Effects of dietary zinc upon tissue zinc and percent unsaturated plasma-zinc binding capacity

Invited review: Mineral absorption mechanisms, mineral interactions that affect acid-base and antioxidant status, and diet considerations to improve mineral status

Several minerals are required for life to exist. In animals, 7 elements (Ca, P, Mg, Na, K, Cl, and S) are required to be present in the diet in fairly large amounts (grams to tens of grams each day for the dairy cow) and are termed macrominerals. Several other elements are termed microminerals or trace minerals because they are required in much smaller amounts (milligrams to micrograms each day). In most cases the mineral in the diet must be absorbed across the gastrointestinal mucosa and enter the blood if it is to be of value to the animal. The bulk of this review discusses the paracellular and transcellular mechanisms used by the gastrointestinal tract to absorb each of the various minerals needed. Unfortunately, particularly in ruminants, interactions between minerals and other substances within the diet can occur within the digestive tract that impair mineral absorption. The attributes of organic or chelated minerals that might permit diet minerals to circumvent factors that inhibit absorption of more traditional inorganic forms of these minerals are discussed. Once absorbed, minerals are used in many ways. One focus of this review is the effect macrominerals have on the acid-base status of the animal. Manipulation of dietary cation and anion content is commonly used as a tool in the dry period and during lactation to improve performance. A section on how the strong ion theory can be used to understand these effects is included. Many microminerals play a role in the body as cofactors of enzymes involved in controlling free radicals within the body and are vital to antioxidant capabilities. Those same minerals, when consumed in excess, can become pro-oxidants in the body, generating destructive free radicals. Complex interactions between minerals can compromise the effectiveness of a diet in promoting health and productivity of the cow. The objective of this review is to provide insight into some of these mechanisms.

Effect of microbial phytase on zinc bioavailability and cadmium and lead accumulation in growing rats

This study evaluated the effect of increasing levels of dietary microbial phytase on the bioavailability of zinc and the accumulation of cadmium and lead in growing rats. Five groups of seven albino rats (initial average weight 47 g) were housed individually and fed phytate-rich diets (7 g/kg) based on maize, soya bean meal, corn starch and soya bean oil over a 4-week experimental period. The basal diet contained 24 mg zinc (native concentration), 10 mg lead as Pb(CH3COO)2.3H2O and 5 mg cadmium as CdCl2 per kg and was supplemented with 0, 250, 500, 1000 and 2000 U phytase from Aspergillus niger per kg diet. Supplementation of microbial phytase significantly increased apparent zinc absorption. Differences in zinc bioavailability due to supplementation of the diet with microbial phytase were evident in zinc concentration in plasma, femur and testes as well as in the percentage unsaturated plasma zinc binding capacity and the activity of the zinc metalloenzyme alkaline phosphatase. Cadmium concentrations in liver and kidneys were not significantly altered in response to the different dietary treatments. There was a tendency for femur lead concentration to be increased in response due to the phytase supplementation.

Zinc oxide and amino acids as sources of dietary zinc for calves: effects on uptake and immunity

Calf starter diets were formulated to contain 60 ppm of Zn, 150 or 300 ppm of Zn in the form of Zn-Met and Zn-Lys, or 300 ppm of Zn in the form of ZnO to compare relative bioavailability and effects on immunity. Holstein heifer calves were weaned at wk 5 and fed experimental starter diets from wk 6 to 12. Feed intake, body weight, Zn concentrations in liver and serum fractions, and mineral concentrations in serum were measured to determine the effects of treatment. In addition, peripheral blood lymphocyte blastogenesis, interleukin-2 production, cytotoxic activity, and the ability of blood neutrophils to phagocytose and kill bacteria were assessed at wk 0, 2, 4, and 6 of the trial. Feed intakes and body weight gains were similar among calves. Concentrations of Zn in serum were elevated in calves fed 300 ppm of Zn as Zn-Met and Zn-Lys but not in calves fed ZnO. Concentrations of Zn in liver were significantly elevated by 300 ppm of Zn in the form of Zn-Met and Zn-Lys (360 micrograms/g) but not by the other Zn treatments or by the control (245 micrograms/g). No treatment had an effect on the concentrations of Lys and Met in serum; however, concentrations of Lys did decrease in serum as the age of the calves increased. There was no significant treatment effect on mitogen-induced lymphocyte blastogenesis, interleukin-2 production, lymphocyte cytotoxicity, or phagocytic and intracellular killing ability of blood neutrophils. These data indicated greater absorption and retention of Zn when administered in the form of Zn-Met and Zn-Lys than that when ZnO was administered to young calves. However, there was no advantage to the immune function of extra dietary Zn.

