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                                                                    EXHIBIT 10.4

                               FIRST AMENDMENT TO

                                 LOAN AGREEMENT

                             AND SECURITY AGREEMENT

                                 BY AND BETWEEN

                                FITNESSAGE, INC.

                                       AND

                    NATURAL ALTERNATIVES INTERNATIONAL, INC.

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                                 FIRST AMENDMENT
                                       TO
                      LOAN AGREEMENT AND SECURITY AGREEMENT

        This First Amendment to Loan Agreement and Security Agreement effective
this 6th day of December, 1999, amends that certain Loan Agreement and Security
Agreement dated November 11, 1999, by and between FitnessAge, Inc., a Nevada
corporation ("Corporation") and Natural Alternatives International, Inc. a
Delaware corporation ("Lender").

        The parties to the Loan Agreement wish to amend Sections 2, 3.2 and 6.8
of the above referenced Loan Agreement to read as follows:

SECTION 2.     SECURITY

        As security for the performance and payment of all obligations and
indebtedness of the Corporation to the Lender, the Corporation agrees that at
all times prior to the performance and payment of all such obligations and
indebtedness, the Lender shall have a perfected security interest, superior to
all other liens in all the rights, title, and interest of the Corporation in
Custom Nutrition, LLC, a Delaware limited liability company, including, without
limitation, rights held by the Corporation as a member, manager or creditor of
Custom Nutrition, LLC, as well as the allocable interest of the Corporation in
any amount received by Custom Nutrition, LLC from the Corporation as
contribution to Custom Nutrition LLC by the Corporation of proceeds received by
the Corporation from Bally Total Fitness Holding Corporation or its affiliates
(Bally") pursuant to Section 6.8 hereinbelow, whether such interests are now
owned or hereafter acquired and wherever the same may be located, and shall
include the proceeds, products, and accessories of any kind to any thereof. The
interests described above are referred to herein as the "Collateral". In this
connection, the Corporation has executed and delivered to the Lender
concurrently herewith the Security Agreement attached hereto and incorporated
herein by this reference ("Security Agreement").

        3.2 Subsequent Loan Funding. Upon the later of (i) November 23, 1999 or
(ii) satisfaction or waiver by the Lender of each of the conditions set forth in
Section 9 of this Agreement, the Lender shall deliver to the Corporation funds
in the amount of $350,000, less expenses of Lender including its legal fees
incurred in connection herewith. If the conditions set forth in Section 9 of
this Agreement are not satisfied, or waived by Lender (in its sole discretion)
by December 6, 1999, the amounts due under the Note(s) shall be all due and
payable and the Corporation shall be in default of this Agreement.

        6.8 Operation of Custom Nutrition, LLC. During the term of the Loan and
as additional consideration for Lender entering into the Loan, Corporation shall
contribute to Custom Nutrition LLC all revenue Corporation receives from Bally
in connection with the distribution offer or sale by Bally of nutritional
products, and Corporation will take all action necessary or appropriate in
connection with the operation of Custom Nutrition, LLC to cause 40% of such
amounts to be

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distributed and paid to Lender. In addition Corporation agrees to take all
action necessary or appropriate in connection with the operation of Custom
Nutrition, LLC to cause the remaining 60% of such amounts to be deposited in an
escrow account of Custom Nutrition, LLC for the benefit of Lender to be held as
Collateral for the performance by the Corporation of its obligations pursuant to
the terms of this Agreement, the Notes and the Security Agreement. Upon payment
in full of all amounts due under the Loan Corporation and Lender shall take all
action necessary or appropriate in connection with the operation of Custom
Nutrition, LLC to cause any remaining portion of funds in the escrow to be
distributed and paid to Corporation.

        The parties to the above referenced Security Agreement wish to amend
Section 1(i) to read as follows:

        1. Definitions of Terms Used Herein.

               (i) "Bally Proceeds" shall mean all of the gross receipts of the
Corporation from Bally Total Fitness Holding Corporation or its affiliates
("Bally") in connection with or resulting from the offer sale and distribution
by Bally of nutritional products, which amount Debtor has undertaken to cause to
be contributed and transferred to Custom Nutrition, LLC, and 60% of which funds
Debtor has undertaken to cause Custom Nutrition LLC to place in an escrow
account, the contents of which account are to be held as security for the
amounts to become due and owing on the Notes (as defined below) pursuant to this
Agreement and the Loan Agreement between Debtor and Secured Party dated November
11, 1999 (the "Loan Agreement"), a copy of which is incorporated herein by this
reference.

