Brief Summary:

This randomized double-blind placebo-controlled crossover study seeks to evaluate the antidepressant effect of L-leucine, an essential amino acid, in patients with Major Depressive Disorder (MDD).

Detailed Description:

This is a pilot phase II clinical trial of L-leucine to test its efficacy in reducing depressive symptoms in MDD patients, especially those who exhibit increased inflammation. The determination of increased inflammation will be done post-hoc. During the screening visit, all study participants will provide demographic information and complete self-report assessments and clinician evaluations and examinations. Blood and urine tests will also be performed. All participants who meet eligibility criteria and are willing to proceed with the study will enter this 6-week study after being randomized to two-week course of either L-leucine or placebo. In this cross-over study, participants will be crossed over to the second treatment after 2 weeks of washout. The study period will last 42 days (6 weeks) from the baseline visit. Both L-leucine and placebo will be provided as an effervescent mixture powder. Investigators hypothesize that MDD subjects will have greater reduction in depression severity on leucine as compared to placebo.