Selenium Supplementation in Moderate-Severely Active Ulcerative Colitis Patients Treated With Advanced Therapies

Inclusion Criteria: * Known or newly diagnosed moderate to severe UC (as defined by the modified Mayo score of 5-9; confirmed by clinical, endoscopic, and/or histopathological evidence prior to screening as per standard of care) who are either being started on or are being switched to a different FDA approved advanced therapy * The acceptable list of advanced therapies is (anti-TNF, anti-IL23, anti-integrin). For example, anti-tumor necrosis factor - infliximab, adalimumab, golimumab, certolizumab; anti-IL12/23 - ustekinumab, mirikizumab, risakizumab, guselkumab; anti-integrin - vedolizumab * Mayo endoscopic sub-score ≥2 (moderate to severe) * Mayo rectal bleeding sub-score ≥1 (moderate to severe) * Mayo stool frequency sub-score ≥2 (moderate to severe) * Age 18-85 and able to fully participate in all aspects of the trial Exclusion Criteria: * Pediatric patients defined by being younger than 18 years of age * Patients who are currently pregnant, expecting to participate in getting pregnant during the study period through natural or assisted techniques (in-vitro fertilization, intra-uterine insemination, intracytoplasmic sperm injection, embryo transfer, planned egg or sperm donor), or are currently lactating. * Women with childbearing potential will be required to use highly effective birth control if not surgically sterile or postmenopausal for ≥ 2 years for the duration of the active intervention period. Highly effective forms of birth control include those that alone or in combination result in a low failure rate (i.e., less than 1 percent per year) when used consistently and correctly. This would include combined pill and progestin-only pill, evra patch, nuvaring, depo-provera, paragard, mirena, implanon, female sterilization, male sterilization. * The criteria for being considered postmenopausal would be the following: twelve months of spontaneous amenorrhea or; six months of spontaneous amenorrhea with serum FSH levels 40 mIU/mL or; six weeks post-surgical bilateral oophorectomy with or without hysterectomy. * Male subjects are considered of reproductive potential unless surgically sterile (e.g., vasectomy) and should not participate in activities of reproductive potential other than heterosexual intercourse (e.g., they should not participate in in-vitro fertilization or other reproductive assistance techniques) during the active intervention period of 12 weeks. Male subjects of reproductive potential that have female partners of reproductive potential should commit to the use of recommended contraception in the partnership during the active intervention period of 12 weeks, and be informed of the recommendations for pregnancy screening during the intervention phase of the trial. Additionally, male subjects (regardless of reproductive potential) that have partners that are currently pregnant should commit to condom use during intercourse to prevent transmission of the drug product through semen during the active intervention period of 12 weeks. * If participants become pregnant during the intervention period, they will be withdrawn to avoid risks to the fetus. If participants become pregnant during the follow-up observational period, they will be permitted to remain in the study as no active intervention is being administered. Research related assessments and visits will be tailored to those recommended during pregnancy by the treating provider(s). * Medical conditions that may predispose to toxicity including a history of type 2 diabetes mellitus, hypothyroidism, acute or chronic kidney disease, history of kidney transplant, history of infertility. * Abnormal baseline labs for renal function, thyroid function, or hepatic function: Renal function panel including creatinine (results should fall within normal lab reference ranges below 1.3 mg/dL for males and 1.1 mg/dL for females). Thyroid function tests with thyroid stimulating hormone (TSH; results should fall within normal lab reference ranges of 0.5 to 5.0 mIU/L). Hepatic function panel (results should fall within normal lab reference ranges) including alanine aminotransaminase (below 55 U/L for males and 45 U/L for females), aspartate aminotransferase (below 40 U/L for males and 32 U/L for females), total bilirubin (below 1.2 mg/dL), direct bilirubin (below 0.3 mg/dL), alkaline phosphatase (below 120 IU/L for males and 104 IU/L for females). * Any patient taking blood thinners, cholesterol-lowering drugs, antioxidants, warfarin, or any other immune system-dependent medications that may interact with selenium. Specifically, they should not be taking any of the following: alendronate, baloxavir marboxil, cinoxacin, ciprofloxacin, deferiprone, delafloxacin, dimercaprol, eltrombopag, enoxacin, etidronate, gatifloxacinm gemifloxacin, grepafloxacin, ibandronate, levofloxacin, lomefloxacin, moxifloxacin, nalidixic acid, norfloxacin, ofloxacin, patiromer, penicillamine, risedronate. sodium polystyrene sulfonate, sparfloxacin. tiludronate, trientine, trovafloxacin, vadadustat * Allergies to components/compounds used to formulate selenium or placebo supplements. * Known or suspected diagnosis of Crohn's colitis, indeterminate colitis, ischemic colitis, radiation colitis, diverticular disease associated with colitis, microscopic colitis or infectious colitis (Clostridium difficile, cytomegalovirus (CMV), any other pathogenic illness felt by the investigator to be the source of colitis). * Concern for impending need for hospitalization or urgent colectomy as determined by the treating provider(s) and/or investigator performing screening/evaluation for enrollment. * Unwillingness or inability to be compliant with selenium supplementation, adjustments in diet if necessary, or complete study-related visits/biospecimen collection.