or amino acids on emptying rates is far from clear. Cooke & Moulang (1972) measured the gastric volume 20 min after ingestion of a 350 ml test meal and found no difference in the rate of gastric emptying between glucose and amino acid solutions of similar molar concentration and osmolality; the amino acids tested were: glycine, alanine, leucine, lysine and methionine. Using a scintigraphic technique, Fisher et al. (1987) found similar gastric emptying rates for iso-energetic carbohydrate and protein meals. However, Barker et al. (1978) measured the gastric volume 15–25 min (constant for each subject) after ingestion of 750 ml solutions of glucose, glycine and diglycine, and found that the slowing of gastric emptying was about 10% greater for glycine than it was for glucose. McHugh & Moran (1979) used the serial test meal method in monkeys with chronic indwelling gastric cannulae and found similar rates of gastric emptying of iso-energetic solutions of glucose, casein hydrolysate and medium-chain triglyceride oil. Maerz et al. (1994) used gamma-camera imaging in rats and found similar half emptying time for iso-energetic solutions of glucose, casein hydrolysate and a lipid suspension.
The reasons for these contradictory results may be due to the different techniques used to measure gastric emptying rate, and the different volumes and sample times used. The exponential nature of the rate of gastric emptying of most solutions (Hunt & Spurrell, 1951; Hunt & McDonald, 1954; Rehrer et al. 1989; Vist & Maughan, 1994) means that measurements made at a single time point, as in some of these studies, may produce misleading results, particularly when solutions of a low energy content are studied. When measurements are made at only a single time point, a linear rate of emptying is inevitably assumed. The possibility of species differences, perhaps related to the habitual diet, must also be considered.
Calbet & MacLean (1997) used a double sampling gastric aspiration method, which allows serial measurements to be made and the time course of emptying to be followed, and they showed that a mixed glucose and whey protein hydrolysate solution was emptied at a similar rate to an iso-energetic mixed glucose and pea protein hydrolysate solution. They also showed that a glucose-only solution of half the energy content was emptied faster, and that a mixed glucose and milk protein solution of twice the energy content was emptied more slowly. However, that study did not address the question of whether gastric emptying is determined by the total energy content irrespective of energy source or is also influenced by the source of energy. Soluble protein tends to become insoluble at the low pH normally present in the stomach. However, soy protein hydrolysates are now available that stay in solution over a wide range of concentrations and pH, making it possible to test the properties of protein using liquids. Soy protein hydrolysates were used in the present study to investigate the effect of energy content on the rate of gastric emptying, energy delivery and fluid availability from two energy sources, namely glucose and protein.
In addition to the measurement of gastric emptying using a gastric aspiration technique, an isotopic tracer for water was added to ingested solutions. By measuring the appearance of this tracer in the circulation, an indication of the combined effects of gastric emptying and intestinal absorption on unidirectional water flux can be obtained. The accumulation in the circulation of a tracer added to the test drink has previously been used as an index of water absorption from ingested solutions (Pinson, 1952; Davis et al. 1987; Leiper & Maughan, 1988). The rate of tracer accumulation has been shown to follow known patterns of gastric emptying and intestinal absorption (Davis et al. 1987), and to be reproducible (Lambert & Maughan, 1992).
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Methods |
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Subjects were asked to refrain from strenuous exercise and from consumption of alcohol for 24 h prior to each of the experimental trials, and reported to the laboratory in the morning after an overnight fast. A nasogastric tube (French levine, 14 gauge, Vygon, Ecouen, France) was positioned in the stomach as described by Hassan & Hobsley (1970). Tests were conducted 1 week apart with one solution tested each day in a randomized order, and subjects remained seated throughout the study. The 600 ml test drink contained 15 mg l–1 of phenol red as a non-absorbable marker. Solutions were instilled rapidly into the stomach via the nasogastric tube; this procedure was completed within 1 min. Although the test drink was instilled, this will be referred to as ingestion.
