Mo. Rev. Stat. § 456.4-419
2. The following provisions apply to any exercise of the authority granted by subsection 1 of this section:
(2) Unless the exercise of such power is limited by an ascertainable standard, no trustee of the first trust may exercise such authority to make a distribution from the first trust if:
(3) Except if participating in a change that is needed for a distribution to any such beneficiary under an ascertainable standard, no trustee shall exercise such authority to the extent that doing so would have the effect either of:
(5) The exercise of such authority may not reduce any income interest of any income beneficiary of any of the following trusts:
(L. 2011 S.B. 59)