The Relationship between Zinc Intake and Serum/Plasma Zinc Concentration in Children: A Systematic Review and Dose-Response Meta-Analysis

Nu ien s 2012, 4, 841-858; doi:10.3390/nu4080841 nu ien s ISSN 2072-6643 www.mdpi.com/jou nal/nu ien s Re iew The Rela ionship be ween Zinc In ake and Se um/Plasma Zinc Concen a ion in Child en: A Sys ema ic Re iew and Dose-Response Me a-Analysis Vic o ia Hall Mo an 1, *, Anna-Louise S amme s 2 , Ma isol Wa hon Medina 2 , Suja a Pa el 2 , Fiona Dykes 1 , Olga W. Sou e ein 3 , Ca la Dullemeije 3 , Ca men Pé ez-Rod igo 4 , Lluis Se a-Majem 5 , Ma iela Nissensohn 5 and Nicola M. Lowe 2 1 Ma e nal & In an Nu i ion & Nu u e Uni , Uni e si y o Cen al Lancashi e, P es on PR1 2HE, UK; E-Mail: [email p o ec ed] 2 In e na ional Ins i u e o Nu i ional Sciences and Food Sa e y S udies, Uni e si y o Cen al Lancashi e, P es on PR1 2HE, UK; E-Mails: [email p o ec ed] (A.-L.S.); [email p o ec ed] (M.W.M.); [email p o ec ed] (S.P.); [email p o ec ed] (N.M.L.) 3 Di ision o Human Nu i ion, Wageningen Uni e si y, PO Box 8129, 6700 EV Wageningen, The Ne he lands; E-Mails: [email p o ec ed]l (O.W.S.); Ca la.Dullem[email p o ec ed] (C.D.) 4 Communi y Nu i ion Uni , Bilbao Ci y Council, Bilbao 48001, Spain; E-Mail: [email p o ec ed] 5 Depa men o Clinical Sciences, Uni e si y o Las Palmas de G an Cana ia, Las Palmas de G an Cana ia 35016, Spain; E-Mails: [email p o ec ed]pgc.es (L.S.-M.); [email p o ec ed] (M.N.) * Au ho o whom co espondence should be add essed; E-Mail: [email p o ec ed]; Tel.: +44-1772-893830; Fax: +44-1772-892998. Recei ed: 4 May 2012; in e ised o m: 27 June 2012 / Accep ed: 10 July 2012 / Published: 26 July 2012 Abs ac : Recommenda ions o zinc in ake du ing childhood a y widely ac oss Eu ope. The EURRECA p ojec a emp s o consolida e he basis o he de ini ion o mic onu ien equi emen s, aking in o accoun ela ionships among in ake, s a us and heal h ou comes, in o de o ha monise hese ecommenda ions. Da a on zinc in ake and bioma ke s o zinc s a us epo ed in andomised con olled ials (RCTs) can p o ide es ima es o dose- esponse ela ionships which may be used o unde pinning zinc e e ence alues. This sys ema ic e iew included all RCTs o appa en ly heal hy child en aged 1–17 yea s published by Feb ua y 2010 which p o ided da a on zinc in ake and bioma ke s o zinc s a us. OPEN ACCESS Nu ien s 2012, 4 842 An in ake-s a us eg ession coe icien ( 󰆹) was calcula ed o each indi idual s udy and calcula ed he o e all pooled  󰆹 and SE ( 󰆹) using andom e ec s me a-analysis on a double log scale. The pooled dose- esponse ela ionship be ween zinc in ake and zinc s a us indica ed ha a doubling o he zinc in ake inc eased he se um/plasma zinc s a us by 9%. This e idence can be u ilised, oge he wi h cu en ly used balance s udies and eple ion/deple ion s udies, when se ing zinc ecommenda ions as a basis o nu i ion policies. Keywo ds: zinc; child en; se um zinc; sys ema ic e iew; dose- esponse; die a y ecommenda ions; EURRECA 1. In oduc ion Subop imal die a y zinc in ake is inc easingly ecognised as an impo an public heal h issue. Al hough he lack o gene ally accep ed bioma ke s o zinc s a us has impeded es ima ion o he global p e alence o zinc de iciency, based on in o ma ion ega ding he amoun o zinc p esen in na ional ood supplies, i has been es ima ed ha he isk o low die a y in ake o abso bable zinc and consequen zinc de iciency a ec s be