DIET

Food sources of Zn

Animal products provide most of the zinc in omnivores. Zinc intake correlates with protein intake, though the source of protein has an effect:

• eggs, milk, poultry (chicken – typically 1 mg/100g)
• fish have a lower content than red meat or shellfish (oyster – typically 75 µg/100g).

Cereals are the main plant source – especially whole-grain cereals.

Nuts and legumes are also good sources.

Photo Foods high in zinc

Dietary Supplement Fact Sheet: Zinc

Bioavailability

15-45% of Zn taken in with food is absorbed. Absorption of zinc depends on other food components: Low molecular weight compounds, especially amino acids histidine, cysteine and methionine, bind Zn, increase solubility and improve absorption. High MW compounds that bind Zn can form poorly soluble compounds that hinder absorption. Phytate is particularly important.

Phytate (myo-inositol hexaphosphate) is a hexose sugar with all 6 carbons phosphorylated. It occurs in whole-grain bread, eaten as a staple food in some middle-eastern countries, and can cause severe deficiency of Zn (+ other minerals)

Figure Phytate (myo-inositol hexaphosphate)

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Meat, liver, eggs and seafood are good sources of Zn because of the absence of compounds that inhibit absorption and the presence of compounds that enhance absorption.

Zinc excretion

Most occurs via gastrointestinal tract: very little in urine. The amount excreted in the gut varies with Zn intake. Other significant losses: skin, hair, sweat; semen (up to 1 mg/ejaculation); menstruation (up to 0.5 mg per period).

Homeostasis

If dietary intake falls, there follows a decrease in endogenous losses – i.e. more of the zinc secreted into the gut is reabsorbed. Thus the tissue concentrations are maintained.

Recommended intake

Human nutrient requirements can be assessed on the basis of

(a) lowest intake consistent with homeostasis

(b) amount required to replace endogenous losses (factorial approach)

(c) intake needed to maintain normal function.

In the case of Zn, there are no reliable functional indicators of Zn status. And a balanced level of Zn in the body can be achieved over a wide range of intakes, from 3 to 30 mg/day. So the factorial approach is used.

Problems with estimating Zn status:

• Serum and blood concentrations are maintained within a narrow range and do not represent tissue levels
• Hair analyses are difficult to interpret, though there seems to be an association between low concentration in hair and low intake.
• Urine – not sensitive – only a minor excretory route.
• Metallothionein mRNA level in monocytes? Promising.
• Response to Zn supplementation

Recommendations

Zinc and pregnancy

• For many women, intake during pregnancy is below the recommended levels.
• Women with low plasma zinc in the first trimester tend to have a higher frequency of pregnancy complications (e.g. 8x higher risk of low birth weight).
• Serum Zn at birth tends to be lower in women giving birth to children with neural tube defect.

Toxicity of zinc

Acute - e.g. contamination from galvanised containers;

• >200 mg/day → emetic
• Chronic → secondary Cu deficiency

The USEPA has an oral reference dose for Zn of 0.3 mg/kg/day. This is the maximum daily dose that is likely to be without appreciable risk of harmful effects over a lifetime. In other words, higher doses are likely to increase the chance of harmful effects. The oral reference dose is below the RDA for age groups >15. But no health hazards are known from ingesting the RDA for Zn at any age.