The bioavailability of Zn depends also on the dietary enhancers and inhibitors, and host related factors. Zn in food is absorbed in ionic form through a carrier mediated transport process hence, its absorption can easily be interacted with other molecules in the intestine Soluble low molecular weight organic substances such as amino acids and organic acids act as Zn binding ligands to facilitate absorption, while other organic compounds that form insoluble complexes with Zn could inhibit its absorption ^46.
The absorption of Zn is affected by the amoimt and type of dietary protein.
Fractional Zn absorption increases linearly with the increase in protein content of a meal However, different types of proteins affect Zn absoiption differently depending on the composition of the diet particularly on its content of the phytic acid. For mstance, bovine serum albumin and the isolates of dephytinised soybean protein reduced the absorption of Zn when added to non-phytic acid meal However, when animal proteins were added to meals containing inliibitory factors
such as phytic acid increased absorption of Zn was obseived The reason for this could be due to the ability of animal proteins to prevent Zn from forming insoluble Zn-phytate complexes. In a recent study by Rosado (2005) milk and yogurt were found to increase Zn absorption by 70% and 78% respectively when taken with a meal which were related to the amino acid released from milk and yogurt proteins that keep Zn soluble and more available.
The influence of proteins on the absoiption of Zn and Fe were found to be different. For instance addition of bovine seirim albumin to the same test meal reduced Fe absoiption by 50% compared to 20% reduction on Zn absorption Soybean isolate produced also different responses when applied to the same test meal with more reduction on the absoiption of Fe than Zn Rosado et al (2005) reported no effect on the bioavailability of Fe when milk and yoghurt were added to a high phytate, while the absorption of Zn was increased significantly.
The inhibitory effect of phytic acid on Zn absorption is reported in several studies These studies demonstrated negative effects of phytic acid on the absorption of Zn even when tlie content of phytic acid was as low as 50 mg/1. Adams et al. (2002) conducted a human study to compaie the fractional absorption of Zn fr'om a diet prepaied from maize low in phytic acid with that prepared from high phytic acid maize. The former diet which contained 60% less phytic acid had 43%
increased fractional Zn absorption than the high phytic acid meal.
In a recent review, Lonnerdal (2002) discussed studies tliat have reported the effect of reducing phytic acid fr om diets on the absorption of Zn. The review presented studies which used different methods of removing phytic acid from composite meals. However, the levels of Zn absorption after the phytic acid removal were consistently approaching 45% which was similar to that from a non-phytic acid containing diet (46%). These observations confiim the inhibitory effect of phytic acid on the absorption of Zn and that any reduction in the dietary phytic acid, regardless of method used is likely to have a positive effect on the absoiption of Zn
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The effect of the elemental interaction on the absorption of Zn has also been studied and discussed. There are reports that Ca reduces the absorption of Zn in high phytic acid meals The foimula Ca x Phytate/Zn has been postulated to be used as a predictor of Zn bioavailability. This is because Ca has a tendency to form Ca-Zn- Phytate complexes that are even less soluble than Zn-phytate complexes The levels of Ca in a diet affected the phytic acid; zinc ratio that impairs the absorption of Zn. A ratio of 12:1 was found not to affect the absorption of Zn when Ca level was 0.75%, however, when the Ca level was increased to 1.75%, a ratio of less than 6:1 was foimd to be effective
The reports on the interaction of dietary Fe and Zn in gastrointestinal tract aie inconsistent. Studies involving pregnant women and children, discussed in section 2.1, have reported a negative influence of Fe on Zn absorption especially when Fe was given as supplements. However, those studies involving health adults have demonstrated less interaction particularly at low doses of Fe. The ratio of Fe/Zn of 2.5:1 reported by Solomon (1981) to inhibit the absorption of Zn was found to have no effect on the absorption of Zn in water even when provided in a fasting state.
Instead, it was a higher ratio of 25:1 that decreased the absorption of Zn horn 59% to 34% hi another study by Velberg et al. (1984) a ratio of 10:1 in a solution of ZnCh was found to reduce the absoiption of Zn from 61% to 34%. However, when the same ratio was provided with turkey, no significant decrease was observed.
These obseivations indicate that Fe supplement to healthy adults have a negative effect on the absoiption of Zn only if it is taken into an empty stomach. Furthermore, the interaction of Fe on Zn became less pronounced when the concentration of Zn was raised to a dose similar to that found in natural diets Since the Fe dose provided in all tliese studies is far higher than the content of Fe found in natural foods, then the competitive interaction between Fe and Zn in healthy adults is not likely to occur in ordinary dietary conditions However, population groups with higher requirements for Zn, such as infants, adolescents, pregnant and lactating mothers may be more sensitive to Fe-Zn interactions.