[Zinc--update of an essential trace element]

Since the recognition of zinc as an essential trace element in man and animals there has been a remarkable progress in our knowledge of the role of zinc in nutritional physiology, biology and medicine during the last few decades. Highlights in zinc research, mechanisms and homeostatic regulation of zinc absorption, sources of zinc intake, dietary factors and mineral interactions affecting zinc bioavailability are reviewed in the present paper. This is followed by an overview of the biochemical functions of zinc in enzymes, gene expression, endocrinology, immunology and oxidative stress. General signs and metabolic consequences of zinc deficiency as well as excessive intake and toxicity of zinc are summarized. Furthermore, national and international dietary zinc recommendations and different methods to determine the zinc status are discussed.

Enhancement of zinc utilization from phytate-rich soy protein isolate by microbial phytase

A study with three groups, each with 11 male, individually housed albino rats (initial average weight = 50 g) was undertaken to examine the effect of microbial phytase (added to a diet containing phytate) on the availability of zinc. The rats were fed diets on the basis of soy protein isolate and corn starch over a 3-week period. All diets contained 15-16 mg Zn/kg diet and 0.40% PA. Thus, molar PA:Zn-ratios of 26:1 were obtained. Group I (control) was fed the phytase-free basal diet. In groups II (pair-fed to group I) and III, 1,000 U of microbial phytase (Aspergillus niger var. van tighem) per kg diet were added. Some rats fed the phytase-free basal diet (control) showed typical symptoms of zinc deficiency, including cyclic changes in food intake, anorexia and partial alopecia. By the addition of 1,000 U microbial phytase the apparent absorption of zinc (percent of intake) significantly increased from 33% (control) to 63% (1,000 U, pair-fed) and 66% (1,000 U, ad lib.). Similar positive effects of the phytase-supplementation were observed for three zinc status parameters in plasma, zinc-concentration, percent unsaturated zinc-binding capacity, activity of alkaline phosphatase and the zinc-concentration in femur and testes. The present study shows that an addition of microbial phytase to phytate-rich diets based on soy protein isolate considerably improves the availability of zinc in growing rats.

[Estimation of the zinc requirement for broilers using their ability for selective zinc absorption and by dose-response relations]

Broiler chicks are able to select an adequate dietary Zn-concentration in choice-feeding. The zinc requirement should be ascertained by this method of self-selection. 72 day-old chicks were divided into 9 groups and were kept over 5 weeks in individual cages. By gradually supplementing ZnSO4.7H2O to a semisynthetic basal diet with isolated soybean protein 5 diets were made with Zn-concentrations of 14, 22, 30, 38 and 50 ppm, which respectively were fed to one group. The four other groups had to choose between two rations: 14/38; 14/50; 22/38; 22/50. Selective zinc intake was represented by a significant increase of dietary zinc concentration compared to random selection (mean of both diets). Feed intake, zinc intake and weight gain were measured daily and the end total feed consumption, live weight, feed conversion rate, plasma-zinc concentration and plasma-zinc binding capacity at the end of the experiment. In comparison optimal dietary zinc levels were estimated by dose-response relations, both by sigmoid growth curves and by broken-line model. With 30 ppm Zn chickens reached a high fattening level with daily weight gains of 60 g. Feed intake and growth rate were markedly reduced at a dosage of 14 and 22 ppm Zn, feed conversion rate was tendentially decreased below a level of about 34 ppm, whereas above zinc binding capacity reached a plateau. Birds self-selected a dietary zinc level of 32 ppm, which were adequate for the criteria feed intake and weight gains. Zinc requirements of about 40 ppm were assessed by conventional methods.

