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運動員貧血是一種常見的現象,主要與鐵代謝異常有關。運動引起的血漿容量擴張和鐵需求增加是其主要原因,尤其在耐力運動員中較為普遍。此外,營養不良、腸胃吸收問題、運動引起的損失(如流汗、血尿)以及炎症反應等也會加劇貧血情況。女性和青少年運動員更容易受到影響,及早檢測鐵質狀況有助於改善運動表現和健康。
Anemia in Sports: A Narrative Review
運動中的貧血:敘述性評論
Damian MT, Vulturar R, Login CC, Damian L, Chis A, Bojan A. Anemia in Sports: A Narrative Review. Life (Basel). 2021;11(9):987. Published 2021 Sep 20. doi:10.3390/life11090987
https://pmc.ncbi.nlm.nih.gov/articles/PMC8472039/
Abstract
Recent years have brought about new understandings regarding the pathogenesis of anemia in sports. From hemodilution and redistribution considered to contribute to the so-called “sports anemia” to iron deficiency caused by increased demands, dietary restrictions, decreased absorption, increased losses, hemolysis, and sequestration, to genetic determinants of different types of anemia (some related to sport), the anemia in athletes deserves a careful and multifactorial approach. Dietary factors that reduce iron absorption (e.g., phytate, polyphenols) and that augment iron’s bioavailability (e.g., ascorbic acid) should be considered. Celiac disease, more prevalent in female athletes, may underlie an unexplained iron deficiency anemia. Iron loss during exercise occurs in several ways: sweating, hematuria, gastrointestinal bleeding, inflammation, and intravascular and extravascular hemolysis. From a practical point of view, assessing iron status, especially in the athletes at risk for iron deficiency (females, adolescents, in sports with dietary restrictions, etc.), may improve the iron balance and possibly the performance. Hemoglobin and serum ferritin are measures that are easily employable for the