Gastric emptying was measured using a modification of the double sampling gastric aspiration technique of George (1968) as described by Beckers et al. (1988). Gastric samples were collected at 10 min intervals for 1 h after ingestion. One small modification to the technique was that the dye concentration (g l–1) in the 5 ml aliquot added at each sampling point was increased progressively (0.25 at the 10 and 20 min sample points; 0.50 at the 30 and 40 min sample points; 1.0 at the 50 min sample point; 2.0 at the 60 min sample point) in order to improve the accuracy of measurement (Hurwitz, 1981). From the concentration of dye in the samples, the volume in the stomach at the times of sampling was calculated as described by Beckers et al. (1988).
Four test drinks were studied: a 60 g l–1 soy protein hydrolysate solution (6P); a 120 g l–1 soy protein hydrolysate solution (12P); a 70 g l–1 glucose solution (7G); and a 23 g l–1 glucose solution (LG). The soy protein hydrolysate (Nova, Denmark) also contained approximately 9% carbohydrate and fibre and approximately 6% moisture (from the manufacturer's data sheet), so to make a glucose solution iso-energetic with the 60 g l–1 soy protein hydrolysate solution, 70 g l–1 glucose was needed (Table 1). The dilute glucose solution LG was made to have the same osmolality as 6P. The measured osmolality of the solutions (mosmol kg–1) was: 6P, 321 ± 24; 12P, 506 ± 35; 7G, 608 ± 20; LG, 311 ± 5.
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The soy protein hydrolysate also contained chloride and small amounts of sodium and potassium. In order to minimize the differences between the solutions, electrolytes were added such that all solutions contained the same electrolytes in the same concentrations (Table 1).
Phenol red was analysed spectrophotometrically after dilution (1: 20) with an NaOH–NaHCO3 (250: 500 mmol l–1, pH 9.7) buffer. The glucose content of solutions was measured enzymatically with a glucose test kit (GOD-Perid kit, Boehringer Mannheim, Lewes, UK). The pH of the gastric samples was measured within 2.5 h of sampling (pH meter 140, Corning Ltd, Halstead, Essex, UK), and the osmolality by freezing point depression (Osmomat 030, Gonotec, YSI, Farnborough, UK). Sodium and potassium concentration in the samples were measured using flame photometry (Clinical Flame Photometer 410c, Corning Ltd); chloride concentration was determined using a coulometric titrator (PCLM 3; Jenway, Dunmore, Essex, UK).
The isotopic tracer measurements were carried out simultaneously with the gastric emptying measurements. After passing the gastric aspiration tube, subjects immersed one hand in hot water (42 °C) for 10 min to arterialize the venous blood (Forster et al. 1972). An indwelling butterfly cannula (21 g) was introduced into a superficial dorsal hand vein and two 5 ml arterialized venous blood samples were collected 2 min apart. The average concentration of deuterium in these two samples was taken as the background level of deuterium in the circulation, and this value was subtracted from the concentration measured in subsequent samples. The test solution, containing 12 g of 99.8% deuterium oxide (Norsk Hydro, Norway), was served at room temperature, and was instilled into the stomach as described above. Arterialized venous blood samples were collected at 2, 5, 10, 15, 20, 30, 45 and 60 min following ingestion of each of the solutions. The hand remained immersed in water maintained at 42 °C throughout the trial. Whole blood samples were frozen, and the water content was collected by vacuum distillation. The deuterium content in the blood water was measured by infra-red spectrophotometry (Miran 1FF; Buck Scientific, Harold Wood, Essex, UK).
Results were tested for normality of distribution by correlation of n scores (Minitab). The normally distributed results were analysed by a two-factor repeated measure ANOVA followed by a least significance difference comparison of the means where significant effect was found. The time for half the original test solution volume to empty from the stomach (t1/2) was calculated as described by Elashoff et al. (1982); these values were not normally distributed and as such were tested using the Kruskal–Wallis non-parametric one-way analysis of variance and pairwise differences were assessed using Wilcoxon's matched-pairs signed ranks test where appropriate. Results are reported as mean ±S.E.M and median (range) as appropriate. Significance level in all cases was taken as P P 0.05.
