这个句子在语法上是不正确的。在英语中,定语从句的连接词应该根据先行词来判断。在这个句子中,先行词是"reason",是一个抽象名词,因此应该使用关系代词"that"或"for which"。修正后的句子应该是:"The reason that/for which I do not eat is because I have already taken it." 或者 "The reason why I do not eat is because I have already taken it." 这样才是符合英语的语法规则的。