Gastric Emptying For Specific Foods May Be A Key To Managing Deadly Illnesses In The Elderly

Bethesda, MD., April 2003 The rate of gastric emptying is a major measure of the glucose and cardiovascular responses to oral carbohydrates. The former is key to the dietary management of people with diabetes mellitus, in whom strict control of blood glucose has been shown to reduce microvascular complications. As cardiovascular responses are relevant to the prevention of postprandial hypotension (subnormal arterial blood pressure following a meal), this can be an important clinical problem. Both Type 2 diabetes and postprandial hypotension occur most frequently in the elderly.

Now, a team of Australian physiologists has produced evidence that the gastric emptying, or nutrient absorption, for specific foods, may hold the answer for dietary management of deadly illnesses affecting the elderly population.

Background

An individuals blood glucose levels after eating are influenced by a number of factors but it is now recognized that gastric emptying accounts for at least 35 percent of the variance in peak postprandial glucose levels after oral glucose (75 grams) in both healthy individuals and patients with Type 2 diabetes. Controlling gastric emptying by dietary and pharmacological means in order to minimize postprandial glucose represents a new approach to glycemic control. Studies in rodents have established the importance of early insulin release in the control of postprandial glucose excursions in that a small, early increase in blood/portal insulin levels is more effective than a larger, later increase in reducing blood glucose levels. Thus, whereas slowing of nutrient absorption may benefit Type 2 diabetes patients, it is possible that modest acceleration of the initial gastric emptying rate of carbohydrates would have a beneficial effect on overall glycemia in Type 2 diabetes. This would also be true among healthy subjects because it would to an increase in early insulin release, particularly if the subsequent emptying of carbohydrates is slower.

Postprandial hypotension, defined as a fall in systolic blood pressure of approximately 20 mmHg after a meal, occurs frequently in older persons and in patients with diabetes mellitus and autonomic neuropathy and is associated with a number of clinical conditions including loss of consciousness, falls, stroke, angina, and increased mortality. Ingestion of a carbohydrate, particularly large amounts, induces the greatest cardiovascular response, particularly after a meal while fat, protein, or water have relatively little effect.

The mechanisms mediating the fall in postprandial blood pressure are poorly defined. It is believed that digestive blood flow, release of gut hormones, and sympathetic nervous activity are thought to play a role. Recent studies indicate that the magnitude of the fall in blood pressure in both Type 2 diabetes and healthy older subjects is greater when gastric emptying is relatively more rapid. Furthermore, in healthy older subjects, the fall in blood pressure and increase in heart rate are greater during intraduodenal glucose infusion. These observations suggest that the postprandial fall in blood pressure and rise in heart rate may be related to the early phase of gastric emptying.

A New Study

The aims of a new study were to evaluate the effects of ingestion of a solid, noncarbohydrate meal on gastric emptying and intragastric distribution of, and the glycemic and cardiovascular responses to, a liquid glucose load in healthy, older subjects. The study objective was to assess whether a solid meal would accelerate the initial emptying phase and intragastric distribution of a liquid leading to (1) an overall reduction in postprandial glycemia consequent to an increase in early insulin secretion and (2) a greater postprandial fall in blood pressure and increase in heart rate.

The authors of "Solid Meal on Gastric Emptying of, and Glycemic and Cardiovascular Responses to, Liquid Glucose in Older Subjects, are Melanie K. Berry, Antonietta Russo, Judith M. Wishart, Anne Tonkin, Michael Horowitz, and Karen L. Jones, all from the University of Adelaide, Royal Adelaide Hospital, Adelaide, South Australia, Australia. Their findings appear in the April 2003 edition of the American Journal of PhysiologyGastrointestinal and Liver Physiology, one of 14 journals published monthly by the American Physiological Society (APS).

Methodology

Twelve healthy older subjects (six male, six female), age 72.9 + 2.1 years; body mass index (BMI) 25.3 + 0.5 kg/m2, were enrolled in the study. Each subject underwent concurrent measurements of gastric emptying, blood glucose, serum insulin, blood pressure, and heart rate on two days separated by at least three days. After exclusions, ten subjects were studied. Each subject had measurements of gastric emptying, blood glucose, serum insulin, blood pressure, and heart rate after ingestion of a glucose drink with (mixed meal) or without (liquid only) a solid meal.. None of the research subjects had evidence of postprandial hypotension or cardiovascular autonomic neuropathy.

Results

Gastric emptying of liquid was initially slightly more rapid after the mixed meal compared with liquid only at five minutes and much slower after 120 minutes. The time to peak blood glucose was less and blood glucose subsequently lower after the mixed meal. The increase in serum insulin was greater after the mixed meal. Blood pressure fell in the first 30 min, with no difference between the two meals. Increase in heart rate after both meals was greater after the mixed meal.

The results also revealed:

the presence of a 300-g noncarbohydrate solid meal has discrepant effects on early and subsequent emptying of a nutrient liquid meal, so that gastric emptying of a 50-g glucose drink is initially slightly faster and then much slower compared with when the liquid is consumed alone;

acceleration of the early liquid emptying induced by the solid meal is associated with a change in intragastric distribution, so that relatively more liquid is retained in the distal stomach;

the more rapid, early emptying of glucose is associated with an earlier peak in blood glucose, a greater serum insulin response, and an overall reduction in postprandial glycemia;

and whereas the initial postprandial fall in blood pressure was not affected by the presence of a solid meal, the increase in heart rate was greater.

Conclusions

These observations are consistent with the concept that the early phase of gastric emptying is a major determinant of postprandial glycemia as well as the cardiovascular response to a meal and have implications for the dietary management of Type 2 diabetes and postprandial hypotension.

Although the subjects were healthy volunteers, the observations are consistent with evidence that modulation of gastric emptying by dietary or pharmacological means could be used to optimize blood glucose control in Type 2 diabetes. The novel concept presented is that dietary strategies should be directed at the stimulation of a greater initial insulin response by accelerating the early emptying of carbohydrates and, subsequently, slowing it to delay glucose absorption. Studies in patients with Type 2 diabetes are required to evaluate this further, however.

The findings also indicate that in healthy older subjects, the rate of delivery of glucose to the small intestine is a determinant of the postprandial fall in blood pressure and increase in heart rate. In this research, the initial fall in blood pressure after an oral glucose load was not affected by a solid meal, whereas it may have been expected that more rapid, early liquid emptying would induce a greater fall in blood pressure. It appears that the initial absorption rate of glucose is a fundamental determinant of the cardiovascular response to carbohydrate, although the mechanisms mediating these effects remain uncertain. Accordingly, dietary strategies for the prevention of postprandial hypotension should perhaps focus on delaying the initial gastric emptying and/or small intestinal absorption of carbohydrate. Studies are now required to address these hypotheses.

Source: April 2003, edition of the American Journal of PhysiologyGastrointestinal and Liver Physiology, one of 14 journals published monthly by the American Physiological Society (APS).

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.

Source: American Physiological Society