Sentence Correction

Recent studies suggest that an intake of vitamin E in excess of that found naturally in a balanced diet may actually increase the risk of developing certain illnesses, despite the claims of thousands of people who swear by it as a dietary supplement.
1.Recent studies suggest that an intake of vitamin E in excess of that found naturally in a balanced diet may actually increase the risk of developing certain illnesses
2.Recent studies suggest that taking vitamin E in excess of that found naturally in a balanced diet actually increases the risk of developing certain illnesses
3.Certain illnesses may be at a higher risk of development if vitamin E is taken in excess of a balanced diet, according to recent studies
4.According to recent studies, the intake of vitamin E, if in excess of that found naturally in a balanced diet, may actually increase developing certain illnesses
4.Vitamin E, recent studies suggest, if in excess of that found naturally in a balanced diet, may actually increase the development risk of certain illnesses

OA:A

I have a doubt about the underlined portion of this sentence.
What does the pronoun it refer to here ?
Is it the intake of Vitamin C ?
If this is true, then is the phrase the intake of Vitamic C the subject of the first part of the sentence ?

Can it refer to vitamin C ?

Thanks in advance.