<PAGE>   1
                                                                  Exhibit 10.45

                                                                  EXECUTION COPY

                                SECOND AMENDMENT

            SECOND AMENDMENT, dated as of November 12, 1999 (this "Amendment"),
to the Amended and Restated Credit and Guarantee Agreement, dated as of November
15, 1996 (as amended, supplemented or otherwise modified from time to time, the
"Credit Agreement"), among Twinlab Corporation, a Delaware corporation
("Holdings"), Twin Laboratories Inc., a Utah corporation (the "Borrower"), the
several banks and other financial institutions parties to the Credit Agreement
(the "Lenders"), The Bank of New York, as co-agent for the Lenders thereunder
(in such capacity, the "Co Agent"), and The Chase Manhattan Bank, as
administrative agent for the Lenders thereunder (in such capacity, the
"Administrative Agent").

                              W I T N E S S E T H:

            WHEREAS, Holdings, the Borrower, the Lenders, the Co-Agent and the
Administrative Agent are parties to the Credit Agreement;

            WHEREAS, the Borrower has requested that the Administrative Agent,
with the consent of the Required Lenders, amend certain provisions of the Credit
Agreement; and

            WHEREAS, the Administrative Agent and the Lenders are willing to
agree to the requested amendments on the terms and conditions contained herein;

            NOW THEREFORE, in consideration of the premises and the mutual
covenants contained herein, the parties agree as follows:

            1. Definitions. Unless otherwise defined herein, terms defined in
the Credit Agreement shall have their defined meanings when used herein.

            2. Amendment to Subsection 4.23 of the Credit Agreement. Subsection
4.23 of the Credit Agreement is hereby amended by deleting the date "September
30, 1999" therein and substituting, in lieu thereof, "November 30, 1999".

            3. Amendment to Subsection 7.1 of the Credit Agreement. Subsection
7.1 of the Credit Agreement is hereby amended by deleting such subsection in its
entirety and substituting, in lieu thereof, the following:

            "7.1 Financial Condition Covenants.

            (a) Leverage Ratio. Permit the ratio of (i) Consolidated Total Debt
      at the last day of any fiscal quarter ending during any "Test Period" set
      forth below to (ii)

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                                                                               2


Consolidated EBITDA for the period of four consecutive fiscal quarters ending on
such date to be greater than the amount set forth opposite such period below:

<TABLE>
<CAPTION>
             Test Period Ending                  Leverage Ratio
             ------------------                  --------------
             <S>                                 <C>
             12/31/96                            4.25 to 1.00
             12/31/97                            4.00 to 1.00
             12/31/98                            3.75 to 1.00
             12/23/99 and thereafter             3.00 to 1.00
</TABLE>

            (b) Interest Coverage Ratio. Permit for any period of four
      consecutive fiscal quarters ending on the last day of any fiscal year the
      ratio of (i) Consolidated EBITDA for such period to (ii) Consolidated Cash
      Interest Expense for such period to be less than the amount set forth
      opposite such period below:

<TABLE>
<CAPTION>
             Test Period Ending                  Interest Coverage Ratio
             ------------------                  -----------------------
             <S>                                 <C>
             12/31/96                            1.75 to 1.00
             12/31/97                            2.20 to 1.00
             12/31/98                            2.60 to 1.00
             12/23/99 - 1/08/00                  2.90 to 1.00
             12/23/00- 1/08/01                   3.00 to 1.00
             12/23/01 and thereafter             3.00 to 1.00"
</TABLE>

            4. Amendments to Subsection 7.2 of the Credit Agreement. (a)
Subsection 7.2(c) of the Credit Agreement is hereby amended by deleting the
amount "$5,000,000" therein and substituting, in lieu thereof, "$10,000,000".

            (b)Subsection 7.2(1) of the Credit Agreement is hereby amended by
deleting such subsection in its entirety and substituting, in lieu thereof, the
following:

            "(1) so long as no Default or Event of Default is continuing at the
      time thereof, or would occur as a result thereof, the incurrence of
      Indebtedness (including without limitation Indebtedness in respect of
      Sale/Leaseback Transactions or Financing Lease Obligations) in an
      aggregate principal amount not to exceed $15,000,000 at any time
      outstanding to finance (or to reimburse the Borrower or its Subsidiaries
      for amounts expended to finance), on terms reasonably satisfactory to the
      Required Lenders, the expansion, renovation or relocation of their
      manufacturing, warehouse or office facilities (including, without
      limitation, any equipment related thereto), provided that, at the time of
      incurrence of such Indebtedness, the Borrower shall have provided
      projections to the Lenders showing that after giving effect thereto it
      will be in compliance with all covenants set forth in subsection 8.1 on a
      pro forma basis for each future year during the remaining term of this
      Agreement;"

