On study entry, clinical and laboratory assessments will be carried out with the subject on their usual home diet. A complete history and physical exam will be performed. An echocardiogram must be obtained within the past year or it will be performed on the day of study entry. Following analysis of their diet and a negative pregnancy test, subjects will receive a modified diet containing triheptanoin (up to 2 grams/kg/24 hours), or continued on their previously established triheptanoin dose; not to exceed RDA for fat, substituted for their MCT oil and/or natural fat. This will be given by g-tube or orally divided into 2 or more doses. The dose will be adjusted on the basis of safety laboratory monitoring at specific time points and for adverse symptoms. The remainder of their diet will be modified to maintain appropriate caloric intake and balance. Total calories appropriate for RDA will be prescribed.