@@SitaDechen 先生說得完全正確。成年人(N Engl J Med.1992;326(18):1173)或小孩(J Clin Endocrinol Metab.2014;59(4):1132)因喝(廠家錯誤配方)加強（fortified)維生素D及鈣牛奶造成維生素D急性中毒(平均血液濃度達到173 ng/ml, 約正常值上限的4.3倍)。其實意外或蓄意攝取巨量維生素非常普遍。就維生素D為例，因治療消化道吸收不良，特殊飲食設計，或以增強免疫為名，動轍四五萬單位吞食，算是很常見的事。部份原因是被"維生素不會過量中毒"的宣言誤導。The photosynthesized pre-Vit D is hydroxylated to D3 in liver 。This process can be reversed by photoconversion via a negative feedback regulatory mechanism to breakdown excessive 25(OH) Vitamin D to inactive metabolites。The regulatory feedbak limits the maximal photosynthesis of Vitamin D to just about double that of top normal range, (40 ng x2= approximately at 80 ng/ml)。That is why photosynthesis of Vitamin D is self-limiting and rarely cause lntoxication. External supplement of Vitamin D is another story. It is also true that hepatic hydroxylation of pre-vitamin D2 to D3 can be regulated and broken down to inactive metabolites, but megadose of vitamin ingestion can overwhelm this negative feed back mechanism, and makes it too ineffective to prevent further production of 25(OH)Vitamin D in liver, and active form of 1,25(OH)Vitamin D in kidney。This is why hypervitaminosis D or intoxication comes to being。
因為維生素D會促進腸胃道吸收磷酸鹽及鈣, 有可能造成血磷酸及鈣過量，造成中毒反應。各國衛生機構都有安全上限及推薦範圍(針對骨骼健康的目的)，大都畧同。如美國國家醫學院(私立科學智庫，National Institute of Medicine)建議一般人到70歲是每日400-600單位，七十五歲以上或孕婦可高達600-800單位。安全上限是4000單位(特殊病理狀况可能例外)。過量中毒(hypervitaminosis D)的徵狀完全是高血鈣(hypercalcemia)及高血磷(hyperphosphatecemia)的生理反應，如多尿(排鈣及磷)，結石，厭食，嘔心，衰弱乏力，昏迷，癲癇。一般通論，自然攝取的維生素，皆可由天生置入的安全閘閥節制，過量或剩餘水溶性維生素可直接(溢出)由腎管排出。脂溶性可存積於體內造成中毒現象。以日曬合成的維生素產量有限，肝臟置入安全管制機置，反轉日光合成(photoconversion)25(OH)Vitamin D 回到前沿的pre-vitamin(D2)或無生理效果的代謝產物。這個機置不足以應付巨額的補充藥丸，所以會造成維生素D中毒。凡事不能光看表面就湊下定論，read the fine prints。