evaluation of patients’ iron status. Cutoff values should probably be further assessed with respect to the sex, age, and type of sport. A healthy gut microbiome influences the iron status. Athletes at risk of iron deficiency should perform non-weight-bearing, low-intensity sports to avoid inducing hemolysis.
Keywords: sports anemia, iron metabolism, hepcidin, genetic causes of anemia
摘要
近年來,對運動性貧血的發病機制有了新的認識。從血液稀釋與再分配被認為是所謂「運動性貧血」的成因,到因需求增加、飲食限制、吸收減少、損失增加、溶血及隔離引起的缺鐵性貧血,再到與運動相關的遺傳性貧血,各種因素都顯示運動員的貧血需以審慎且多角度的方式加以探討。飲食中的植酸、酚類等降低鐵吸收的因子,以及如維生素 C 等能增加鐵生物可用性的因子,都需列入考量。女性運動員中更常見的乳糜瀉,可能是無法解釋的缺鐵性貧血的潛在原因。在運動中,鐵的流失可透過多種途徑發生,例如流汗、血尿、胃腸道出血、炎症反應,以及血管內外的溶血。從實務角度看,對於有缺鐵風險的運動員(如女性、青少年、從事飲食受限的運動項目者),評估鐵狀態可能有助於改善鐵平衡及提升表現。血紅素和血清鐵蛋白是評估鐵狀態的簡便指標,其界限值應根據性別、年齡及運動類型進一步檢討。健康的腸道微生物群亦影響鐵的狀態。有缺鐵風險的運動員應進行非負重、低強度的運動,以避免引發溶血。
關鍵詞:運動性貧血、鐵代謝、鐵調素、貧血遺傳因素
引言
運動員,依定義為健康的個體,但因體能訓練、生理與心理壓力及環境條件等因素,他們的血液及生化指標常偏離常規值[1]。在運動性貧血的發病機制中,多數與鐵代謝相關。近年來,對此複雜議題的理解有所進展。從被認為導致「運動性貧血」的血液稀釋及再分配,到因需求增加、飲食限制、吸收減少、損失增加、溶血及隔離引起的缺鐵性貧血,再到與運動相關的遺傳性貧血,顯示運動員的貧血需以審慎且多角度的方式探討。
運動性貧血
運動員的血紅素濃度通常低於一般人群,此現象被稱為「運動性貧血」,但實際上是錯誤命名,因其實為假性貧血[2]。耐力訓練引起的運動性血漿量增加,導致血細胞比容(Hct)、血紅素(Hb)及紅血球數量(RBC)下降,此變化在密集訓練幾天內即可發生[3,4,5]。同時,運動刺激紅血球生成,雖然血紅素的絕對質量增加,但此機制無法趕上血漿擴張的速度[4]。以靜脈樣本測得的血紅素濃度下降,可定義為相對性或稀釋性貧血,其總血紅素量及紅血球量正常[6]。運動人群的鐵參數界限值仍具爭議[7]。隨機、安慰劑對照試驗顯示,對缺鐵女性運動員每天補充 100 毫克硫酸鐵(FeSO4)可改善鐵狀態,並可能提升運動表現[7]。健康的腸道微生物群也影響鐵的狀態[8]。
瑞士運動醫學會的共識指出,基線的 Hb、Hct、平均紅血球體積(MCV)、平均紅血球血紅素(MCH)及血清鐵蛋白有助於監測缺鐵狀態[6]。健康男性及 15 歲以上女性運動員的血清鐵蛋白值低於 15 μg/L 表示鐵庫空,15 至 30 μg/L 則表示鐵庫偏低。6 至 12 歲兒童及 12 至 15 歲青少年的建議界限值分別為 15 和 20 μg/L[6]。在成年精英運動員中,因需求增加,界限值應提升至 50 μg/L[6]。檢測應於基線及每年進行兩次[6]。
訓練後,部分運動員的血紅素值低於正常範圍,這是耐力訓練引起的血漿量擴張所致[5]。生理上的年齡差異亦須考量[9]。青少年及青春期前運動員參與競賽的比例逐漸增加,其成長突增期、荷爾蒙變化、炎症反應及鐵狀態應列入考量[9]。訓練對成長、新陳代謝、酵素及血液指標的影響,取決於訓練負荷、類型及開始年齡,可能正面亦可能負面[9]。運動員及非運動員的血液指標會隨時間變化[10]。