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                                                                               3


            5. Amendment to Subsection 7.8 of the Credit Agreement. Subsection
      7.8 of the Credit Agreement is hereby amended by deleting such subsection
      in its entirety and substituting, in lieu thereof, the following:

            "7.8 Limitation on Capital Expenditures. Make or commit to make any
      Capital Expenditures other than Capital Expenditures in an aggregate
      amount not to exceed (i) $6,000,000 for the Borrower and its Subsidiaries
      during the fiscal year of the Borrower ending on or about December 31,
      1999; provided, that the Borrower and its Subsidiaries may make, or commit
      to make, additional Capital Expenditures in the amount of up to
      $12,500,000 in the aggregate, to be used to finance the expansion,
      renovation or relocation of their manufacturing, warehouse or office
      facilities (including, without limitation, any equipment related thereto);
      (ii) $20,000,000 for the Borrower and its Subsidiaries during the fiscal
      year of the Borrower ending on or about December 31, 2000; and (iii)
      $12,000,000 per annum for the Borrower and its Subsidiaries during any
      subsequent fiscal year of the Borrower; provided, that, commencing with
      the fiscal year of the Borrower ending on or about December 31, 2000, (a)
      Capital Expenditures not in excess of $2,000,000 permitted to be made
      during any fiscal year (and not carried over from a prior fiscal year) and
      not made during such fiscal year may be carried over and expended during
      the next succeeding fiscal year and (b) Capital Expenditures made during
      any fiscal year shall be first deemed made in respect of amounts carried
      over from the prior fiscal year and then deemed made in respects of
      amounts permitted for such fiscal year."

            6. Amendment to Subsection 7.10 of the Credit Agreement. Subsection
7.10 of the Credit Agreement is hereby amended by deleting such subsection in
its entirety and substituting, in lieu thereof, the following:

            "7.10 Limitation on Optional Payments and Modifications of Debt
      Instruments and other Obligations. (a) Make any optional payment or
      prepayment on or redemption, defeasance or purchase of any Senior
      Subordinated Notes except (i) in accordance with the provisions of
      subsection 3.1 (b)(i)(u), (ii) with the proceeds of Indebtedness permitted
      under subsections 7.2(e) (in connection with the incurrence of Refinancing
      Indebtedness under subsection 7.2(o)) and 7.2(o), (iii) as long as no
      Default or Event of Default has occurred and is continuing, in an
      aggregate cash amount not to exceed $15,000,000 unless the ratio of
      Consolidated Total Debt to Consolidated EBITDA (calculated on the terms
      set forth in subsection 7.1(a)) for the most recent completed four fiscal
      quarters prior to any such optional payment, prepayment, redemption,
      defeasance or purchase is less than or equal to 2.0 to 1.0 and then in an
      aggregate cash amount not to exceed $30,000,000, and (iv) so long as (x)
      no Default or Event of Default has occurred and is continuing, (y) the
      amount available for borrowing by the Borrower under the Revolving Credit
      Commitments is not less than $15,000,000 and (z) the ratio of Consolidated
      Total Debt to Consolidated EBITDA (calculated on the terms set forth in
      subsection 7.1(a)) for the most recent completed four fiscal quarters
      prior to any such optional payment, prepayment, redemption, defeasance or
      purchase is less than or equal to 1.5 to 1.0, in an aggregate cash amount
      not to exceed $40,000,000; (b) amend, modify or change, or

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                                                                               4


      consent or agree to any amendment, modification or change to any of the
      terms of the Senior Subordinated Note Indenture (other than any such
      amendment, modification or change which (i) would extend the maturity or
      reduce the amount of any payment of principal thereof or would reduce the
      rate or extend the date for payment of interest thereon or (ii) does not
      in any way adversely affect the interests of the Administrative Agent or
      the Lenders hereunder, thereunder or under the other Loan Documents or
      (iii) is of a technical or clarifying nature); (c) designate any
      Indebtedness having a principal amount in excess of $20,000,000 as "Senior
      Debt" under and as defined in the Senior Subordinated Note Indenture
      without the consent of the Administrative Agent; or (d) amend, modify or
      change, or consent or agree to any amendment, modification or change to
      the articles of incorporation (or such similar charter documents) of the
      Borrower or any Subsidiary in any material respect."

