The Role of Zinc and Vitamin A in Persistent Diarrhea Among Infants and Young Children

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 and serum A in of diarrhea but not in those of other infections A reduction in mortality through in vitamin A status may occur by a decrease in diarrhea incidence or by reduction in its severity by preventing and persistent diarrhea. These clinical effects are Vitamin A the and functional of epithelial that as a against It at multiple in the immune system. In the deficient there is in the response to and activity, and cell are decreased In the mucosal immune response to cholera toxin is impaired Vitamin A also plays a role in the of cell and in the of cell in the which has a bearing on intestinal epithelial during and after acute enteric infections and on the absorption of and other it was shown that although vitamin A deficiency by caused little in the the atrophy by infection was greater in deficient than in An between vitamin A deficiency and increased diarrhea morbidity has been reported It has also been shown that diarrhea to Clinical trials to assess vitamin impact on diarrhea-related morbidity have been two periodic administration of large dose of vitamin A may decrease acute and persistent diarrhea in 3 to months. In if the effects of vitamin A appear early its administration during an episode of acute diarrhea may decrease the duration of the its and the risk of its persistent diarrhea. Vitamin A Supplementation During Acute Diarrhea In the placebo-controlled trial in administration of of vitamin A to children with acute diarrhea did not in significant reduction in duration of or the risk of It is that this lack of was caused by inadequate absorption, because results in earlier studies have shown that more than two of the administered large dose is absorbed during acute diarrhea This study involved subjects and as such could have only effects with significant impact In a trial in India by et administration of of vitamin A to children 1 to 5 years of age with acute diarrhea resulted in a reduction in persistent episodes that days or These results are difficult to because there was no impact on other diarrheal duration or frequency and In the subgroup of children who were not breast there however, a significant in nearly all including that of persistent diarrhea This subgroup impact seems given that breast feeding has been shown to against severe vitamin A deficiency a large dose of vitamin A is well during acute diarrhea, but evidence of substantial clinical impact on the outcome of the treated episode is Vitamin A Supplementation and Diarrhea Prevention Several double-blind, placebo-controlled trials have evaluated the impact of vitamin A supplementation on morbidity in two of the trials in this review produced results showing a significant reduction in severe The most striking impact was observed in where vitamin A supplementation resulted in a substantial reduction in the incidence and prevalence of severe the investigators did not results on persistent diarrhea Results in the trial significant reduction in associated with severe but no impact on incidence or duration of These results that vitamin A has little effect on diarrhea but there is some evidence that it may severe watery or persistent diarrhea. several trials resulted in no reduction in severe diarrhea or persistent episodes were observed after vitamin A administration is to be In results of studies of the impact of vitamin A on the impact was observed in where breast feeding in trial subjects were the It has been postulated that of breast feeding may have to the results in the The supplementation with vitamin A 6 months, at 6 months of age in countries in which deficiency is a public health there is insufficient for supplemental vitamin A to children. other deficiency has received the as a factor in the of chronic diarrhea, particularly in as and an important role in and and in amino cells are to in Clinical of deficiency appear in the and in the epithelial cell and the In experimental deficiency, villus atrophy and significant diarrhea have been within the of cells have also been intestinal secretory response to cholera toxin is also reported to be deficiency has been observed in a number of with of chronic diarrhea and including and It is such a deficiency is the of these or is a factor that intestinal In addition of to treatment is reported to improve intestinal structure and and to of diarrheal The effects of administration of have been observed in of developing countries as well as in from countries who have acquired this disorder Clinical of treatment with in children with has not been It is also not administration of to malnourished children during acute diarrhea reduces such as persistent diarrhea. These should be resolved through with may be deficient in other that are important to to malnourished children with acute or persistent diarrhea in developing countries are to be identified. and have been proposed as for research, given their important role in function. there is no evidence that their deficiency or diarrhea in children. Until more is a approach is to approximately daily of all and micronutrients to malnourished children during nutritional A approach may be in all children who have persistent or chronic diarrhea associated with poor growth. The of of decreased absorption or with the of other trace is a of is to an excess of zinc. A zinc intake of as little as has been shown to status, by a in activity plasma and plasma levels and have been observed after daily doses of to but these are 20 to times than the for The daily for zinc are 5 of elemental zinc for and 10 mg for children aged 1 to 6 years The has for to and to use micronutrient for children with severe and persistent diarrhea has not been by the because of mortality in the most malnourished children with its The is to it only after assessment of nutritional status. and have been from the formulation because they with other a of and vitamin A have so been the main focus of to reduce micronutrient Although more research is there is evidence to the that zinc deficiency should be the