The role of zinc and vitamin A in persistent diarrhea among infants and young children.

Abstract

Recent research has increased our understanding regarding the genesis of persistent diarrhea in developing countries. Although the incidence of acute diarrhea in childhood is primarily determined by the intensity of exposure to pathogens in an environment, it now appears that host factors largely determine the duration and severity of the illness and its mortality risk(1). Malnutrition is clearly the most important risk factor for the development of persistent diarrhea and its complications(2). Malnourished children have multiple nutritional deficiencies. In clinical and epidemiologic studies, it has been difficult to delineate the contribution of individual micronutrient deficiency to the excess diarrheal morbidity in the malnourished host. In results of experimental studies, however, dietary deficiency of some micronutrients has been shown to increase susceptibility to infection and to have direct adverse effects on the gastrointestinal tract structure and function(1,3-15). In humans, it has been difficult to evaluate the health consequences of deficiency of such micronutrients as zinc, because of the absence of reliable laboratory markers of poor status (16). The scenario has now changed considerably, with the availability of results from several large placebo-controlled supplementation trials of individual micronutrients among children residing in developing countries. This review covers vitamin A and zinc in some detail, in that these have been the focus of clinical studies; the other nutrients are discussed briefly. ZINC The zinc atom has a unique combination of properties that renders it useful in biologic systems. Zinc is an essential component of more than 200 enzymes pervading all metabolic pathways (17,18). The role of zinc in such enzymes can be structural and catalytic. It helps in the stabilization of biologic membranes in pharmacologic doses(19,20). The major role of zinc is in multiplicative cell growth. Zinc has a fundamental role in gene replication, activation, repression, transcription, and translation(17,18). The biologic actions of zinc have an important bearing on various components of the immune system. Zinc deficiency, both acquired and inherited, is associated with lymphoid atrophy, decreased cutaneous delayed hypersensitivity responses, lower thymic hormone activity, a decreased number of antibody-forming cells, and impaired T-killer-cell activity. Reduced activity of thymic hormone, which is involved in the differentiation of T cells, has been proposed as a marker of mild zinc deficiency(20-23). Zinc also plays an important role in intestinal structure and function. In animal models, zinc deficiency is associated with mild villus atrophy and decreased brush border disaccharidase activity(1,3,13). In zinc-deprived animals, the secretory response to cholera toxin is enhanced, and this phenomenon is reversible by dietary replenishment (13). In malnourished Bangladeshi children with acute diarrhea, zinc supplementation resulted in a rapid reversal of the abnormal lactulose-mannitol ratio(14). Mannitol is absorbed across epithelial cells and lactulose through the intraepithelial space. In mucosal damage, the former is less absorbed than the latter. Given these biologic effects of zinc deficiency, it seems highly plausible that zinc has an important role in determining the risk of acquiring enteric infection, in the severity of the resultant pathophysiologic disturbances, and in the recovery process. Effect of Diarrhea on Zinc Nutriture When dietary zinc intake is low, there is a compensatory increase in intestinal zinc absorption, but it is insufficient to maintain normal status. This may be particularly so during acute diarrhea when zinc absorption is impaired (16). Fecal zinc losses have been consistently noted to increase during diarrhea and to be more pronounced during persistent diarrhea (24-26). The zinc-depleting effects of diarrhea are most distinctly seen in adults receiving parenteral nutrition: Intravenous zinc required to achieve positive zinc balance averaged 13 mg in patients with ongoing diarrheal fluid losses, compared with 2.5 mg in patients without such losses (27). The increased fecal losses are reflected in low serum or plasma zinc levels in patients with diarrhea. Hypozincemia is also more pronounced in persistent than in acute diarrhea (28-32). The low serum concentration of zinc during diarrhea normalizes after recovery. Zinc-depleting effects of acute and persistent diarrhea are likely to be more pronounced in developing countries in which subclinical zinc deficiency is common (33,34). Zinc Deficiency as a Cause of Diarrhea The most striking evidence that zinc deficiency causes diarrhea is seen in acrodermatitis enteropathica, a congenital disorder characterized by immunosuppression, villus atrophy, severe diarrhea, and growth failure. These improve significantly after zinc supplementation(35). Until recently, knowledge of the prevalence and geographic distribution of mild to moderate zinc deficiency and its relationship with acute and persistent diarrhea or with other infectious diseases was limited. An important obstacle has been the lack of a