|
|
| Zinc
in Nutrition |
Zinc
(Zn) is an essential trace element for all animals. It is
found in all organs and tissues of the body, with bone, muscle,
liver, kidney, and skin accounting for the majority of body
Zn. Most of the Zn in foods and feeds is bound, primarily
to protein or amino acids, nucleic acids, or phytate. Zinc
must be released from these complexes before absorption from
the gut can occur.
Zinc Absorption
Released Zn2+ in the lumen in the small intestine can enter
enterocytes via either active (carrier mediated) or passive
(paracellular) absorption. Once inside the enterocyte, Zn2+
can be bound to either CRIP (cysteine-rich intestinal protein)
or metallothionein. CRIP is thought to serve as a shuttle
for transporting Zn to the basolateral membrane where Zn,
in turn, is released for active transport into the portal
blood plasma. Plasma albumen then picks up the Zn and transports
it to various sites in the body. Much of the Zn that gets
bound to metallothionein in enterocytes is transported back
to the gut lumen instead of to the portal blood. Thus, CRIP
(and perhaps other nonspecific zinc binding proteins) is considered
a positive factor in Zn absorption, whereas metallothionein
is considered a negative factor.
Zinc Excretion
Zinc absorption from food and feeds averages about 20%. The
true absorption efficiency is thought to be only 10% for plant-based
foods but 30% for animal-based foods. Thus, in terms of Zn
excretion from the body, most occurs via feces. Moreover,
when turnover of body Zn occurs, much of this Zn also ends
up in the gut via pancreatic and biliary secretions. Nonetheless,
some Zn is excreted in the urine, and major trauma (injury,
burns, etc,) can markedly increase Zn excretion via the kidney.
Zinc Functions
There are few, if any, essential nutrients that have a greater
array of functions than Zn. The list of Zn-containing or Zn-dependent
enzymes continues to increase and now numbers in excess of
200. Thus, Zn is involved in virtually all of life's processes
and has major catalytic roles in protein, lipid, carbohydrate,
and nucleic acid metabolism. It is therefore required for
growth, reproduction, appetite, vision, wound healing, heme
synthesis, immunocompetency, and hormone activity (growth
hormone, insulin, sex hormones, corticosteroid hormones).
A partial list of well-characterized Zn-dependent enzymes
is listed below.
| Enzyme |
Function |
| Carbonic
Anhydrase |
Primarily
in RBC; essential for rapid disposal of CO2 produced
in cells; also important for delivery of O2 to
cells |
| Alkaline
Phosphatase |
Hydrolyzes
phosphate monoesters |
| Alcohol
Dehydrogenase |
Converts
alcohols to aldehydes, e.g., conversion of retinol
to retinal is important for vision |
| Carboxypeptidase
A |
A
pancreatic enzyme needed for protein digestion |
| Aminopeptidase |
Protein
digestion |
| Aminolevulinic
Acid Dehydratase |
Heme
synthesis |
| Superoxide
Dismutase (SOD) |
Removal
of superoxide (O2-) radicals |
| Phospholipase
C |
Releases
PO3 from phospholipids |
| Betaine-Homocysteine
Methyltransferase |
Conversion
of homocysteine to methionine |
| Methionine
Synthase |
Folate
and B-12-dependent conversion of homocysteine
to methionine |
| Polymerases,
Kinases, Nucleases, Transferases, Phosphorylases,
Transcriptases |
Multitude
of functions, including DNA and RNA synthesis
and degradation |
|
Commercial Uses of Zinc
Zinc is mined in ores that normally contain 3 to 6% Zn and
8 to 12% lead (Pb). Processing procedures are used to separate
Zn concentrates (mostly ZnS) from Pb concentrates. Further
processing is used to produce impure ZnO and SO2. The ZnO
is then used to produce other Zn salts or Zn metal; the SO2
is converted to H2SO4. In producing Zn metal, impure ZnO is
reacted with H2SO4 to produce ZnSO4. After several purification
steps (to remove other metals), pure ZnSO4 is obtained, and
this is used to produce Zn metal that is essentially 100%
pure Zn. Of the Zn products used in the U.S., a considerable
portion is imported from either Mexico or Canada. Over twice
as much Zn is consumed in the U.S. as that produced in the
U.S. Well over 98% of Zn used in the U.S. is for the automotive
and construction industries, leaving less than 2% for use
in the agricultural and food industries.
Zinc Supplements
The primary sources of Zn used as supplements for animal feeds
are feed-grade (FG) ZnO (72% Zn) and FG ZnSO4·H2O (36% Zn),
and each product has roughly a 50% market share. Other sources
of supplemental Zn include a Zn-methionine complex, Zn proteinates,
and tetrabasic Zn chloride [Zn5Cl2(OH)8]. Zinc supplements
for human foods, including Zn tablets or Zn salts for vitamin-mineral
tablets, are generally provided in the form of purified ZnO,
ZnSO4·7H2O, ZnCO3, or Zn complexes.
