<PAGE>   1
                       TWIN LABORATORIES INC., as Issuer,


                                       and


                           THE GUARANTORS NAMED HEREIN


                                       and


                 STATE STREET BANK AND TRUST COMPANY, as Trustee


                          THIRD SUPPLEMENTAL INDENTURE


                         Dated as of September 15, 1998

                                       to


                                    INDENTURE


                  Dated As of May 7, 1996 as supplemented as of
                     December 1, 1997 and as of May 14, 1998




                   10-1/4% Senior Subordinated Notes due 2006
<PAGE>   2
            THIRD SUPPLEMENTAL INDENTURE, dated as of September 15, 1998 by and
among TWIN LABORATORIES INC., a Utah corporation (the "Company"), ADVANCED
RESEARCH PRESS, INC., a New York corporation ("Advanced"), TWINLAB CORPORATION,
a Delaware corporation formerly named TLG Laboratories Holding Corp. ("Holding
Company"), CHANGES INTERNATIONAL OF FORT WALTON BEACH, INC., a Florida
corporation ("Changes"), BRONSON LABORATORIES INC., a Delaware Corporation
("Bronson"), HEALTH FACTORS INTERNATIONAL, INC., a Delaware corporation ("Health
Factors"), and PR NUTRITION, INC., a California corporation ("PR Nutrition"), as
Guarantors, and State Street Bank and Trust Company, a Massachusetts bank and
trust company (successor to Fleet National Bank), as Trustee (the "Trustee").

            WHEREAS, the Company, Advanced, Holding Company and Fleet National
Bank, as Trustee executed an Indenture, dated as of May 7, 1996 (the
"Indenture"), in respect of $100,000,000 aggregate principal amount of the
Company's 10-1/4% Senior Subordinated Notes due 2006 (the "Securities");

            WHEREAS, State Street Bank and Trust Company is the successor to
Fleet National Bank, as Trustee under the Indenture;

            WHEREAS, on December 1, 1997 the Company, Advanced, Holding Company,
Changes and Trustee executed the First Supplemental Indenture dated as of
December 1, 1997 (the "First Supplemental Indenture");

            WHEREAS, on May 14, 1998, the Company, Advanced, Holding Company,
Changes, Bronson, Health Factors and Trustee executed the Second Supplemental
Indenture dated as of May 14, 1998 (the "Second Supplemental Indenture");

            WHEREAS, the term Indenture as hereinafter used in this Third
Supplemental Indenture shall mean the Indenture as amended by the First
Supplemental Indenture and the Second Supplemental Indenture;

            WHEREAS, Section 11.3 of the Indenture requires, under circumstances
specified in Section 11.3, that certain Subsidiaries of the Company execute and
deliver to the Trustee a supplemental indenture pursuant to which such
Subsidiaries of the Company shall be named as additional Subsidiary Guarantors;
and

            WHEREAS, all conditions and requirements necessary to make this
Third Supplemental Indenture a valid, binding and legal instrument in accordance
with its terms have been performed and fulfilled and the execution and delivery
hereof have been in all respects duly authorized.
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            NOW, THEREFORE, in consideration of the above premises, each party
agrees, for the benefit of each other party and for the equal and ratable
benefit of the Holders of the Securities, as follows:

                                   ARTICLE I

                                   AMENDMENTS

            Section 1. The Company, Advanced, Holding Company, Changes, Bronson,
Health Factors and PR Nutrition and the Trustee hereby amend the Indenture and
agree that PR Nutrition shall be a Subsidiary Guarantor, a Guarantor and a
Future Subsidiary Guarantor for all purposes under the Indenture and each of the
terms "Subsidiary Guarantor", "Guarantor" and "Future Subsidiary Guarantor"
shall for all purposes under the Indenture specifically include PR Nutrition.


                                   ARTICLE II

                            MISCELLANEOUS PROVISIONS

            Section 2.1. Terms Defined. For all purposes of this Third
Supplemental Indenture, except as otherwise defined or unless the context
otherwise requires, terms used in capitalized form in this Third Supplemental
Indenture and defined in the Indenture have the meanings specified in the
Indenture.

            Section 2.2. Indenture. Except as amended hereby, the Indenture and
the Securities are in all respects ratified and confirmed and all their terms
shall remain in full force and effect.

