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Department of Nutrition, University of California at Davis, Davis, CA 95616-8669
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONDietary factors that influence...Future directionsREFERENCES |
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KEY WORDS: • zinc • zinc absorption • zinc bioavailability • inhibitors • phytate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONDietary factors that influence...Future directionsREFERENCES |
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), supplementation or
fortification with zinc has been associated with increased linear
growth (Brown et al. 1998
), reduction in diarrheal
disease (Black 1998
), enhanced immune function
(Shankar and Prasad 1998
) and improved pregnancy outcome
(Goldenberg et al. 1995
). Although the cause of
suboptimal zinc status in some cases may be inadequate dietary intake
of zinc, inhibitors of zinc absorption are likely the most common
causative factor. Many staple foods in developing countries, including
cereals, corn and vegetables, are relatively good sources of zinc, but
even if net zinc intake appears adequate by most recommendations,
compromised zinc status is common (Gibson et al. 1998
).
It is therefore important to identify and evaluate dietary factors that
affect zinc absorption. With such knowledge, better advice may be given
with regard to avoiding or limiting components with inhibitory effects
on zinc absorption and choosing foods or dietary components that
enhance zinc absorption. Furthermore, agricultural and food processing
methods that reduce the content of inhibitors of zinc absorption may be
developed and put into common use. |
Dietary factors that influence zinc absorption |
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TOPABSTRACTINTRODUCTIONDietary factors that influence...Future directionsREFERENCES |
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The amount of zinc in a meal will, in itself, affect zinc absorption
(Sandström and Cederblad 1980
). With increasing
amounts of zinc in a meal, fractional zinc absorption (%) will
decrease. This has, for example, been shown by Sandström and Cederblad (1980)
, who administered human adults
radiolabeled zinc solutions in water and measured zinc absorption by
whole-body counting. When 40 µmol was administered, 73 ± 10% of the dose was absorbed, whereas 46 ± 13% was absorbed
when 200 µmol was administered. The amount of zinc absorbed, however,
increased from 29 ± 4 to 92 ± 26 µmol. It is likely that
the reduced fractional absorption of zinc at higher doses is due to
saturation of the transport mechanisms for zinc. Zinc absorption has
been shown to consist of a specific, saturable carrier-mediated
component and a nonspecific, unsaturable diffusion-mediated
component (Menard and Cousins 1983
, Steel and Cousins 1985
). The capacity of the saturable transport
mechanism to absorb zinc has been investigated in experimental animals
and humans. Coppen and Davies (1987)
found that when
dietary zinc was increased from 5 to 40 mg/kg, fractional zinc
absorption decreased, whereas the amount of zinc absorbed increased
linearly at higher dietary levels, which would be consistent with a
diffusion process. In humans, intestinal perfusion studies with
solutions containing increasing concentrations of zinc showed linear
increases in zinc absorption from 0.1 to 1.5 mmol/L zinc, with rates
leveling at higher concentrations (Lee et al. 1989
).
Sandström (1992)
calculated that the zinc
concentration in the duodenum after a meal in a human is likely to be
<2 mg/L (30 µmol/L); thus, in the presence of dietary ligands, the
concentration of "free" zinc is likely to be considerably lower.
Therefore, zinc is likely to predominantly be transported via the
saturable, specific transport mechanism. It is likely that zinc
absorption also is homeostatically regulated by intestinal excretion of
zinc; considerably lower quantities of endogenous zinc were excreted
when infants were fed a low zinc formula than when a formula with a
regular level of zinc was fed (Ziegler et al. 1989
).
When evaluating the effect of dietary factors on zinc absorption in
humans, it is important to consider methodological constraints. The
absorption of zinc from water solutions is quite different from that
from meals, and effects on zinc uptake that may be observed for water
solutions may not be found or may be considerably diminished when meals
are consumed (Sandström 1992
,
Sandström et al. 1985
). A compilation of data from
zinc absorption studies in humans shows that the amount of zinc
absorbed from single meals usually levels off at ~18–20 µmol,
whereas zinc absorption from water solutions can reach levels of
80–100 µmol (Lönnerdal 1997a
,
Sandström 1992
). These studies clearly illustrate
that different considerations must be made when zinc is administered as
a supplement, apart from meals, and when zinc is part of a meal or
added to food as a fortificant.
