|SCN News, Number 09 - Focus on Micronutritients (ACC/SCN, 1993, 70 p.)|
Roger Shrimpton, Senior Programme Coordinator, UNICEF, Jakarta, Indonesia
While more attention presently is on three micronutrients, many others may be important. Among the likely candidates in zinc. We invited the following article from Dr R Shrimpton, who has been involved in research on this topic for many years.
There are forty or more substances known to be essential in the human diet, but of these only three micronutrients, vitamin A, iron and iodine are thought to be commonly deficient. There is a growing suspicion however that zinc might also be included in this category. This is based on two separate considerations. On the one hand the pervasive nature of zinc dependent enzymes in metabolic processes. On the other that zinc supplementation is beneficial in many disease states. In malnourished children vitamin A status has been improved and immune response corrected, and even the duration of diarrhoeal disease seems to be reduced by zinc supplements. Could it not be therefore that zinc deficiency is associated with the aetiology of protein energy malnutrition?
Human zinc deficiency was first reported in the Middle East in the early sixties. There was much speculation then that deficiency might be widespread among children in developing countries. Although much more has been learned about the vital importance of zinc in human nutrition in the last thirty years, there is still no reliable indicator of zinc deficiency. Proof of deficiency still depends on getting a response to supplementation. Many zinc supplementation trials have been carried out in children and pregnant and lactating women during the last thirty years. What new perspective have these studies brought to our knowledge on zinc? Is it really something that we only need a trace of and get easily from any common diet? What evidence is there today that deficiency is a public health problem? This article tries to answer some of these questions.
How Essential is Zinc?
Calling zinc a trace element is perhaps a misnomer left over from the days it was hard to detect. It is certainly present in more than a trace in all tissues. In the late forties McCance and Widdowson showed that the adult human body contains about two grams of zinc. Sixty percent of body zinc is in muscle, 20% in bone, 5% in blood and liver and 3% in skin and the gastrointestinal tract. Scoular and Macy did balance studies in preschool children in the early forties showing that five milligrams were retained out of an intake of 16 mg a day. Such a retention, five times greater than iron for example, seemed to speak against the classification of zinc as a trace element. With the development of atomic absorption spectrophotometry in the late fifties, investigation of the importance of zinc in human nutrition was made much easier.
Zinc is the most abundant trace metal inside most cells. The exception is red blood cells where iron has its special oxygen-carrying function. Even the macro element calcium is less abundant than zinc in all other cells except bone cells. Zinc is not limited, as are calcium and iron, to a few functional roles. Zinc is a functionally essential component of more than 200 enzymes, pervading all metabolic pathways. The role of zinc in such enzymes can be either structural and/or catalytical. Zinc also helps to stabilize membrane structures. It protects their integrity by the reduction of free radical formation, thus preventing lipid peroxidation.
The paramount importance of zinc to an organism is in multiplicative cell growth. Zinc has a fundamental role in gene replication, activation and repression, is critical for transcription and translation, and affects nucleic acid metabolism. Growth of young rats on a zinc deficient diet stops within twenty-four hours, probably due to the lack of gene regulatory proteins. These contain a common structure - the zinc finger, which are loops of chains of amino acids, held together at the base by a zinc atom. Gene regulatory proteins may contain eleven such fingers, which reach down into the grooves of the DNA helix and promote transcription. Zinc also mediates the activity of growth hormone. When growth hormone attaches to its specific receptor sites on a cell membrane, it needs a zinc atom to make the connection. The resulting complex has been called a zinc sandwich.
Symptoms of severe deficiency in rats include loss of hair and gross skin lesions. In older rats there are testicular atrophy and failure of spermatogenesis, congenital malformations and difficult births with excessive bleeding in pregnant females. These symptoms have been found in humans with acrodermatitis entropathica, a genetic defect in zinc absorption mechanisms. Before it was discovered that zinc supplements resolved the problem, children with the defect died of lung and intestinal infections before they reached two years of age.
