Test meal studies

Stimulated mainly by the work of Trowell, David Jenkins and co-workers carried out a series of test meal experiments. Eight type two diabetics were given two test meals of bread and marmalade (Jenkins et al., 1976a), one of which contained a supplement of the fibers guar and pectin. Plasma glucose was significantly lower from 30-90 min after the high 

More recently, Jenkins and co-workers have carried out in vitro studies to learn more of these mechanisms. Lentils, soya beans, and wholemeal 

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T33. Tulin, M., Guttman, J., and Almy, T. P., The secretion of gastric mucin following an alcohol test meal Studies on normal subjects and on patients with peptic ulcer. Gastroenterology 9, 191-197 (1947). 

In studies on test meals, Walker et al. (32) discovered that the calcium of Swiss chard, which has a high oxalic acid content, was poorly absorbed. Children excreted more calcium during the 6 hours after a test meal of milk, Pumpkin leaves, cassava leaves, or pigweed leaves than after Swiss chard. All supplements contained 200 mg calcium. 

In the studies on humans there appeared to be decreased calcium balances when 200 g or more of spinach per day was included in the diet. In two of the studies in which women were fed spinach, calcium intakes were below the Recommended Dietary Allowance of 800 mg/day (37). Some studies were conducted for short period of a week or less, which may not be sufficient time to adjust to a change in diet. From measurement of calcium excretion in urine after a test meal, it was shown that the calcium in oxalate-containing vegetables was less well-absorbed than that of milk or of vegetables not containing oxalic acid. However, this would not necessarily affect calcium balance, since the total amount of calcium in the diet would have to be considered. The effect of a combination of oxalic acid and fiber on calcium bioavailability should be further investigated. 

Results from some recent studies (19-36) on the effects of fiber are summarized in Table I. For the most part, the results are from multi-day balance studies. However, Turnland et al. (36) used the stable isotope fecal monitoring method to assess zinc utilization and Simpson et al. (24) measured iron absorption from a single test meal. 

In a series of studies, Cook and co-workers (48,49,50) observed a marked decrease in non-heme iron absorption when soy protein was added to beef or protein mixtures or when soy protein was consumed as the primary protein source in test meals and compared to the effects of including egg albumen or beef alone in the test meal (Table III) Hallberg and Rossander (38J, however, found less of an effect when total iron absorption (heme plus non-heme iron) was considered. Likewise, in a study in which reconstituted textured soy was used to replace 30% of the beef, the absorptions of total iron were not greatly altered (51). In a study by Stekel et al. (52), replacing part of the beef with soy isolate or adding hydrated isolate to beef decreased non-heme iron absorption from 12.4% to 9.2 and 9.3%, respectively. In a related study, non-heme and total iron absorptions were low when soy isolate was the major protein source. 

Series of studies with test-meals in which the major protein sources were (g protein and source) (a) 14g, beef, (b) 7g, beef,... 

As noted above, disagreement has often been observed among different studies on the effects of fiber, phytic acid and protein source on mineral utilization. Some possible reasons include (a) estimates of absorption from single meals (with or without previous consumption of the same foods used in the test meal which may also affect results) may not always be equivalent to results from multi-day balance studies, (b) in balance studies, the failure to allow sufficient time (e.g., 1-2 weeks or more) for adaptation may alter the findings, (c) variations in the compositions of meals or diets, including mineral levels, between studies may influence the results obtained, and (d) the persons used as subjects vary and this may have an affect. In addition, in the fiber studies, the levels, types, and particle size of fiber fed have varied widely and levels of other possibly confounding components (e.g., caffeine, tanins, oxalates) may have differed. 

In another study in which the test meal procedure was used,59 "true achlorhydria" was found in about 3 per cent of the younger men, but the incidence increased up to 23 per cent for men between 60 and 69 years old. Corresponding values for women were about the same for the younger group, but the incidence was 26 to 28 per cent in women between 60 and 70. Presumably the lack of hydrochloric acid would show a much higher incidence in older people if the fasting juice or the first collection of histamine juice were analyzed. 

Some (Geracioti and Liddle, 1988 Pirke et ah, 1994), but not all (Philipp et ah, 1991) studies have shown that bulimics have reduced plasma CCK after a test meal. Lower-than-average basal levels of CSF CCK have also been found in women with BN (Lydiard et... 

Hedge et al. (1995) studied 5-HT4 receptor mediated stimulation of gastric emptying in rats using a specially prepared semi-solid test meal containing charcoal. 

Voigt et al. (1995) studied the involvement of the 5-HTia receptor in CCK induced satiety by recording food intake during a 2 hours test meal in food deprived and in freely feeding rats. 

