Leucine for Depression Study (L-DEP)

Eligibility Details: Inclusion Criteria: Participants will be required to be in good general health (as evaluated during the phone and in-person screening sessions) and aged 18 to 65 years. Exclusion Criteria: Following a structured telephone interview, prospective participants with the following conditions will not advance to the in-person screening session: presence of chronic mental or physical illness, history of allergies, autoimmune, liver, or other severe chronic diseases, current use of prescription medications such as steroids, NSAIDs, immune modifying drugs, opioid analgesics, and psychotropics, or previous history of fainting during blood draws. These inclusion and exclusion criteria will be examined in detail and confirmed in the in-person screening session by the study physician. Furthermore, any participant who has any of the following conditions will be ineligible for the study. Medical Conditions: (1) presence of co-morbid medical conditions not limited to but including maple syrup urine disease (a contraindication to leucine treatment), cardiovascular (e.g., history of acute coronary event, stroke) and neurological diseases (e.g., Parkinson's disease), as well as pain disorders; (2) presence of co-morbid inflammatory disorders such as rheumatoid arthritis or other autoimmune disorders; (3) presence of an uncontrolled medical condition that is deemed by the investigators to interfere with the proposed study procedures, or to put the study participant at undue risk; (4) presence of chronic infection, which may elevate proinflammatory cytokines; (5) presence of an acute infectious illness in the two weeks prior to the screening session. Psychiatric Disorders: (6) an Axis I psychiatric disorder as determined by the Research Version of the Structured Clinical Interview for DSM-5 (SCID-5-RV) including a current major depressive disorder (a prior history of depression is not an exclusion criterion, which will be considered for a pre-planned sensitivity analysis); (7) lifetime history of suicide attempt or inpatient psychiatric admission; (8) current suicidal ideation assessed by the Columbia Suicide Severity Rating Scale (C-SSRS); (9) current depressive symptoms assessed by the PHQ-9 (≥ 5)). Medication and Substance Use: (10) current and/or past regular use of hormone-containing medications including steroids; (11) current and/or past regular use of non-steroid anti-inflammatory drugs; (12) current and/or past regular use of immune modifying drugs that target specific immune responses such as TNF antagonists; (13) current and/or past regular use of analgesics such as opioids; (14) current and/or past regular use of psychotropic medications, including antidepressants, anxiolytics, antipsychotics, hypnotics, sedatives, and barbiturates; (15) current and/or past regular use of cardiovascular medications, including antihypertensive, antiarrhythmic, antianginal, and anticoagulant drugs; (16) current smoking or excessive caffeine use (>600 mg/day) because of the known effects on proinflammatory cytokine levels; (17) evidence of recreational drug use from urine test. Health Factors: (18) BMI > 35 because of the effects of obesity on proinflammatory cytokine activity and also on risk for sleep disordered breathing; or (19) any abnormalities on screening laboratory tests.