We can conclude that zinc is not only essential, but because it is involved in so many important process may even be first limiting. This means it is the critical limiting factor in the diet. Zinc is especially needed in times of rapid growth. This is due not only to effects on gene replication and nucleic acid metabolism but also as a mediator of growth hormone action. The consequences of zinc deficiency are likely therefore to be extensive, if not catastrophic for the organism.
Why Is It So Difficult To Diagnose Zinc Deficiency?
The very essentiality of zinc makes it difficult to detect deficiency. Most zinc in cells is tied up in a very functional way, such that zinc concentrations vary little in the same sort of cells. When there is enough zinc to form a new cell then it will be formed with a normal zinc content. When there is not enough zinc then no new cell is formed. Except in a few types of cells in the intestine and the liver, zinc is not stored nor does it accumulate. Although there is no store for zinc, it can be mobilized by catabolizing cells. Cells cannot become depleted of zinc without losing functionality.
While severe zinc deficiency is easily recognized, detecting sub clinical zinc deficiency states in humans continues to be a challenge for nutritional science. In rats there is a spectrum of clinical signs from the severest to the mildest levels of deficiency. On a purified animal protein diet skin and hair signs of severe deficiency are produced by diets containing less than 2ppm. Growth is affected at 8ppm, and zinc repletion occurs with maximal concentrations in hair, serum, bone and liver at 15ppm. Attempts to use any of these indicators in humans to define mild deficiency have not been successful.
Zinc intake tells us little about zinc status. A large amount of zinc in the diet is not a guarantee of sufficiency. The first evidence of zinc deficiency in humans appeared in Egyptian and Iranian populations subsisting on bread made from unleavened whole wheat flour. The zinc intake from these diets is high at 15mg a day, but it is not available because of a high phytate content. Phytate, the phosphorus storage compound of plant seeds, binds zinc and other bivalent ions in insoluble complexes, making them unavailable to human and other monogastric species. Iron, when present in large amounts in the diet, also inhibits zinc absorption.
Circulating and tissue levels of zinc do not necessarily reflect zinc status. Low hair zinc suggests mild zinc deficiency, but high hair zinc is found in both replete and severe deficiency states. Low levels of zinc in blood also do not give conclusive evidence of zinc deficiency. In the acute phase response to injury and infection, plasma zinc levels fall to less than 50% of premorbid levels. During pregnancy zinc is also redistributed and circulating levels reduced. Many zinc-containing enzymes in blood have been shown to be reduced in simple zinc deficiency. However, all studies so far have failed to show the sensitivity and specificity necessary for definitive diagnosis of deficiency in the various physiological states of infection and pregnancy.
Circulating zinc levels also do not reflect zinc balance status. It is possible to have normal circulating zinc levels in the face of a heavy negative zinc balance. When there is catabolism of muscle cells or haemolysis of blood cells, for example, large amounts of zinc are released into the circulation and lost through the urine. Over time the body can become very depleted of its zinc content by such mechanisms. Patients with sickle cell anaemia have signs of deficiency, including reduced immune response and decreased fragility of red blood cell membranes that respond to zinc supplementation.
Proof of zinc deficiency status depends on observing a beneficial effect of zinc supplements. If a sub optimal zinc status exists, then zinc supplements should improve some body function. Since it is very sensitive to zinc deficiency, growth is the outcome that is most expected to benefit from zinc supplementation. Zinc supplementation benefits children being rehabilitated from severe malnutrition. Does that mean zinc deficiency is involved in the aetiology of protein energy malnutrition?
What is the Evidence that Zinc Supplements Benefit Children?
Zinc supplementation studies on the growth of children have produced mixed results. Eight controlled supplementation trials provide useful information, as summarized here. In the early studies in the Middle East three attempts were necessary before a growth effect was produced. The first attempt in Egypt failed because they did not correct for the simultaneous deficiencies of other nutrients. The first Iranian experiment corrected for this midway and eventually managed to influence sexual maturation but not growth. The second Iranian study produced a growth effect by restricting the subjects to thirteen years-old boys going through their adolescent growth spun. Small growth effects have also been shown in growth-retarded American and Canadian children with low hair zinc levels. Interpretation of these results is difficult as the children have similar zinc intakes to those with no growth retardation and normal hair zinc levels.