reliable method for assessment of human zinc nutritional status in field conditions, particularly where acute infections are common. Of the many laboratory assays for the measurement of tissue zinc concentration and functional indexes of zinc status, determination of plasma zinc concentration, despite many limitations, remains the assay of choice, particularly when evaluating the zinc status of population groups. Plasma zinc concentrations below 10.5 μmol/l in the postabsorptive state or below 9 μmol/l in the post-prandial state indicate zinc deficiency. It is important to note that plasma levels of zinc can decrease in systemic infection temporarily as a part of the acute phase response. Use of stable isotope techniques to evaluate zinc homeostasis appear promising, but these techniques will be difficult to apply on a large scale under field conditions. Results of well-designed, large supplementation trials have now demonstrated that zinc deficiency is common in children of many developing countries and that improving zinc status through daily supplementation substantially reduces acute and persistent diarrhea. These studies may be placed in two categories: in one type of supplementation trial, zinc was administered daily during acute diarrhea to assess its impact on the supplemented episodes. The other group of studies involved supervised or unsupervised daily administration of zinc for several months to assess impact on the incidence of acute and persistent diarrhea. Zinc Supplementation During Acute and Persistent Diarrhea The results of published trials are summarized in Table 1. In an initial study in children with acute diarrhea, Sachdev et al. observed a significant reduction in duration of diarrhea after zinc supplementation in children with low rectal mucosal zinc(36). These preliminary data have now been confirmed in findings in a large, double-blind, placebo-controlled, community-based trial in India (33). In reported results, children who received 20 mg elemental zinc daily during acute diarrhea had a 23% reduction(95% confidence interval [CI] 12% to 23%) in the risk of continued diarrhea. When zinc supplementation was initiated within 3 days of the onset of diarrhea, there was a 39% reduction each in the proportion of episodes lasting more than 7 days (95% CI 7,61%) and the mean frequency of watery stools (95% CI 6,70%) in the zinc group. The reduction in severity and duration of diarrhea were greater in stunted children. It is noteworthy that 37% of children in this trial had low (<9 μmol/l) plasma concentrations of zinc. Sachdev et al. also reported reduced duration in children with persistent diarrhea who received supplemental zinc; but the trial sample size was small, and the results were not statistically significant(37). Bangladeshi children with persistent diarrhea who were supplemented with zinc recovered earlier from diarrheal symptoms, and the risk of fatal outcome during or immediately after the episode was also substantially reduced (14). The practical implications of these findings are that in deficient populations, administration of 10 to 20 mg elemental zinc daily to young children early in acute diarrhea, as an adjunct to fluid therapy, is likely to shorten duration of the episode and to reduce substantially the risk of persistent diarrhea. That persistent diarrhea can be prevented is very important in designing treatment programs, because of the high rate of associated fatality (38) and the adverse nutritional impact (39) on patients of protracted diarrhea, and because of families' satisfaction with a treatment approach that relies primarily on oral rehydration and discourages antimicrobial therapy. Before adoption of such a recommendation, zinc supplementation trials in acute diarrhea must be conducted in other regions, given the considerable differences in foods and feeding practices among developing countries. In future studies, the benefit of zinc supplementation during acute diarrhea should also be evaluated in the following subgroups of children: well-nourished, wasted, stunted, breast-fed or not, aged less than 6 months, and normal or low plasma concentrations of zinc. For these trials, daily zinc doses of 20 to 40 mg in children more than 6 months old and approximately 10 mg in those aged less than 6 months seem to be safe and likely to offer benefit. Ultimately, if it were considered desirable to supplement all or subgroups of children with zinc during acute diarrhea, an appropriate method of administration would have to be identified. The zinc supplement could be given to parents by health-care workers, but the compliance with such a regimen must be examined first. The other options may be to fortify oral rehydration salts solution (ORS) with zinc, either attached piggyback to the ORS packet or premixed with the rehydration salts. Zinc fortification of ORS may pose problems in controlling the doses delivered and may not be successful because in developing countries, many episodes of diarrhea are now treated at home with readily available fluids without rehydration salts. These questions must be resolved through operational research. Although a decision to use zinc during acute diarrhea in developing countries must await more clinical research, there is