[The effect of a supplement of microbial phytase on zinc availability]

A study of 35 (5 x 7) male, individually housed, albino rats (initial average weight = 50 g) was undertaken to examine the effect of an addition of microbial phytase to a diet containing phytate on the availability of zinc. The rats were fed a semisynthetic diet based of egg white and cornstarch over a 3-week period. All diets were supplemented with 20 mg Zn/kg. Group I (control) was fed the basal diet free of phytic acid (PA) and phytase. By replacing cornstarch by Na-phytate (0.5% in group II and 1.0% group III), molar phytate: Zn ratios of 25 and 50:1 were obtained, respectively. In groups IV (0.5% PA) and V (1.0% PA) 1000 U of microbial phytase were added. A molar phytate:Zn ratio of 25 (group II) and 50:1 (group III) resulted in a dose-dependent depression of growth and feed efficiency ratio. These negative effects of the addition of PA could be completely counteracted by the supplementation of 1,000 U of phytase in group IV and partially so in group V. Similarly, the apparent absorption and retention of Zn, Zn-concentration in femur and testes and different Zn-status-parameters in plasma (Zn-concentration, percent unsaturated plasma-Zn binding capacity, activity of alkaline phosphatase) were improved by adding 1,000 U microbial phytase/kg diet. The present study shows that an addition of microbial phytase to phytate-rich diets considerably improves the availability of Zn in growing rats.

[Zn-binding capacity of serum. A parameter for diagnosing marginal Zn deficiency (author's transl)]

In two experiments with male Sprague-Dawley rats, the influence of varying the dietary Zn content on per cent Zn-binding capacity of the serum was to be measured. In the first experiment, young rats were given, after a 14-day Zn depletion, the following diets with 1.3, 4, 6, 8, 10, 12, 20, and 100 mg Zn/kg DM for 2 weeks. Per cent Zn-binding capacity of the serum was 90% in the extreme deficiency range and decreased to less than 60% with increasing dietary Zn content, whereas the serum Zn content rose steadily with the Zn supply. Following Zn injection, per cent Zn-binding capacity of the serum significantly decreased to values around 70% in all animals of the groups given 1.3--12 mg dietary Zn. At 20 and 100 mg/kg of diet, per cent Zn-binding capacity of the serum was not affected by Zn injection. In the second experiment, per cent Zn-binding capacity of the serum was also found to depend upon Zn supply without prior depletion in rats of three different age groups with life weights of 50, 100, and 300 g. Under these experimental conditions, per cent Zn-binding capacity of the serum proved to be a good indicator for diagnosing alimentary Zn supply.

Toxicity of ammonium molybdate added to drinking water of calves

Molybdenum, as ammonium molybdate, was added to the drinking water of 5-wk-old calves to establish the minimum toxic concentration. A basal diet with 13 ppm copper and .29% sulfur was fed ad libitum for 21 days. The concentration of copper in liver was reduced with 50 ppm added molybdenum in water but not with 1 or 10 ppm. However, copper in plasma was elevated with 50 ppm added molybdenum in water while changes in ceruloplasmin concentration were nonsignificant. The calculated percent copper as ceruloplasmin copper in plasma decreased from 61% to 43% with all additions of molybdenum. Apparently uptake of plasma copper by tissues was reduced by molybdenum decreasing the bioavailability of copper. These data indicate the difficulty of detecting molybdenum-induced hypocuprosis from plasma copper and ceruloplasmin without data on tissue copper. With dietary levels of 13 ppm copper and .29% sulfur, the minimum toxic concentration of molybdenum in drinking water for calves is between 10 and 50 ppm, and the critical copper-to-molybdenum ratio is less than .5. Molybdenum in water may be less toxic to calves than molybdenum in fresh forages.

Biological availability of zinc from inorganic sources with excess dietary calcium

The effects of chemical form of supplemental zinc and elevated dietary calcium on intestinal absorption of zinc were measured. Calves and rats were fed diets low in zinc, and zinc availability was the percentage increase of zinc in plasma with dietary supplementation of zinc. Availabilities of zinc supplied as zinc chloride, zinc sulfate, zinc oxide, and zinc carbonate were comparable in both calves and rats. Elevated amounts of dietary calcium as ground limestone reduced absorption of zinc in rats fed soy-protein but had no effect on absorption of zinc in the lactating cow. Thus, for cows consuming large amounts of calcium, an increase in the concentration of zinc beyond 40 ppm of the diet appears unnecessary.