More recently studies have been carried out in apparently healthy but growth-retarded children in developing countries outside the Middle East. These studies in Thailand, Guatemala and the Gambia found no growth response to zinc supplements.
There were other non-growth effects of zinc supplementation. In the Gambian and Guatemalan studies zinc supplementation affected body composition. In Guatemala there was an increase in arm skin fold thickness. Arm circumference decreased during the study, but this decrease was smaller in the zinc supplemented group. The Gambian study found a positive effect on arm circumference, and found less malarial infection in the zinc supplemented group. In Thailand, supplementation with zinc improved dark adaption times and the integrity of conjunctival epithelia, even though Vitamin A status was not improved. Serum alkaline phosphatase activity also increased in Thailand, but not in Guatemala, Egypt or Iran.
Comparison between these studies and extrapolation of the results to other populations is very difficult due to lack of information and differing methodologies. The level of supplementation, the type of placebo and the method of administration of the supplement varied greatly. An increase in plasma zinc levels in the zinc supplemented but not the placebo control group was found only in Egypt, Thailand, and Guatemala. The effects were not consistent in Iran. In USA there was no increase in either group. In the Gambia there was no information on zinc levels before supplementation, so we don't know whether levels rose in either group. Initial plasma zinc levels in children were lowest in the Middle East and USA studies. Few authors reported the results of analysis of standard reference materials in their laboratories. Dietary zinc intakes are hard to compare due to the differing age groups and/or lack of information, and differing availabilities.
It is perhaps not surprising that zinc did not affect growth in most of the studies, as they were done at the wrong time, i.e. too late. Except for the second Iranian study the children were not going through their peak growth spurts. Most growth retardation in developing countries occurs before twelve to eighteen months of age. Zinc needs during this period are high on a body weight or energy basis, since this is the age when growth is mostly by cell multiplication. Growth velocities plateau at eighteen months, and multiplicative growth becomes less important. The children studied, although growth-retarded, had normal growth rates for their age. What the studies so far have been trying to test is whether zinc can trigger catch up growth, or make children grow at a rate faster than expected for their age. Unfortunately there are no reports of zinc supplementation studies in children between the ages of six and eighteen months, when growth faltering occurs in developing countries. Equally important perhaps would be to understand the influence of maternal zinc status on child growth during these early formative years.
What is the Evidence that Zinc Supplementation Benefits Women and their Offspring?
Studies of the effects of maternal zinc supplementation on growth of offspring are few and have shown mixed results. Seven controlled studies were reviewed for this article. Three studies of zinc supplementation during pregnancy sought an effect on foetal growth, but found none. These studies were carried out in developed countries. Cross sectional studies have suggested a relationship between birth weight and plasma zinc levels, but only in women with low plasma zinc levels. Studies of zinc supplementation during pregnancy in tropical countries have not been reported. In the tropics zinc requirements are likely to be higher and intakes lower. Circumstantial evidence suggests linkages between the high rates of foetal malformation and maternal zinc deficiency in the Middle East.
A growth effect was found in Amazonian mothers who were zinc supplemented during lactation. Boys of zinc supplemented mothers gained half a kilogram more than those of non zinc supplemented mothers during five months of exclusive breast feeding. Other studies of zinc supplementation in lactating mothers have not studied growth of the infants.
Zinc supplementation studies in women have mostly investigated the effect on breast milk zinc levels and not growth. The results have been mixed. A supplement of 15 mg a day in the Amazon and in the USA produced the same milk zinc levels. In both studies the fall in milk zinc over time was less, suggesting that beyond six months of lactation supplementation would produce a difference in breast milk zinc levels. A study in Indiana, using a higher zinc dose of 25mg a day, did increase milk zinc levels. The trial was neither blinded nor randomized however. A randomized and blinded study in Maryland USA with a similar zinc supplementation level found no effect. In the Amazonian mothers, supplementation with zinc improved maternal vitamin A status and doubled milk vitamin A levels. The infants being solely breast fed by zinc supplemented mothers had less diarrhoeal episodes. The possible link of zinc deficiency and toxaemia of pregnancy has been suggested by cross sectional studies, but not yet confirmed by supplementation trials.