        All remaining terms of both the Loan Agreement and the Security
Agreement remain in full force and effect.

Corporation:                            Lender:

FitnessAge, Inc.                        Natural Alternatives International, Inc.
a Nevada corporation                    a Delaware corporation

By:  /s/ Michael L. Jeub                By:  /s/ Mark A. LeDoux
    ----------------------------            ------------------------------------
    Michael L. Jeub,                        Mark A. LeDoux,
    President                               Chief Executive Officer<PAGE>   1
                                                                    EXHIBIT 10.5

NEITHER THIS PROMISSORY NOTE NOR THE COMMON SHARES INTO WHICH IT IS CONVERTIBLE
HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 OR ANY STATE SECURITIES
LAW. THE COMPANY WILL NOT TRANSFER THIS PROMISSORY NOTE OR THE UNDERLYING COMMON
SHARES UNLESS: (i) THERE IS AN EFFECTIVE REGISTRATION COVERING SUCH PROMISSORY
NOTE OR SUCH COMMON SHARES, AS THE CASE MAY BE, UNDER THE SECURITIES ACT OF 1933
AND APPLICABLE STATE SECURITIES LAWS, OR (ii) IT FIRST RECEIVES A LETTER FROM AN
ATTORNEY, ACCEPTABLE TO THE BOARD OF DIRECTORS OR ITS AGENTS, STATING THAT IN
THE OPINION OF THE ATTORNEY THE PROPOSED TRANSFER IS EXEMPT FROM REGISTRATION
UNDER THE SECURITIES ACT OF 1933 AND UNDER ALL APPLICABLE STATE SECURITIES LAWS.

                                FITNESSAGE, INC.

                       CONVERTIBLE SECURED PROMISSORY NOTE

                                NOVEMBER 11, 1999

        FOR VALUE RECEIVED, FitnessAge, Inc., a Nevada corporation ("Company"),
promises to pay to the order of Natural Alternatives International, Inc., a
Delaware corporation, or any subsequent holder of this Convertible Secured
Promissory Note ("Note") hereinafter collectively referred to as the "Holder,"
payable at the Holder's offices at 1185 Linda Vista Drive, San Marcos,
California 92069, or such other place as may be designated in writing by notice
to the Company from the Holder, the principal sum of $400,000.00 receipt of
which is acknowledged, with interest thereon during the period that any portion
of the principal balance due under this Note remains unpaid and outstanding at
the rate of Twelve Percent (12%) per annum, compounded monthly. The principal
hereof, together with all accrued and unpaid interest, shall be paid in full on
or before November 10, 2000.

1.  WAIVER

        The Company and any and each other person or entity liable for the
payment or collection of this Note expressly waives demand and presentment for
payment, notice of nonpayment, protest, notice of protest, notice of dishonor,
bringing of suit and diligence in taking any action to collect amounts called
for under this Note and in the handling of property at any time existing as
security in connection with this Note, and shall be directly and primarily
liable for the payment of all sums owing and to be owing on this Note,
regardless of and without any notice, diligence, act or omission as or with
respect to the collection of any amount called for under this Note or in
connection with

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any right, lien, interest or property at any and all times had or existing as
security for any amount called for under this Note.

2.  COSTS OF COLLECTION

        The Company agrees to pay all reasonable costs, including reasonable
attorneys' fees, incurred by the Holder in collecting or enforcing payment of
this Note in accordance with its terms.

3.  NO SUBORDINATION

        (a) This Note shall, to the extent and in the manner hereinafter set
forth be subject in all cases to the provisions of any subordination agreement
between the holder(s) of other indebtedness of the Company and the Holder, and
otherwise shall be of first priority and shall not at any time be subordinated
or subject in right of payment to the prior payment of any other indebtedness of
the Company, whether now existing or hereafter created, with the sole exception
of the $1.6 million in loans due in December 1999.

