Indeed, the standards regulating a trustee’s exercise of discretion as to beneficiary distributions is generally seen as the exercise of fiduciary duty:
A trustee’s discretionary power with respect to trust benefits is to be distinguished from a power of appointment. The latter is not subject to fiduciary obligations and may be exercised arbitrarily within the scope of the power.
It is the fiduciary nature of the exercise of discretion that guarantees review and regulation by the Courts: “[N]o language, however strong, will entirely remove any power held in trust from the reach of a Court of Equity.”
 Restatement (Third) § 50 cmt. a.