Few studies considered why zinc supplementation in women had no effect, and possible reasons for geographical differences. Many studies did not even report dietary or circulating zinc levels. None have considered the influence of recent sexual practices on the zinc status of the women. This is relevant because men can transfer one milligram of zinc to women by sexual intercourse. The common practice of restricting food intake in the last weeks of pregnancy will mobilize zinc from muscle and guarantee that zinc is available for the birthing process.
It is difficult to understand why variability in milk zinc values is large, both within and between populations, yet supplements have no effect. Milk zinc concentrations are highest soon after birth and decrease with time. Geographical differences in early lactational milk zinc levels may be related to differences in maternal weight gain during pregnancy. Weight gain during pregnancy is on average 13 kg in women from developed countries and seven kg in poor women from developing countries. Most of this difference is in the maternal tissues, not the foetus. After birth the catabolism of uterine tissue could contribute to circulating zinc levels and perhaps influence milk zinc levels. In the face of high free circulating zinc levels supplementary zinc may not be absorbed.
Conclusions, and Research Needs
There is still no evidence that zinc deficiency is widespread but under recognized. We know that it is critically essential for multiplicative cell growth, but we still don't know how to detect deficiency other than by doing supplementation trials.
The zinc supplementation trials in children reported to date have not looked at age groups when multiplicative cell growth is greatest. The zinc supplementation in children aged two to twelve years of age in developing countries did not improve growth. Other effects were observed, suggesting that appetite may be improved, contributing to increased fat accretion. Further studies are required during the first eighteen month period of life, when growth failure occurs in developing countries.
More supplementation studies are required in women tropical countries, especially in Asia. Lowenstein commented fifty years ago that the zinc intakes of Asian rice and fish eating populations were low at six milligrams a day. These intakes are the same as those of Amazon women already shown to be deficient during lactation. Breastmilk vitamin A levels are commonly low in poor women from tropical countries. There is a need to investigate whether zinc supplementation would raise vitamin A levels in breast milk outside the Amazon. There is no evidence that zinc supplementation benefits birth weight in humans, but studies are still needed in tropical countries.
Zinc deficiency studies in monkeys have produced effects on birth weight but only in male infants. The greatest effects of deficiency in young monkeys born to mildly zinc deficient mothers only appeared in the second year of life. Then the offspring were smaller, had smaller appetites and grew slower even though eating a normal diet. Zinc supplementation studies are needed in tropical populations, starting in pregnancy and continuing into infancy. The relationship between maternal zinc deficiency and maternal mortality should be investigated. The causes of maternal mortality include excessive bleeding and hypertension, both of which are known to be associated with zinc deficiency.
Studies to date have not really looked at the risks associated with not being zinc replete. These considerations are more relevant now that the importance of zinc as an antioxidant has been realized. It is likely that cancer is more common in non zinc replete people. From animal studies we know that the teratogenic effects of lead for example are only found in non zinc replete sheep. The toxic effects of cadmium, such as high blood pressure, can also be reduced by zinc supplementation. In many tropical countries the combination of a chronic sub clinical zinc deficiency and an environmental toxin could combine to produce cleft palate and still births for example.
When developing future study protocols, supplementation trials should lake a broad ecological view of possible zinc sources. The practice of earth eating is common in pregnant women. The betel nut chewing by Asian women, and the special clays sold all over Africa for women to eat should be contemplated. In the Amazon it was customary for indigenous tribes to cremate their family members and put the ashes into a pot. Every day they would put a little of the ashes into their drinks to remember their ancestors. Many of these practices undoubtedly contribute to maintaining fragile mineral balances, who knows maybe even for zinc. As time goes by modern wisdom tends to erode such primitive practices.