Long-term zinc intake, i.e., zinc status, can also affect absorption of
dietary zinc. Although the long-term use of zinc supplements does
not appear to cause any down-regulation of zinc absorption compared
with normal, healthy subjects not taking any supplements
(Sandström et al. 1990
), low zinc intake and zinc
status do affect zinc absorption. Istfan et al. (1983)
fed young men a formula diet containing either 1.5 or 15 mg zinc/d and
measured zinc absorption in a fasted state after 6 d. Zinc
absorption was 92% from the low zinc diet and 81% from the high zinc
diet. Wada et al. (1985)
performed similar stable
isotope studies in young men and found that zinc absorption from the
diet was 53% when the zinc intake was 5.5 mg/d and that it decreased
to 25% when 16.5 mg/d was fed. Similarly, August et al. (1989)
found that young adult subjects absorbed 64 ± 5%
of zinc from the diet when it contained 2.8–5 mg/d but only 39 ± 3% when it contained 12.8–15 mg/d. Differences were also found in
elderly subjects (43 ± 7% versus 21 ± 1%), but as can be
seen, the extent of zinc absorption was lower in this age group. We
used infant rhesus monkeys and radioisotopes and found that zinc
absorption rapidly increased from ~40% when a formula with the
current level of zinc fortification (4 mg/L) was fed to ~60% when a
formula with lower zinc content (1 mg/L) was fed (Polberger et al. 1996
). Thus, it appears that feeding low zinc diets
increases zinc absorption in all age groups and that homeostatic
mechanisms up-regulate zinc absorption and retention. Previous zinc
intake may therefore have an effect on studies on zinc bioavailability.
Protein quantity and quality.
The amount of protein in a meal is positively correlated to zinc
absorption (Sandström et al. 1980
). When compiling
results from several studies with humans to whom various protein
sources and amounts had been administered, fractional zinc absorption
increased in a linear fashion with increasing protein content
(Sandström 1992
). It should also be emphasized
that protein is a major source of dietary zinc that results in an
increased zinc intake with increased protein content of the meal. Thus,
in general, increased dietary protein leads to increased zinc intake
and a higher bioavailability of the zinc provided.
The type of protein in a meal will also affect zinc bioavailability.
Animal protein (e.g., beef, eggs, cheese) has been shown to counteract
the inhibitory effect of phytate on zinc absorption from single meals
(Sandström and Cederblad 1980
), but this may be
due to amino acids released from the protein that keep the zinc in
solution (see later) rather than a unique effect of animal protein as
such. Casein in milk has been shown to have a negative effect on zinc
absorption. We found that zinc absorption was higher with
milk-based infant formula than with cow’s milk
(Sandström et al. 1983
). Because the milk formula
was whey adjusted, we hypothesized that casein had a negative effect on
zinc absorption compared with whey protein. When two milk formulas,
which were manufactured to differ only in their whey-to-casein ratio
(60:40 versus 20:80) were compared, zinc absorption in human adults was
significantly higher from the whey-predominant formula than from
the casein-predominant formula (32 versus 21%)
(Lönnerdal et al. 1984
). It is likely that
phosphorylated serine and threonine residues on partially undigested
casein subunits bind zinc and reduce zinc bioavailability in a manner
similar to what has been shown for iron (Hegenauer et al. 1979
, Hurrell et al. 1989
). Casein
phosphopeptides
(CPP),2
however, which are formed during the digestion of casein, may affect
zinc absorption in a manner different from casein as such. These
smaller peptides contain phosphorylated threonine and serine residues
and will, depending on the closeness of the negatively charged
phosphate groups, bind mineral ions such as zinc
(Lönnerdal 1997b
). It has been shown in vivo that
CPP are formed, that they bind calcium and that they are capable of
keeping calcium in soluble form in the intestinal lumen. CPP are now
