Nowadays it is widely argued that professional sports are damaging to people's health in that they involve grueling training sessions as an integral part of the occupation.

in 在这里用作介词，表示"在...方面"或"就...而言"。 that 引导从句，在这里与 in 搭配构成"in that"短语，表示"因为"或"在于"，用于说明原因或解释前文观点。 in that 作为一个固定搭配，相当于"because"或"since"，用于正式语体中表达因果关系。