            7. Amendment to Subsection 7.12 of the Credit Agreement. Subsection
7.12 of the Credit Agreement is hereby amended by deleting such subsection in
its entirety and substituting, in lieu thereof, the following:

            "7.12 Limitation on Changes in Fiscal Year. Permit the fiscal year
      of the Borrower to end on a day not falling within the period from and
      including December 23 to and including January 8."

            8. Amendment to Subsection 12.16 of the Credit Agreement. Subsection
12.16(a) of the Credit Agreement is hereby amended by deleting such subsection
in its entirety and substituting, in lieu thereof, the following:

            "(a) The obligations arising under the Guarantee and Collateral
      Agreement shall be unconditional and binding on each of Holdings and the
      Borrower pursuant to the terms therein; provided that if either (i) on the
      last day of any period of four fiscal quarters the ratio of Consolidated
      Total Debt to consolidated EBITDA (as calculated according to the
      provisions of subsection 7.1(a)) is less than 1.5 to 1.0 or (ii) ratings
      of BBB- and Baa3 or above are attained by the Borrower or Holdings from
      Standard & Poor's Ratings Group and Moody's Investors Services,
      respectively, the Collateral pledged (but not the Guarantee contained) in
      such Guarantee and Collateral Agreement shall, upon the prior written
      consent of the Administrative Agent and the Required Lenders (in each
      case, which consent shall not be unreasonably withheld), be released; and
      provided further that if such performance criteria at any time thereafter
      are not maintained, any such Collateral that has been released shall again
      be pledged in favor of the Administrative Agent, for the ratable benefit
      of the Lenders, pursuant to the Guarantee and Collateral Agreement."

            9. Conditions to Effectiveness. This Amendment shall be effective on
the conditions that (a) the Administrative Agent shall have received
counterparts hereof, duly executed and delivered by Holdings and the Borrower
and consented to by the Required Lenders and the Grantors under the Guarantee
and Collateral Agreement dated as of May 7, 1996 (the "Guarantee and Collateral
Agreement") among Holdings, the Borrower, the Subsidiary Guarantors named
therein and the Administrative Agent; (b) the Administrative Agent shall have

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received, for the account of each Lender which executes and delivers this
Amendment, an amendment fee in the amount of $7,500 per Lender; and (c) no
Default or Event of Default shall have occurred and be continuing on the date
hereof after giving effect to this Amendment. The date on which all of the above
conditions are met shall be the date of effectiveness of this Amendment (the
"Amendment Effective Date").

            10. Representations and Warranties. In order to induce the
Administrative Agent and the Lenders to enter into this Amendment, Holdings and
the Borrower hereby represent and warrant to the Administrative Agent and the
Lenders that the representations and warranties of Holdings, the Borrower and
the other Loan Parties contained in the Loan Documents are true and correct in
all material respects on and as of the Amendment Effective Date (after giving
effect hereto) as if made on and as of the Amendment Effective Date (except
where such representations and warranties expressly relate to an earlier date in
which case such representations and warranties were true and correct in all
material respects as of such earlier date); provided that all references to the
"Credit Agreement" in any Loan Document shall be and are deemed to mean the
Credit Agreement as amended hereby.

            11. Notice of Effectiveness. The Administrative Agent shall promptly
advise the Lenders and the Borrower that this Amendment has become effective.

            12. Applicable Law and Jurisdiction. This Amendment has been
executed and delivered in New York, New York, and the rights and obligations of
the parties hereto shall be governed by, and shall be construed and enforced in
accordance with, the laws of the State of New York.

            13. Counterparts. This Amendment may be executed by the parties
hereto in any number of separate counterparts and all of said counterparts taken
together shall be deemed to constitute one and the same instrument.

            14. Successors and Assigns. This Amendment shall be binding upon and
inure to the benefit of Holdings and the Borrower and their respective
successors and assigns, and upon the Administrative Agent and the Lenders and
their respective successors and assigns. The execution and delivery of this
Amendment by any Lender prior to the Amendment Effective Date shall be binding
upon its successors and assigns and shall be effective as to any loans or
commitments assigned to it after such execution and delivery.

