Single-meal Studies Research Articles

The effect of calcium on iron absorption

The experimental and epidemiological evidence demonstrating that Ca inhibits Fe absorption was reviewed, with the objectives of estimating the potential impact of variations in Ca intake on dietary Fe bioavailability and of providing some guidelines for predicting the effects on Fe status of recent recommendations for higher dietary Ca intake. In animal models Ca salts reduced both haem- and non-haem-Fe absorption, the effect being dependent on the amount of Ca administered rather than the Ca:Fe molar ratio; dairy products had a variable effect; factors other than Ca may have been important. In single-meal human absorption studies, both haem- and non-haem-Fe absorption was inhibited by Ca supplements and by dairy products, the effect depending on the simultaneous presence of Ca and Fe in the lumen of the upper small intestine and also occurring when Ca and Fe were given in the fasting state. The quantitative effect, although dose dependent, was modified by the form in which Ca was administered and by other dietary constituents (such as phosphate, phytate and ascorbic acid) known to affect Fe bioavailability. The mechanism by which Ca influences Fe absorption has not been elucidated. The effects of factors that modulate Fe bioavailability are known to be exaggerated in single-meal studies, and measurements based on several meals are more likely to reflect the true nutritional impact. The results of most multiple-meal human studies suggest that Ca supplementation will have only a small effect on Fe absorption unless habitual Ca consumption is very low. Outcome analyses showed that Ca supplements had no effect on Fe status in infants fed Fe-fortified formula, lactating women, adolescent girls and adult men and women. However it should be noted that the subjects studied had adequate intakes of bioavailable Fe and, except in one study, had relatively high habitual Ca intakes. Although cross-sectional analyses in Europe have shown a significant inverse correlation between Ca intake (derived primarily from dairy foods) and Fe stores, the quantitative effect was relatively small. The general conclusion is that dietary Ca supplements are unlikely to have a biologically significant impact on Fe balance in Western societies unless Ca consumption is habitually very low; however, increased consumption of dairy products may have a small negative effect that could be functionally important in pregnancy if Fe supplements are not taken. It is uncertain whether the inverse relationship between consumption of dairy products and Fe status is due entirely to increased Ca intake; substitution of milk proteins for meat may also have negative effects on Fe balance.

Effect of milk and fermented milk on iron absorption in ileostomy subjects

This study was undertaken to determine whether milk with its high calcium content adversely affects the absorption of nonheme iron from the diet as greatly as single-meal studies indicate. Nine ileostomy subjects ate a low-fiber, low-phytate diet for 8 consecutive weeks. During the first and eight weeks they drank a 250-mL soft drink with three main meals and an evening snack each day (0.16 g Ca/d). During the two intervening 3-wk periods, they drank the same amount of low-fat milk or fermented low-fat milk (Verum; Hälsofil, Norrmejerier, Umeä, Sweden) according to a formally randomized crossover design (1.4 g Ca/d). During the last 2 d in each of the four periods, apparent iron absorption (balance) from a test diet together with that period's beverage was measured and the plasma ferritin concentration was determined. There was no decrease in apparent iron absorption during the milk diet periods.

Effects of aspartame ingestion on the carbohydrate-induced rise in tryptophan hydroxylation rate in rat brain

Effects of aspartame (aspartyl-phenylalanine-methylester) on increases in brain-tryptophan level and hydroxylation rate following a high-carbohydrate, protein-free meal were tested. After an overnight fast, rats consumed a protein-free meal containing one of several levels of aspartame. Blood and brain amino acid levels and the in vivo rate of tryptophan hydroxylation in brain were estimated at intervals thereafter. Ingestion of the meal alone increased brain-tryptophan level and hydroxylation rate. Aspartame did not modify these effects, except at doses of 530 mg/kg body weight or more. Results suggest a threshold dose of aspartame can be identified for the rat in single-meal studies above which suppression of carbohydrate-induced increases in brain-tryptophan level and serotonin synthesis occurs. This dose, however, is large and, when corrected for species differences in metabolic rate, is unlikely to be ingested by a human subject as a single load.