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.

a. 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

b. 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

c. 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

d. 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

e. 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

Why is A better than B.

Also in the non underlined portion of the sentence 'it' should refer to Vitamin E.
B is a better choice as the subject is Vitamin E and 'it' refers to vitamin E - "swear by it as a dietary supplement", whereas in choice A 'it' is refereed to "intake of vitamin E" and this is the subject of the sentence in A.