血液指標的變化數據仍具爭議,取決於訓練類型及時長[11]。運動可能導致白血球以外的血液指標急性下降[12]。相反,有研究指出,巴西足球運動員的紅血球濃度、Hb 及 Hct 隨訓練時間增加,可能由於血漿量減少所致[3]。在足球運動員中,訓練一年期間,21% 的運動員 Hct 降低,4% 的 Hb 減少[13]。阿拉伯青少年運動員的血液指標(Hb、Hct、MCV、MCHC、血清鐵蛋白)每年的變化幅度通常不大,且年齡較大的運動員數值高於年齡較小者[9]。
高強度運動會引起血液細胞數量的持續變化,增加炎症指標[14],並增強血小板的黏附性及聚集性,促進凝血酶生成及凝血因子活性[15]。
缺鐵性貧血
鐵是氧結合蛋白的重要組成部分,對於運動表現至關重要[16]。缺鐵會影響氧氣的運輸與傳遞至組織,進而影響運動表現。鐵同時參與電子傳遞鏈中的能量代謝、DNA 合成、粒線體的氧化磷酸化及 ATP 的生成[17,18]。缺鐵性貧血影響高達 52% 的女性青少年運動員[6],以及 30–50% 從事耐力運動的運動員[19]。雖然此情況在女性運動員中最常見(15–35%),但 5–15% 的男性運動員群體也存在缺鐵問題[20]。在高訓練量的運動員中,運動誘發的缺鐵性貧血尤其普遍,例如長跑及中跑運動員、橄欖球選手等[19]。高負荷訓練通常伴隨重阻力訓練;而爆發型運動則使用較輕的負荷,以爆發性的方式進行[21]。
3.1 鐵的代謝
鐵代謝涉及十二指腸上皮細胞的吸收、紅系前體細胞的利用、肝細胞及組織巨噬細胞中的儲存與再利用(圖 1)[19]。鐵穩態的主要調節因子為鐵調素(Hepcidin),其合成在紅血球生成增加時受到抑制,以促進鐵釋放進入血液循環[17]。鐵調素由肝臟產生,並降解鐵轉運蛋白通道(Ferroportin),減少巨噬細胞回收鐵的能力,從而降低鐵的可用性[22]。然而,壓力與炎症會促進鐵調素的表達[17]。無論是阻力訓練還是耐力訓練,運動誘發的鐵調素及白介素-6(IL-6)變化模式相似[17]。基線的鐵蛋白水平及運動後 IL-6 的升高,是運動中鐵調素反應增加的關鍵因素[17]。
圖 1圖 1. 細胞鐵代謝概覽(改編自 Kowdley 等,2019;van Hasselt 等,2016 [23,24])。
圖例:
- DMT1:二價金屬轉運蛋白 1
- dcytb:三價還原酶十二指腸細胞色素 B
- Ho:血基質氧合酶
- HFE:鐵穩態調節因子
- HIF:缺氧誘導因子(HIF-1 和 HIF-2)
人體對鐵的吸收、流失及儲存進行嚴格調控[7,16]。運動中缺鐵的主要機制包括鐵需求增加、鐵流失加劇,以及由於鐵調素(Hepcidin)突增而阻斷鐵吸收[6]。
鐵是血基質(對血紅素和肌紅蛋白結構重要)及其他金屬蛋白(如鐵硫蛋白簇)合成的必需營養素。鐵硫蛋白簇在粒線體代謝中發揮關鍵作用,近期多種鐵硫蛋白合成相關的遺傳缺陷進一步證明了這一點。由於人體中鐵的多重用途,每日需要至少 20 毫克的鐵,其中僅 1–2 毫克來自腸道吸收(膳食鐵),其餘部分由再利用提供。未結合的鐵具有毒性,因此其穩態受到嚴密調控[23,24]。
飲食中的鐵主要有兩種形式:(1)血基質鐵,通過血基質氧合酶(Ho)釋放;(2)非血基質鐵,主要以三價鐵(Fe3+)形式存在。為促進難溶的三價鐵穿過腸細胞膜腔部,三價鐵(Fe3+)需由三價還原酶十二指腸細胞色素 B(dcytb)還原為二價鐵(Fe2+),然後透過二價金屬轉運蛋白 1(DMT1)進入腸細胞。鐵的主要回收路徑是透過血基質氧合酶(Ho)從紅血球衍生的血基質中移除鐵,這一過程發生於巨噬細胞及腸細胞中。
