The history of energetic efficiency research: Where have we been and where are we going?

The development of energetic efficiency concepts followed a recognized pattern of knowledge evolution that began with novel insights leading to cre- ative new concepts. The second phase integrated con- cepts from other fields to create new applicable princi- ples. The third phase was the adoptive or dissemination phase, yielding solutions to industry or societal prob- lems. It is our contention that animal energetics has been in the adoptive phase for approximately 100 yr. Concepts developed during the early phase of nutri- tional energetics included the concept that life is a com- bustion process, the laws of thermodynamics, and the Law of Hess. Subsequent efforts established relation- ships between gas exchange and heat production and established the concept that food not only functions as fuel, but also as a body-building material. Much of the research effort for the last 100 yr has been to 1) devise bases for evaluation of foods that could be related to energy requirements and energy expenditures and 2) establish causes of energy expenditures. Much of the effort has focused on general and broadly applicable