        (b) No payment on account of principal, premium, if any, or interest on
any other indebtedness of the Company shall be made if, at the time of such
payment or immediately after giving effect thereto: (i) there shall exist a
default in the payment of principal, premium, if any, or interest with respect
to this Note, or (ii) there shall have occurred an event of default with respect
to any other indebtedness, or in the instrument under which the same is
outstanding, permitting the holders thereof to accelerate the maturity of such
other indebtedness, and such event of default shall not have been cured or
waived or shall not have ceased to exist.

        (c) Upon: (i) any acceleration of the principal amount due on this Note;
or (ii) any payment or distribution of assets of the Company of any kind or
character, whether in cash, property or securities, to creditors upon any
dissolution or winding up or total or partial liquidation or reorganization of
the Company, whether voluntary or involuntary or in bankruptcy, insolvency,
receivership or other proceedings, all principal, premium, if any, and interest
due or to become due upon this Note shall first be paid in full, or payment
thereof provided for in money or money's worth, before any other creditor of the
Company shall be entitled to retain any assets so paid or distributed in respect
to such other debt (for principal, premium, if any, or interest); and upon any
such dissolution or winding up or liquidation or reorganization or any payment
or distribution of assets of the Company of any kind or character, whether in
cash, property or securities, to which the holder of any other indebtedness
would be entitled, except for these provisions, shall be paid by the Company or
by any receiver, trustee in bankruptcy, liquidating trustee, agent or other
person making

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such payment or distribution, or by such other creditor if received by it,
directly to the Holder(s) of this Note or their representatives, to the extent
necessary to pay this Note in full.

4.  VOLUNTARY CONVERSION

        (a) The Holder shall have the right, at the Holder's option exercisable
at any time to convert this Note into such number of fully paid and
nonassessable shares of Common Stock (subject to adjustment as set forth below)
as shall be obtained by dividing the principal amount outstanding hereunder,
plus any accrued but unpaid interest, by the Conversion Price (as hereinafter
defined). The Conversion Price shall be equal to the lesser of (i) $1.00 per
share or (ii) $0.25 above the purchase price of a share of Common Stock sold by
the Company in the aggregate amount of $500,000 provided such sale is on or
before January 31, 2000, or if the event described in this section 4(a)(ii) does
not occur then the Conversion Price shall be $0.75 per share.

        (b) The Conversion Price shall be adjusted from time to time as follows:

               (i) In case the Company shall hereafter (A) subdivide its
        outstanding shares of Common Stock into a greater number of shares; (B)
        combine its outstanding shares of Common Stock into a smaller number of
        shares; or (C) issue by reclassification of its Common Stock any shares
        of capital stock of the Company, the Conversion Price in effect
        immediately prior to such action shall be adjusted so the Holder shall
        be entitled to receive the number of shares of Common Stock or other
        capital stock of the Company which it would have owned immediately
        following such action had this Note been converted immediately prior
        thereto. An adjustment made pursuant to this subsection (i) shall become
        retroactively effective as of immediately after the record date in the
        case of a dividend or distribution and shall become effective
        immediately after the effective date in the case of a subdivision,
        combination or reclassification. If, as a result of an adjustment made
        pursuant to this subsection (i), the Holder shall become entitled to
        receive shares of two or more classes or series of capital stock, the
        Board of Directors of the Company, in good faith (whose determination
        shall be conclusive and shall be described in a notice given to the
        Holder) shall determine for accounting purposes the allocation of the
        adjusted Conversion Price between or among shares of such classes of
        capital stock or shares of Common Stock and other capital stock.