進入細胞後,鐵可能以結合鐵蛋白的形式儲存,也可能通過基底側腸細胞轉運蛋白(Ferroportin)輸出至血液循環。該蛋白負責腸細胞及巨噬細胞中鐵的輸出。此輸出過程涉及銅依賴性鐵氧化酶——釉藍蛋白(Hephaestin),它將二價鐵重新氧化為三價鐵,進而聯繫鐵與銅的吸收[23,24,25,26,27,28]。在循環系統中,三價鐵(Fe3+)與無鐵轉運蛋白(Apo-Transferrin)結合,形成含鐵轉運蛋白(Holo-Transferrin)。釉藍蛋白及銅藍蛋白(Ceruloplasmin)共同影響鐵轉運蛋白輸出二價鐵至血液的能力[29]。
鐵調素由肝細胞合成,是調節循環系統鐵水平的核心,控制鐵穿過腸細胞及巨噬細胞的轉運。缺氧是鐵調素代謝的重要調節因子,缺氧誘導因子(HIF-1 和 HIF-2)可抑制鐵調素活性,這些因子在適應低氧環境的反應中至關重要,能增加紅血球生成所需的鐵的生物可用性。鐵調素的主要刺激因子包括鐵、炎症/感染以及內質網或營養壓力[27]。鐵調素的合成受到多種蛋白調控,包括由 HFE 基因編碼的鐵穩態調節因子(HFE)、基質蛋白酶 2(Matriptase-2)、幼年血鐵蛋白(Hemojuvelin)及轉運蛋白受體 2(Transferrin Receptor 2)。
3.2 非遺傳性影響鐵代謝的因素
3.2.1 鐵的吸收
腸道對鐵的吸收主要受其生物可利用性影響。素食飲食會降低鐵吸收,而為提升表現而進行的慢性碳水化合物限制,可能也會調節鐵代謝[30]。飲食中的鐵與植酸、草酸、磷酸鹽、酚類等化合物形成複合物,尤其在植物性飲食中含量較高,因而降低了鐵的吸收。然而,某些分子如維生素 C,則可促進鐵吸收。膳食中鐵的生物可利用性似乎比鐵的攝取總量更為重要。為改善腸道對鐵的吸收,應減少降低吸收的因素(如植酸、酚類等),並增加提升鐵生物可利用性的因素(如維生素 C 等)[31,32]。發酵性碳水化合物的存在能刺激細菌生成丙酸及其他短鏈脂肪酸,進而提高礦物質吸收。研究顯示,強化鐵的穀物如麵粉及其衍生食品因含有高濃度植酸,會降低鐵吸收[33]。
3.2.2 運動中的鐵流失
運動中鐵的流失途徑包括流汗、血尿、胃腸道出血、炎症反應,以及血管內外的溶血[34,35]。流汗是運動中調節體溫的重要機制,可能導致每升汗液流失高達 2.5 微克的鐵[36,37]。血尿通常見於跑者,因反覆運動導致膀胱後壁與固定的膀胱頸接觸而造成膀胱挫傷[38]。其他導致跑者血尿的可能機制包括腎小球通透性增加、腎缺血、足底衝擊引發的溶血,或這些因素的結合[38,39]。一般而言,運動後的血尿與蛋白尿為暫時性,其原因包括缺氧、乳酸累積、氧化壓力及荷爾蒙變化[39]。蛋白尿及膽紅素尿可能是跑步期間急性腎損傷的潛在指標[40]。腎臟缺氧損傷及腎小球動脈收縮中,兒茶酚胺發揮作用,進而導致血尿[40]。
3.2.3 胃腸道疾病
運動員的胃腸道疾病亦可能影響消化道出血。適度運動對腸道炎症性疾病具有一定的保護作用,並可降低患者疾病活動程度[41]。運動還能減少相關壓力及焦慮[42]。然而,高強度運動可能導致腸道損傷、通透性增加、內毒素血症,以及胃腸道蠕動減慢和吸收不良[43]。運動誘發的胃腸道綜合症是由血流從胃腸道重新分配至工作肌肉,以及交感神經系統活動增加抑制腸神經系統功能引起[43]。該綜合症可能導致吸收不良、糞便血流失、腸道微生物群改變及系統性炎症反應[43]。透過在耐力運動中保持適度的水分攝取(避免過度補水)、根據個人體質攝取碳水化合物、運動前飲食調整(如非乳糜瀉患者採用無麩質飲食)、避免使用非甾體抗炎藥(NSAIDs)及使用多種抗氧化膳食補充劑,可緩解此問題[43,44]。乳糜瀉可能是無法解釋的鐵限制性貧血的成因,在女性運動員中更為常見[45,46]。此外,舞者或體操運動員中常見的過度活動症候群,包括 Ehlers–Danlos 綜合症等,也可能是潛在因素。
Ehlers–Danlos 綜合症是異質性結締組織疾病的統稱,其特徵為皮膚彈性過度、關節過度活動及軟結締組織脆弱性徵象。該疾病中最常見但並非唯一的血管型,可能導致血腫及其他血管併發症[47]。此外,直腸膨出、痔瘡併發出血及憩室穿孔的發生率亦有所增加[48]。其消化系統症狀可能與腸躁症重疊,也可能是由消化道結構異常(如內臟下垂、裂孔疝、巨結腸、憩室)或腸神經纖維受累導致的自律神經功能障礙引起[49]。
