A century of scientific investigations into the ways that air reacts with carbon, led to the discovery of the conservation of mass and the energy equivalence of foods. Atwater synthesized existing knowledge, most of it based on his own investigations, to define the caloric equivalence of protein, fat, and carbohydrates; 4, 8.9, 4 per gram. His work was a continuation of a series of government research investments. This year we recognize the Centennial of the first such investment by the United States. The USDA supported research which resulted in Atwater's discovery of energy equivalence continues to guide today's applied nutrition programs.