            15. Continuing Effect. Except as expressly amended hereby, the
Credit Agreement as amended by this Amendment shall continue to be and shall
remain in full force and effect in accordance with its terms. This Amendment
shall not constitute an amendment or waiver of any provision of the Credit
Agreement not expressly referred to herein and shall not be construed as an
amendment, waiver or consent to any action on the part of Holdings or the
Borrower that would require an amendment, waiver or consent of the
Administrative Agent or the Lenders except as expressly stated herein. Any
reference to the "Credit Agreement" in the Loan Documents or any related
documents shall be deemed to be a reference to the Credit Agreement as amended
by this Amendment.

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                                                                               6


             IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed and delivered by their respective duly authorized officers as of the
day and year first above written.

                                  TWINLAB CORPORATION

                                  By: /s/ Ross Blechman
                                     -------------------------------------------
                                     Name:  ROSS BLECHMAN
                                     Title: PRESIDENT


                                  TWIN LABORATORIES INC.

                                  By: /s/ Ross Blechman
                                     -------------------------------------------
                                     Name:  ROSS BLECHMAN
                                     Title: PRESIDENT


                                  THE CHASE MANHATTAN BANK as
                                  Administrative Agent, Issuing Bank, Swing Line
                                  Lender and as a Lender

                                  By:
                                     -------------------------------------------
                                     Name:
                                     Title:

Consented to:

THE BANK OF NEW YORK, as Co-Agent and as a Lender


By:
   ------------------------------
Name:
Title:

<PAGE>   7
                                                                               6


             IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed and delivered by their respective duly authorized officers as of the
day and year first above written.


                                  TWINLAB CORPORATION

                                  By:
                                     -------------------------------------------
                                     Name:
                                     Title:


                                  TWIN LABORATORIES INC.

                                  By:
                                     -------------------------------------------
                                     Name:
                                     Title:


                                  THE CHASE MANHATTAN BANK as
                                  Administrative Agent, Issuing Bank, Swing Line
                                  Lender and as a Lender

                                  By: /s/ Dawn Lee Lum
                                     -------------------------------------------
                                     Name:  DAWN LEE LUM
                                     Title: VICE PRESIDENT

Consented to:

THE BANK OF NEW YORK, as Co-Agent and as a Lender


By:
   ------------------------------
Name:
Title:

<PAGE>   8
                                                                               6


            IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed and delivered by their respective duly authorized officers as of the
day and year first above written.

                                  TWINLAB CORPORATION

                                  By:
                                     -------------------------------------------
                                     Name:
                                     Title:


                                  TWIN LABORATORIES INC.

                                  By:
                                     -------------------------------------------
                                     Name:
                                     Title:


                                  THE CHASE MANHATTAN BANK as
                                  Administrative Agent, Issuing Bank, Swing Line
                                  Lender and as a Lender

                                  By: /s/ Dawn Lee Lum
                                     -------------------------------------------
                                     Name:  DAWN LEE LUM
                                     Title: VICE PRESIDENT

Consented to:

THE BANK OF NEW YORK, as Co-Agent and as a Lender


By: /s/ Mary S. McGovern
   -----------------------------
  Name:
  Title:

<PAGE>   9
                                                                               7


BANKBOSTON, N.A. formerly known as
THE FIRST NATIONAL BANK OF BOSTON, as a Lender

By: /s/ Richard D. Hill, Jr.
   -----------------------------------
  Name:   RICHARD D. HILL, JR.
  Title:  Managing Director


DRESDNER BANK AG, NEW YORK BRANCH
AND GRAND CAYMAN BRANCH, as a Lender

By:
   -----------------------------------
  Name:
  Title:

By:
   -----------------------------------
  Name:
  Title:


U.S. BANK NATIONAL ASSOCIATION, as a Lender

By:
   -----------------------------------
  Name:
  Title:


EUROPEAN AMERICAN BANK, as a Lender

By:
   -----------------------------------
  Name:
  Title:

<PAGE>   10
                                                                               7


THE FIRST NATIONAL BANK OF BOSTON, as a Lender

By:
   -----------------------------------
  Name:
  Title:


DRESDNER BANK AG, NEW YORK BRANCH
AND GRAND CAYMAN BRANCH, as a Lender

By: /s/ Anthony C. Caraballo
   -----------------------------------
  Name:  ANTHONY C. CARABALLO
  Title: VICE PRESIDENT