wide agreement among clinicians that zinc and other micronutrients should be administered during persistent diarrhea in infancy and in other forms of chronic diarrhea. Clinical studies are required to identify which of the micronutrients are critical to include in a single formulation for use in malnourished children with persistent diarrhea. Zinc Supplementation and Diarrhea Prevention A vicious cycle of diarrhea-zinc deficiency-diarrhea has been postulated(40). Although it is well established that diarrhea leads to excess zinc losses, until recently, little evidence was available to show that zinc deficiency increases the risk of developing acute or persistent diarrhea. Results of placebo-controlled clinical trials have now clearly demonstrated that in developing countries, increasing zinc intake through daily supplementation leads to substantial reduction in acute diarrhea, dysentery, and persistent diarrhea(33,34,36,37,41,42). The salient characteristics of these trials and their main results are summarized in Table 2. All trials were double-blind, placebo-controlled, and conducted in low socioeconomic communities. The subjects were aged between 6 months and 3 years; the daily zinc dose varied between 10 and 20 mg in periods ranging from 6 to 12 months. Although not all outcomes were reported in each study, the percentage of reduction in incidence of acute diarrhea ranged between 14% and 37%; persistent diarrhea reduction occurred in approximately 20% of patients(33,34,41,42). In the Indian study, several subgroup effects were reported(33). The reduction in dysentery was greater in boys(38%) and in those with low serum concentrations of zinc (50%). The occurrence of reduction in persistent diarrhea was greater (49%) in children older than 1 year-an important statistic, given that nearly 80% of persistent diarrhea-related deaths in several developing countries occur in children 1 to 5 years of age (38). Incidences of reduction of persistent diarrhea were also more frequent in those with lower initial serum concentrations of zinc (73%). Several features in the Mexican trial are noteworthy (34). Although, only 20% of the trial children had low levels of zinc, there was a 37% reduction in incidences of diarrhea, indicating that even marginal deficiency increases diarrhea-related morbidity. Supplementation with iron in addition to zinc did not improve the results of supplementation with zinc alone. Lastly, despite the substantial reduction in diarrhea, there was no significant impact on linear growth velocity. The investigators postulated that deficiency of other micronutrients may have prevented a growth response to zinc. A meta-analysis by Brown of data in 25 studies has confirmed that in all children studied, zinc supplementation had a statistically significant effect of small magnitude on linear growth and weight gain. In children who were stunted at baseline, there was a moderately significant effect of zinc supplementation on linear growth(43). In Vietnam, the incidence of diarrhea was three times less and that for respiratory infections, 2.5 times less in zinc-supplemented children. Zinc supplementation also led to a 2.9-fold reduction in the risk of recurrent diarrhea, but the results for persistent diarrhea were not reported (42). It is important to recognize that the magnitude of reduction in acute and persistent diarrhea resulting from zinc supplementation was at least as large as may be expected from the wide application of vaccines against the important enteric pathogens, or by other nonvaccine interventions, including exclusive breast feeding, safe water supply, and sanitation(45). The diarrhea prevention effect of improved zinc intake in children is of considerable public health significance. Some degree of zinc deficiency is common in children residing in most developing countries, because many staple plant foods are poor sources of micronutrients in that they contain inadequate amounts of some micronutrients and possibly, significant concentrations of antinutrients, including phytic acid, tannins, and other polyphenols that interfere with absorption or use of zinc in the body(43,45,46). Animal protein-containing foods are better sources of zinc, and zinc's bioavailability is greater from them because they contain such promoters of micronutrient availability as the amino acids methionine, cysteine, and lysine. A dietary zinc:phytate molar ratio of more than 15 indicates poor bioavailability; such high ratios have been documented in children in many developing countries(43). Routinely providing supplemental zinc to children in developing countries is impractical. The possibility of fortifying appropriate foods in these settings should be considered. Another alternative may be to increase the zinc content and bioavailability from plant foods, using approaches that increase the mineral content and density of promoter substances and decrease antinutrient factors. This approach is particularly attractive, because it has been shown recently that breeding for trace element-dense seeds improves plant nutrition as well as yield (45). VITAMIN A Vitamin A is an essential nutrient needed in small amounts for normal functioning of the visual system, growth and development, maintenance of