Administration of about 1 mg of Zn would appear to be acceptable compromise in studies with adult subjects consuming about 15 mg/day of natural zinc. However, it might be necessary, or desirable, to enrich the diet or a test meal with a second isotope (, J). Since Zn cannot be measured by neutron activation, Zn is a potential candidate for this second tracer. However, the natural abundance of this isotope is high (18.6 atomic %) so that the feasibility of its application at physiological intake levels must first be established. In situations where Zn is used as a label, Zn serves as the reference isotope because the precision of measurement of Zn is better than for Zn. 

Studies on the effect of oxalic acid in spinach on calcium balance in humans have shown a small decrease or no effect on calcium balance (12). However, when subjects were given test meals of either Swiss chard (13) or amaranth (, 1, which are rich in oxalic acid, urinary excretion of calcium indicated that the absorption of calcium from these sources was less than that of an equal amount of calcium from milk. Absorption of calcium from milk was also reduced when given along with amaranth (14). 

A4. Anderson, R. K., and Fogelson, S. J., Secretion of gastric mucin in man. Comparative study in normal subjects and in patients with peptic ulcer in response to alcohol test meal. /. Clin. Invest. 15, 169-172 (1936). 

The following study illustrates the range in availability of iron whenexmsumed with various foods. The subjects consumed a dose of radioactive iron ( Fe or Fe) that had been dispersed In the test food. The test foods included starch (Experiment 1), com meal mush (Expt. 2), com meal mush with meat (Expt. 3), bread (Expt. 4), and bread and meat Expt. 5). The availability of the iron was determined by measuring the amount of radioactivity incorporated into the red blood cells 2 weeks after consumption of the test meal. The results, listed in Table 10.12, show that iron s availability was lowest when mixed with the com meal and that availability was improved when meat was consumed with the com meal. The meat seemed not to affect the availability of the iron in the bread. It is not clear why the effect of the meat differed in Experiments 3 and 5. 

The following study illustrates the effects of different foods on the absorption of zinc. Subjects ate a test diet consisting of a mil made with whole wheat flour or white flour, along with other foods, as indicated in Table Itl.18. The roll contained 1.0 mg of radioactive zinc ( Zn). The total amount of zinc in the test meal was about 3.,t mg. Two weeks later, the amount of radioactive zinc remaining in the... 

Food can affect both the absorption and the clearance of theophylline. One study showed that the absorption of a modified-release formulation of theophylline was very slow after an overnight fast, in contrast to absorption after a test meal (86). The effect may be dose-related. More specifically, dietary protein significantly affects theophylline clearance a low-protein diet reduces theophylline clearance and a high-protein diet increases it. The implications for clinical practice have not been elaborated, but dietary extremes are contraindicated in patients taking theophylline (51). 

The second study also involved rats. All of the rats were raised on a diet containing 25% protein for 2 weeks. The protein was casein. On the day of the experiment, the rats were fed test diets containing 9% protein (O), 25% protein ( ), or 50% protein (A). The rats were sacrificed at various times after the test meals, as indicated in Figure 8.16, and used for assays of an enzyme used for BCAA catabolism. Muscle was used as the source of enzyme. The results demonstrate that the enzyme activity was clearly higher in rats fed diets containing elevated amoimts of protein. 

When the products of gastric digestion reach the duodenum, bicarbonate secreted by the pancreas begins to neutralize the stomach acid. The concentration of bicarbonate in pancreatic juice ranges from about 70 to about 150 meq/H (21). In a study involving human subjects, Murthy et al. (22) found the pH of duodenal aspirates to range from 4.7 to 7.2 following administration of a Lundh Test meal. 

Radioisotope techniques have allowed precise measurement of dietary iron absorption. Initial studies utilized test meals of individual food which had been intrinsically labeled with radios active iron prior to harvesting (1,2). Utilization of these single food meals allowed a rank order to be established among the tested foods. Subjects could serve as their own controls when an identical reference dose was given to each subject. 

This reference dose of ferrous sulfate ascorbic acid was absorbed at a higher rate in iron deficient subjects than in iron replete subjects a ratio of the absorption of the test meal to the reference dose allowed comparisons to be made between individual subjects. As these early studies were limited to study of single food items an effort was made to extend the technique by developing designs utilizing extrinslcally tagged test meals (3,4). Utilization of these techniques has given evidence that dietary iron forms two separate pools in the gut, one a pool of heme iron and the other a pool of nonheme iron. The predominate source of iron in human diets is in the form of nonheme iron (5),... 

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