Disclaimer: "Hard" here is not a difficulty indicator. Please do not edit the title.
This is a Word Web. Divide the 16 words into four groups of four such that they can collocate with one same word. Then find the answer that collocates with the four derived word. (-ed) means the derived word has an -ed suffix in those cases. Have fun!
(-ed) Coffee Copper Dietary Health Hormone Ion Iron Mental Mining Nutritional Sports Staffing Test Treatment Vitamin (-ed) Water
Dietaryis not in the same group as
Testis not in the same group as
This puzzle is a part of Ooohh, it all makes sense now! :D