PI decided not to pursue this study.

Traumatic brain injury (TBI) is a leading cause of death and disability in young people. It has been called the "signature wound" of the Iraq war because of its frequency among troops. TBI is associated with many chronic disabilities. Physical alterations include reduced exercise tolerance and profound muscle weakness, whereas psychological alterations include diminished sense of well-being, depression, fatigue and anxiety. Muscle and brain tissues rely upon circulating blood amino acids as precursors for metabolic functions. The investigators have shown that even one year after injury, plasma valine, an essential amino acid (EAA), was markedly reduced in patients with TBI compared to healthy controls. The investigators speculate that low plasma valine concentration contributes to chronic fatigue after TBI, since valine and tryptophan compete for the same transporter into the brain, and a low plasma valine concentration will allow more tryptophan to be transported. As a consequence, increased brain tryptophan will increase serotonin production, which may significantly contribute to the development of fatigue. Thus, the investigators will test if restoring valine concentration in persons with TBI may reduce fatigue perception and improve physical and neuropsychological function. Further, the investigators have previously shown that EAA intake has an anabolic effect in healthy young and elderly individuals. However, no data are currently available in persons recovering from TBI. Thus,the investigators will also test if EAA and/or valine can improve muscle mass in patients with TBI.

Inclusion Criteria:

1. Presence of traumatic brain injury
2. Age 18-65 years
3. Ability to sign informed consent
4. >3 months post-injury, <36 months post-injury
5. Ambulatory or require minimal to moderate assistance for safe ambulation

Exclusion Criteria:

1. Subjects with cardiac abnormalities considered exclusionary by the study physicians
2. Subjects with uncontrolled metabolic disease, including liver or renal disease
3. Subjects with cancer or recently (6 months) treated cancer other than basal cell carcinoma
4. Any subject currently on a weight-loss diet or a body mass index >34 kg/m2
5. Recent anabolic or corticosteroids use (within 3 months)
6. Dementia
7. Inability to tolerate an upright position
8. Postural reflexes prohibiting ambulation and inability to follow 2-step commands
9. Any other condition or event considered exclusionary by the PIs and covering physician