               (ii) In case the Company shall hereafter issue options, rights or
        warrants to holders of its outstanding shares of Common Stock generally
        entitling them (for a period expiring within 45 days after the record
        date mentioned below) to subscribe for or purchase shares of Common
        Stock at a price per share less than the Conversion Price on the record

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        date mentioned below, the Conversion Price shall be adjusted so that the
        same shall equal the price determined by multiplying the Conversion
        Price in effect immediately prior to the date of issuance of such
        options, rights or warrants by a fraction of which the numerator shall
        be the number of shares of Common Stock outstanding on the date of
        issuance of such options, rights or warrants plus the number of shares
        which the aggregate offering price of the total number of shares of
        Common Stock offered pursuant to such options, rights or warrants would
        purchase at such current Conversion Price per share of Common Stock, and
        of which the denominator shall be the number of shares of Common Stock
        outstanding on the date of issuance of such options, rights or warrants,
        plus the number of additional shares of Common Stock offered for
        subscription or purchase pursuant to such options, rights or warrants.
        Such adjustment shall become retroactively effective as of immediately
        after the record date for the determination of stockholders entitled to
        receive such options, rights or warrants.

               (iii) No adjustment in the Conversion Price shall be required
        unless such adjustment would require an increase or decrease of at least
        1% of such price; provided, however, that any adjustments which by
        reason of this subsection (iii) are not required to be made shall be
        carried forward and taken into account in any subsequent adjustment. All
        calculations shall be made to the nearest cent or to the nearest 1/100th
        of a share, as the case may be.

               (iv) In the event that at any time as a result of an adjustment
        made pursuant to subsection (i) above, the Holder shall become entitled
        to receive any shares of the Company other than shares of Common Stock,
        thereafter the Conversion Price of such other shares so receivable upon
        conversion of this Note shall be subject to readjustment from time to
        time in a manner and on terms as nearly equivalent as practicable to the
        provision with respect to Common Stock contained in this Note.

               (v) No adjustment in the Conversion Price need be made under
        subsection (ii), if the Company issues or distributes to the Holder the
        shares, rights, options, or warrants referred to in such subsection that
        the Holder would have been entitled to receive had the Note been
        converted prior to the happening of such event or the record date with
        respect thereto.

        (c) If at any time the Company shall be recapitalized by reclassifying
its outstanding Common Stock into shares with a par value, if the Company or a
successor corporation shall consolidate or merge with or convey all or
substantially all of its or of any successor corporation's property and assets
to any other corporation or corporations, or if the Company or a successor
corporation shall distribute Common Stock or other assets pursuant to, without
limitation, any spin-off, split-off or other distribution of assets, the Holder
shall thereafter have the right to receive

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upon the basis and on the terms and conditions specified in this Note in lieu of
the Common Stock theretofore issuable upon the conversion of this Note, such
shares, securities or assets as may be issued or payable with respect to, or in
exchange for, the number of shares of Common Stock theretofore issuable upon the
conversion of this Note had such conversion taken place immediately prior to
such recapitalization, consolidation, merger, conveyance or distribution.

        (d) If at any time the Company shall dissolve, liquidate or wind up its
affairs, the Holder may thereafter receive upon conversion hereof in lieu of
each share of Common Stock that it would have been entitled to receive the same
kind and amount of any securities or assets as may be issuable, distributable or
payable upon any such dissolution, liquidation or winding up with respect to
each share of Common Stock.

        (e) In the event (i) the Company shall issue any shares of Common Stock,
options or rights to subscribe for shares of Common Stock, or any securities
convertible into or exchangeable for shares of Common Stock, (ii) the Company
shall take a record of the holders of its Common Stock for the purpose of
entitling them to receive a dividend payable otherwise than in cash or any other
distribution in respect to the Common Stock pursuant to, without limitation, any
spin-off, split-off or distribution of the Company's assets, or (iii) the
Company shall take a record of the holders of its Common Stock for the purpose
of entitling them to subscribe for or purchase any shares of any class or to
receive any other rights; or (iv) of any reclassification or other
reorganization or recapitalization of the shares which the Company is authorized
to issue, consolidation or merger of the Company with or into another
corporation, or conveyance of all of substantially all of the assets of the
Company; then, and in such event, the Company shall send to the Holder, at least
30 days prior thereto, a notice stating the date or expected date on which such
event is to take place. Such notice shall specify the date or expected date if
any is to be fixed, as of which holders of Common Stock of record shall be
entitled to exchange their Common Stock for securities or other property
deliverable upon such reclassification, reorganization, consolidation or merger,
as the case may be.