Feed-grade ZnSO4·H2O is made from impure ZnO, but FG ZnO products
are manufactured by several methods. Feed-grade ZnSO4·H2O
contains impurities (mostly sodium) that amount to about 0.4%
of the product, but FG ZnO can range from 8% (Waelz process)
to 2% (Hydrosulfide process and French process) impurities.
The major impurities in FG ZnO are iron (mostly Fe2O3), calcium,
sodium, manganese, magnesium and aluminum (in descending order).
In animal feeds, Zn supplements are generally incorporated
into trace-mineral premixes. Also, however, FG ZnO is used
to provide pharmacologic levels of Zn (1,500 to 3,000 mg Zn/kg
of diet) in diets for newly weaned pigs where it functions
as a growth promoter. Most of the research published on Zn
as a growth promoter for young pigs has shown that ZnO (mostly
Waelz process) is more effective than ZnSO4·H2O for this purpose.
Recent evidence, however, suggests that TBZC is just as effective,
or more so, than ZnO for growth promotion in young pigs.
Zinc has long been used in clinical applications for humans.
Zinc oxide, for example, is used as an ointment for enhancement
of wound healing. Also, efficacy for control of diarrhea has
been shown for Zn supplementation of infants in underdeveloped
countries. Pharmacologic Zn supplements also are used by clinicians
to treat the copper toxicity problems of Wilson's Disease.
Zinc Bioavailability
No source of Zn, whether a food source or a supplement, is
100% absorbed from the gut. In animal nutrition, "relative"
rather than "true" utilization (absorption) is estimated by
use of animal bioassays. These bioassays generally involve
feeding graded levels (below the Zn requirement) of Zn from
a standard Zn source such as analytical-grade ZnSO4·7H2O.
Zinc from this standard is assigned a relative bioavailability
value (RBV) of 100%, and other sources of Zn being investigated
are given values relative to the ZnSO4·7H2O standard. Chick
or rat bioassays generally involve a Zn-depletion period followed
by a test period during which Zn sources are added to a Zn-deficient
diet based on either soy or egg white. Both soy products (soy
isolates or concentrates) and egg white are severely deficient
in bioavailable Zn. Either weight gain or bone Zn accumulation
can be used as dependent response criteria, and both slope-ratio
(weight gain or bone Zn regressed on supplemental Zn intake)
and standard-curve methodology can be used to estimate RBV
of the Zn sources under investigation. The purpose of these
bioassays is to obtain RBV values for Zn supplements or feed
ingredients that will apply to practical situations in which
these same supplements would be added to conventional corn-soybean
meal diets.
In assessing Zn RBV of Zn supplements, whether inorganic Zn
salts or organic Zn complexes, a Zn deficient soy diet or
egg white diet will result in similar RBV estimates. However,
when phytate-containing feed ingredients are evaluated, an
egg white basal diet will yield RBV values that are roughly
twice as high as those determined using a soy-isolate basal
diet. This occurs because the phytate contained in soy isolates
or concentrates (i.e., in the basal diet) reduces the utilization
of Zn in the inorganic Zn standard (e.g., ZnSO4·7H2O) far
more than it reduces the Zn contained in the phytate-containing
feed ingredient. Thus, Zn RBV values in chicks for soybean
meal have been estimated at 78% and 40% using Zn-deficient
soy concentrate and egg white diets, respectively. Because
Zn additions, in practice, are made to phytate-containing
corn-soybean meal diets, the RBV value for soybean meal determined
using a phytate-containing soy concentrate basal diet is the
correct value that can be extrapolated to practice.
Several dietary factors can affect the absorption efficiency
of Zn. Zinc absorption is enhanced by: 1) low Zn intake, and
2) Zn consumption with cysteine or cysteine-containing peptides
(e.g., glutathione); also with ascorbic acid or other organic
acids such as citric acid and picolinic acid. The cysteine-glutathione
effect is thought to account for the high RBV of Zn in meat
and meat by products. Factors that reduce Zn absorption are:
1) excess Zn intake, 2) consumption of Zn with phytate, particularly
when excess calcium is also consumed, 3) consumption of Zn
with oxalates (e.g., foods such as spinach, berries, chocolate)
or tannins (e.g., tea), and 4) major trauma (stress) such
as pain, surgery and burns.
Research from the University of Illinois and the University
of Florida has provided Zn RBV estimates for several Zn supplements
relative to reagent-grade ZnSO4·7H2O. In general, Zn complexes
(e.g., citrates, proteinates) have produced RBV values close
to 100%, and Zn-methionine has yielded RBV values for chicks
of greater than 100%. Various sources of FG ZnSO4·H2O have
produced Zn RBV estimates ranging from 85 to 100%, but FG
ZnO products have varied greatly in RBV. Several estimates
have been made for Waelz-processed ZnO, with RBV values varying
from lows of 34% to highs of 50%. On the other hand, FG ZnO
produced by either the hydrosulfide or French process have
yielded Zn RBV estimates that averaged about 90%, similar
to that found for reagent-grade ZnO. Chlorides and carbonates
of Zn are well utilized. Tetrabasic Zn chloride (TBZC), for
example, was found to have a Zn RBV value of 107%. Zn metal
products were reported to have low Zn RBV values, ranging
from 36% (Zn metal fume) to 67% (Zn metal dust).
