Inclusion Criteria for Main Study:
* Males and females, from 0 (including newborns) to \< 18 years of age at time of randomization
* Confirmed diagnosis of LC-FAOD
* Have a caregiver(s) willing and able to assist in all applicable study requirements
* Have a legally authorized representative willing and able to provide written informed consent after the nature of the study has been explained and prior to any research-related procedures, and the study participant to be able to provide age-appropriate written assent
* Have ANY ONE of the following significant clinical manifestations of LC-FAOD:
* At least 2 in the prior year, or 3 in the prior 2 years, of severe major episodes of metabolic decompensation (eg, hypoglycemia, rhabdomyolysis, or exacerbation of cardiomyopathy, requiring ER/urgent care unit visits or hospitalizations)
* Recurrent symptomatic hypoglycemia (clinical symptoms of hypoglycemia) requiring intervention
* Susceptibility to hypoglycemia after short periods of fasting (less than 4 to 12 hours, depending on age)
* Evidence of functional cardiomyopathy requiring ongoing medical management or clinical manifestation of heart failure
* Sibling(s) with the same pathogenic variant who presented with MCEs
* Participant with pathogenic variants that are known or suspected to be associated with absent or severely reduced enzyme activity or with severe disease manifestations.
* From the period following informed consent to 5 days after the last dose of study drug, females of childbearing potential and fertile males must consent to use highly effective contraception. If female, agree not to become pregnant. If male, agree not to father a child or donate sperm
Inclusion Criteria for Liver Substudy:
* Enrollment in the Main Study of Study UX007-CL302
* Age \> 2 years
* Liver fat content ≥ 2% and \< 20% PDFF as assessed by 1 H-MRS
* Body mass index \< 95th percentile
* Able to comply with instructions (remaining still during scan) and requirements (eg, constraints on recent meals, no metallic items or implanted devices in the body, no recent contrast agents) for liver 1 H-MRS scan
Exclusion Criteria for Main Study:
* Enrolled in a clinical study involving concurrent use of an investigational drug product within 30 days before Screening
* Use of a prohibited medication (eg, valproate products or pancreatic lipase inhibitors) within 30 days before Screening, or unwilling to avoid a prohibited medication or other substance that may confound study objectives
* Treatment with triheptanoin within 60 days of Screening
* History of known hypersensitivity to triheptanoin or MCT or its excipients that, in the judgement of the Investigator, places the subject at increased risk for adverse effects
* Caregiver unwilling or unable to sign informed consent, or release of medical records, or follow study procedures
* Have any comorbid conditions, including unstable major organ-system disease(s), that in the opinion of the Investigator places the subject at increased risk of complications, interferes with study participation or compliance, or confounds study objectives or interpretation of results. History of metabolic decompensation(s) with metabolic acidosis, hyperammonemia, and/or liver enzyme elevations does not constitute an exclusion criterion unless in the opinion of the Investigator places the subject at increased risk of complications, interferes with study participation or compliance, or confounds study objectives or interpretation of results.
* Have a diagnosis of pancreatic insufficiency
* Pregnant, breastfeeding, or planning to become pregnant (self or partner) at any time during the study
Exclusion Criteria for Liver Substudy:
* Acute or chronic liver disease other than LC-FAOD that presents with increased risk of liver fat (eg, hepatic cirrhosis, viral toxic or drug hepatitis, diabetes mellitus) and/or metabolic syndrome
* Need for anesthesia/sedation to perform liver 1 H-MRS