commercially available and may be used in human nutrition. However,
although several studies have documented a positive effect of CPP on
calcium absorption, there are less data on zinc absorption. In a recent
study (Hansen et al. 1996
), we found that the addition
of CPP to phytate-containing solutions significantly increased
calcium and zinc absorption in suckling rat pups as well as in human
intestinal cells (Caco-2) in culture. A human study on zinc absorption
with a high phytate infant gruel showed that CPP addition had no
effect, whereas a stimulatory effect was found for a rice-based
gruel with very low phytate content (Hansen et al. 1997a
). Thus, the effect of CPP may be dependent on the
composition of the meal, particularly its phytate content. This may
also explain the lack of an effect of CPP on zinc absorption from bread
meals (Hansen et al. 1997b
). It is also possible that a
beneficial effect of CPP is more pronounced with liquid meals (e.g.,
formula) than with solid foods (e.g., gruel, bread). Finally, CPP may
potentially be more active when being formed during digestion than when
fed preformed; any effect on zinc absorption may in fact depend on the
digestion and release of amino acids from the CPP, which in themselves
may positively affect zinc absorption (Hansen et al. 1997b
).
Studies on the effect of various protein sources are often confounded
by the fact that the proteins often contain other constituents that may
affect zinc absorption. Soy protein isolates, for example, which are
often used in composite meals, normally contain considerable amounts of
phytate (discussed later). In a recent study with radioisotopes,
Davidsson et al. (1996)
studied zinc absorption from
liquid meals to which different proteins were added. In their study,
phytate had been removed from the soy protein through treatment with
phytase, and the effect of soy protein as such could be studied. When
comparing the addition of 30 g of demineralized bovine whey or
casein, bovine serum albumin (BSA), egg albumen and soy protein, zinc
absorption was found to be significantly reduced compared with the
protein-free meal when BSA or soy protein was added. The authors
compared their results with those of similar studies on iron absorption
from liquid meals and found that the inhibitory effect of BSA and soy
protein on zinc absorption was less pronounced than that observed for
iron (Hurrell et al. 1989
). No significant inhibitory
effect of bovine casein was found in their study. However, they used
casein, which had been demineralized by the use of an ion exchanger,
whereas in our study, we did not use demineralized casein because the
product was an infant formula containing minerals. It is possible that
the treatment itself affected the structure, composition and
mineral-binding capacity of the protein source.
The dephytinized soy protein isolate studied by Davidsson et al. (1996)
was found to have a negative effect on zinc absorption,
which is consistent with previous observations on iron absorption.
These results should be viewed with some caution, however. We studied
zinc absorption from infant formula based on dephytinized soy protein
isolate in infant rhesus monkeys and found that zinc absorption
increased compared with regular soy formula and became virtually
identical to that from milk-based infant formula
(Lönnerdal et al. 1988
). Thus, the soy protein
itself did not appear to have any significant inhibitory effect on zinc
absorption. In addition, when we added phytate to milk formula, zinc
absorption decreased significantly and became very similar to that from
soy formula, again suggesting that soy protein itself has no
significant inhibitory effect. It appears from the study by
Davidsson et al. (1996)
that the phytase-treated soy
protein isolate that was used received no further treatment before
inclusion in the test meal. Therefore, this protein source must have
contained a considerable amount of inorganic phosphate, which is
released by the enzyme treatment. It is possible that the high amount
of phosphate had a negative effect on zinc absorption, rather than the
soy protein itself.
Phytate and fiber.