3.2.4 炎症反應
炎症可能參與運動性貧血的形成,因無論運動類型或強度如何,運動後白介素-6(IL-6)均會增加[50]。反覆的高強度運動在動物實驗中誘發多系統炎症反應[14]。IL-6 的升高可能觸發鐵調素增加,進而降低消化道鐵的吸收[50,51]。隨機對照試驗中,高訓練量運動員的運動後蛋白質、碳水化合物及維生素 D3 和 K2 的補充,未能顯著減弱鐵調素反應[51]。在炎症性貧血患者中,炎症是鐵調素的活化因子,而運動前的鐵狀態是鐵調素的主要調節因子[26,52]。缺氧是另一個鐵調素調節因子,缺氧誘導因子(HIF-1 和 HIF-2)可抑制鐵調素活性,增加鐵的生物可利用性以促進紅血球生成[26]。
3.2.5 其他鐵流失因素
其他流失因素包括女性運動員常見的月經過多或需服藥以維持表現的月經症狀[53]。月經週期對運動員表現的影響是近期一個重要研究領域[54,55]。口服避孕藥被用來控制月經週期並矯正經血過多[45,56]。然而,口服避孕藥會增加業餘女性運動員血液中的氧化壓力標誌物及 C 反應蛋白(CRP)水平[56]。在女性運動員中,生理參數不能僅依據年齡及體重從高水平運動員中推導而得[45]。舞蹈及長跑等運動中的稀發性月經及閉經發生率範圍為 3.4% 至 70%[45,57]。耐力運動員中閉經現象常見,並與較高的心血管訓練量相關[57]。
運動相關溶血性貧血
運動誘發的溶血是指紅血球在運動過程中破裂和破壞的現象[58]。跑步時的血管內溶血多因足底衝擊所致,尤其在涉及跑步或競走的運動中,由於地面衝擊力而發生[59,60]。跑者的紅血球壽命僅為非運動員的 40%[58]。此外,跑步者的膀胱挫傷也會導致血尿[38]。在耐力運動中,甚至非創傷性運動(如耐力游泳)中,溶血可能引發高膽紅素血症,原因包括肌肉收縮及腎臟血管收縮,導致紅血球在微血管中受壓[40,58,60]。蛋白尿與膽紅素尿是跑步期間急性腎損傷的潛在指標[40]。
溶血的原因包括地面衝擊引起的機械性損傷、反覆肌肉收縮、血管收縮,以及代謝紊亂(如高溫、脫水、缺氧、低張性、剪切應力、乳酸中毒、氧化損傷、蛋白水解作用、兒茶酚胺增加及溶血卵磷脂升高)[58]。運動適應還會導致脂質分布變化,包括游離膽固醇減少及溶血卵磷脂增加,進而提升滲透脆性[58,61]。其他因素如紅血球先天性異常、酸中毒及高溫也可能加劇溶血[58]。血紅素結合蛋白及其他清除蛋白可清除運動誘發溶血產生的小量游離血紅素[58]。尿液試紙檢測可識別易感急性腎損傷的運動員[62]。低強度連續騎行中溶血的減少表明,負重支撐的低強度活動對溶血具有保護作用[20]。
遺傳、運動與貧血
α-肌動蛋白-3(由 ACTN3 基因編碼)是光譜蛋白家族的一員,在肌肉收縮中具有結構、代謝及信號傳遞功能[63]。它是一種位於 Z 線的肌節支架蛋白,與 α-肌動蛋白-2 一起將肌動蛋白絲錨定於肌肉收縮裝置中[63]。ACTN3 基因的多態性(R577X, rs1815739)會影響代謝途徑及肌肉表型:XX 表型與骨骼肌及鐵代謝效率更高相關[34]。一項馬拉松研究顯示,大多數跑者在比賽後紅血球、血紅素(Hb)及血球比容(Hct)減少,同時血尿、肌紅蛋白、紅血球分布寬度、平均紅血球血紅素濃度、平均紅血球血紅素、膽紅素、促紅血球生成素及肌酐均增加[34]。賽後鐵、轉鐵蛋白水平及轉鐵蛋白飽和度立即升高,但於 15 天內逐漸下降[34]。持久運動後血液參數下降僅出現在 ACTN3 的 RR 和 RX 基因型中,而 XX 基因型則未見此情況[34]。577X 等位基因的純合子約佔全球人口的 20%,完全缺乏 α-肌動蛋白-3[63]。有趣的是,XX 表型在耐力運動員中更為普遍[63]。α-肌動蛋白-3 缺乏對力量型運動和短跑不利,但對耐力運動有益[63]。
