Background and Relevant Pharmacokinetics
Zinc is an essential trace element known to play an important role in all human living cells. The human body contains approximately 2 g zinc in total, with 60% found in skeletal muscle and 30% in bone mass, although it is found in all body tissues and fluids. Dietary intake of zinc by healthy adults is 6-1 5 mg/day, but less than half of this is absorbed. It is now known that zinc absorption is influenced by many factors and adequate dietary intake is not necessarily indicative of adequate zinc status. High amounts of zinc in a meal cause a fractional decrease in zinc absorption and foods with high phytate content (e.g. wholegrains, corn and rice) significantly reduce zinc absorption due to the formation of strong and insoluble complexes. Concerns have also been raised over the potential of calcium, iron, copper and cadmium to reduce zinc absorption. Alternatively, the amount of animal protein in a meal positively correlates to zinc absorption and the amino acids histidine and methionine, and various organic acids present in foods, such as citric, malic and lactic acids, can also increase absorption. As such, zinc is best absorbed from animal food sources.
Zinc sulfate and gluconate are the most commonly used forms in supplements.
Meat, liver, eggs, seafood (especially oysters and shellfish) are the best sources, but zinc is also found in nuts, legumes, wholegrains, miso, tofu, brewers’ yeast, mushrooms, green beans and pumpkin seeds.
Zinc: Deficiency Signs and Symptoms
Severe deficiency is rarely seen in industrialised countries, but marginal deficiency and inadequate intakes are not uncommon. According to a large national survey of over 29,000 people conducted in the USA, only 55.6% had adequate zinc intakes (based on total intakes of >77%of the 1989 US RDI levels). Young children aged 1-3 years, female adolescents and older people aged >71 years had the lowest percentage of ‘adequate’ zinc intake, and were identified at greatest risk of deficiency. Others at risk are alcoholics (especially those with liver disease), pregnant and lactating women, teenagers experiencing rapid growth, malnourished individuals including those with anorexia nervosa, people with severe or chronic diarrhea, malabsorption syndromes or inflammatory bowel diseases, and strict vegetarians.
SIGNS AND SYMPTOMS OF DEFICIENCY
• Anorexia and impaired sense of taste.
• Slowed growth and development, and delayed sexual maturation.
• Delayed sexual maturation, hypogonadism and hypospermia, and menstrual problems.
• Skin rashes.
• Chronic and severe diarrhea.
• Immune system deficiencies and increased susceptibility to infection.
• Impaired wound healing due to decreased collagen synthesis.
• Night blindness; swelling and clouding of the corneas.
• Erectile dysfunction.
Zinc deficiency in pregnancy is associated with the following:
• Increased maternal morbidity, pre-eclampsia and toxaemia.
• Prolonged gestation.
• Inefficient labour.
• Atonic bleeding.
• Increased risk of abortion and stillbirths.
• Low birthweight infants.
• Diminished attention in the newborn and poorer motor function at 6 months.
This can result from inadequate dietary intake of zinc; however, inhibition of zinc absorption is a common causative factor. Strict vegetarians are at risk of deficiency if their major food staples are grains and legumes because the phytic acid in these foods will impair dietary zinc absorption.
Zinc deficiency develops in some people with cirrhosis, malabsorption syndromes, sickle cell anaemia, conditions of increased zinc loss, such as severe burns or major surgery, chronic diarrhea or diabetes, HIV and AIDS, and during prolonged parenteral nutrition. Additionally, strenuous exercise and elevated ambient temperatures increase zinc losses through perspiration. A congenital disorder known as acrodermatitis enteropathica causes severe zinc deficiency.
Clinical note— Measuring zinc status is difficult
Currently, there is no universally accepted single measure of zinc status in humans. The most commonly used approach is the measurement of serum zinc levels, but this is a poor measure of marginal zinc deficiency because zinc is primarily found intracellularly and only a small portion is found in the circulation where it is mainly bound to plasma protein. Plasma zinc levels are affected by the homeostatic system and other factors, such as diurnal rhythm, stress, infection, starvation and plasma protein levels. As a consequence, it is not an accurate reflection of dietary zinc intake or true zinc status. Estimation of hair zinc levels is another tool used, with low hair zinc levels indicative of zinc depletion. This method is costly and also subject to inaccuracies caused by variable hair growth and the presence of zinc in some shampoos. The zinc taste response test (also known as the Bryce-Smith taste test) is a popular measure among naturopathic practitioners. It relies on patients detecting a taste after oral administration of 10 ml_ of a zinc sulfate solution. Delayed taste perception or lack of taste is seen to indicate a zinc deficiency state. It is based on the theory that sense of taste is primarily influenced by the zinc dependent enzyme, gustin, in the saliva. As such, low zinc status should substantially affect taste discrimination. This method is not particularly accurate and hampered by variations in patients’ subjective sense of taste and the fact that agents other thans zinc influence taste perception. Clinical studies with zinc taste tests have confirmed the inconsistency of the results. Ultimately, this leaves the diagnosis of marginal deficiency up to a practitioner’s clinical suspicion.