This paper is a bit old but I am posting it to keep it in focus for a discussion. Please notice that low fat vegan diets often tend to be "low energy"!
Mattson MP, Cutler RG, Camandola S. Energy intake and amyotrophic lateral sclerosis. Neuromolecular Med. 2007;9(1):17-20
AbstractRoy Walford, a physician and scientist who pioneered research on the anti-aging effects of caloric restriction and subjected himself to a low-energy diet, recently died from amyotrophic lateral sclerosis (ALS). Information from his case, epidemiological findings, and recent controlled studies in mouse models of ALS suggest that low-energy diets might render motor neurons vulnerable to degeneration, whereas high-energy diets are ameliorative. This contrasts with the effects of low-energy diets on various neuronal populations in the brain that respond adaptively, activating pathways that promote plasticity and resistance to disease. One reason that motor neurons might be selectively vulnerable to low-energy diets is that they are unable to engage neuroprotective responses to energetic stress response involving the protein chaperones, such as, heat-shock protein-70 [*].
*) In addition to that, ketone bodies (often very low on low-energy diets) would have been protective against neural damage caused by the stress response, see:
Ketone bodies protect neurons from stress hormone-induced damage
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This video http://www.youtube.com/watch?v=K-PzhyTlODc illustrates what kind of diet did Dr. Walford followed for about 10 years before his death (see around 7m into video).