The effect of increasing levels of various protein sources on zinc absorption from a legume-based meal was studied in humans with the use of a radionuclide technique. The meals were extrinsically labelled with 65Zn and absorption was determined from measurements of the whole-body retention of the isotope. The mean fractional zinc absorption for the 13 meals was 24.7 +/- 6.9% and was only influenced by the protein content of the meal to a limited extent (r = 0.45). However, the amount of zinc absorbed from the meals was strongly correlated with both the protein (r = 0.85) and zinc content (r = 0.86): 5.9 +/- 1.7 mumol of zinc was absorbed from the basal bean meal which had the lowest protein content; the addition of low zinc chicken doubled the protein content and increased zinc absorption to 10.3 +/- 2.0 mumol; the addition of zinc-rich beef also doubled the protein content, however, zinc absorption was increased to 15.9 +/- 4.7 mumol. It is concluded that the zinc content of the main protein source of the diet determines the amount of zinc absorbed to a large extent. However, relatively small amounts of animal protein can significantly improve the value of a legume-based meal as a source of zinc.
The absorption of zinc from meals based on 60 g of rye, barley, oatmeal, triticale or whole wheat was studied by use of extrinsic labelling with 65Zn and measurement of the whole-body retention of the radionuclide. The cereals were prepared in the form of bread or porridge and were served with 200 mL of milk. The oatmeal flakes were also served without further preparation. The absorption of zinc was negatively correlated to the phytic acid content of the meal with the highest absorption, 26.8 +/- 7.4%, from the rye bread meal containing 100 mumol of phytic acid and the lowest, 8.4 +/- 1.0%, from oatmeal porridge with a phytic acid content of 600 mumol. It is concluded that food preparation that decreases the phytic acid content improves zinc absorption.
Colonic retention of zinc and calcium was studied after installation during colonoscopy of 30 mumol of zinc and 6.4 mmol of calcium labeled with 65Zn and 47Ca, and measurement of the whole-body retention of the radionuclides. After cecal installation in nine patients, retention (day 13) of zinc was 3.5 +/- 2.1% (mean +/- SD) and of calcium 3.5 +/- 2.7%. The calculated mean absorption was 4.1% for zinc and 14.1% for calcium. Application at the hepatic flexure in four patients resulted in a mean retention (day 13) of 1.2% for zinc and 0.6% for calcium. Under prevailing conditions, colonic absorption of zinc is relatively small, compared to the uptake after oral administration. Colonic absorption of calcium could, however, account for a substantial part of the total calcium uptake.
The absorption of zinc from soy protein-containing meals was studied in healthy human subjects by means of extrinsic labelling with 65Zn and whole body counting. Three types of soy protein, a soy flour, a soy concentrate and a soy isolate, were used in two types of meals: one consisting of rice and meat sauce and the other of white or whole-meal bread. Thirty and 100% of the protein of the meat sauce and 50% of the protein of bread was replaced with the soy protein products. Replacement of 30% of the protein in meat sauce had a marginal effect on the percentage absorption of zinc, whereas the absolute amount of zinc absorbed was lower due to the lower zinc content in soy products than in meat. After total replacement of the meat protein, a significantly lower percentage and absolute absorption of zinc was observed. A lower absorption of zinc was observed when soy protein was added to white bread. The absorption of zinc from the whole-meal bread was low and did not change when 50% of the flour protein was replaced by soy protein. We conclude that the effect of soy protein on zinc absorption depends on the degree of replacement, the phytic acid and zinc content of the soy product and the protein content of the meal.
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