2.8.3 Bioavalability of Cu
The bioavailability of Cu in the duodenum is also a dependent of the nutritional status of the host. It has been reported that the absorption of the total ingested Cu in a body of a healthy person is 30-63% while it reaches up to 70% for Cu deficient person ^^6. Lonnerdal (1996) in his review on the bioavailability of Cu, indicated that Cu as has been explain for Fe is absorbed more from the animal- protein diet (41%) than from the plant protein-diet (34%).
However, the data in a review by Lonnerdal (2002) concluded that phytic acid has no effect on the absorption of Cu, The studies using adult humans, infant rhesus monlceys and suckling rats all consistently supported the findings that, unlike Fe and Zn, Cu absorption is not influenced by dietary phytic acid He further suggested that the reason for the lack of effect of phytic acid on the absoiption of Cu could be due to high solubility of Cu-phytate complex at the pH of the gastrointestinal fract.
Several studies have shown the inhibition of Cu absorption by Fe and Zn.
Excessive concentrations of Zn have been reported to reduce the absorption of Cu in the duodenum However, the inhibition of Zn to the absorption of Cu is dose related. Agust et al, found that Zn/Cu ratio of 2:1, 5:1, and 15:1 had no effect on the absoiption of Cu. However, very high doses of Zn have been successfully used to treat Wilson disease, an inborn Cu disorder that result in Cu toxicity High levels of dietary Fe may also affect Cu absorption Lonnerdal (1996) in his review presented two infant studies, which showed the inhibition of Cu absoiption by excess . intake of Fe.
High intake of sugar is found to increase the absorption of Cu in humans This might be due to the fact that high intake of sugar increases Cu requirements because of its involvement in the metabolism of sugar.
2.8.4 Bioavailability of Mn
The absorption of Mn for adult humans is generally low; it is reported to be less than 5% ^^6. Because of that, studies on the absorption of Mn that involve
humans are few ^^6. Therefore the model that is going to be discussed here will include results from animal studies. Results fr'om animal studies have indicated that the absoiption of Mn, as other essential elements, is an age dependant. In a research involving rats, adults absorbed only 3 - 4 % of administered dose of Mn compared to 20% absorbed by young rats The bioavailability of Mn depends on the type of food; for instance, it is absorbed more from human milk than cows’ mille i?*. It is also reported that the absorption of Mn is independent of its amount in the diet or the body burden of the element
The absorption of Mn is also influenced by other dietary factors. Studies involving human subjects have revealed an inhibitory effect of phytate on the absorption of Mn although the effect is not as high as that for Fe and Zn Low fractional absorption of Mn (0.7%) was found in infant formula based on soy isolate containing phytic acid. However, when phytic acid was enzymatically degraded from the foimula, a significant increase in the absorption was found from 0.7% to 1.6%
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Interaction of Mn and other metals in the gastrointestinal tract has been shown to reduce its bioavailability. Ca is reported to be an inhibitory factor in the absorption of Mn i?*. The absorption and retention of Mn from foods with low Fe is relatively high Using mice as a model, it was shown that if milk is supplemented with Fe both the retention and absoiption of Mn is reduced
2.8.5 Bioavailability of Cr
The absoiption of Cr in the form of Cr^^ is less than 3% irrespective of the dose while hexavalent Cr which is toxic is better absorbed The body retention of Cr is age dependent as uiinary Cr contents were found to be higher for elders than yoimg subjects
The bioavailability of Cr^^ is reduced by its ability to undergo hydration at neutral or alkaline pH and precipitates as insoluble Cr(0H)3. However, when present, amino acids compete with the hydroxide ions in solution to form Cr-amino acid soluble complexes of low moleculai* weight which are more available for absoiption
The absorption of dietaiy Cr is also enhanced by vitamin C (ascorbic acid) and vitamin B3 (nicotinic acid) taken together with a meal
The absorption of Cr is hindered by a number of nutritional factors including high content of sugar and interaction of Cr with other metals. Diets high in sugar are reported to stimulate Cr losses. Kzlovsky et al. (1986) provided 37 subjects with high sugar diets for 12 weeks. Twenty seven out of the total subjects had their urinaiy Cr losses increased from 10% to 30% compared to subjects consuming a normal sugar diet. In animal studies, the absorption of Cr is also interfered with Zn and Fe. On separate occasions, oral dose of Fe and Zn were found to decrease the absorption of Cr in rats