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Results |
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Although the stomach was washed and emptied prior to the instillation of the test drinks, a variable amount of gastric residue was inevitably present, causing the total volume in the stomach to be greater than the volume of test drink instilled. Swallowed saliva and gastric secretions were added to the gastric volume during the course of the study and are included in the measured total volume. The total fluid volume present in the stomach at each sample point is shown in Fig. 1. At all sample points, the total volume in the stomach was greater after ingestion of the 120 g l–1 soy protein hydrolysate solution than after ingestion of each of the other solutions. At the 10 min sample point, no other differences in total volume were observed. At the 20, 30, 40 and 50 min sample points after ingestion, total volume in the stomach was lower after ingestion of the dilute glucose solution (LG) than after ingestion of any of the other solutions. At the 10, 20 and 30 min sample points, there was no difference in the total volume in the stomach after ingestion of the two iso-energetic solutions (6P, 7G). At the 40, 50 and 60 min sample points, however, the total volume in the stomach was greater after ingestion of the 60 g l–1 soy protein hydrolysate solution than after ingestion of the iso-energetic glucose solution.
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The volume of test drink remaining in the stomach, i.e. the volume after correction for the initial residual volume and the volume of swallowed saliva and gastric secretions, was calculated separately and is shown in Table 2. At all sample points, there was a greater volume of test drink remaining in the stomach after ingestion of 12P than after ingestion of any of the other solutions. At the 20, 30 and 40 min sample points, the volume of test drink in the stomach was smaller after ingestion of LG than after ingestion of any of the other solutions. At the 10, 20, 30 and 40 min sample points after ingestion, there was no difference in the volume of test drink remaining in the stomach between the two iso-energetic solutions (6P, 7G), but at the 50 and 60 min sample points, the volume of test drink in the stomach was greater after ingesting 6P than after ingesting 7G.
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Energy delivery to the small intestine
Both the amount of soy protein hydrolysate and the amount of glucose delivered to the small intestine are reported as kilojoules emptied into the small intestine; the cumulative energy delivery to the small intestine is shown in Fig. 2. At all sample points, the dilute glucose solution delivered less energy to the small intestine than the two protein hydrolysate solutions, even though the volume emptied was greater. At the 10 and 20 min sample points after ingestion, there was no difference in energy delivery between the two glucose solutions (7G and LG). At the 10, 20, 30, 40 and 50 min sample points there was no difference between solutions 6P, 12P and 7G. At the 60 min sample point, the energy delivered to the small intestine was greater after ingestion of 12P than after ingestion of 6P. There was no significant difference in energy delivery between the two iso-energetic solutions (6P, 7G) during the first 50 min after ingestion. At the 60 min sample point, the energy delivered to the small intestine was greater after ingestion of 7G than after ingestion of 6P. The energy delivery from 12P and 7G was similar at all sample points.
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The pH of the glucose solutions was greater than that of both protein-containing solutions before ingestion (Table 1). After mixing with the gastric residue, which took approximately 1 min, the pH of the two glucose solutions decreased, but that of the protein-containing solutions remained essentially unaltered. The pH of the solutions 1 min after ingestion was: 6P, 4.6 ± 0.1; 12P, 4.6 ± 0.1; 7G, 2.7 ± 0.3; LG, 3.1 ± 0.9. At this and at all subsequent sample points, the pH of the two protein solutions was higher than that of the two glucose solutions. At the 30, 40 and 50 min sample points, the pH of the gastric contents was higher after ingestion of 12P than after ingestion of 6P. The pH of the solutions 60 min after ingestion was: 6P, 3.9 ± 0.3; 12P, 4.4 ± 0.2; 7G, 1.7 ± 0.3; LG, 1.8 ± 0.4.
Osmolality
These solutions contained deuterium oxide as a tracer for assessment of the net water absorption from these solutions. The mean osmolality (mosmol kg–1) of the solutions before adding deuterium oxide was: 6P, 321; 12P, 506; 7G, 608; LG, 311 (Table 1). Deuterium oxide depresses the freezing point of aqueous solutions, and after adding 12 g of deuterium oxide, the measured osmolality (mosmol kg–1) was: 6P, 278 ± 14; 12P, 454 ± 23; 7G, 558 ± 4; LG, 262 ± 4. Because of the possibility of varying concentration of deuterium oxide in the gastric aspirate, only estimates of the osmolality of the gastric contents could be made in this experiment.