It was shown early in animal studies that phytate has an inhibitory
effect on zinc absorption (O’Dell 1969
, Vohra and Kratzer 1964
). The phosphate groups in inositol
hexaphosphate can form strong and insoluble complexes with cations such
as zinc, and because the gastrointestinal tract of higher species lack
any significant phytase activity, phytate-bound minerals will be
excreted in the stool. The finding of zinc deficiency in human subjects
in the Middle East suggested that phytate can affect zinc status in
humans as well (Halsted et al. 1972
), and subsequent
studies confirmed that this was the case. Because staple foods in most
part of the world contain phytate (e.g., corn, cereals, rice, legumes),
it is obvious that both zinc and iron status may be compromised in
significant portions of the population. By using radioisotopes and
whole-body counting, we found very low zinc bioavailability from
soy-based infant formula compared with milk formula and human milk
(Sandström et al. 1983
). As discussed briefly
previously, the addition of phytate to milk formula reduced zinc
absorption to a level similar to that from milk formula, supporting the
belief that the low zinc absorption from soy was due to its phytate
content. When the phytate was removed from soy protein isolate by a
precipitation process, zinc absorption was significantly improved
(Lönnerdal et al. 1988
). An accompanying
experiment in suckling rat pups using increasing concentrations of
phytate suggested that the absorption of zinc is inversely correlated
to the phytate concentration of the diet and that there is no threshold
level that must be surpassed to observe an effect, as has been
suggested for iron. Thus, any reduction in dietary phytate content is
likely to result in an improvement in zinc absorption.
There are several methods available to reduce the phytate content of
various foods. Leavening of bread was shown early to decrease its
phytate concentration (Nävert et al. 1985
), and
fermentation in general also achieves the same effect, resulting in
enhanced zinc absorption (Gibson et al. 1998
,
Svanberg and Sandberg 1988
). Germination and milling can
also reduce the phytate content of legumes and cereals (Gibson et al. 1998
,Svanberg and Sandberg 1988
).
Recently, the treatment of foods with food-grade commercial phytase
or the addition of phytase to the diet has been shown to effectively
reduce the phytate content of various foods, with a subsequent
beneficial effect on mineral absorption (Sandberg et al. 1996
, Türk and Sandberg 1992
). Finally,
plant breeding and genetic engineering can be used to produce low
phytate cultivars of cereals and legumes with improved mineral
bioavailability (Mendoza et al. 1998
).
Phytate in food is composed of a mixture of different phosphorylated
forms of inositol phosphate (Sandberg and Ahderinne 1986
); the hexaphosphate is usually the major form, but
pentaphosphates, tetraphosphates and triphosphates are also present.
Because various types of processing can alter the proportions of these
inositol phosphates, it is important to evaluate their individual
effects on zinc absorption. We found that the hexaphosphate and
pentaphosphate forms inhibited zinc absorption in a rat pup model,
whereas the tetraphosphate and triphosphate forms had no significant
effect (Lönnerdal et al. 1989
). Subsequent studies
in human subjects confirmed these findings (Sandström and Sandberg 1992
). It thus becomes evident that "total
phytate" of a meal or a diet is too crude of a measure when
evaluating zinc bioavailability; instead, methods that specifically
quantify the various forms of inositol phosphates are needed when
assessing the effects on zinc absorption (Sandberg and Ahderinne 1986
).
Fiber is often implied as having a negative effect on zinc absorption.
However, this is usually due to the fact that most fiber-containing
foods also contain phytate. Knudsen et al. (1996)
recently reported low zinc absorption from a fiber-rich diet, but
the diet was also high in phytate. Reducing the phytate content of
bread by leavening considerably increased zinc absorption to a degree
similar to that from white bread (low fiber), suggesting that fiber in
itself has no or little effect on zinc absorption (Nävert et al. 1985
). Studies on isolated fiber components such as
-cellulose (Turnlund et al. 1982
) show no significant
inhibitory effect on zinc absorption. It is therefore unlikely that
fiber has any negative effect on zinc nutrition of humans.
Calcium.
It appears unlikely that calcium per se has a negative effect on zinc
absorption. We added calcium to cow’s milk formula to a level of
~1300 mg/L and found no significant difference in zinc absorption
from the formula with the regular level of calcium (500 mg/L) through
the use of radioisotopes in human adults and paired observations
(Lönnerdal et al. 1984
). Similarly, Spencer et al. (1984)
and Dawson-Hughes et al. (1986)
added large amounts of calcium to a meal and found no effect on zinc
absorption in human adults. It also appears that the long-term use
of calcium supplements has no effect on zinc status; Gambian women who
were given 1000 mg calcium/d had plasma zinc concentrations similar to
those of unsupplemented women (Yan et al. 1996
).
