Answer:
The main idea is that too much of everything is bad.
Step-by-step explanation:
The main idea is the central message that the author is trying to pass across.
The author begins the text with the main idea and every other text following it are arguments that contribute to explaining and defending the main idea.
The author begins by saying that no matter how good and beneficial something is, there is always a side effect of doing too much of it.
He goes ahead to buttress his point by proving that Vitamin C, which is a healthy supplement can have bad side effects like nausea and diarrhea when more than the approved daily recommendation is exceeded.