Deuterium accumulation in the circulation
All concentrations have been corrected for baseline levels of deuterium in the circulation before ingestion of the test solution containing deuterium oxide. The highest concentration of deuterium in the blood after each treatment for each subject was taken as the peak concentration of deuterium (Cmax), the time at which the Cmax occurred was taken as the time (tmax) to peak concentration, and the deuterium accumulation rate (p.p.m. min–1) was calculated by linear regression of the deuterium concentrations measured before ingestion of the test solution to Cmax for each subject.
Peak concentration of deuterium in the circulation
The peak concentration of deuterium in the blood (Cmax) was greater after ingestion of LG than after ingestion of any of the other solutions (Table 3). The Cmax level was higher (P= 0.03) for 7G than for 12P. There was a tendency for Cmax to be greater after ingestion of 6P than the other soy protein solution 12P but no statistically significant difference was detected (P= 0.06). Similarly, whereas there was a trend for Cmax to be greater following intake of 7G than following intake of the iso-energetic solution 6P, no statistical difference could be detected (P= 0.07).
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The time to reach peak concentration of deuterium in the circulation (tmax) was shorter after ingestion of LG and 7G than after ingestion of 6P (P= 0.04) and 12P (P= 0.02) (Table 3). There was no difference (P= 0.33) in tmax between the two soy protein hydrolysate solutions (6P and 12P). tmax was not different (P= 0.1) between LG and 7G.
Rate of accumulation of deuterium in the circulation
The rate of deuterium accumulation in the circulation was faster (P= 0.01) for LG than for 6P and 12P, but not faster (P= 0.06) than for 7G (Table 3). The rate of accumulation was faster (P= 0.02) after ingestion of 7G than after ingestion of 12P, but not faster (P= 0.1) than after ingestion of the iso-energetic 6P. The two soy protein hydolysate solutions had a similar (P= 0.25) rate of accumulation of deuterium in the circulation.
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Discussion |
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The receptor/receptors that initiate feedback mechanisms from the small intestine to regulate the rate of gastric emptying have not been identified or understood, but several hypotheses have been postulated. The osmoreceptor postulated by Hunt & Pathak (1960) appears to be the most popular. This osmoreceptor would be located at or close to the site of absorption. Both glucose and amino acids are actively absorbed in the small intestine, but by different transport processes. In the present study the osmolality of iso-energetic solutions of glucose and protein was different; a receptor responding to absorption rates may react equally to transport of the two. There are a variety of transport mechanisms for carbohydrates and amino acids, and with so many different transporters, it is likely that there are at least as many different receptors regulating the gastric emptying response to nutrients. The osmoreceptor postulated by Hunt & Pathak (1960) is envisaged as a volume receptor: intracellular volume would alter because of water flux following on from solute absorption, but this would require certain assumptions to be met. This osmoreceptor must be located at or close to the site of absorption and must respond equally to any actively absorbed particle. However, to give the same rate of emptying for iso-energetic solutions with different osmolality, it would have to be able to distinguish between the osmotic consequences of the active transport of solute and the water flux resulting from the osmotic gradient itself. Vist & Maughan (1995) compared the emptying rates of solutions containing glucose and glucose polymer so as to vary both energy density and osmolality independently: solutions with different energy content but similar osmolality emptied at different rates, but where the energy content was similar, even large differences in osmolality had relatively little effect on the emptying rate. It is apparent from these results that the effects of osmolality were small in comparison with those of energy density.
Even in the presence of actively transported solutes such as glucose and amino acids, hypertonic solutions will promote a net efflux of water into the intestinal lumen (Gisolfi et al. 1990; Rehrer et al. 1992), whereas net water uptake is observed from isotonic or hypotonic solutions, suggesting that any role of a simple osmoreceptor is unlikely to be a major one. Throughout the gastric emptying measurement period the pattern of differences in the estimated osmolality of the stomach aspirates appeared to be similar to that of the original solutions. At the end of the 60 min emptying period the calculated mean (±S.D.) osmolality of the aspirates of LG was 283 ± 24, of 7G was 531 ± 12, of 6P was 300 ± 18 and of 12P was 487 ± 9 mosmol kg–1. However, the osmolality of the solutions in contact with the absorptive mucosa is not known, and as such it is possible that rapid hydrolysis of the protein hydrolysates at the brush border might greatly alter the osmolality of these solutions presented to intestinal osmoreceptors. The recognized effects of luminal hypertonicity in producing a net efflux of water into the intestinal lumen, nevertheless, must affect the response of mucosal osmoreceptors, suggesting that osmolality has only a limited effect on the control of gastric emptying of liquids. The ability of different energy substrates to exert an effect on emptying in proportion to their energy density remains difficult to explain.