The calcium content of the diet may, however, affect zinc absorption
from phytate-containing meals. It has been postulated that the
formula [Ca] x ([phytate]/[Zn]) ratio can be used as a
predictor of zinc bioavailability (Fordyce et al. 1987
).
The reason for this is that calcium has the propensity to form
complexes with phytate and zinc that are insoluble and consequently
have an inhibitory effect on zinc absorption. Although there certainly
are studies that support this concept, the interaction is complex, and
it is possible that this ratio may be of limited predictive value. For
example, we added calcium to a soy-based infant formula (1300
versus 550 mg/L) and found that zinc absorption increased significantly
compared with regular soy formula, even though an increased [Ca] x
[phytate]/[Zn] ratio would predict lower zinc absorption. We
hypothesized that a larger proportion of calcium bound to phytate in
the gastrointestinal tract, thereby making more zinc available for
absorption (Lönnerdal et al. 1984
). Several
authors have suggested that the phytate-to-zinc molar ratio can be used
to estimate zinc bioavailability from the diet (Davies and Olpin 1979
, Lo et al. 1981
, Morris and Ellis 1980
), and it is possible that this ratio in general may have
more predictability than the ratio that includes calcium as a variable.
Iron.
The potential interaction between iron and zinc has caused concern.
Iron deficiency is the most common single-nutrient deficiency in
the world, and many fortification and supplementation programs have
been launched to improve iron nutrition. However, with an increasing
awareness that marginal zinc deficiency also may be common, a negative
impact of iron provision on zinc absorption and status could further
exacerbate zinc status. It is therefore important to critically
evaluate any possible interactions between iron and zinc. It was found
by Solomons and Jacob (1981)
that high doses of
inorganic iron decreased zinc uptake as measured by changes in plasma
zinc over the next 4 h after an oral dose. Human adults were
administered 25 mg of zinc (as ZnSO4) in water
solution, and iron was added at 25, 50 or 75 mg. Plasma zinc uptake was
reduced significantly when 25 mg of iron was added, and this effect was
magnified when the dose level was increased to 50 or 75 mg. However,
the dose of zinc that was used was very high to detect sizable
increases in plasma zinc after the meal, and it was administered in a
fasting state, which may amplify the effect. We used a dose of zinc
similar to that obtained from most meals and studied zinc absorption by
using radiolabeled zinc and whole-body counting
(Sandström et al. 1985
). We found a significant
reduction in zinc absorption in the fasting state when iron was added
to the zinc dose in water solution at a 25:1 molar ratio but not at a
2.5:1 ratio, which is similar to the ratio used in the study by
Solomons and Jacob (1981)
. Thus, the interaction appears
much less pronounced when zinc intake is closer to a
"physiological" level. We also studied the effect of adding
histidine to the test dose as a model of a meal containing ligands that
bind zinc. In this case, no significant negative effect on zinc
absorption was found even when the ratio between iron and zinc was
25:1. Furthermore, when the same dose of zinc was added to a meal and
iron was added at a ratio of 2.5:1 or 25:1, no inhibitory effect on
zinc absorption was observed. Thus, it appears that the effect of iron
on zinc is exerted only at a very high ratio of iron to zinc and in
water solution. This suggests that iron fortification will not affect
zinc absorption and that if iron and zinc are administered in a
supplement and this is given apart from meals, an inhibitory effect
will be found only when the iron-to-zinc ratio is very high, which is
unlikely if pharmaceutical preparations are made with the purpose of
eliminating both iron and zinc deficiencies. That iron fortification of
foods is unlikely to affect zinc absorption was demonstrated recently
by Davidsson et al. (1995)
, who examined the effect of
iron fortification of bread (65 mg/kg), weaning cereal (500 mg/kg) and
infant formula (12 mg/L) in human adults with the use of stable
isotopes. No significant negative effect on zinc absorption was found
compared with the same foods without iron fortification. Similar
negative results were obtained by Fairweather-Tait et al. (1995)
, who studied the effect of iron fortification of a
weaning food on zinc absorption in infants with the use of stable
isotopes.