ween one- hi d and one-hal o he wo ld’s popula ion [1] and a es o de iciency may app oach 73% in some egions [2]. Al hough se e e zinc de iciency is uncommon in Eu opean popula ions, ma ginal de iciency is likely o be much mo e p e alen [3], wi h associa ions o immune sys em dys unc ion and es ic ed physical de elopmen [4]. Child en a e pa icula ly ulne able o subop imal zinc s a us du ing pe iods o apid g ow h ha c ea e inc eased zinc needs ha may no be me [5,6]. I is es ima ed ha o e 450,000 dea hs pe yea (4.4% o all mo ali ies) among child en be ween six mon hs and i e yea s o age a e a ibu able o zinc de iciency [7]. Zinc de iciency also has an impac o child mo bidi y, impai ing g ow h and con ibu ing o childhood s un ing [8,9]. Physiological equi emen s o zinc peak a he ime o he pube al g ow h spu , he onse o which a ies acco ding o gende . In gi ls he onse o he g ow h spu (OGS) occu s a 10.1 yea s and peak heigh eloci y (PHV) occu s a 12.0 yea s. In boys OGS and PHV occu a 11.8 and 14.2 yea s, espec i ely [10]. E en a e he g ow h spu has ceased, adolescen s may equi e addi ional zinc o eple e issue zinc pools deple ed du ing pube y [11]. Ma ginal zinc s a us du ing he pube al g ow h spu has been associa ed wi h slowe skele al g ow h, ma u a ion, and educed bone mine alisa ion [12–14]. As nea ly a hi d o o al skele al mine al is accumula ed in he 3–4 yea pe iod immedia ely a e he onse o pube y [15] subop imal zinc in ake may ha e long- e m consequences on bone heal h. Al hough a sensi i e and speci ic bioma ke has ye o be iden i ied, a ecen sys ema ic e iew concluded ha plasma (o se um) zinc concen a ion was esponsi e o bo h zinc supplemen a ion and deple ion and is he mos widely used bioma ke o zinc [16]. Howe e , me a-analy ic me hods ha e no ye been used o model zinc s a us as a unc ion o zinc in ake le els. Unde s anding he ela ionship be ween die a y in ake and mic onu ien s a us is essen ial o de i ing die a y ecommenda ions. The ecommenda ions o zinc in ake du ing childhood a y widely ac oss Eu ope and compa isons a e di icul due o di e ences in ca ego isa ion. Fo example be ween 4 and 6 age ca ego ies a e used Nu ien s 2012, 4 843 o desc ibe mic onu ien ecommenda ions in childhood wi h di e en age cu -o poin s being used by di e en Eu opean coun ies [17,18]. Recommenda ions o zinc in ake di e be ween males and emales a he age o 15 yea s in mos coun ies, bu also di e a he age o 10 yea s in some coun ies. Zinc in ake alues ange om 2.9 o 10.0 mg/day in child en aged 5 yea s, 5.7 o 15.5 mg/day (boys) and 4.6 o 15.0 mg/day (gi ls) in child en aged 10–15 yea s [17]. The EURRECA p ojec a emp s o consolida e he basis o he de ini ion o mic onu ien equi emen s ac oss Eu ope, aking in o accoun ela ionships among in ake, s a us and heal h ou comes, in o de o ha monise hese ecommenda ions [19]. This pape p esen s a sys ema ic e iew o he da a om all a ailable andomised con olled ials (RCTs) mee ing EURRECA’s quali y s anda d [20], which in es iga ed zinc in ake and bioma ke s o zinc s a us, and combines hese s udies in me a-analyses o model zinc concen a ions in se um o plasma as a unc ion o zinc in ake. 2. Me hods 2.1. Sea ch S a egy This esea ch was conduc ed wi hin he amewo k o he Eu opean Mic onu ien Recommenda ions Aligned (EURRECA) Ne wo k o Excellence ha aims o iden i y he mic onu ien equi emen s o op imal heal h in Eu opean popula ions (EURRECA [21]). This e iew was pa o a wide e iew p ocess o iden i y s udies assessing he e ec o zinc in ake on di e en ou comes (bioma ke s o zinc s a us and heal h ou comes). The wide sea ches we e pe o med o li e a u e published up o and including Feb ua y 2010 using MEDLINE, Embase, and Coch ane using sea ch e ms o “s udy designs in humans” and “zinc” and “in ake OR s a us”. Bo h indexing and ex e ms we e used and languages included we e es ic ed o hose spoken in he EURRECA Ne wo k (English, Du ch, F ench, Ge man, Hunga ian, I alian, No wegian, Polish, Spanish, G eek, and Se bian). The ull O id MEDLINE sea ch s a egy can be ound in Table 1. Re e ence lis s o e ie ed a icles and published li e a u e e iews we e also checked o ele an s udies. Au ho s we e con ac ed o eques missing da a o cla i y me hods o esul s. The sea ch p ocess is illus a ed in Figu e 1. Table 1. Sea ch s a egy: MEDLINE Feb ua y 2010 [22]. No. Sea ch Te m Resul s 1 andomised con olled ial.p . 280,821 2 con olled clinical ial.p . 79,998 3 andomised.ab. 196,604 4 placebo.ab. 117,891 5 clinical ials as opic.sh. 146,242 6 andomly.ab. 145,491 7 ial.ab. 203,467 8 andomised.ab. 38,423 9 6 o 3 o 7 o 2 o 8 o 1 o 4 o 5 734,511 10 (animals no (human and animals)).sh. 4,482,479 11 9 no 10 642,665 12 (coho * o “case con ol*” o c oss-sec ional* o “c oss sec ional” o case-con ol* o p ospec i e o “sys ema ic e iew*”).mp. 768,885 Nu ien s 2012, 4 844 Table 1. Con . 13 exp me a-analysis/ o expmul icen e s udy/ o ollow-up s udies/ o p ospec i e s udies/ o in e en ion s udies/ o epidemiologic s udies/ o case-con ol s udies/ o exp coho s udies/ o longi udinal s udies/ o c oss-sec ional s udies/ 1,013,635 14 13 o 12 1,203,767 15 14 no 10 1,154,385 16 11 o 15 1,599,094 17 ((zinc o Zn o zinc sulpha e o zinc glucona e o zinc ace a e o me hionine o zinc iso ope*) adj3 (in ake* o die * o supplemen * o deple * o s a us o se um o plasma o leukocy e o concen a ion* o expos* o o i * o u ine o hai )). i,ab. 16,681 18 Nu i ional Suppo / o Die a y Supplemen s/ o nu i ional equi emen s/ o B eas eeding/ o exp in an ood/ o bo le eeding/ o in an o mula/ 63,098 19 exp Nu i ional S a us/ o exp De iciency Diseases/ o supplemen a ion/ o die supplemen a ion/ o die a y in ake/ o exp die es ic ion/ o exp mine al in ake/ o Die / o Food, Fo i ied/ o nu i ion assessmen / o Nu i i e Value/ 176,014 20 (in ake* o die * o supplemen * o deple * o s a us o se um o plasma o leukocy e o concen a ion* o expos* o o i * o u ine o hai ). i,ab. 3,166,092 21 18 o 19 o 20 3,263,114 22 zinc/ 41,027 23 22 and 21 20,745 24 23 o 17 26,943 25 24 and 16 2410 2.2. C i e ia o he Conside a ion o S udies o This Re iew Included s udies we e RCTs in appa en ly heal hy child (human) popula ions aged om 1 o 17 yea s ha supplied zinc supplemen a ion ei he as capsules o pa o a o i ied meal. I supplemen al zinc was p o ided as a componen o a o i ied meal, s udies we e only conside ed accep able i zinc was he only cons i uen ha was di e en be ween ea men g oups. Only s udies ha measu ed zinc s a us as se um o plasma zinc we e included; and hose ha epo ed su icien da a o had su icien da a ob ainable om he au ho s o es ima e  󰆹 and SE ( 󰆹) o he assumed linea ela ion on he log e –log e scale. S udies we e excluded i hey we e a g oup RCT (communi y ial), o we e commen a ies, e iews, o duplica e publica ions om he same s udy. S udies we e excluded i child en we e hospi alised, had se e e p o ein-ene gy malnu i ion o a ch onic disease o i supplemen al zinc was p o ided o less han 6 weeks. 