            Section 2.3. Governing Law. THIS THIRD SUPPLEMENTAL INDENTURE SHALL
BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW
YORK, AS APPLIED TO CONTRACTS MADE AND PERFORMED WITHIN THE STATE OF NEW YORK,
WITHOUT REGARD TO PRINCIPLES OF CONFLICT OF LAWS.

            Section 2.4. Successors and Assigns. All agreements of the Company,
Advanced, Holding Company, Changes, Bronson, Health Factors and PR Nutrition in
this Third Supplemental Indenture and the Securities shall bind their respective
successors and assigns.

            Section 2.5. Multiple Counterparts. This Third Supplemental
Indenture may be executed in any number of counterparts, each of which shall be
an original; but such counterparts shall together constitute but one and the
same instrument.


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            Section 2.6. Effectiveness. The provisions of this Third
Supplemental Indenture shall become effective immediately upon its execution and
delivery by the Trustee in accordance with the provisions of Article IX of the
Indenture.

            Section 2.7. Trustee Disclaimer. The Trustee accepts the amendment
of the Indenture effected by this Third Supplemental Indenture and agrees to
execute the trust created by the Indenture as hereby amended, but only upon the
terms and conditions set forth in the Indenture, including the terms and
provisions defining and limiting the liabilities and responsibilities of the
Trustee, which terms and provisions shall in like manner define and limit its
liabilities and responsibilities in the performance of the trust created by the
Indenture as hereby amended, and, without limiting the generality of the
foregoing, the Trustee shall not be responsible in any manner whatsoever for or
with respect to any of the recitals (other than the second recital) or
statements contained herein, all of which recitals or statements are made solely
by the Company, Advanced, Holding Company, Changes, Bronson, Health Factors and
PR Nutrition or for or with respect to (i) the validity, efficacy or sufficiency
of this Third Supplemental Indenture or any of the terms or provisions hereof,
(ii) the proper authorization hereof by the Company, Advanced, Holding Company,
Changes, Bronson, Health Factors and PR Nutrition by corporate action or
otherwise, (iii) the due execution hereof by the Company, Advanced, Holding
Company, Changes, Bronson, Health Factors and PR Nutrition, or (iv) the
consequences (direct or indirect and whether deliberate or inadvertent) of any
amendment herein provided for, and the Trustee makes no representation with
respect to any such matters.

            Section 2.8 Headings. The headings of the Articles and Sections of
this Third Supplemental Indenture have been inserted for convenience of
reference only, are not to be considered a part hereof and shall in no way
modify or restrict any of the terms or provisions hereof.


                     [REMAINDER OF PAGE INTENTIONALLY BLANK]


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                                   SIGNATURES

            IN WITNESS WHEREOF, the parties hereto have caused this Third
Supplemental Indenture to be duly executed, all as of the date first written
above.


TWIN LABORATORIES INC.,                  TWINLAB CORPORATION,
as Issuer

By:/s/Ross Blechman                        By:/s/Ross Blechman
    _________________________                 ___________________________
    Name:Ross Blechman                        Name:Ross Blechman
    Title:Chairman, CEO & President           Title:Chairman, CEO & President
                      
    

ADVANCED RESEARCH PRESS,                 CHANGES INTERNATIONAL OF
INC.                                     FORT WALTON BEACH, INC.

By:/s/Ross Blechman                        By:/s/Ross Blechman
   ___________________________               ___________________________
   Name:Ross Blechman                        Name:Ross Blechman
   Title:Executive Vice President            Title:Chairman


                     
BRONSON LABORATORIES, INC.               HEALTH FACTORS INTERNATIONAL,
                                         INC.

By:/s/Ross Blechman                     By:/s/Ross Blechman
   ___________________________              ___________________________
   Name:Ross Blechman                      Name:Ross Blechman
   Title:Chairman, CEO & President         Title:Chairman, CEO & President

                  

                                      -4-
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STATE STREET BANK AND TRUST              PR NUTRITION, INC.
COMPANY, AS TRUSTEE


By:/s/Elizabeth Hammer                          By:/s/Ross Blechman
   ___________________________                  ______________________
   Name:Elizabeth Hammer                        Name:Ross Blechman
   Title:Vice President                         Title:Chairman & CEO

                          
                                                 
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