It is hypothetically possible that the administration of iron
supplements during a longer period of time could cause mucosal loading
with iron, which in turn may affect mechanisms of zinc uptake and
transport. However, we gave iron drops (30 mg/d) to infants for 6 mo
and found no effect on zinc status as assessed by plasma zinc
(Yip et al. 1985
). Furthermore, we gave 50 mg iron/d to
human adults for 2 wk and found no significant effect on zinc
absorption using whole-body counting (Sandström et al. 1985
). Thus, it appears that the long-term use of iron
supplements does not impair zinc absorption or zinc status.
Copper.
Although it is unlikely that the level of copper in the diet or in a
supplement would be substantially elevated, it is important to evaluate
the potential interaction between copper and zinc, particularly because
an interaction has been described in experimental animals, albeit at
very high ratios. August et al. (1989)
studied the
effect of the addition of 2 mg of copper (corresponding to the
Estimated Safe and Adequate Daily Dietary Intake for adults) to
human adults consuming a diet with 0.4–1.0 mg copper and 12.8–15 mg
of zinc per day by using stable isotopes. No negative effect on zinc
absorption was detected, suggesting that modestly increased intakes of
copper do not interfere with zinc absorption when zinc intake is
satisfactory. It remains to be studied whether increased copper intake
affects zinc absorption when zinc intake is low. Such a situation may
exist in countries where the drinking water is contaminated with copper
and the dietary intake of zinc is low.
Cadmium.
It is known that toxic levels of cadmium can inhibit zinc absorption. This is beyond the scope of the present article, but it is important to recognize that some foods, such as cereals, contribute a small but significant amount of cadmium to our daily diet. The extent to which, if any, these nontoxic levels of cadmium affect zinc absorption in humans is not well known.
Low-molecular-weight ligands and chelators.
When zinc forms a complex between a low-molecular-weight ligand or chelator and that complex can be absorbed, it is likely that the net effect on zinc absorption will be positive, because the solubility of zinc is increased. Ligands/chelators (e.g., EDTA), amino acids (e.g., histidine, methionine) and organic acids (e.g., citrate) have therefore been used in efforts to enhance zinc bioavailability.
EDTA.
Early studies in turkeys and chicks demonstrated that the negative
effect of phytate on zinc absorption could be overcome by the addition
of EDTA to the diet (Kratzer et al. 1959
, O’Dell 1969
). Studies in human subjects using Na-Fe-EDTA confirmed
these results (Solomons et al. 1979
). The basic
principle behind this effect is that EDTA can help to solubilize zinc
from more or less insoluble phytate-zinc complexes and form
stronger complexes due to its high binding constant to zinc. However,
this positive effect is not always observed, and in some cases a
negative effect on zinc absorption has been noted. It was realized that
the ratio of EDTA to inhibitors such as phytate and other cations
competing for complex formation, such as Ca2+,
Mg2+ and Fe2+, was critical
when it comes to the effect being positive or negative. It has
subsequently been found that the Zn-EDTA complex is transported
intact from the lumen into the enterocyte but not across the
basolateral membrane (Hempe and Cousins 1989
). The fact
that no Zn-EDTA complex could be detected in plasma supported this
notion and was further in agreement with findings by O’Dell (1969)
that parenteral EDTA administration increases zinc
excretion and that Zn-EDTA in blood is not utilizable. Not only the
addition of EDTA to the diet should be considered; Davidsson et al. (1994)
recently described a positive effect on zinc
absorption of the addition to cereal-based diets of Na-Fe-EDTA
compared with ferrous sulfate. The authors suggested that the
Na-Fe-EDTA complex dissociates, at least partially, at the lower pH of
the stomach and that Zn-EDTA complexes are formed. This in turn may
help zinc to stay soluble but also to be taken up by the enterocyte in
the presence of inhibitors, such as phytate. Inside the enterocyte,
dissociation of the Zn-EDTA complex may allow zinc to become
complexed to other, transferable ligands or to be transported in free
form across the basolateral membrane. It is evident that the
interaction between EDTA and zinc as well as other cations and other
low-molecular-weight chelators (Desrosiers and Clydesdale, 1989
) and its consequences for metal ion transport warrant
further studies.