Micronutrients TBZC®
The product trade name is derived from Tetra-Basic Zinc Chloride,
which can be thought of as a hybrid between zinc chloride
(strongly acidic) and zinc hydroxide (strongly alkaline),
in which 80% of the acidity has been neutralized. The result
is a salt that is totally insoluble in water, non-hygroscopic,
unreactive in most foods or feedstuffs, and yet highly bioavailable.
Since this compound is neutral and water insoluble, it has
excellent palatability and very low interactions with other
ingredients in a food mixture compared to zinc chloride, zinc
sulfate or chelated forms of the metal.
Five bioavailability studies have all indicated that TBZC
has a higher bioavailability relative to zinc sulfate, with
values ranging from 102 to 111%. Four studies comparing TBZC
to zinc oxide as a growth promotant all indicate improved
weight gain and feed conversion at lower levels using TBZC.
Testing in vitro has shown better antimicrobial acitvity
with TBZC than both zinc sulfate and zinc oxide.
Suggested References
Baker, D. H. and C. B. Ammerman. 1995. Zinc
bioavailability. In: Bioavailability of Nutrients for Animals:
Amino Acids, Minerals and Vitamins (Eds. C. B. Ammerman, D.
H. Baker and A. J. Lewis). Academic Press, pp. 367-399.
Batal, A.B., T.M. Parr, and D.H. Baker. 2001. Zinc bioavailability
in tetrabasic zinc chloride and the dietary zinc requirement
of young chicks fed a soy concentrate diet. Poultry Sci. 80:87-91.
Cao, J., P.R. Henry, C.B. Ammerman, R.D. Miles, and R.C. Littel.
2000. Relative bioavailability of basic zinc sulfate and basic
zinc chloride for chicks. J. Appl. Poultry Res. 9:513-517.
Edwards, H.M., III and D.H. Baker. 1999. Bioavailability of
zinc in several sources of zinc oxide, zinc sulfate, and zinc
metal. J. Anim. Sci. 77:2730-2735.
Edwards, H.M., III., and D.H. Baker. 2000. Zinc bioavailability
in soybean meal. J. Anim. Sci. 78:1017-1021.
Hahn, J. D. and D. H. Baker. 1993. Growth and plasma zinc
responses of young pigs fed pharmacologic levels of zinc.
J. Anim. Sci. 71:3020-3024.
Hill, G.M., G.L. Cromwell, T.D. Crenshaw, C.R. Dove, R.C.
Ewan, D.A. Knabe, A.J. Lewis, G.W. Libal, D.C. Mahan, G.C.
Shurson, L.L. Southern, and T.L. Veuum. 2000. Growth promotion
effects and plasma changes from feeding high dietary concentrations
of zinc and copper to weanling pigs (regional study). J. Anim.
Sci. 78:1010-1016.
Hortin, A. E., P. J. Bechtel and D. H. Baker. 1991. Efficacy
of pork loin as a source of zinc, and effect of added cysteine
on zinc bioavailability. J. Food Sci. 56:1505-1508. Mavromichalis,
I., C.M. Peter, T.M. Parr, D. Ganessunker, and D.H. Baker.
2000. Growth promoting efficacy in young pigs of two sources
of zinc oxide having either a high or a low bioavailability
of zinc. J. Anim. Sci. 78:2896-2902.
Mavromichalis, I., D.M. Webel, E.N. Parr, and D.H. Baker.
2001. Growth promoting efficacy of pharmacologic doses of
tetrabasic zinc chloride in diets for nursery pigs. Can. J.
Anim. Sci. 81:387-391.
O'dell, B.L., J.M. Yohe, and J.E. Savage. 1964. Zinc availability
in the chick as affected by phytate, calcium and ethylenediaminetetracetate.
Poultry Sci. 43:415-419.
Sandoval, M., P.R. Henry, C.B. Ammerman, R.D. Miles, and R.C.
Littel. 1997. Relative bioavailability of supplemental inorganic
zinc sources for chicks. J. Anim. Sci. 75:3195-3205.
Wedekind, K. J., A. E. Hortin and D. H. Baker. 1992. Methodology
for assessing zinc bioavailability: efficacy estimates for
zinc-methionine, zinc sulfate and zinc oxide. J. Anim. Sci.
70:178-188.
Wedekind, K.J., A.J. Lewis, M.A. Giesemann, and P.S. Miller.
1994. Bioavailability of zinc from inorganic and organic sources
for pigs fed corn-soybean meal diets. J. Anim. Sci. 72:2681-2689.
|
|