2.3. Selec ion o A icles O 4719 iden i ied a icles in he wide sea ch on zinc in ake, s a us and p io i y heal h ou comes in all popula ions, 2557 we e excluded based upon sc eening o he i le and abs ac . Two independen e iewe s sc eened 10% o he abs ac s in duplica e and any disc epancies we e discussed be o e sc eening he emaining e e ences. Following subdi ision in o app op ia e popula ion g oups he ull ex s o he 328 manusc ip s we e assessed o de e mine inclusion and exclusion by wo independen e iewe s and disag eemen s ec i ied h ough discussion. 302 s udies we e excluded because hey did no mee he inclusion c i e ia. O he emaining 26 s udies, 8 s udies we e excluded as hey had no Nu ien s 2012, 4 845 in es iga ed he ela ion be ween zinc in ake and zinc ela ed bioma ke s, bu ela ed ei he in ake o s a us di ec ly o a heal h endpoin . Fo he pu pose o his pape , 18 RCTs emained. Table 2 p esen s he cha ac e is ics o he included s udies. Figu e 1. S udy selec ion p ocess o sys ema ic e iew (* some pape s epo ed mo e han one ela ionship). 2.4. Da a Ex ac ion Fo each o he iden i ied manusc ip s, da a was ex ac ed independen ly by wo e iewe s in o a s anda dised da abase. Ex ac ed da a included popula ion cha ac e is ics, dose o elemen al zinc in in e en ion and placebo supplemen s, du a ion o he s udy, die a y in ake o zinc, and mean concen a ion o zinc in plasma o se um a he end o he in e en ion pe iod. Se um/plasma zinc concen a ions we e con e ed o µmol/L when applicable. Nu ien s 2012, 4 846 Table 2. Summa y o included ials epo ing he e ec o die a y zinc in ake on se um/plasma zinc s a us in child en. Fi s Au ho , Yea , Coun y Pa icipan s T ea men G oups (n) Mean Zn In ake (mg/day) Mean (SD) Plasma/Se um Zn (µmol/L) Du a ion Zinc S a us Bioma ke [Analy ical Me hod] Main Resul s Mahloudji, 1975, I an [23] Males & emales aged 6–12 yea s Fe only (12); Fe + 20 mg/day Zn (13) 5.65; 25.65 8.95 (1.80) 8.50 (1.93) 8 mon hs Plasma Zn [AAS] No signi ican di e ence be ween plasma Zn o he supplemen ed and placebo g oups Hambidge, 1979, USA [24] Males & emales aged 33–90 mon hs Male placebo (15); Male Zn FM 2.57 mg/day (20); Female placebo (14); Female Zn FM 2.57 mg/day (11) 6.3; 9.27; 6.3; 9.27 11.06 (2.23) 11.85 (2.23) 10.61 (1.81) 11.96 (1.81) 9 mon hs Plasma Zn [AES] Plasma Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (gi ls and combined sexes only p < 0.05) Wal a ens, 1983, USA [25] Males & emales aged 2–6 yea s Placebo (16); 10 mg/day Zn (16) 4.6; 15.9 11.32 (2.14) 10.86 (2.14) 12 mon hs Plasma Zn [AES] No signi ican di e ence be ween plasma Zn o he supplemen ed and placebo g oups Gibson, 1989, Canada [26] Males aged 59–95 mon hs Placebo (21); 10 mg Zn/day (18) 6.4; 16.7 15.8 (3.5) 17.9 (3.4) 6 mon hs Se um Zn [AAS] No signi ican co ela ion be ween se um Zn and die a y Zn le els Ca an, 1993, Gua emala [27] Males & emales, mean age 81.5 (±7.0) mon hs 1 Placebo (74); 10 mg Zn/day (71) 5.65; 15.65 14.9 (2.1) 16.2 (2.9) 25 weeks Plasma Zn [AAS] Plasma Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.01) F iis, 1997, Zimbabwe [28] Males and emales aged 11–17 yea s Placebo (121); 30–50 mg/day Zn (122) 5.65; 45.65 2 10.89 (2.5) 11.71 (2.4) 12 mon hs Se um Zn [AAS] The decline in zinc concen a ion was signi ican ly lowe