26 C.F.R. § 1.651(b)-1
In computing its taxable income, a simple trust is allowed a deduction for the amount of income which is required under the terms of the trust instrument to be distributed currently to beneficiaries. If the amount of income required to be distributed currently exceeds the distributable net income, the deduction allowable to the trust is limited to the amount of the distributable net income. For this purpose the amount of income required to be distributed currently, or distributable net income, whichever is applicable, does not include items of trust income (adjusted for deductions allocable thereto) which are not included in the gross income of the trust. For determination of the character of the income required to be distributed currently, see § 1.652(b)-2 . Accordingly, for the purposes of determining the deduction allowable to the trust under section 651, distributable net income is computed without the modifications specified in paragraphs (5), (6), and (7) of section 643(a), relating to tax-exempt interest, foreign income, and excluded dividends. For example: Assume that the distributable net income of a trust as computed under section 643(a) amounts to $99,000 but includes nontaxable income of $9,000. Then distributable net income for the purpose of determining the deduction allowable under section 651 is $90,000 ($99,000 less $9,000 nontaxable income).
26 C.F.R. §1.651(b)-1