A human eating the Human-Human Fruit would probably gain no major new powers, but they would still get the usual Devil Fruit downside: they would lose the ability to swim.

What most sources say

In One Piece discussions, the standard view is that the Human-Human Fruit is meant to give non-humans a human form, so a human user would not get much practical benefit. Oda has also been paraphrased as joking that a human might become “enlightened,” which is usually taken as a light, non-literal answer rather than a confirmed power boost.

Likely outcome

  • No meaningful transformation, because the user is already human.
  • No hybrid form, since that form would be redundant for a human user.
  • Possible “wiser” or “enlightened” effect in the joke/theory sense, but not a clearly established canon ability.
  • Loss of swimming ability, like any Devil Fruit user.

Why people debate it

Some fans argue it could make a person an “ideal” or “peak” human, but that idea is speculative and not confirmed by the series. Others point out that if the fruit behaves like a normal Zoan, it would simply be wasted on a human, with only the standard drawback remaining.

Practical takeaway

If a human ate it, the safest canon-based answer is: almost nothing useful happens, except they become unable to swim.

TL;DR: A human would not really gain a new power from the Human-Human Fruit; they’d mostly just lose the ability to swim.