        (f) The Company will at all times reserve and keep available out of its
authorized shares, solely for issuance upon the conversion of this Note, such
number of shares of Common Stock as from time to time shall be issuable upon the
conversion of this Note.

        (g) In order to exercise the conversion privilege, the Holder shall
deliver this Note to the Company accompanied by a written request for conversion
executed by the Holder. Such conversion shall be deemed to have been effected
immediately prior to the close of business on the day on which such conversion
request and Note shall have been received by the Company, and at such time the
rights of the Holder to receive principal and interest shall cease, and the
Holder shall be treated for all purposes as the record holder of such Common
Stock at such time. As promptly as practicable after the receipt of such
conversion request and this Note, the Company shall cause to be issued and

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delivered to the Holder a certificate or certificates for the number of shares
of Common Stock issuable upon conversion of this Note. Such certificate or
certificates shall bear such legends required, in the opinion of counsel for the
Company, under applicable securities law.

        (h) It is specifically agreed that this Note may be converted in part
only by the Holder and upon such conversion in part, the Company will deliver to
the Holder another Note in this form for the proportionate part of this Note not
converted.

        (i) No fractional shares of Common Stock will be issued in connection
with any conversion under this Note, but in lieu of such fractional shares, the
Company shall make a cash refund therefor equal in amount to the product of the
applicable fraction multiplied by the Conversion Price then in effect.

        (j) The Holder and all holders of shares of Common Stock issued upon
conversion of this Note ("Conversion Shares") are entitled to certain rights to
registration of such Conversion Shares by the Company under an Investor Rights
Agreement. Reference is made to the Investor Rights Agreement for a more
complete statement of the registration rights of the Holder and the holders of
Conversion Shares. Copies of the Investor Rights Agreement are on file at the
office of the Company.

5.  OPTIONAL PREPAYMENT

        This Note is pre-payable at any time upon thirty (30) days written
notice, in whole or in part, by the Company without penalty provided, however,
that upon notice of prepayment by the Company, Natural Alternatives
International, Inc. or Holder shall have 30 days after the receipt of notice
herein, to exercise their conversion privileges as set forth herein, including
under the heading "Voluntary Conversion." Prepayments shall be applied first to
accrued but unpaid interest and then to principal.

6.  AMENDMENT

        This Note may not be amended in any respect except by a written
agreement executed by the person to be charged with the amendment.

7.  SECURITY

        This Note is secured by all of the rights title and interest of the
Company in Custom Nutrition, LLC, a Delaware limited liability company,
including without limitation any interest of the Company as a Manager, Member or
creditor of Custom Nutrition, LLC and the Company's allocable share of all gross
revenue received by Custom Nutrition, LLC from Bally Total Fitness

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Holding Corporation or its affiliates as set forth in the Loan Agreement between
the parties hereto, dated November 11, 1999, and shall include the proceeds,
products and accessories of any kind to any thereof, pursuant to and with the
priorities referenced in the Security Agreement executed by the Company of even
date herewith.

8.  APPLICABLE LAW

        This Note shall be governed by and construed and enforced in accordance
with the laws of the State of Delaware. It is the intention of the Company and
Holder to conform strictly to the usury laws now in force in any the state whose
laws may apply to this transaction. Accordingly, notwithstanding anything to the
contrary in this Note or in any instrument securing the same, it is agreed that
if a court of competent jurisdiction applies the laws of any state other than
Delaware in construing this Note or the enforcement thereof, then in that event
the aggregate of all charges that constitute interest under the laws of that
state that are contracted for, chargeable or receivable under this Note or any
other such instrument shall under no circumstances exceed the maximum amount of
interest permitted by laws of that state, and any excess, whether occasioned by
acceleration of maturity of this Note or otherwise, shall be deemed a mistake in
calculation and canceled automatically and, if theretofore paid, shall be either
refunded to the Company or credited on the principal amount of this Note, at the
election of the Holder.

DATED: November 11, 1999          FitnessAge, Inc.,
                                  a Nevada corporation

                                  By:  /s/ Michael L. Jeub
                                      ------------------------------------
                                      Michael L. Jeub, President

                                  By:  /s/ David G. Forster
                                      ------------------------------------
                                      David G. Forster, Chief Financial Officer

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