Amino acids.
Histidine is good chelator of zinc, and clinical studies in human
subjects have shown a positive effect of histidine on zinc absorption
as measured by the increase in plasma zinc (as area under the curve)
(Schölmerich et al. 1987
). However, the molar
ratio of histidine to zinc is of importance because it may have a
strong effect on zinc metabolism; high doses of histidine were used
earlier to induce experimental zinc deficiency in human subjects,
because they enhance the urinary excretion of zinc (Henkin et al. 1975
). That this may occur with more modest excesses of
histidine was indicated in a clinical trial in which we found that
infants fed formula with a high protein concentration had lower plasma
zinc concentrations than infants fed formula with less protein
(Lönnerdal and Chen 1990
). Because infants fed the
high protein formula had significantly higher plasma histidine
concentrations than those fed formula with less protein, we suggested
that they also had higher urinary losses of histidine and zinc. It is
important to emphasize that the net effect of an amino acid on zinc
absorption and retention must be considered; studies on cells or
perfused intestines may well show a positive effect of amino acids such
as histidine on zinc uptake (Wapnir et al. 1983
), but if
urinary losses are also increased, there may be no or a negative effect
on overall zinc retention.
Methionine complexes of zinc have been used to improve zinc absorption
from the diet, but besides improved carcass quality in steers
(Greene et al. 1988
), there is limited evidence for
this. Hempe and Cousins (1989)
found that the
zinc-methionine complex reduces zinc absorption in ligated rat
duodenal loops, which is in agreement with similar
observations on intestines from swine and poultry (Hill et al. 1987
). Because the proportion of absorbed
65Zn was similar in rats fed
ZnC12 or zinc-methionine, they concluded that
the zinc-methionine complex is not absorbed intact. Thus, the
association of zinc to methionine may be too weak to make the complex
"survive" passage in the upper gastrointestinal tract, and
dissociation of zinc and reassociation with other ligands may occur.
Organic acids.
Various organic acids that are present in foods, such as citric, malic
and lactic acid, have been shown to facilitate the absorption of iron
(Gillooly et al. 1983
). These acids, however, are less
effective than ascorbic acid for stimulating iron absorption, and it is
believed that the effect is due to weak chelation, which may help keep
the metal ion in solution or facilitate its uptake by the mucosal cell.
Although these acids may enhance iron absorption less than ascorbic
acid, it should be emphasized that in many foods they are present in
substantial concentrations. Citrate, for example, is present in human
milk at 3–5 mmol/L and therefore binds a significant part of zinc in
breast milk (Lönnerdal et al. 1980
). The addition
of organic acids, such as citrate, to foods can therefore under some
conditions enhance zinc absorption (Pabon and Lönnerdal 1992
).
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TOPABSTRACTINTRODUCTIONDietary factors that influence...Future directionsREFERENCES |
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FOOTNOTES |
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2 Abbreviations used: BSA, bovine serum albumin; CPP, casein phosphopeptides.
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TOPABSTRACTINTRODUCTIONDietary factors that influence...Future directionsREFERENCES |
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2. Black R. E. Therapeutic and preventive effects of zinc on serious childhood infectious diseases in developing countries. Am. J. Clin. Nutr. 1998;68(suppl.):476S-479S[Abstract]
3. Brown K. H., Peerson J. M., Allen L. H. Effect of zinc supplementation on children’s growth: a meta-analysis of intervention trials. Bibl. Nutr. Dieta 1998;54:76-83
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