D-beta-hydroxybutyrate, the principal "ketone" body in starving man, displaces glucose as the predominating fuel for brain, decreasing the need for glucose synthesis in liver (and kidney) and accordingly spares its precursor, muscle-derived amino acids. Thus normal 70 kg. man survives 2-3 months of starvation instead of several weeks, and obese man many months to over a year. Without this metabolic adaptation, H. sapiens could not have evolved such a large brain. Recent studies have shown that D-beta-hydroxybutyrate, the principal "ketone", is not just a fuel, but a "superfuel" more efficiently producing ATP energy than glucose or fatty acid. In a perfused rat heart preparation, it increased contractility and decreased oxygen consumption. It has also protected neuronal cells in tissue culture against exposure to toxins associated with Alzheimer's or Parkinson's. In a rodent model it decreased the death of lung cells induced by hemorrhagic shock. Also, mice exposed to hypoxia survived longer. These and other data suggest a potential use of beta-hydroxybutyrate in a number of medical and non-medical conditions where oxygen supply or substrate utilization may be limited. Efforts are underway to prepare esters of beta-hydroxybutyrate which can be taken orally or parenterally to study its potential therapeutic applications.