epithelial cell integrity, immune function, and reproduction. Until recently, vitamin A research was focused on preventing xerophthalmia and blindness in developing countries. The emphasis is now on defining the effects of moderate and possibly mild subclinical deficiency on mortality and morbidity. In a meta-analysis of the data in a large number of vitamin A supplementation trials conducted in developing countries, it was concluded that improving vitamin A status by periodic supplementation significantly reduces overall mortality in children aged 6 months to 5 years, although results of two studies included in the evaluation were negative(47). The few studies that examined cause-specific mortality produced results showing that deaths caused by diarrhea and measles were reduced, but there was no effect on pneumonia-related mortality(47). The mortality reduction was also observed in Ghana, where the prevalence of xerophthalmia was low, suggesting that moderate deficiency may also have important health consequences(48). It appears that vitamin A stimulates the acute-phase response, evidenced by raised levels of C-reactive protein and serum amyloid A protein, in cases of diarrhea but not in those of other infections (1,49). A reduction in diarrhea-specific mortality through improvement in vitamin A status may occur by a decrease in diarrhea incidence or by reduction in its severity by preventing dehydration and persistent diarrhea. These clinical effects are biologically plausible. Vitamin A maintains the physical and functional integrity of epithelial tissues that serve as a barrier against infection. It acts at multiple sites in the immune system. In the deficient state, there is impairment in the antibody response to bacterial polysaccharides and proteins, cytotoxic activity, and natural killer cell numbers are decreased (50). In the rat model, mucosal immune response to cholera toxin is impaired (51). Vitamin A also plays a role in the production of cell glycoprotein and in the regulation of cell division in the intestine(52-54), which has a bearing on intestinal epithelial renewal during and after acute enteric infections and thereby on the absorption of water, electrolytes, and other nutrients. Recently, it was shown that although vitamin A deficiency by itself caused little change in the jejunal epithelium, the jejunal atrophy induced by rotavirus infection was greater in deficient than in replete animals (55). An association between vitamin A deficiency and increased diarrhea morbidity has been reported (56,57). It has also been shown that diarrhea predisposes to xerophthalmia(58). Clinical trials to assess vitamin A's impact on diarrhea-related morbidity have been designed around two distinct hypotheses. Firstly, periodic administration of large dose of vitamin A may decrease acute and persistent diarrhea in 3 to 4 months. In addition, if the effects of vitamin A appear early enough, its administration during an episode of acute diarrhea may decrease the average duration of the episode, its severity, and the risk of its progression into persistent diarrhea. Vitamin A Supplementation During Acute Diarrhea In the first placebo-controlled trial in Bangladesh, administration of 60,000 μg of vitamin A to children with acute diarrhea did not result in significant reduction in stool output, average duration of episodes, or the risk of persistence (54). It is unlikely that this lack of efficacy was caused by inadequate absorption, because results in earlier studies have shown that more than two thirds of the administered large dose is absorbed during acute diarrhea (59-61). This study involved 83 subjects and as such could have detected only effects with significant impact (54). In a recent trial in India by Bhandari et al., administration of 60,000μg of vitamin A to 900 children 1 to 5 years of age with acute diarrhea resulted in a 70% reduction in persistent episodes that lasted 14 days or more. These results are difficult to interpret, because there was no impact on other outcomes-e.g., average diarrheal duration or stool frequency and consistency. In the subgroup of children who were not breast fed, there was, however, a significant improvement in nearly all possible outcomes, including that of persistent diarrhea (62). This subgroup impact seems plausible, given that breast feeding has been shown to protect against severe vitamin A deficiency (63,64). Overall, a large dose of vitamin A is well tolerated during acute diarrhea, but definitive evidence of substantial clinical impact on the outcome of the treated episode is lacking. Vitamin A Supplementation and Diarrhea Prevention Several double-blind, placebo-controlled trials have evaluated the impact of periodic, large-dose vitamin A supplementation on morbidity in diarrhea(65-70). Only two of the six recent trials covered in this review produced results showing a significant reduction in severe diarrhea(67,69). The most striking impact was observed in Brazil, where vitamin A supplementation resulted in a substantial reduction in the incidence and prevalence of severe diarrhea; the investigators did not report results on persistent diarrhea (69). Results in the Ghana trial showed significant reduction in clinic visits associated with severe