in he Zn supplemen ed g oup compa ed o he placebo g oup (p < 0.02) Rosado, 1997, Mexico [29] Males & emales aged 18–36 mon hs Placebo (55); 20 mg Zn/day (54) 5.65; 25.65 14.4 (4.45) 16.8 (5.88) 12 mon hs Plasma Zn [AAS] Plasma Zn inc eased signi ican ly in he Zn supplemen ed g oup o e he 12 mon hs pe iod (p < 0.01) Ruz, 1997, Chile [30] Males & emales aged 27–50 mon hs Placebo (33); 10 mg/day Zn (36) 6.4; 17.1 17.7 (1.9) 17.6 (2.2) 6 mon hs Plasma Zn [AAS] No signi ican di e ence be ween plasma Zn o he supplemen ed and placebo g oups Sands ead, 1998, China [31] (3 egions) Males & emales aged 6–9 yea s Chonqing MN, no Zn (35); 20 mg/day Zn + MN (35); Quindgdao MN, no Zn (36); 20mg/day Zn + MN (36); Shanghai MN, no Zn (37); 20 mg/day Zn + MN (37) 5.65; 25.65; 5.65; 25.65; 5.65; 25.65 19.83 (4.12) 23.6 (4.12) 20.42 (4.08) 22.97 (4.08) 17.9 (2.75) 17.97 (2.75) 10 weeks Plasma Zn [AAS] Plasma Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.05) in Chonqing and Quindgdao g oups. Nu ien s 2012, 4 847 Table 2. Con . Cla k, 1999, UK [32] Pe ipube al emales, mean age 12.2 (±0.3) yea s Placebo (19); 15 mg Zn/day (23) 6.6; 21.6 12.6 (1.0) 16.7 (4.9) 6 weeks Se um Zn [no me hod gi en] Se um Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.001) Smi h, 1999, Belize [33] Males & emales aged 22–66 mon hs Placebo (10); 70 mg Zn/day (12) 5.65; 75.65 11.7 (0.68) 13.5 (0.68) 6 mon hs Se um Zn [AAS] Se um Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.001) Munoz, 2000, Mexico [34] Males & emales aged 18–36 mon hs Placebo (54); 20 mg/day Zn (47) 5.65; 25.65 14.3 (4.7) 16.8 (5.6) 6 mon hs Plasma Zn [AAS] Se um Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.0001) Lopez de Romana, 2005, Pe u [35] Males & emales aged 3–4 yea s Fe FM (12); Fe + 3 mg/day Zn FM (10); Fe + 9 mg/day Zn FM (12); 4.71; 8.72; 15.7 11.87 (1.88) 11.65 (1.25) 12.60 (1.51) 70 days Plasma Zn [ICP-MS] No signi ican di e ences in plasma Zn we e ound be ween ea men s Sil a, 2006, B azil [36] Males & emales aged 12–59 mon hs 3 Placebo (30); 10 mg/day Zn (28) 5.65; 15.65 8.0 (0.58) 13.4 (0.25) 4 mon hs Se um Zn [AAS] Se um Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.05) Sands ead, 2008, USA (Mexican Ame icans) [37] Males & emales aged 6–7 yea s MN, no Zn (25); 20 mg/day Zn + MN (25) 5.65; 25.65 15.4 (1.5) 15.6 (1.2) 10 weeks Plasma Zn [AAS] Mean plasma Zn inc eased signi ican ly in bo h g oups compa ed o baseline (p < 0.05) Wuehle , 2008, Ecuado [38] Males & emales aged 12–30 mon hs Placebo (56); 3 mg Zn/day (50); 7 mg Zn/day (52); 10 mg Zn/day (54) 5.65; 8.65; 12.65; 15.65 10.6 (1.6) 12.3 (1.6) 13.3 (1.7) 14.0 (1.7) 4 6 mon hs Plasma Zn [ICP-MS] The mean change in plasma zinc concen a ions om baseline inc eased p og essi ely wi h highe doses o supplemen al Zn (p < 0.001) de Oli ei a, 2009, B azil [39] Pubescen males, mean age 13 ( ± 0.4) yea s Placebo (26); 22 mg Zn/day (21) 5.65; 27.65 16.9 (2.1) 18.7 (3.5) 12 weeks Plasma Zn [ICP-MS] Plasma Zn signi ican ly highe in Zn supplemen ed compa ed o placebo (p < 0.05) Ucka de, 2009, Tu key [40] Males & emales aged 8–9 yea s Placebo (109); 15 mg/day Zn (109) 5.65; 20.65 19.19 (1.80) 19.50 (2.41) 10 weeks Se um Zn [CS] Bo h supplemen ed and placebo g oups had signi ican ly highe se um Zn a ollow up (p < 0.05) AAS, a omic abso p ion spec oscopy; AES, a omic emission spec oscopy; ICP-MS, induc i ely coupled plasma mass spec ome y; CS, calo