The amount of fast twitch fibers in the body has increased. However, it ain't actually increased but it is converted from slow twitch fibers to fast twitch fibers
closed as unclear what you're asking by Nathan Tuggy, Glorfindel, stangdon, Varun Nair, SovereignSun Apr 12 '17 at 11:39
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The italicized "it" can be included or omitted. The word "but" has to have parallel structure between the comparisons. With the italicized "it," the comparison is between two independent clauses (The two items being compared could stand alone as sentences). Without it, then two subordinate clauses are being compared. The sentence the way it is now needs a comma since there are two independent clauses:
The amount of fast twitch fibers in the body has increased. However, it ain't actually increased, but it is converted from slow twitch fibers to fast twitch fibers.
Both instances of "it" are pronouns without an antecedent. I would suggest writing the sentence like this:
The amount of fast twitch fibers in the body has increased. However, the total number of fibers hasn't actually increased, but the new fast twitch fibers are converted from slow twitch fibers.