illnesses but no impact on incidence or duration of diarrhea(67). These results imply that vitamin A has little effect on diarrhea incidence, but there is some evidence that it may prevent severe watery or persistent diarrhea. Why several well-designed trials resulted in no reduction in severe diarrhea or why persistent episodes were observed after vitamin A administration is yet to be explained. In results of studies of the impact of vitamin A on morbidity, the maximum impact was observed in Brazil, where breast feeding rates in trial subjects were the lowest. It has therefore been postulated that different patterns of breast feeding may have contributed to the varying results in the trials. The World Health Organization recommends routine supplementation with vitamin A once every 6 months, beginning at 6 months of age in countries in which deficiency is a public health problem. Currently, there is insufficient justification for administering supplemental vitamin A to children. OTHER MICRONUTRIENTS Among other micronutrients, folate deficiency has received the greatest attention as a possible contributory factor in the pathogenesis of chronic diarrhea, particularly in tropical malabsorption syndromes. Folates function as coenzymes and play an important role in purine and pyrimidine synthesis and in amino acid metabolism (71). Rapidly proliferating cells are especially sensitive to abnormalities in DNA synthesis. Clinical manifestations of folate deficiency appear first in the hematopoietic system and then in the epithelial cell surfaces and the gonads. In experimental folate deficiency, villus atrophy and significant diarrhea have been noted. Megaloblastic changes within the nuclei of crypt cells have also been described. Further, intestinal secretory response to cholera toxin is also reported to be heightened (72). Folate deficiency has been observed in a number of disorders with symptoms of chronic diarrhea and malabsorption, including celiac disease and tropical malabsorption syndrome. It is unclear whether such a deficiency is merely the result of these disorders or is a factor that perpetuates intestinal dysfunction. In adult tropical malabsorption syndromes, addition of folic acid to treatment regimens is reported to improve intestinal structure and function and to hasten resolution of diarrheal symptoms (72,73). The effects of administration of folates have been observed in residents of developing countries as well as in visitors from developed countries who have acquired this disorder (73). Clinical efficacy of treatment with folic acid in children with tropical enteropathy has not been shown. It is also not known whether administration of folic acid to malnourished children during acute diarrhea reduces such complications as persistent diarrhea. These issues should be resolved through controlled intervention trials. Children with malnutrition may be deficient in other micronutrients. Those that are important to provide to malnourished children with acute or persistent diarrhea in developing countries are yet to be identified. Among these, selenium, manganese, and magnesium have been proposed as candidates for further research, given their important role in cellular function. Currently, there is no evidence that their deficiency initiates, causes, or perpetuates diarrhea in children. Until more information is available, a reasonable approach is to provide approximately 2 recommended daily allowances (RDA) of all vitamins and micronutrients to malnourished children during nutritional rehabilitation. A similar approach may be taken in all children who have persistent or chronic diarrhea associated with poor physical growth. The potential hazards of overdoses of micronutrients' causing decreased absorption or interference with the metabolism of other trace elements is a concern. Absorption of copper is especially sensitive to an excess of zinc. A zinc intake of as little as 50 mg/day has been shown to influence copper status, indicated by a decline in erythrocyte copper-zinc superoxide dismutase activity (74,75). Low plasma copper and plasma ceruloplasmin levels and anemia have been observed after daily doses of 450 to 660 mg/day, but these are 20 to 30 times higher than the 2 RDA for zinc(76,77). The recommended daily allowances for zinc are 5 mg/day of elemental zinc for infants and 10 mg for children aged 1 to 6 years (78). The World Health Organization has developed tentative guidelines for low-cost, simple to manufacture, and easy to use micronutrient formulations for children with severe malnutrition and persistent diarrhea (43). Iron has not been recommended by the World Health Organization because of apprehension about higher mortality in the most severely malnourished children with its use: The recommendation is to add it only after assessment of nutritional status. Iodine and manganese have been omitted from the formulation because they react negatively with other micronutrients. From a global perspective, deficiencies of iodine, iron, and vitamin A have so far been the main focus of efforts to reduce micronutrient malnutrition. Although more research is needed, there is strong evidence to support the opinion that combatting zinc deficiency should be accorded the same priority.