ic spec opho ome y; MN, mic onu ien s; FM, o i ied meal; 1 all pa icipan s also ecei ed MN supplemen s; 2 child en weighing <29.5 kg we e gi en 30 mg Zn/day and hose >29.5 kg we e gi en 50 mg Zn/day, his igu e is an a e age o he wo doses; 3 all pa icipan s also ecei ed Fe o i ied milk; 4 geome ic means. Nu ien s 2012, 4 848 2.5. Da a Syn hesis One s udy ha included wo zinc- ea ed g oups and wo con ol g oups (males and emales) was ea ed as wo independen es ima es in he analysis [24] and one s udy ha included h ee zinc- ea ed g oups and h ee con ol g oups ( om di e en egions o China) [31] and was ea ed as h ee independen es ima es in he analysis. Whe e s udies p o ided ou come da a o wo o mo e zinc- ea ed g oups hey we e included as sepa a e es ima es in he me a-analysis [35,38]. In one s udy zinc s a us was measu ed a 6 mon hs and 12 mon hs in he same popula ion [26] and only he measu e a 6 mon hs was used in he analysis (whe e n was he la ges ). I die a y in ake o zinc (in addi ion o he in e en ion) was no epo ed we impu ed a alue o 5.65 mg/day, he mean die a y in ake le el o he RCTs (n = 7) ha did epo die a y zinc in ake. As mean baseline se um/plasma zinc concen a ions we e in equen ly epo ed, he se um/plasma zinc concen a ions in he con ol g oup o he RCTs we e used as a p oxy o he baseline se um/plasma zinc concen a ions o ou analyses. 2.6. P e-Speci ied Po en ial Fac o s Modi ying he Associa ion A pooled me a-analysis was pe o med combining he e idence om all he a ailable RCTs. In addi ion, we in es iga ed whe he age, dose o zinc, du a ion o he supplemen a ion, and ype o supplemen (zinc only s. zinc wi h o he mic onu ien s) we e a iables ha modi ied he associa ion. 2.7. S a is ical Analyses As we wan ed o es ima e he dose- esponse ela ion be ween zinc in ake and se um/plasma zinc, he da a p esen ed in he s udies had o be ans o med o a common s a is ic, namely a eg ession coe icien ( 󰆹) and he s anda d e o (SE ( 󰆹)) o his eg ession coe icien . The ans o ma ions used o de i e his common single-s udy es ima e om he a ailable summa y s a is ics pe s udy ha e been desc ibed elsewhe e [41]. In sho , we es ima ed an in ake-s a us eg ession coe icien ( 󰆹) o each indi idual s udy, based on he assump ion o a linea ela ion on he log e –log e -scale (na u al loga i hm o in ake s. na u al loga i hm o s a us). This shape o his linea ela ionship on he log e –log e -scale co esponds o a mono onic conca e unc ion on he o iginal scale o β < 1. This shape is assumed o be ealis ic o he biological ela ionship be ween zinc in ake and plasma/se um zinc concen a ions. As he ue dose- esponse cu e is unknown, his app oxima ion p o ides a p ac ical me hodology o es ima e he dose- esponse ela ionship. We calcula ed he o e all pooled  󰆹 and SE ( 󰆹) using andom e ec s me a-analysis, which es ima es he be ween-s udy a iance using he me hod o De Simonian and Lai d and used his es ima e o modi y he weigh s used o calcula e he summa y es ima e. Residual he e ogenei y be ween s udies was e alua ed using he I 2 s a is ic. P e-speci ied po en ial ac o s ha could modi y he associa ion we e explo ed using s a i ied andom e ec s me a-analyses. The s a is ical ans o ma ions o ob ain  󰆹’s and SE ( 󰆹)’s we e pe o med using GenS a e sion 13-SP2 (VSN In e na ional L d. [42]) and he me a-analysis was pe o med using STATA e sion 11.0 (College S a ion, TX, USA), wi h s a is ical signi icance de ined as p < 0.05. Nu ien s 2012, 4 849 2.8. Assessmen o Risk o Bias in Included S udies In o de o assess he quali y o he included s udies and he isk o bias, indica o s o in e nal alidi y we e collec ed du ing da a ex ac ion (Table 2). Based on he indica o s wo independen e iewe s assessed he o e all isk o bias and disag eemen s esol ed by discussion. The c i e ia o judging hese indica o s we e adap ed om he Coch ane Handbook o Sys ema ic Re iews [43]. 