Growth failure is a major complication of Rett syndrome. Evidence supports the assumption that dietary energy insufficiency relative to energy needs is the primary cause of growth failure. Altered partitioning of energy balance, i.e., increased energy expenditure due to involuntary motor activity, is the suspected mechanism of the greater energy needs. Thus, the long-term OBJECTIVE of this proposal is to identify the mechanism by which the partitioning of energy balance in Rett girls is altered and to estimate its contribution to the overall energy requirement of these children. The SPECIFIC AIMS of this proposal are: 1) to determine whether total daily energy expenditure in girls with Rett syndrome is higher than that in healthy girls, 2) to determine whether the components of energy expenditure, measured by sleeping and quietly and actively awake metabolic rates, in girls with Rett syndrome are increased compared with those in healthy controls, 3) to determine whether dietary energy intakes in girls with Rett syndrome are sufficient to meet the energy demands of involuntary motor activity compared with those of healthy girls, 4) to determine whether dietary energy is an amount that exceeds measured total daily energy expenditure is sufficient to reverse a) growth failure, measured by height and weight velocities, skinfold thicknesses, and muscle circumferences, and b) the abnormalities in the components of energy expenditure measured by sleeping and quickly and actively awake metabolic rates, after nutritional intervention in girls with Rett syndrome, and 5) to determine whether the absolute amount of proportion of energy expended in repetitive motor activity increases after nutritional rehabilitation in girls with Rett syndrome. To accomplish these goals, the components of energy balance will be studied in two groups of SUBJECTS: 1) girls with Rett syndrome and 2) healthy controls. Rett girls also will be studied after one year of nutritional rehabilitation via enteral tube feedings. METHODS: Total daily energy expenditure and intakes will be determined by deuterium and 180 isotope dilution techniques and measured dietary intakes, respectively. Sleeping and quietly as well as actively awake metabolic rates will be determined concurrently by whole body indirect calorimetry. Gross motor movement, heart rate, and sleep patterns during calorimetry will be monitored respectively, by telemetry, radar detection, and polysomnography. Physical activity patterns will be determined from 24-hour activity records. Body composition and stature will be determined by whole body potassium counting, electrical conductivity, anthropometry, and stadiometry. SIGNIFICANCE: This information will enhance our understanding of the partitioning of dietary energy balance during conditions of altered physical activity and growth and will lead to a more rational approach to nutritional intervention and reversal of growth arrest in girls with Rett Syndrome.