By: /s/ John R. Morrison
   -----------------------------------
  Name:  JOHN R. MORRISON
  Title: VICE PRESIDENT


U.S. BANK NATIONAL ASSOCIATION, as a Lender

By:
   -----------------------------------
  Name:
  Title:


EUROPEAN AMERICAN BANK, as a Lender

By:
   -----------------------------------
  Name:
  Title:

<PAGE>   11
                                                                               7


THE FIRST NATIONAL BANK OF BOSTON, as a Lender

By:
   -----------------------------------
  Name:
  Title:


DRESDNER BANK AG, NEW YORK BRANCH
AND GRAND CAYMAN BRANCH, as a Lender

By:
   -----------------------------------
  Name:
  Title:

By:
   -----------------------------------
  Name:
  Title:


U.S. BANK NATIONAL ASSOCIATION, as a Lender

By:
   -----------------------------------
  Name:
  Title:


EUROPEAN AMERICAN BANK, as a Lender

By: /s/ Jennifer J. Raman
   -----------------------------------
  Name:  Jennifer J. Raman
  Title: Assistant Vice President

<PAGE>   12
                                                                               8


ERSTE BANK DER OESTERREICHISCHEN SPARKASSEN AG,
GRAND CAYMAN ISLAND BRANCH, as a Lender

By: /s/ Rima Terradista             /s/ John S. Runnion
   ------------------------
  Name: RIMA TERRADISTA             JOHN S. RUNNION
  Title: Vice President             FIRST VICE PRESIDENT


ZIONS FIRST NATIONAL BANK, as a Lender

By:
   -------------------------------------------
  Name:
  Title:


ADVANCED RESEARCH PRESS, INC., as a Grantor

By:
   -------------------------------------------
  Name:
  Title:


CHANGES INTERNATIONAL OF FORT WALTON BEACH, INC., as a Grantor

By:
   -------------------------------------------
  Name:
  Title:


HEALTH FACTORS INTERNATIONAL, INC., as a Grantor

By:
   -------------------------------------------
  Name:
  Title:

<PAGE>   13
                                                                               8


ERSTE BANK DER OESTER REICHISCHEN SPARKASSEN AG,
GRAND CAYMAN ISLAND BRANCH, as a Lender

By:
   -------------------------------------------
  Name:
  Title:


ZIONS FIRST NATIONAL BANK, as a Lender

By: /s/ P. Boyd Hales
   -------------------------------------------
  Name:  P. Boyd Hales
  Title: Vice President


ADVANCED RESEARCH PRESS, INC., as a Grantor

By:
   -------------------------------------------
  Name:
  Title:


CHANGES INTERNATIONAL OF FORT WALTON BEACH, INC., as a Grantor

By:
   -------------------------------------------
  Name:
  Title:


HEALTH FACTORS INTERNATIONAL, INC., as a Grantor

By:
   -------------------------------------------
  Name:
  Title:

<PAGE>   14
                                                                               8


ERSTE BANK DER OESTERREICHISCHEN SPARKASSEN AG,
GRAND CAYMAN ISLAND BRANCH, as a Lender

By:
   -------------------------------------------
  Name:
  Title:


ZIONS FIRST NATIONAL BANK, as a Lender

By:
   -------------------------------------------
  Name:
  Title:


ADVANCED RESEARCH PRESS, INC., as a Grantor

By: /s/ Ross Blechman
   -------------------------------------------
  Name:  ROSS BLECHMAN
  Title: PRESIDENT


CHANGES INTERNATIONAL OF FORT WALTON BEACH, INC., as a Grantor

By: /s/ Ross Blechman
   -------------------------------------------
  Name:  ROSS BLECHMAN
  Title: PRESIDENT


HEALTH FACTORS INTERNATIONAL, INC., as a Grantor

By: /s/ Ross Blechman
   -------------------------------------------
  Name:  ROSS BLECHMAN
  Title: PRESIDENT

<PAGE>   15
                                                                               9


BRONSON LABORATORIES INC., as a Grantor

By: /s/ Ross Blechman
   -------------------------------------------
  Name:  Ross Blechman
  Title: President


PR NUTRITION, INC., as a Grantor

By: /s/ Ross Blechman
   -------------------------------------------
  Name:  Ross Blechman
  Title: President


TWINLAB FSC INC., as a Grantor

By: /s/ Ross Blechman
   -------------------------------------------
  Name:  Ross Blechman
  Title: President