Increasing the carbohydrate content of a solution will generally increase the energy delivery to the small intestine (Hunt & Knox, 1968; Hunt & Stubbs, 1975; Foster et al. 1980; Hunt et al. 1985; Murray, 1987). In spite of the slower rate of emptying of 7G compared with LG, because of its greater carbohydrate content, 7G delivered a greater amount of carbohydrate to the small intestine. As expected, the two iso-energetic solutions (6P, 7G) delivered similar amounts of energy to the small intestine for the first 50 min after ingestion. Doubling the energy content from 60 to 120 g l–1 soy protein hydrolysate did not result in a greater energy delivery to the small intestine during the first 40 min after ingestion.
The similar rate of energy delivery to the small intestine after ingestion of 7G, 6P and 12P may be explained if the rate of gastric emptying was regulated to ensure a constant rate of energy delivery to the small intestine as suggested by Brener et al. (1983). Although the concentrated solution (12P) appeared to be emptied in an almost linear fashion, the other three more dilute solutions were emptied in a more exponential pattern. The rate of energy delivery was similar between these three solutions, but it was not constant and tended to decrease as the volume in the stomach, and hence the rate of emptying, decreased. Others (Hunt et al. 1985; Rehrer et al. 1990; Vist & Maughan, 1994, 1995; Calbet & MacLean, 1997) have also shown that the energy delivery to the small intestine is not constant: increasing either the energy content or the volume of the ingested solution increases the rate of energy delivery to the small intestine. The 60 g l–1 soy protein hydrolysate solution and the 23 g l–1 glucose solution had similar osmolality but were not emptied at the same rate, verifying previous findings that the osmolality of the ingested solution is of less importance than energy content for regulation of the rate of gastric emptying of energy-containing solutions (Hunt et al. 1985; Vist & Maughan, 1995; Calbet & MacLean, 1997).
The rate of accumulation of deuterium in the circulation is a measure of the difference between the rate at which the tracer moves from the small intestine into and out of the circulation into the extravascular space (Maughan & Leiper, 1990). As the concentration in the circulation increases and the rate at which the tracer equilibrates with the extravascular space approaches the rate at which the tracer enters the circulation, the rate of accumulation slows, and if the rates are similar, a distinctive peak in the concentration of tracer in the circulation may not be seen. The results of deuterium accumulation in the circulation show a faster rate of fluid availability after ingestion of the 23 g l–1 glucose solution than after ingestion of the other three more concentrated solutions. This correlates well with the gastric emptying results, in which, after the first 10 min of rapid emptying, LG was emptied more rapidly than any of the other solutions. The results also suggest that fluid availability after ingesting solutions is strongly influenced by the rate of gastric emptying.
Both the slowly emptying 12P and 6P solutions resulted in a slower rate of accumulation of tracer in the circulation than the two glucose solutions. The time to peak concentration, the concentration of deuterium in the blood, the peak value, and the rate of accumulation were greater after ingestion of 7G than after 12P, indicating a more rapid availability of fluid after ingestion of 7G than after ingestion of 12P. Although the rate of energy delivery to the small intestine from these two solutions was similar, the slower emptying of 12P will have resulted in a smaller volume and hence a smaller amount of deuterium available for absorption in the small intestine. The measured concentration of deuterium in the circulation was not significantly different between the two iso-energetic solutions (6P, 7G), which correlates well with the similar rates of gastric emptying for these two solutions. The time to peak concentration was slightly slower (P= 0.04) for 6P than for 7G, and there was a tendency for the peak deuterium value to be less (P= 0.07) after ingesting the protein solution that after the iso-energetic glucose solution; however, the accumulation rates were similar (P= 0.1). This suggests that the rate of intestinal absorption of water from the 6% protein solution may have been marginally slower than from the 7% glucose solution.
The results of this study show that iso-energetic solutions of glucose and soy protein hydrolysate used were emptied from the stomach at similar rates, and that the unidirectional intestinal water flux following ingestion of these two solutions is similar.
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References |
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