HFE 基因的突變可能與受影響個體的體能提升有關。例如,80% 的成功法國運動員攜帶異型合子的 HFE 突變(C282Y、H63D 或 S65C),顯示鐵供應的增加可能促進了運動表現[65]。第 1 型(或典型遺傳性)血色素沉著症是一種常染色體隱性遺傳病,特徵為鐵在各器官中的緩慢但持續積累,通常在 40 至 50 歲時臨床表現明顯。北歐人口中高達 0.5% 為 HFE 基因的 C282Y 突變純合子,但只有 5% 的男性及不到 1% 的女性純合子最終發展為肝纖維化或肝硬化。H63D 和 C282Y 的復合異型合子則與鐵過載相關[23]。
某些藥物可能通過螯合、吸收不良或溶血作用影響鐵代謝,例如抗腫瘤藥物 Tiaprine[66]。此外,部分藥物會導致藥物誘發的免疫性溶血性貧血(DIIHA),例如抗生素(頭孢曲松)、利福平、高劑量青黴素治療(>10 天)及抗炎藥(雙氯芬酸、美芬酸)[67,68,69]。這類藥物大多僅在個別病例中引發 DIIHA,其發生率約為每百萬至兩百萬人中 1 例[69]。
地中海貧血特徵或鐮刀型細胞病影響全球數百萬人,在某些人群中更為常見。對於持續且原因不明的貧血運動員,應在首次評估或後續檢查時考慮這些疾病的可能性[70,71]。
其他考量
運動員鐵狀態的調控策略多樣,包括飲食(宏量營養素)、性激素、環境壓力(如高原訓練導致的缺氧)、運動類型等[20]。
高原環境可促進對缺氧的適應,透過增加紅血球數量提升耐力運動員在海平面下的表現[72,73]。適應高原環境需要額外的鐵攝入,特別是在冬季運動中[74,75]。促紅血球生成素誘導的紅血球或血紅素質量增加,是對缺氧的適應性反應[72]。除此之外,缺氧還會引發血管新生、葡萄糖運輸及糖酵解變化、pH 波動、乳酸耐受性增加、粒線體適應等其他反應[73]。高原訓練每日需增加 100–200 毫克的元素鐵攝入[72],且可在賽季中適當安排高原訓練以提升運動表現[76]。
運動已知會增加催乳激素(PRL)以及下丘腦–垂體–腎上腺軸的激素(如 ACTH 和生長激素 GH)的水平[77,78]。超過一半的運動員體內催乳激素水平偏高[79]。
近期針對健康男性的禁食與運動研究顯示,禁食會促進與鐵攝取相關基因的表達,並抑制與鐵儲存及輸出的基因表達[80]。間歇性禁食可能使足球運動員的血紅素、鐵蛋白及轉鐵蛋白水平均下降,雖統計上顯著,但平均值仍處於正常範圍內[81]。
值得注意的是,除了創傷性失血對貧血的影響外,一些接觸性運動(如拳擊)中發生的出血,可能導致腦組織內游離鐵積累,進而引發鐵介導的氧化壓力及神經退化。為減少神經元損失,研究正探討鐵螯合策略或增加膳食中維生素 E 的抗氧化作用,以減輕此類長期後果[71,82]。
高水平男女運動員可能受 RED-S 症候群(運動相對能量不足症候群)影響。該症候群由國際奧委會於 2014 年定義為由於熱量攝入不足及/或能量消耗過多所導致的健康與表現受損的綜合症[83]。RED-S 是基於女性運動員三聯症模型調整而來,該三聯症的特點是低能量可用性,對生殖及骨骼健康產生負面影響[78]。此狀況還可能影響血液參數、免疫力、新陳代謝、蛋白合成、成長與發育、內分泌、消化、心血管及心理功能[83]。RED-S 與過度訓練症候群(OTS)具有相似性,兩者均源於下丘腦–垂體軸,並受低碳水化合物及能量可用性影響[78]。低能量可用性可能部分誘發鐵缺乏,並加劇其影響[84]。青少年及年輕女性運動員中,鐵蛋白降低及缺鐵性貧血與低能量可用性的相關指標呈現正相關[84]。
結論
除了需求增加外,鐵吸收減少、鐵隔離與流失,以及其他運動員貧血的成因,均如表 1 所示。
表 1. 運動員貧血的成因從實務角度來看,評估鐵狀態——尤其是對於有缺鐵風險的運動員(如女性、青少年及從事飲食受限運動者)——在訓練季的開始及期間非常重要。血紅素及血清鐵蛋白是評估鐵狀態的簡便指標,其界限值應根據性別、年齡及運動類型進一步檢討。健康的腸道微生物群會影響鐵的狀態[8]。在血清鐵蛋白水平正常或偏高的情況下,長期補鐵並不建議。高原訓練期間則需補充鐵。有缺鐵風險的運動員應進行非負重、低強度的運動,以減少額外的溶血風險。
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