POWER OF ATTORNEY

     Know all by these present, that the undersigned hereby constitutes and
appoints each of John P. Butler and Nicole R. Hadas signing singly, as the
undersigneds true and lawful attorney-in-fact to:

     (1)     prepare, execute in the undersigneds name and on the undersigneds
             behalf, and submit to the U.S. Securities and Exchange Commission
             (the SEC) a Form ID, including amendments thereto, and any other
             documents necessary or appropriate to obtain codes and passwords
             enabling the undersigned to make electronic filings with the SEC of
             reports required by Section 16(a) of the Securities Exchange Act of
             1934, as amended (the Act) or any rule or regulation of the SEC;

     (2)     execute for and on behalf of the undersigned, in the undersigneds
             capacity as an officer, director and/or shareholder of Akebia
             Therapeutics, Inc. (the Company), Forms 3, 4 and 5 in accordance
             with Section 16(a) of the Act, and the rules thereunder;

     (3)     do and perform any and all acts for and on behalf of the
             undersigned that may be necessary or desirable to complete and
             execute any such Form 3, 4 or 5 and timely file such form with the
             SEC and any stock exchange or similar authority; and

     (4)     take any other action of any type whatsoever in connection with the
             foregoing that, in the opinion of such attorney-in-fact, may be of
             benefit to, in the best interest of, or legally required by, the
             undersigned, it being understood that the documents executed by
             such attorney-in-fact on behalf of the undersigned pursuant to this
             Power of Attorney shall be in such form and shall contain such
             terms and conditions as such attorney-in-fact may approve in such
             attorney-in-facts discretion.

     The undersigned hereby grants to each attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all the acts such attorney-in-fact shall
lawfully do or cause to be done by virtue of this Power of Attorney and the
rights and powers herein granted.  The undersigned acknowledges that each of
the foregoing attorneys-in-fact, in serving in such capacity at the request of
the undersigned, is not assuming any of the undersigneds responsibilities to
comply with Section 16 of the Act.

     This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4 and 5 with respect to the
undersigneds holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to each
of the foregoing attorneys-in-fact.

     IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of June 25, 2020.

                                         By:   /s/ David A. Spellman
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                                         Name: David A. Spellman
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