Long Term Efficacy and Safety of Orlistat for Type 1 Hyperlipoproteinemia

Inclusion criteria: * Age ≥ 8 years * Type I hyperlipoproteinemia confirmed by bi-allelic disease-causing variants in any one of the T1HLP genes (LPL, APOC2, APOA5, LMF1, GPIHBP1, or GCKR). * Subjects with digenic inheritance with heterozygous disease-causing variants in two different T1HLP genes. * Subjects who have a fasting TG greater than or equal to 750 mg/dL at the end of run-in period of 8 weeks will be eligible for randomization. * Subjects who do not have confirmed genetic mutation for Type 1 hyperlipoproteinemia but have a fasting TG greater than or equal to 750 mg/dL at the end of run-in period of 8 weeks will be eligible for randomization. * Well controlled diabetes mellitus with hemoglobin A1c \< 8% * Off orlistat for a period of 2 months * Patients on Olezarsen and Plozasiran (APOC3 antisense oligonucleotide) can enroll if on the drug for more than 3 months * Following methods of contraception for males and females of childbearing age will be employed Males: Being in this research may damage your sperm, which could cause harm to a child that you may father while on this study. If you take part in this study and are sexually active, you must agree to use a medically-acceptable form of birth control. Medically-acceptable forms of birth control include: (1) surgical sterilization (vasectomy), or (2) a condom used with a spermicide (a substance that kills sperm). Females: If you are part of this study while pregnant or breast-feeding an infant, it is possible that you may expose the unborn child or infant to risks. For that reason, pregnant and breast-feeding females cannot participate in the study. If you can become pregnant, a blood or urine pregnancy test will be done, and it must be negative before you participate in this study. If you take part in this study and you are sexually active, you and any person that you have sex with must use medically-acceptable birth control (contraceptives) during the study. Medically-acceptable birth control (contraceptives) includes: (1) surgical sterilization (such as hysterectomy or "tubes tied"), (2) approved hormonal contraceptives (such as birth control pills, patch or ring; Depo-Provera, Implanon), (3) barrier methods (such as condom or diaphragm) used with a spermicide (a substance that kills sperm), or (4) an intrauterine device (IUD). If you do become pregnant during this study, you must tell the researchers immediately. Exclusion Criteria: * Secondary hypertriglyceridemia due to diabetes, renal disease, alcoholism and drug therapy such as estrogens and estrogen analogues, steroids, HIV-1 protease inhibitors, retinoic acid derivatives, interferons, or lasparaginase. * On lomitapide or participating in clinical trial of volanesorsen. * On cyclosporine * Having serum TSH outside of the normal range if on levothyroxine supplementation. * Use of levothyroxine to suppress TSH in individuals with thyroid cancer. * Pregnant or lactating women * Significant liver disease (elevated transaminases \> 2 times upper limit of normal) * Alcohol abuse (\> 7 drinks or 84 g per week for women and \> 14 drinks or 168 g per week for men) * Severe anemia (hematocrit \< 24%) * Illicit drug use (cocaine, marijuana, LSD, etc.) * Major surgery in the past three months * Congestive heart failure * Serum creatinine greater than 2.5 mg/dL * Cancer within the past five years * Gastrointestinal surgery in the past * Current therapy with anti-coagulants, digoxin and anti-arrhythmics * Chronic malabsorption syndromes * Cholestasis * Acute illnesses such as acute pancreatitis in the last 8 weeks * Previous history of renal calcium oxalate stones