3. Resul s Twen y- ou es ima es o zinc in ake and se um/plasma zinc s a us in 18 RCTs wi h child en we e eligible o me a-analysis. All s udies we e RCTs published be ween 1975 and 2009 which epo ed zinc in ake and plasma/se um zinc concen a ions. The 24 es ima es included 1722 pa icipan s in o al wi h sample sizes anging om 10 o 122. The median du a ion o he ials was 24 weeks ( ange 6–52 weeks). In 11 s udies zinc was supplemen ed alone a doses anging om 3 o 70 mg/day and in 7 s udies pa icipan s also ecei ed o he mic onu ien s. Zinc was p o ided wi h i on supplemen s [23] o i on o i ied milk [36], as pa o a o i ied meal [24,35], and wi h o he mic onu ien s [27,31,37]. The zinc dose anged om 10 o 20 mg/day when combined wi h o he i amin/mine als and 2.57 o 9 mg/day when p o ided in o i ied meals. Mos s udies (n = 7) p o ided he zinc supplemen s in he o m o zinc sulpha e, bu o he s used zinc ci a e [32], zinc glucona e [33], zinc ca bona e [23], zinc me hionine [29,34], amino acid chela e [27], and elemen al zinc in a sy up [28,40]. S udies we e conduc ed in La in Ame ica (n = 9), No h Ame ica (n = 4), Asia (n = 3), A ica (n = 1) and Eu ope (n = 1). Habi ual zinc in akes anged om 4.6 o 7.1 mg/day (whe e da a was p o ided) and ages o child en anged om 2 o 17 yea s. Mos s udies included, bu did no di e en ia e be ween, males and emales, bu one s udy p o ided sepa a e male and emale da a [24], one included only emales [32], and one only males [39]. The majo i y o s udies (n = 13) epo ed ha zinc supplemen a ion signi ican ly inc eased zinc plasma/se um s a us o signi ican ly educed he decline in zinc se um alues compa ed o placebo. O hese, wo s udies also epo ed inc eased plasma/se um zinc concen a ions in he placebo g oup. Fi e s udies ailed o ind a signi ican ela ionship be ween zinc supplemen a ion and zinc s a us [23,25,26,30,35], ou o which p o ided zinc supplemen s o o i ied meals wi h a zinc concen a ion o 10 mg/day o less. Ou me a-analysis o a ailable s udies sugges ed ha zinc supplemen a ion was associa ed wi h inc eased se um/plasma zinc concen a ions. Combining he 18 RCTs in one me a-analysis yielded an o e all pooled β-coe icien o 0.12 (95% CI 0.04, 0.20; p < 0.005; I 2 97.6%) (Figu e 2). Since we applied a base-e loga i hmic ans o ma ion on he zinc in ake and se um/plasma zinc concen a ion be o e calcula ion o he s udy-speci ic  󰆹’s, he o e all  󰆹 ep esen s he di e ence in he log e ans o med p edic ed alue o se um/plasma zinc s a us o each one-uni di e ence in he log e ans o med alue in zinc in ake. The e o e, an o e all  󰆹 o 0.12 means ha o e e y doubling in zinc in ake, he di e ence in zinc se um o plasma concen a ion is 2  (2 0.12 = 1.09), which is 9%. This means ha a pe son wi h a zinc in ake o 14 mg/day has a zinc se um/plasma concen a ion ha is 9% highe han a pe son who has a zinc in ake o 7 mg/day. Nu ien s 2012, 4 856 20. 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