His diet mainly consisted of vegetables and fruits.

consisted 是动词 consist 的过去式。consist 由前缀 con-（表示"共同、一起"）和词根 sist（来自拉丁语 sistere，意为"站立"）组成，字面意思是"站在一起"，引申为"由...组成"的意思。consisted of 表示"由...构成/组成"这个状态发生在过去。